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thesis entitled
ENVIRONMENTAL AND PHYSIOLOGICAL FACTORS INFLUENCING
PREMATURE CYATHIA ABSCISSION IN

EUPHORBIA PULCHERRIMA NILLD.
presented by

Steven H. Miller

has been accepted towards fulfillment
of the requirements for

M.S. deefimin Horticulture

Keyw/KQ/vééw

Major professor

 

 

Royal D. Heins

Dan: August 23, 1984

 

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ENVIRONMENTAL AND PHYSIOLOGICAL EACTORS INFLUENCING
PREMNTURE CYATHIA.ABSCISSION IN
EUPHORBIA.PULCHERRIMA ‘WILLD.

By

Steven H. Miller

A.THESIS

Subuitted to
Michigan State University
in partial fulfillmnt of the requirements
for the degree of

MASERCFSCIM

Department of Horticulture

1984

IiNIRflflENEKLIflE)PHEEKEOGHfluaFWCHI§3INEUflQKING
PREWHMRE<3ENEUKAEHHBSE11IN
EUREEBLAIKECHENHMA VflILD.

By

Steven H. Miller

low irradiance levels, high temperatures, and water stress all
promoted cyathia abscission in poinsettia. Low irradiance appeared to be
the primary environmental factor promoting abscission. Compared to
plants maintained under normal daylight (ND) at 16°C night temperature
(Nl') abscission was 62% greater on plants placed under 75% shade 4 weeks
after the start of short days. Increasing the NT from 16° to 21° while
simultaneously moving plants to shade only increased abscission an
additional 10%. Repeatedly water stressing plants to - 0.6 MPa usually
promoted abscission when plants were grown under ND, but not when placed
under shade. Leaf removal on plants with intact bracts promoted
abscission prior to anthesis, while bract removal on plants with intact
leaves decreased abscission. Carbohydrate levels increased in leaves on
plants with bracts removed but carbohydrates decreased when bracts were

present.

I want to thank all of the individuals who in many ways helped
me in this research project.

My special thanks goes to my major professor Dr. Royal Heins
for his help and encouragement throughout this project.

My appreciation also goes out to the members of my guidance
committee: Drs. William H. Carlson, Arthur C. Cameron, and Norman B.
Good for their helpful suggestions and assistance.

A special thank you goes out to Mrs. L. Kent for her assistance
in typing the tables.

I also want to thank Paul Ecke Jr. and Dr. David Hartley of Paul
Ecke Poinsettias, for their help and alpport on this project. Financial
support and plant material was provided by Paul E‘cke Poinsettias,
Ehcinitas, CA. Plant material was also donated by California - Florida

Plant Corporation, Frenont, CA.

ii

midance Carmittee:
'Ihe paper format was adopted for this thesis in accordance with
departmental and university requirements. Sections I and II are to be

submitted to the Journal of the American Society for Horticultural
Science; and Section III to Scientia Horticulturae.

iii

TABmoroamaM-s
Page
LISTOFTABLES................. vi
LISTCFFIGURES.................viii
LI'I'ERA’I'UREREVIEN

Anatomical Aspects of Abscission . . . . . . . . . . . 1
Description of Abscission Zone . . . . . . . . . . . 1
Separation Layer. . . . . . . . . . . . . . . . 3
Protective Layer. . . . . . . . . . . . . . . . 4

Hormonal Ragulation. . . . . . . . . . . . . .

Auxin . . . . . . . . . . . . . . . . . . .
Ethylene . . . . . . . . . . . . . . . . . .
Abscisic Acid . . . . . . . . . . . . . . . .
Gibberellins . . . . . . . . . . . . . . . . .
qmmins O I O O O O O O O O O O O O O O O

B imml Wht 1m 0 O O O O O O O O O O O O O
auums . O O. O O O O O O O O O O O O O O
mtms O O O O O O O O O O O O O O O O 0

Physiological Regulation . .

Raspiration . . . . . . . . . . . . . . . . .
carbohydrate Metabolism . . . . . . . . . . . . . 13
Carbohydrate Translocation . . . . . . . . . . . . 14
Environmental Regulation... . . . . . . . . . . . . l7
Irradiance . . . . . . . . . . . . . . . . . l7
TEmperature . . . . . . . . . . . . . . . . . 18
Water Stress . . . . . . . . . . . . . . . . . 19

Literature Cited . . . . . . . . . . . . . . . . 22
SECTION I
ENVIRONMENTAL EACTORS INFLUENCING PREMATURE CYATHIA.ABSCISSION
IN POINSETTIA.'ANNETTE HEGG DARK RED'

Abstract . . . . . . . . . . . . . . . . . . . 38
muwmt im 0 O O I O O O O O O r O 0 O O O O O O 3 9

Page

Materials and Methods 40

Results....IIII.IIIII.....44
DiMSSim O. O O O O O O O O O O O O O O O O O 47
Literature Citfi O O O O O O O O O O O O O O O O 72

SECTION'II
PHYSIOLOGICAL FACTORS INFLUENCING PREMATURE CYATHIA.ABSCISSION
IN POINSETTIA.'ANNETTE HEGG DARK RED'

msumt O O C O 74

qumtim O O O O O O O O O O O O O O O O O O 75
mter ials w mm 0 O O O O O O O O O O O O O O 77
ksults O O O O O O O O O O O O O O O O O O O 81
DiMSSim C O O O O O O O O O O O O O O O O O 83
Literature Cited . . . . . . . . . . . . . . . . 94
SECTION] III
VARIATION IN CULETVAR.SENSITIVTTYIRD
CYATHIA ABSCISSION
msumt O O O O O O O O O O O O O O O O O O O 96
Intrwmtim O O O O O O O O O O O O O O I O O O 97
Materials and Methods . . . . . . . . . . . . . . . 98
ESUJ-ts am DiMSSim O O O O O O O O O O O O O O 100
Literature Cited . .. . . . . . . . . . . . . . 115

Table

2.

3.

LISTG'TABLES

Page
SEIZTICN I

Mean percent cyathia abscission 50 to 80 days after the start

of short days influenced by irradiance and/or tenperature
modifications inposed 2, 4, 6, or 8 weeks after the start of
short days. The irradiance change consisted of reducirg

irradiance by 75%, the tenperature charge consisted of raisirg

the night tenperature from 16°C to 210C. Ebrpt. 1. . . . . 51

Mean percent cyathia abscission on poinsettia 'Annette Hegg

mrk fhd' 65 days after the start of short days (SD) initially
grown at 16°C night tenperature (NT) and normal daylight

(ND) and then noved to 21°C NT (tenperature charge) and/or

75% shade (irradiance change) at 2, 4, 6, or 8 weeks after the
startofSD.Expt.l............... 52

Mean final dry weight (g) and analysis of variance of

poinsettia 'Annette Hegg Dark Red' initially grown at 16°C

night tenperature (NT) and norml daylight (ND) and then

noved to 21° NI' (tenperature change) and/or 75% shade

(irradiance charge) at 2, 4, 6, or 8 weeks' after the start
ofshortdays................53

Mean percent cyathia abscission 70 days after the start of

short days for poinsettia 'Annette Hegg Dark Red' grown at

16°C or 18°C night tenperature (NI) under normal daylight

from 0 to 6 weeks of short days. Plants were then mved to a

130, 160, 180, or 210 NT greenhouse and/or placed under 75%

shade. Half of the plants at each tenperature and irradiance
treatments were water stressed to ca. -.6 MPa. Ehcpt. 2. . . 54

Analysis of variance 55 to 80 days after the start of short

days (SD) for poinsettia 'Annette Hegg Dark Red' grown at

16°C of 18°C night tenperature (NT) under normal daylight

fruit 0 to 6 weeks after the start of short days. Plants

were then noved to a 130, 160, 180, or 210 NT greenhouse

and/or placed under 75% shade. Half of the plants at each
tetrperature and irradiance treatment were water stressed to
ca.-.6MPa.Ehrpt.2..............55

vi

Table

1.-

2.

3.

SEDI‘ICN II
Page

Mean percent cyathia abscission and analysis of variance

55 to 85 days after the start of short days (SD) for

poinsettia 'Annette Hegg Dark Red' with cyathia covered

with an aluminum cover 5, 6, or 8 weeks after the start of
SD.Expt.l.................37

Mean percent cyathia abscission and analysis of variance

55 to 85 days after the start of short days for poinsettia
'Annette Hegg Dark Red' with bracts removed, leaves shaded.

or bracts rennved and leaves shaded at 5, 6, or 8 weeks

after the start of short days. Eatpt. 2. . . . . . . . 88

Percent of the plants reachirg anthesis, man percent ‘
cyathia abscission, and analysis of variance 50 to 85 days
after the start of short days (SD) for poinsettia 'Annette
Hegg Dark Red' grown at 18°C night tenperature with bracts:
leaves, or bracts and leaves renoved at S, 6, 7, or 8 weeks
after the start of short days. Expt. 3. . . . . . . . 89

Mean percent abscission and analysis of variance 56 to 70

days after the start of short days for poinsettia 'Annette

Hegg Dark Red' grown at 16°C or 21°C night tettperature

and spaced at 11, 33, or 65 plants m-Z. Expt. 4. . . . . 90

Mean glucose equivalents per 3 bract or leaf disks sampled

at 6, 8, or 9 weeks after the start of shortdays from

bracts and leaves of poinsettia 'Annette Hegg Dark Red'

grown at 16°C or 21°C night temperatures under normal

daylightor 75% shade. Expt. 5. . . . . . . . . . . 91

SELTICN III

than percent abscission on plants 70 days after the start

of short days and on plants 21 days after anthesis for five
poinsettia cultivars. Plants were grown at 16°C night

temperature until visible bud, then at 16°C, or 21°C until
anthesis. plants were evaluated in the greenhouse or in a
postharvest environment (180C, 10 umol S'lm'z) . . . . 1.05

umber of days to anthesis for five poinsettia cultivars

gram at 16°C until visible bud (ca. 28 days), then at a
16°C or 21°C night tenperature to anthesis. . . . . . 106

vii

LIST or FIGURES

Figure ‘ Page

1.

3.

SEIH‘ICN I

‘menty-five treatments resulting from novirg poinsetia

'Annette Hegg Dark Red' amorg four greenhouse environments:

(1) 16°C night tenperature (NT) and normal daylight (ND) ,

(2) 160 NT and 75% shade (SHD), (3) 210 NT and ND, and (4)

21° NT and SHD at 2, 4, 6, or 8 weeks after the start of
shortdays.£htpt.l..............57

than percent cyathia abscission 49 to 77 days after the

start of short days (SD) for poinsettia 'Annette Hegg Dark

Red' renainirg at 16°C night taiperature (NT) and normal

daylight (ND) or noved to 16° NT and 75% shade (SHD),

21° NT and ND, or 21° NT and SHD at 2 weeks after the start
ofSD.Expt.l................59

than percent cyathia abscission 49 to 77 days after the

start of short days (SD) for poinsettia 'Annette Hegg Dark

lhd' grown at 16°C night tenperature (NT) and nomal

daylight (no shift) or noved to 75% shade (SHD) at 2, 6,
or8weeksafterthestartofSD.Ehtpt.l . . . . . . 61

than percent cyathia abscission 49 to 84 days after the

start of short days (SD) for poinsettia 'Annette Hegg Dark

Ihd' grown at an initial night tenperature (NT) of 16°C or

18°C under norml daylight (ND) from 0 to 6 weeks after

the start of SD. Plants were then noved to a finishing NT

of 16° or 21° under ND or 75% shade (SHD) , or water stressed

(98) to ca. -.6 MPa from 6 WSD until anthesis. Expt. 2. . . 63

than percent cyathia abscission 49 to 84 days after the

start of short days (SD) for poinsettia 'Annette Hegg Dark

Red' initially grown at an 18° night tenperature under

mrmal daylight (ND), from 0 to 6 weeks after the start of

SD then finished at a 130 NT under m or 75% shade (SI-ID) ,

or water stressed (V6) to ca. -.6 MPa. Expt. 2. . . . . . 65

than percent cyathia abscission 49 to 84 days after the

start of short days (SD) for poinsettia 'Annette Hegg Dark

Red' grown at a constant 16°C night tenperature under

mrmal daylight (ND) until 6 weeks of SD, then noved to ND

or 75% shade (SI-ID) and /or water stressed (V5) to ca.
-.6MPa.Ebtpt.2...............67

viii

Figure

7.

l.

l.

4.

than percent cyathia abscission 49 to 77 days after the
start of short days (SD) for poinsettia 'Annette Hegg Dark
Red' rown in a controlled environment chamber (125 uml
s-lm- from 0800 m: to 1600 hr) at 16°C or 21°C night
tenperature (NT) or moved after 5 weeks of SD frat 16° or
21° NT chanber to the 21° or 16° chanber respectively. . .

than percent cyathia abscission 16 days before anthesis to
16 days after anthesis for poinsettia 'Annette Hegg Dark
Heft; grown in a controlled environment chanber (125 umol
s" ‘ fran 0800 hr to 1600 hr) at a constant 210 C night
tenperature (NT) or noved to a 16° NT after 3 or 5 weeks
ofshortdays. .............

SWITCH II

Percent photosynthetically active radiation transmission
into poinsettia 'Annette Hegg Dark Red' canopies at plant
spacings of 11, 33, 55, or 97 plants n-Z. Expt. 4. . . .

SHEEN III

than percent cyathia abscission 56 to 98 days after the
start of short days of five poinsettia cultivars finished
at 21°C night tenperature and evaluated in the greenhouse.

Expt.2..................

than percent cyathia abscission 56 to 98 days after the
start of short days of five poinsettia cultivars finished
at 21°C night tenperature and evaluated in a postharvest
enviromuent.Expt.2..............

than percent cyathia abscission 7 days before anthesis to
35 days after anthesis of five poinsettia cultivars finished
at 21°C night tenperature and evaluated in the greenhouse.
Wt. 20 O O O O O O O O O O O O O O O O 0

than percent cyathia abscission 7 days before anthesis to
35 days after anthesis of five poinsettia cultivars finished
at 210C night tenperature and evaluated in a postharvest
enviromuent.Expt.2.

Page

69

'71

93

108

1.10

1.1.2

1.1.4

LI'IERA’IUREREVIEW

LITERATURE REVIEW
Anatomical Aspects of Abscission

Description of abscission zone. At the base of most plant organs is a
region called the abscission zone where the changes that preceed
abscission occurs. The abscission zone forms during ontogony (55) in
leaves, floral parts, and fruits, but may be induced by several factors
(89, 188). The leaf abscission zone is usually very conspicuous in some
herbaceous species (95, 187) as is the abscission zone for flowers (89,
200) and perianth segments (66). -

Cells in the abscission zone do not differentiate to full
maturity as do cells of adjacent tissues, which defines the zone as a
region of arrested development (8). Cells and structures in tissues
distal to the abscission zone (i.e. fibers, laticifers, resin canals)
may be absent or much less developed in the abscission zone (8).

In the zone, cells are smaller, more densly filled with
cytoplasm (23, 103, 113, 155), with fewer vacuoles and less cell-wall
deposition (8). The limited cell-wall development does not mean the zone
cells are weak. Often the zone is initially as strong as adjacent
tissues but only weakens prior to seperation (46, 120, 174). However,
weakness is sometimes found in the abscission zone. A well defined grove
is frequently formed at the insertion of some absc ising organs which

increases the structural weakness of the zone, however the groves do not

necessarily have any relation to abscission (89). Additional weakness
can also occur in the zone by a swelling of the cell walls prior to
separation (23, 95, 139, 200).

In vascular tissues of the abscission zone, only tracheary
elements may be present (54). In the region where separation occurs,
there is a lack of sclarification in the cells of the pith with short,
broad tracheary elements (185). The concentration of vascular tissue is
in the center rather than the periphery which presumably weakens the
zone (55). In Phaseolus sp. vascular bundles fuse to form a ring (8).
This was also observed in the leaflet abscission zone of Citrus; (100).

In the abscission zone, cell division usually occurs prior to
cell separation (187). However, careful study of the patterns of
separation in the abscission zones of several species led to the
conclusion that cell division was not always an essential aspect of
abscission (8). Gawadi and Avery (61) reported that leaf abscission in
poinsettia occured without cell division. Baird and Webster (15) found
that cell division did not occur in the abscission zone of mature
fruits, nor was cell division found prior to flower abscission in
several solanaceous species (89). However, in cases where cell division
occurs prior to cell separation, the purpose of cell division is for
the development of the separation and primary protective layers (8).
Cell division in the abscission zone commonly occurs in the pith,
cortex, epidermis, and living cells of the vascular tissue (188).'The
result is the formation of several tiers of cells, the distal tier(s)

usually becomes the separation layer and the proximal tier(s) forms the

primary protective layer (8).

Sggration layer. The separation layer is almost always restricted to a
narrow band of cells, often only a single layer thick. These cells
secrete the enzymes necessary for the cell-wall hydrolysis and
separation. Separation layers-in leaves and lateral structures are
usually parallel to the surface of the supporting stem, although
significant variations can exist. In legumes separation always takes
place in a rather abrupt transition region between the pulvinus and the
leaf stalk (26). In Sambucus separation occurs followirg the positional
differentiation of special "target“ cells in the abscission zone that
grow and lose their adhesiveness in response to endogenous ethylene
(134).

Cell separation usually occurs after cell division and can
initiate in any tissue of the separation layer as considerable variation
exists among species (188). In ggl_eu_s leaf abscission (110) the usual
abscission pattern is for separation to begin in the abaxial side of the
petiole through the epidermal and cortical tissue, until the leaf is
supported only by the adaxial part of the cortex and xylem elements. In
leaflet abscission of Phaseolus, (186) separation may start internally
through the pith cells and proceed outward.

Separation of cells in the abscission zone occurs three ways:
mechanical breakage involvirg non-living cells of the vascular tissue
(185), dissolution of the middle lamella (95), and dissolution of the

middle lamella and primary wall (187). Flower abscission follows these

same general patterns (89, 200) but usually happens faster than leaf
abscission (156), and is frequently associated with meristematic
activity in the separation layer (66, 76, 86, 104, 105, 200).

Cell enlargement in the abscission zone often occurs prior to
and after separation (54, 188). In leaf abscission differential
enlargement in the distal and proximal sides of the abscission zone
creates shear forces across the cell walls which result in the leaf
beirg forced off the plant by cell expansion in the proximal side (97).
The differential enlargement in leaves has been associated with
abscission in a number of species (23, 61, 112, 153). However, cell
enlargement is normally not observed before abscission of floral parts
(89, 104, 105).

Protective layer(s). A primary protective layer develops proximal to the
separation layer to protect the new surface from injury arr] water loss
(54). In herbaceous species, the layer usually consists of little more
than the tissues exposed by separation with suberin and lignin deposited
between the outermost cells. There may be little or no cell division
(61). However, in most species, cell division occurs well before
abscission, resulting in several tiers of cells proximal to the
separation layer which form the primary protective layer (8). Next, a
per iderm develops beneath the protective layer, which usually becomes
continuous with the periderm of the stem (187). In some cases, the
outermost cells of the protective layer may collapse shortly after
exposure to the air, but still form what appears to be an effective

protective layer (8) .

One or more secondary protective layers can develop beneath the
primary protective layer in several woody genera (95). In Cornus, Tilia,
and Gleditsia a separation layer can develop distal to the secondary
protective layer by abscission of the primary protective layer (8).
Secondary protective layers across leaf scars usually become continuous
with the periderm of the stem (95). The development of protective layers
often involves expansion of parenchyma tissues which can crush and close
off the functional phloem (8).

Hornpnal lhgulation

The involvement of hormones in abscision was first postulated by
Laibach and Maschmann in 1933 (93). Since then extensive research has
been conducted to determine the role of plant hormones in the abscission
of plant organs. Several hypothesis have been developed which associates
each of the known plant hormones with abscission.

Auxin, Auxin (IAA) was the first of the major plant hormones to be
identified. Mai (107) showed that orchid pollen (known to be a source of
auxin) delayed the abscission of debladed petioles. La Rue (94)
confirmed Mai's work with 99.1.9.9é and Ricinus by applyirg synthetic 1AA.
Other workers also concluded that auxin plays only an inhibitory role in
abscission (110, 175). However, auxin inhibits abscission only when
applied distally to the abscission zone (6). Proximal applications of
auxin accelerate abscission (33, 84, 101, 102, 167, 180). As separation

approaches, the ratio of free-extractible auxin on the proximal verses

distal sides of the abscission zone decreases in bean (158) and cotton
(29). Abscission can be accelerated if distal IAA application is delayed
(33). High concentrations of naphthalene acetic acid (NAA) inhibited
abscission while a low concentration promoted abscission in bean
explants (145). An early application regardless of concentration,
inhibited while a late application promoted abscission of bean explants
(145) and in apple leaf petioles (17). This was the basis for a two-
stage hypothesis (146). In stage I auxins tend to retard or inhibit
abscission and in stage II auxins accelerate abscission (146). Later
research with explants identified the role of auxins and ethylene in
these two stages (83). Stage I is the auxin dependent stage when the
tissue is relatively insensitive to ethylene and a period durirg which
normal auxin inhibition of abscission gradually diminishes and is lost
(8). The length of stage I was found to decrease with leaf age (33).
During stage II, auxin and ethylene accelerate abscission (33, 200).
The active biochemical and structural charges of separation take place
in stage II. Senescirg tissue produces large amounts of ethylene which
generally promotes absc iss ion (184) .

The auxin gradient concept was later developed to summarize the
aspects of auxin physiology and abscission in bean leaf explants (8).
The concept involves two main fluxes of auxin in bean explants: a flow
of auxin from the leaf, and a flow of auxin down the stem. This creates
an auxin concentration gradient across the abscission zone and appears
to be the primary controllirg factor in leaf abscission (8, 9, 84). This

would explain why a high concentration of auxin distal to the abscission

zone tends to delay leaf abscission and a high concentration proximal to
the zone promotes abscission. As the ratio of proximal to distal auxin
concentration increases (as auxin moves across the abscission zone) the
process of abscisssion proceeds (8).
Ethylene. Early work on ethylene grew out of observations that
illuminating gas induced leaf abscission in greenhouse crops.(205).
Shull (159) first observed that ethylene induced leaf abscission in pot
roses. Addicott (6) states that ethylene is not always required for
abscission to develop (24) although it is involved with abscission in
several species.

Much of the recent information on ethylene-induced abscission
has come from working with plant explants. Jackson and Osborne (82)
reported that very little ethylene was released from explant tissues
distal to the abscission zone until immediately after abscission. Bean
explant sensitivity to ethylene depends on the explants stage of
sensitivity (83). During stage I, explants are relatively insensitive to
applied ethylene, however, during stage II, explants begin to abscise as
ethylene levels increase (8, 184). Evidence for stage I explant
insensitivity to ethylene compared to stage II show ethylene-mediated
increases in protein synthesis occured in stage II explants, but not in
stage I explants (3). Ethylene treatment during stage I increased the
effectiveness of a treacment during stage II by reducing break strength
in the explant (2). Exogenous ethylene applied in stage I explants
inhibits polar transport of auxins, increases IAA.oxidase activity, and

decreases the level of diffusible auxin (71, 118, 132, 173). During

stage II, applied ethylene enhances pectinase activity (119), increases
cellulase activity (78), and decreases break strength (37).

Osborne (131) has suggested that the initial stimulus for

abscission is a hormonal imbalance due to environmental changes and
endogenous competition: this hormonal imbalance would mediate localized
senescence of cells in the abscission zone, leading to an increase in
ethylene synthesis which would be the signal for abscission. Ethylene
only affects cells in the abscission zone by promoting synthesis of
hydrolytic enzymes or enzymes involved in growth of cells in the
proximal tissue (131).
Abscisic acid. Okhuma gt a_l_ (129) first demonstrated that abscisic
acidUEno wastanannrdssfluiinxmflator hainmfidlyaflxxfising<xnxcn both;
The activity of ABA was equal to or slightly greater than ethylene (22).
Application of ABA has promoted abscission in leaves (35, 50, 52, 136,
147, 162), branches (152, 194), flowers (7, 111, 137, 178) and fruit
(11, 36, 51). Application of ABA accelerated leaf abscission in explants
(1, 14, 23, 37). Many studies have established or strongly indicated a
hormonal role of ABA in the promotion of abscission (50, 163, 176, 177)
as high levels of endogenous ABA have been found in abscising organs
(47).

It is now however beleived that ABA has little or no direct
effect on leaf abscission of explants or to intact plants (111). The
rise in ABA could be the result of waterstress, which is known to cause
a sharp increase in ABA (86, 195, 201). Applied ABA was only effective

in inducing leaf abscission when a high dosage was applied (111).

However, it is beleived that ABA stimulates leaf abscission due to an
indirect affect of unphysiologically high concentrations of ABA
stimulating ethylene production (111). Abeles Eli _a_l_._ (2) reported that
ABA plus saturatirg levels of ethylene was more promotive of absc ission
than ethylene alone. The abscission promoting effects of ABA occur
during stage II (37) by reducing IAA transport in explants (32) but
there is no evidence that ABA effects abscission if applied durirg stage
I (37, 72). Craker and Abeles (37) observed that ABA increased cellulase
activity in the abscission zone.

Gibberellins. Applications of gibberellins have caused little or no
abscission from intact plants (165, 192) even though it has been tested
on hundreds of plants (8). However, gibberellins have been shown to
accelerate leaf abscission in explants at high concentrations (19, 29,
34), but to retard abscission at low concentrations (103). In most cases
applied gibberellins stimulated fruit development and reduced
abscission of yourg fruits (39, 138, 182).

Applied gibberellins inhibit or delay abscission because the
gibberellins intensify the ability of an organ to function as a nutrient
sink (8). Gibberellins can influence abscission by increasing the
synthesis of IAA in plant tissue promoting abscission (121). However,
this effect may be responsible for the abscission retardation at low
dosages of gibberellin (8). It is likely that gibberellin-induced
increase in auxin intensifies the activity of nutrient sinks (8).
gatok ining. The effect of cytokinins affect on abscission is not well

documented in the literature, although two modes of action have been

10

noted (8). First is the ability of cytokinins to enhance the sink
strength of organs to which it is applied. Cytokinins stimulated growth,
induced parthenocarpic development and decreased fruit abscission (38,
98). Kinetin induced an incease in soluble reducing sugars, soluble
proteins, dryweight, and chlorophyll (202, 203). Cytokinins also delayed
senescence in portions of leaves to which it was applied (96, 120).
Secondly, cytokinins can inhibit abscission (64). Cytokinin
applied directly to the abscission zone inhibited bean explant
abscission (133). However, cytokinin applied to areas distant from the

abscission zone accelerated abscission.

Biochemical Ihgulation

The main biochemical activities related directly to the separation
process are the production, secretion, and action of enzymes which
attack and degrade the middle lamella and cell walls (8). A second
biochemical activity which involves the deposition of cell wall
materials is occurirg simultaneously with the degrading activity (8).
Molisch (114) was the first to suggest the possibility that separation
was the result of enzymatic activity. However, the knowledge of enzymes
accrued slowly and it was not until the last two decades that the
present concepts developed. An important advent was the discovery that
oxygen is an absolute requirement for physiological separation and that
the recognized enzymes of oxidative respiration were also necessary

(28) .

The discovery of how DNA controls the synthesis of an enzyme led
plant physiologists to observe that charges in RNA and protein synthesis
preceded abscission. The activity of DNA and RNA are significant in the
control of abscission, since these changes lead to changes in hormone
levels exported to plant organs. The hormonal changes appear to be the
primary signal for abscisison from the organ to the abscission zone (8).
Principle enzymes involved in degredation of cell walls, particularly
the cellulases and pectinases have received the most attention in the
literature.

Cellulases. Increased cellulase activity has been correlated with
abcission in bean explants (78), $29.9: (65, 140), and other species
(132). Lewis and Varner (99) concluded that increased cellulase activity
in bean leaflet abscission zones was due to de novo synthesis and
detected two forms of cellulase in the abscission zone. Subsequent
investigations disclosed the existence of several isozymes of cellulase
in the abscission zone (63, 99, 141). However, only one of the isozymes
appears closely correlated with abscission (155). In contrast, other
researchers have found that cellulase activity was not associated with

abscission (73). From electron micrograph studies, cell separation was
acheived almost entirely by dissolution of the middle lamella, with
little cellulose breakdown apparent at the time of separation (8).
Cellulase appears to function in the development of the protective

layers, however its involvement in cell separation remains in question

(8) .

12

Pectinases. Molisch (114) was the first to point out that an enzyme

 

might be involved in the dissolution of the middle lamella. The
dissolution of the middle lamella and/or cell walls have been associated
with the abscission of floral parts (89, 104 155, 200).

A reduction in Pectin methlyesterase (PME) activity has been
correlated with increased abscission in M (113), and in flower
abscission of Nicotiana (197). These results suggest that abscission is
facilitated in some way by a reduced ability of PME to esterify pectic
substances (8). In studies with two cultivars of Nicotiana, Yager (198)
found that the cultivar that retained flowers larger had higher levels
of PME. However, a number of workers have reported their inability to
find charges in PME correlated with abscission (2, 73, 113, 140).

Another pectic enzyme, polygalactonuronase (PGU) was found to
rapidly increase immediately prior to the abscission of bean leaflets
(119). Similarly, PGU has been identified in leaflet abscission zones of
C_i_§gi_s_ (143) and in fruit abscission of C_i_t_r_u§ (65). Again, other
workers have not been able to confirm PGU involvement in abscission (18,

73) .

Physiological Regulation

Hegiration. Carns (28) was the first to recognize that oxygen was
an absolute requirement for physiological separation and that the
recognized enzymes of oxidative respiration were necessary for

abscission. Oxidative respiration is essential for abscission to occur

13

by providirg the needed energy for enzymatic processes (8). Lowering the
oxygen level to 5% inhibited abscission in m explants (144).
Measurement of respiration rates in bean explants by oxygen uptake
showed that the abscised explants exhibited a ”climacteric" rise in
respiration while those that did not abscise showed a slow decline in
respiration (29). The loss of carbohydrates in abscisirg apple flowers
was attributed to increased respiration (79).

Oxygen is also needed in the process of ethylene biosynthesis

for the conversion of the intermediate l-aminocyclcpropane-l-carboxylic
acid (4, 5) to ethylene.
Caerate metabolism. Abscission of leaves, flowers, and fruits are
directly related to the carbohydrate status of the plant (8). Abscission
does not occur as readily under high levels of carbohydrates (8, 10,
151). High carbohydrate levels in the plant in general will contribute
to increased vigor of fruits and leaves and will enable such organs to
synthesize hormones which apparently inhibit abscission (8). Decreased
carbohydrate levels promote abscission in fruit (31, 150, 151), leaves
(30, 70, 115, 127) and flowers (92, 166).

The effect of added sugars on organ abscission depends on the
carbohydrate reserves within the plant or explant (20, 26). When the
carbohydrate level is high, the addition of sugars helps maintain the
integrity of cell wall polysaccharides inhibitirg abscission: when the
level is low, additional sugars promote abscission by providing a
substrate for this energy requiring process (10, 20). When carbohydrate

levels were high applications of sucrose strongly retarded leaf

14

abscission in bean explants (20, 26), however, sucrose stimulated leaf
abscission in bean explants when carbohydrate levels were low (170).
Carbohydrate translocation. Primary sources of carbohydrates are usually
leaves (128, 135, 164). Regions that utilize sugars are sinks, such as
growing points of roots and shoots, storage organs (i.e. fruits and
seeds), and vegetative tissues both above and below ground (128).
Translocation of sugars occur down a concentration gradient, the
concentration beirg higher at sources than at sinks (128). Studies have
confirmed this source/sink phenomenon with radioactively labeled l4C
applied to source leaves (154, 168). The labeled 14C tends to
translocate towards developing sinks such as the root, apex of the shoot
or unexpanded leaves (48, 117, 168).

It has been observed that the presence of developing fruits
hastened the senescence of leaves and stems in bean and that removal of
fruits and flowers delayed the onset of leaf senescence (114, 124, 154).
Molisch (114) suggested that formation of flowers and fruits exhausts
organic reserves in the plant. Developing flowers and fruits act as
wmpeting sinks for nutrients from source leaves. Leaves abscise as the
flowers and fruits develop (8). Flowers and fruits are in some degree of
competition with other flowers and fruits on the same plant. Often
first-set fruits (150) and flowers (166) have a clear advantage over
similar later formirg organs which cause them to prematurely abscise (8,
12). In Lilium (49), the youngest flower buds prematurely abscise
presumably as a result of competition for available carbohydrates, the

yourgest buds being the weakest sinks (56). Some species do not produce

15

flowers until there is substantial vegetative growth to support some
reproductive activity (8). Cultivars of Phaseolus that produce fewer
flowers per day have less flower abscission than heavier bearers (166).
This is also true in many fruit species (40, 176, 196).

As discussed above the development of abscission zones in the
plant can be regulated by the hormone status (8). It has been suggested
that translocation of carbohydrates is hormone-directed (8, 135, 154).
Developing parts of the shoot and root (sinks) produce their own
specific hormonal signals (148). Carbohydrate translocation was
promoted to the site of applied benzyladenine (BA) (12, 62, 157, 158),
IAA (21, 62) or gibberellin (GA) (8, 157). Applied ethylene has both
promoted (122) and inhibited (67) carbohydrate translocation. Abscisic
acid has only shown inhibitory effects (8, 122, 194). These studies tend
to support the hypothesis that carbohydrate translocation to a sink is
under some form of hormmal control directly (154) regulatirg growth and
abscission of organs in relation to their ability to attract
carbohydrates (8). Addicott (8) proposed that strong sinks have a high
IAA/GA/CK to moderate ABA ratio and weak sinks have a low IAA/GA/CK to
moderately low ABA ratio. However, others beleive hormones do not
directly affect carbohydrate translocation (62, 122, 142). Light has a
promotirg efect on carbohydrate translocation from source leaf to sink
(168). Studies showed that light can promote vegetative (169) or
reproductive (106, 116) sinks. Greenhouse rose flowerirg is influenced
by the amount of carbohydrate available from photosynthesis for flower

bud development (116). Shading vegetative or reproductive sinks

16

decreased l4C translocation to the sinks (116, 151, 168). Shading
flowers and pods. of soybean reduced their subsequent sink strength and
promoted abscission (76). Decreased carbohydrate translocation caused
fruit abscission (151) and improper flower bud development (91, 106,
116, 171).

Differences in light quality (particularly red and far red light
acting through the phytochrome system) are known to regulate plant
development (59, 160). Curtis (41) and Decoteau and Cracker (45) have
implicated phytochrome as a light sensirg mechanism active in inhibition
of dark-induced leaf abscission in mung bean. Abscission could be
inhibited by red light and the inhibition reversible with far red light.
Red light prevented or delayed flower and fruit abscission in apple (25,
87) and soybeans (75). Red light promoted l4C-sucrose uptake in pea
epicotyles (183) and rose shoots (117) but far red light inhibited
uptake. Adding far red to red further enhanced uptake in rose shoots
over red light alone (117). Far red light inhibited axillary shoot
growth in rose (117), tobacco (88), and tomato (172), thereby promotirg
apical dominance, whereas red light promoted axillary shoots in rose
(117). Mor and Halevy (117) believe far red light promoted greater sink
activity in the apical shoot of the rose, thereby inhibiting axillary
bud growth. Light may effect the unloading process at the sink by

influencirg membrane transport (85).

17

Environmental Regulation

Irradiance. The irradiance intercepted by the plant directly affects
photosynthesis and subsequent carbohydrate supply (8). Irradiance levels
high enough to promote accumulation of carbohydrates increases the vigor
of leaves and fruits enablirg such organs to synthesize hormones which
inhibit abscission (8). High irradiance levels increase the amount of
carbohydrate in source leaves to be translocated to developing
vegetative (169) or reproductive sinks (116, 117). Supplemental lighting
in the lower canopy of soybeans increased pod set, seeds per pod, and
seed yield and decreased flower and pod abscission (76). Additional
irradiance decreased flower bud abscission in M (48).

Low irradiance levels provide low to moderate carbohydrate
levels in the plant which tend to promote abscission (8). Low irradiance
levels are commonly found in tree canopies where leaves are competing
for carbohydrates causirg less competative leaves to abscise (110, 149).
Flower bud abscission in I_ri_s_ (58), Gladiolus (69), figs; (204), and
tomato (90) increased due to poor irradiance conditions in greenhouses
in the winter. In L_i_l_i_um (49) flower bud abscission coincided with peak
ethylene production. As photoperiod decreased, ethylene production
increased (175). Shading decreased the amount of 14C translocated to
shaded areas (76, 116, 151, 168) and reduced the subsequent sink
strergth of flowers and pods in soybean promoting abscission (76).

A shortened photoper iod was found to be an important trigger

for autumnal defoliation of many trees (60). However, increasing the

18

daylergth prevented defoliation in Ac_er_ (130) and Plumaria (123). The
correlation of leaf abscission sensitivity to photoperiod suggests that
leaf abscission in the autumn in some species may be under the control
of phytochrome (8). Curtis (41) and Decoteau and Cracker (45) found that
red light inhibited dark-induced leaf abscission in murg bean: red light
inhibition could be reversed by a brief exposure to far red light.
Temgrature. Early workers had difficulty in distinguishing whether
temperature imposed stresses that served as initiating factors of
abscission or if temperature determined the rate of the abscission
process (8). In extensive work with petal abscission, Fitting (57)
recorded increasing rate of abscission as the temperature increased, as
was more recently observed in petal abscission in the seed geranium
(15). High temperatures also eliminated the effect of Ag" (a known
inhibitor of ethylene action (18)) in inhibitirg petal abscission in the
seed geranium (112). As temperature increases, ethylene production
increases. The ethylene production response to temperature behaves
consistant to an enzyme activated system (27). More recent work defined
temperature response curves which have been determined for explants of
bean (199), cotton (108), and for petal abscission in L_i_r;u_tg lewisii
(190). The characteristic maxima for each species occured at 25°C, 30°:
and 35° respectively. A Q-10 of ca. 2 was found for most temperature
response curves, characteristic of most chemical reactions (8).

Leaf abscission is a common response to cold temperatures.
Severe cold will kill the cells in the abscission zone thus,

physiological separation is not possible (8). Light frosts that do not

l9

injure the abscission zone can cause considerable leaf abscission in
Ci_tru_§ and Gosgypium (8). Dependirg on species, varying degrees of cold
are sufficient to induce abscission, and in some cases cold is the
primary factor initiatirg autumnal defoliation of deciduous trees (8).
Excessively high temperatures also contributes to abscission,
however, some of the high temperature effects may be the result of water
stress rather than temperature itself (8). Weisner (189) observed that
well-watered trees showed little or no leaf abscission at high
temperatures. Leaf abscission that occured was on the lower and
innermost leaves, not the leaves that received the most direct rays
(189), which supports water stress rather than high temperatures that
induced abscission (8). Flower buds, flowers, and yourg fruits abscise
in response to high temperatures in crops such as snap beans (193)-
Water stress. Leaf, flower, or fruit abscission often is in direct
proportion to the severity of a water stress (181). Leaf abscission
varies considerably among species and with leaf age (125, 161). Water
stress induces chemical and biochemical changes that tend to promote
abscission by affectirg hormone levels (8). Water stress increased the
activity of IAA-oxidase (42), decreased diffusible auxin (74), and
decreased auxin transport (43). Itai (80), and Itai and Vaadia (81)
reported a decrease in cytokinin activity in response to water stress.
Water stress also increases endogenous ethylene levels (44, 67). Rapid

increases in ABA levels were found in response to water stress (68, 86,
195, 201).

20

Conclusions

The abscission of organs from the plant body occurs in a region
at the base of the organ called the abscission zone. Cells in the
abscission zone are smaller and more densly filled with cytoplasm. Prior
to abscission, the cells undergo considerable meristmatic activity
forming a separation layer, protective 1ayer(s) , or both. Cells in the
separation layer secrete the hydrolytic enzymes (cellulases and
pectinases) necessary for cell-wall hydrolysis, separation, and
protectve layer formation. Cells can separate three ways: dissolution of
the middle lamella, dissolution of the middle lamella and primary wall,
or mechanical breakage involving non-living cells of the vascular
tissue. A primary protective layer consistirg of periderm usually forms
proximally to the separation layer to protect the newly exposed surface
from injury or water loss. In some woody species a secondary protective
layer may form. Cell enlargement often occurs prior to and after
separation. Differential cell enlargement in the distal and proximal
sides of the abscission zone creates shearing forces across the cell
walls which results in abscission.

The initial stimulus for abscission appears to be a hormonal
imbalance due to environmental charges and endogemus competition. This
hormonal imbalance mediates localized senescence of cells in the
abscission zone, leading to an increase in ethylene synthesis, which

could be the signal for abscission. Ethylene enhances pectinase and

21

cellulase activity. For abscission to occur, all hormones probably
modify the organs sensitivity to ethylene or promote ethylene synthesis.
Auxin both inhibits and promotes abscission dependirg on the stage of
sensitivity of the organ to ethylene and in the auxin concentration
gradient across the abscission zone. Abscisic acid has been shown to
promote abscission as ABA levels increase during water stress which
enhances ethylene production. Gibberellins and cytokinins usually
inhibit abscission by intensifyirg the plant organs ability to function
as a carbohydrate sink, however, if levels of either hormone are low,
ethylene is able to circumvent their inhibitory effect.

Oxidative respiration is essential for abscission to occur by
breaking down energy-rich carbohydrates for the numerous enzymatic
reactions involved.

Hormones play either a direct or indirect effect on controllirg
carbohydrate translocation from source to sink, therefore the level of
carbohydrate at the abscission zone will determine whether the organ
abscises or not.

Low irradiance levels or short photoperiods in log day plants
decreases the sink strength of organs, promotirg ethylene synthesis and
abscission. Irradiance levels sufficiently high to accumulate
carbohydrate increase the vigor of the organ enabling the organ to
synthesize hormones that inhibit abscission. Leaf abscission in some
species appears to be under the control of phytochrome.

High temperatures or water stress appear to induce abscission by

increasirg the rate of ethylene synthesis.

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'Bacaara' roses. II. The effect of environmental factors.
Scientia Hortic. 3:383-391.

Zimnerman, P. W., A. E. Hitcock, and W. Crocker. 1931. The effect
of ethylene and illuminating gas on roses. Contrib. Boyce
Thorpson Inst. 3:459-481.

Section I
Environmental Factors Influencing
Prenature Cyathia Abscission In

Poinsettia 'Annette Hegg Dark md'

Environmental Factors Influencing Premature Cyathia Abscission in
Poinsettia 'Annette Hegg Dark Red'

Steven H. Miller and R. D. Heins]-

thent of Horticulture, Michigan State University:

East Lansg’ , MI. 48824

Additional index words. Euphorbia pulcherrima, center drop, cyathium,
carbohydrate depletion, irradiance, temperature, water stress

Abstract. While low irradiance levels, high temperatures, and water
stress all promoted premature cyathia abscission: in poinsettia 'Annette
Hegg Dark Red', low irradiance appeared to be the primary factor
promoting abscission in the greenhouse. Absc ission was greater in plants
placed under 75% shade at 16°C night temperature (NT) than on plants
placed under normal daylight (ND) at 16° or 21° NT. The earlier plants
were placed under shade after flower initiation, the lower the final
plant dry weight, the earlier cyathia abscission started, and the
greater the severity of abscission. A - 0.6 MPa water stress had no
Received for publication . Michigan Agricultural
Btperiment Station Journal Article No. . The authors appreciate
the helpful discussion and ideasof Dr. William H. Carlson and Dr.
Arthur Cameron on this research project. This project was supported in
part by a grant from Paul Ecke Poinsettias, Encinitas, CA. The cost of
publishing this paper was defrayed in part by the payment of page
charges. Under postal regulation, this paper therefore must be hereby
marked advertisement soley to indicate this fact.

1Research Assistant and Associate Professor respectively.

38

39

consistant effect on abscission of plants grown under shade, while on
plants grown under ND, water stress promoted absc ission as NT' increased
from 13° to 21°. Under controlled experimental conditions at a constant

irradiance, cyathia abscission increased as NI‘ increased.

Introduction

Premature poinsettia cyathia abscission ("center drop") defined
as premature abscission of the poinsettia cyathia prior to normal
marketing, can be a major problem for poinsettia growers in the midwest
and northeastern United States. Abscission of the cyathia in the
greenhouse can occur either before anthesis (pre anthesis) or after
anthesis (post anthesis) While not affecting growers every year, when
the problem occurs, economic losses can be significant. Entire semi-
1oads of poinsettias have been rejected by buyers when cyathia
abscission was found in the shipment (2).

Pre anthesis cyathia abscission has been observed on plants
grown in the greenl'nuse under low irradiance levels (50% shade) (13) and
on plants placed in a controlled environment under low irradiance and
high temperatures before the bracts had completely developed (13, 17).
Post-anthesis cyathia abscission has been observed on plants placed in
postharvest environments (10, ll, 13), and on plants held in a dark
storage (10, 11). The longer the dark storage, the greater the
abscission. However, there have been no systematic investigation into
environmental conditions that promote premature cyathia abscission or

methods suggested to control the problem.

40

In preliminary experiments, low irradiance levels, high night
temperatures late in crop development (finishing temperature) and water
stress were found to promote cyathia abscission (Miller and Heins,
unpublished data). Ethylene at 10 pl 1‘1 had no effect on abscission
when applied at anthesis. Chemical sprays at anthesis of napthalene
acetic acid, 6-benzylamino purine (BA), or silver thiosulphate did not
prevent abscission, however, gibberellic acid (GA-3) or Promaline(GA-4 +
GA-7 +BA) prevented abscission as previously reported (12) but resulted
in undesirable plant quality when applied at anthesis and delayed
flowering when applied earlier in crop development.

The objective of the current studies was to determine the
influence of irradiance, temperature, and water stress on premature

cyathia abscission.

Materials and Methods

Greenhouse experinen .

General exgr imengaL conditions. Iboted cuttings of 'Annette Hegg Dark
mdm' (AHDR) were received on Aug 25, 1983, from Paul Ecke Poinsettias,
Encinitas, CA. One cutting was planted per 10 cm plastic pot (an
experimental unit) in VSP medium (Michigan Peat Co., Houston, TX)
composed of 2 peat : l perlite : l vermiculite (v:v:v) ammended with
dolomitic limestone, superphosphate and trace elements. Plants were

placed in a glass greenhouse and grown single stem at a spacing of 33

41

plants 111'2 under normal daylight (ND) for 1 week. Short days (SD) were
started on Sept 1. Black sateen cloth was pulled over the plants
from 1600 hr to 0800 hr daily.

The initial night temperature (NT) setpoint was 18°C. Day

temperature (UP) and venting temperature setpoints were 3° and 6° above
the NI‘ respectively in all experiments irrespective of NT. Temperatures
were lowered during the first 2 weeks of SD to 160 NP. After 2 weeks of
SD, plants were moved to different greenhouse sections depending on
experimental temperature. Plants were fertilized with 260 mg 1-1 N, 130
mg 1"1 K. and 0.1 mg 1-1 Mo. Cnlormequat was applied at 1500 mg 1-1 as a
foliar spray for height control the day SD started and 1 week later. A
third application of chlormequat at 750 mg 1-1 was applied after 3 weeks
of SD.
Data collection. The date of anthesis and the number of abscised cyathia
were recorded on all plants daily for 25 days past anthesis. The total
nunher of cyathia per plant was determined at the end of each experiment
by counting the number of cyathia greater than 1 mm in diameter still
present plus the number of stubs remaining from previously abscised
cyathia. The percent abscission per plant was calculated by dividing the
number of abscised cyathia by the total number of cyathia formed.

gpt. 1: Influence of temrature 3nd irradiance on abcission. Every 2
weeks, starting 2 weeks after the start of SD until 8 weeks of SD plants
were moved among 4 greenhouse environments: (1) 16° NT and ND, (2) 160

NT and 75% shade, (3) 21° NT and ND, or (4) 21° NT and 75% shade.

42

Treatments are outlined in Figure 1. There were 25 treatments resulting
from plants being moved among environments, 10 plants per treatment.
At the end of the experiment, shoot dry weight was measured on 5 plants
per treatment by removing the above ground portion of the plant and
oren drying the tissue at 60° for 3 days prior to weighing.

Mean percent abscission and mean dry weight were determined for
each treatment and statistical significance was determined by analysis
of variance. The experiment was analyzed as a completely randomized 5 x

5 factorial design.

mt. 2: Influence of finishing tenfirature, irradiance, and water

stress on abscission. Two weeks after the start of SD, plants were

 

placed at initial temperatures of 16° or 18° NT. All plants were then
grown for an additional 4 weeks. Plants were moved from the 16° and 180
initial temperature houses to greenholses with NT of 13°, 16°, 18°, or
21° NT. Plants were kept at these finishing temperatures through data
collection. For each finishing temperature, half of the plants remained
under ND and half were placed under shade. In all temperature and
irradiance treatments, half of the plants were repeatedly water stressed
to a plant water potential of ca. - 0.6 MPa, the other half were watered
daily to maintain a water potential of ca. 0 MPa. Plant water potential
was measured using a pressure bomb (PMS Instrument Co., Corvallis,
Oregon). Ten extra plants were used per treatment to take water
potential readings with an average of 2 plants used each time a reading

was taken. The same 2 plants were not used again. When the average water

43

potential was ca. - 0.6 MPa, all plants in the corresponding treatment
were watered to container capacity. The water stress treatment plants
were repeatedly stressed until all plants in a treatment reached
anthesis. The number of times plants were stressed prior to anthesis
varied from 2 to 5 dependirg on irradiance and finishing temperature.
The experiment was analyzed as a completely randomized design:
split, split, quit plot, with initial temperature as the main plot, and
final temperature, irradiance, and water stress, as subplot factors.
There were 32 treatments (2 initial temperatures X 4 finishing
temperatures X 2 irradiance levels X 2 water stresses), with 10 plants
per treatment. Mean percent abscission was determined daily and

statistical significance determined by analysis of variance.

Controlled environment ngr iment. After planting as described above for
the greenhouse experiments, equal numbers of plants were placed in two
controlled environment chambers and grown silgle stem at a spacing of 33
plants m-2 for 2 weeks under 125 umol s-1 m"2 photosynthetically active
radiation (PAR) (0600 hr to 2200 hr) from 215 W VHO deluxe cool white
fluorescent lamps. Short days were started Sept 9 with an 8 hr light
span from 0800 hr to 1600 hr daily.

The initial NT setpoint was 18°. At the start of SD the NT was
lowered to 16° in one chamber and raised to 21° in the other chamber.
Plants remained at 16° or 21° for 3 weeks when one-third of the plants
at 16° were moved from 16° to 21° and one-third of the plants at 21°

were moved from 21° to 16°. Plants were finished at the new

44

temperatures. Similarly after 5 weeks of SD, another third of the plants
were moved between chambers, with the remainirg third of the plants were
grown continuously at 16° or 21°. Plants remained in respective
chambers durirg data collection. Chlormequat was applied at 1500 mg-1
for height control on Sept 2 and Sept 9. Plants were fertilized and data
was collected as described in the greenhouse experiments.

The experiment was analyzed as a completely randomized 2 X 3
factorial design. There were 6 treatments (2 temperatures X 3 moving
times), with 10 plants per treatment. Mean percent abscission was

determine daily and statistical significance determined by analysis of

variance.

lbsults

m Moving plants from normal daylight (ND) to 75% shade or from
16° to 21° NT resulted in earlier and greater cyathia abscission
compared with plants maintained at 16° under ND (Tables 1, 2).
Abscission was greater on plants under shade at 16° than on plants at
21° under ND (Figure 2). Exposing plants to shade and 21° NT
simultaneously resulted in earlier abscission than on plants under shade
at 16° or the 21° NT alone. cyathia abscission was greater at all times
under shade at either NT compared with plants at 21° NT and under ND
for each date plants were moved (Table l). The earlier plants were moved

to shade the earlier abscission occured and the greater the severity

45

(Figure 3). On some plants moved from ND to shade after 2 or 4 weeks of
SD anthesis never occured as abscission occured before anthesis (Figure
3) and complete abscission of all cyathia occured in a short time span
of 4 to 5 days. A move to shade after 6 or 8 weeks of SD induced less
abscission, and abscission did not start until a minimum of 5 days after
anthesis.

The earlier plants were moved to shade the greater the reduction
in final dry weight (Table 3). Final plant dry weight was significantly
reduced under shade, but moving plants from 16° NT to 21° NT had no

significant effect on dry weight.

Elpt. 2. Cyathia abscission was promoted by increasing the initial
tenprature from 16° to 18° , by increasirg final temperature from 13° to
21°, by reducing irradiance, and by water stressing the plants (Tables
4, 5). Plants grown at a 16° initial temperature had less abscission
than plants grown at 180 initial temperature irrespective of finishing
temperature. In addition as the finishing temperature increased from 13°
to 21°, abscission increased regardless of the initial temperature.
Irrespective of irradiance or water regime, plants grown at a 16°
initial temperature and finished at a 21° finishing temperature had
greater abscission than plants grown at a 180 initial temperature and
finished at a 13° finishing temperature (Table 4).

Plants under shade had greater abscission than plants under ND
in all treatments (Table 4). A significant interaction between

irradiance and finishing temperature which resulted in greater

46

abscission under shade as finishing tenperature increased (Table 5) .
During the period from 55 to 60 days after the start of SD , cyathia
abscission was promoted by high initial temperatures, but not after that
time. Moving plants from 16° to 21° NT did not increase abscission as .
much as moving plants from ND to shade while keeping NT constant at 15°
(Figure 4).

Water stress increased cyathia abscission on plants grown under
ND but had little effect on abscission in plants grown under shade
(Table 4, Figure 5). A greater response to water stress occured when the
NT was lowered from 18° to 13° under ND (Figure 5) than if maintained at
16° (Figure 6).
Controlled environment Er iment. Under controlled conditions, cyathia
abscission was greater on plants grown at a constant 21° NT compared to
a constant 16° NT (Figure 7). One week after anthesis 62% of the cyathia
had abscised at a constant 21° (day 59) compared to only 20% abscission
at a constant 16° (day 77). Plants moved after 5 weeks of SD from 16° to
21° had greater abscission than if moved from 21° to 16°. Plants moved
from 16° to 21° had abscised 38% of the cyathia before anthesis
compared to 3% on plants grown at a constant 21°. However, abscission
was eventually greater on plants grown at a constant 21°.

Comparing treatments at a common physiological stage of
develrpment (anthesis) , plants moved from 210 to 160 after 3 weeks of SD
had less abscission than plants moved at 5 weeks of SD to 16° or the

control ( 21° constant) (Figure 8).

47

Discuss ion

low irradiance levels, high temperatures and water stress all
promoted cyathia absc ission. However, low irradiance appeared to be the
primary factor promoting abscission on plants in the greenhouse. When
plants were moved from ND to shade at 16° NT after 4 weeks of SD, 74% of
the cyathia had abscised 10 days after normal anthesis while only 12% of
the cyathia had abscised at the same time when plants remained under ND
but were moved from 16° to 21° NT (Table 1). Increasing temperature and
decreasing irradiance simultaneously only increased abscission 10% to
84%. In general, the earlier plants were placed under shade, the earlier
abscission occurred. On plants under shade, water stress had no
consistant effect on abscission while under ND, water stress promoted
abscission (Table 4).

Woodhead and Einert (17) reported cyathia to abscise when plants
were placed in a postharvest harvest environment before the bracts were
half—colored. Decreasing the amount of available PAR to the plant (by
shading) decreases photosynthesis and subsequent carbohydrate supply in
the plant which tends to promote the absc ission of leaves, flowers, and
fruits (1).

There was a greater decline in nonsoluable carbohydrate over
time in bracts and leaves in plants grown under shade than on plants
grown under ND (6). Carbohydrate levels are most likely declining in the
cyathia at the same time, especially under shade. A decline in

carbohydrate supply was evident in shade grown plants as they had lower

48

final dry weight compared with plants grown under ND (Table 3).

The developing bracts and cyathia, both being carbohydrate
sinks, compete for available photosynthates. The bracts appeared to be
stronger sinks than the cyathia (6). Cyathia sink strength is further
modified by cyathia shading. On inflorescences growing under shade or
covered by neighboring bracts or leaves, the cyathia frequently do not
fully develop and abscise either before or shortly after anthesis
(personal observation). Abscission may be due to the cyathia's inability
to mobilize sufficient carbohydrates to complete development and
flowering especially when carbohydrates are limiting. Decreased
carbohydrate translocation as a result of low irradiance has caused
fruit abscission (9) and improper flower bud development in rose (7),
tomato (3), iris (4), and Bo_ugainvillea (14).

Temperature also influenced cyathia abscission. As the initial
temperature or finishing temperature increased, cyathia absc ission was
promoted, especially if plants were grown under shade (Tables 1, 2).
Under controlled conditions, cyathia abscission also increased with
increasing finishing temperature (Figure 7). Conversely, there was less
abscission on plants moved from a 21° to 16° finishing temperature after
3 weeks of SD compared to moving at 5 weeks of SD (Figure 8). The
increase in abscission as finishing temperature increased may have been
due to carbohydrate depletion. As temperature increases, respiration
rates increase (8), which decreases the carbohydrate supply in the
plant. Abscission has been reported to increase as the temperature

increased in explants of bean (18), and cotton (5), and in petal

49

abscission in Dim lewisii (15). Flower buds, flowers, and young pods
of snap bean are also known to abscise in response to high temperatures
(16) .

Plants water stressed under ND had greater abscission than
plants growing under shade (Table 4, Figure 5). Under shade, significant
abscission had already occured due to the low irrad iance level at the
time a water stress occured, so there were few additional cyathia to
abscise. However, many cyathia were still present on plants growing
under ND so water stress had the potential for causing more abscission.
These results suggest that premature cyathia abscission is primarily
promoted by low irradiance levels, commonly found in greenhouses as
natural irradiance levels decline in the Fall. Irradiance levels to the
leaves are further reduced as bracts expand or if plant density is high
resulting in shading of the leaves. Decreased carbohydrates available to
the cyathia promote abscission. If temperatures are high, respiration
rates will probably increase, further reducing carbohydrate levels in
the plant. Under water stress, hormonal changes may occur that would
decrease available auxin to the abscission zone and increase endogenous
levels of abscisic acid and ethylene which could promote abscission.

Preventative measures appear to be the only means of controlling
the problem of premature cyathia abscission. Early flower initiation in
late September will allow the cyathia to develq> under higher irradiance
levels. Proper night temperature control in October and early November
will allow lower finishing temperatures to be used. Spacing plants as

they are marketed will allow for greater irradiance penetration to the

50

leaves, resultirg in greater carbohydrate levels potentially available
to the cyathia.

50.

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52

Tab1e 2. Mean percent cyathia abscission on poinsettia
'Annette He 9 Dark Red' 65 days after the start of
short days ?50) initially grown at 16°C night temper-
at re (NT) and normal daylight (ND) and then moved to
21 C NT (temperature change) and/or 75% shade (irradi-
ance change) at 2, 4, 6. or 8 weeks after the start of
SD. Expt. 1.

 

. Weeks after the start of short days

 

Time of irradiance change

 

 

 

Time of 2
temperature change 2 4 6 8 No change
2 78 85 55 10 9
4 94 82 63 4 7
6 88 91 62 12 9
8 86 85 61 0 2
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56

Twenty-five treatments resulting from movirg poinsettia
'Annette Hegg Dark Red' anong four greenhoise environ-
ments: (1) 16°C night tenperature (NT) and normal
daylight (up) [:3 , (2) 16° NI‘ and 75% shade
“"1“""431 210 m and up \\\\\\\\ , and (4) 210 N1‘
and shade - at 2, 4, 6, and 8 weeks after the
start of short days.

T treatments

57

 

v :9 @
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58

Figure 2. Mean percent cyathia abscission 49 to 77 days after the
start of short days (SD) for poinsettia 'Annette Hegg
Dark Red' remaining at 16°C night tenperature (NT) and
normal daylight (ND) or noved to 16° NT and 75% shade
(sap) , 210 NT and ND, or 21° NT andSHD at 2 weeks after
the start of SD. Average day of anthesis for each treat-

ment indicated by "’ . Expt. 1.

59

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NOISSIJSSH 1N33838

Figure 3.

60

Mean percent cyathia abscission 49 to 77 days after the
start of short days (SD) for poinsettia 'Annette Hegg
Dark md' grown at 16°C night tenperature (NT) and
normal daylight (no shift) or noved to 75% shade at

2, 6, or 8 weeks after the start of SD. Average day of
anthesis for each treatment indicated by —>
(282weeks (WKS) ofSD, 686WKSofSD, 888WKS
of SD, NS - No shift). Expt. 1.

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NOISSIJSQU 1N33838

Figure 4.

62

Mean percent cyathia abscission 49 to 84 days after the
start of short days (SD) for poinsettia 'Annette Hegg
Dark Red' grown at an initial night tenperature (M) of
16°C or 18°C under mrmal daylight (ND) from 0 to 6
weeks after the start of short days. Plants were then
moved to a finishing NT of 16° or 21° under ND or 75%
shade (SHD) or water stressed (98) to ca. - 0.6 MPa
firm 6 weeks of SD until anthesis. Day of anthesis was
54 to 55 days after the start of SD for all treatments:

indicated by —> . Expt. 2.

63

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64

man percent cyathia abscission 49 to 84 days after the
start of short days (SD) for poinsettia 'Annette Hegg
Dark Red' initially grown at an 18°C night teuperature
under normal daylight (ND) , from 0 to 6 weeks after the
start of short days ,then finished at a 13° NI‘ under
ND or 75% shade (SHD),or water stressed (V8) to ca.

- 0.6 14% until anthesis. Day of anthesis was 54 to 55

days after the start of SD for all treatments, indicated

by ”.mmz.

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66

Figure 6. Mean percent cyathia abscission 49 to 84 days after
the start of short days (SD) for poinsettia 'Annette
Hegg Dark Red' grown at a constant 16°C night teap-
erature under normal daylight (ND) until 6 weeks of SD,
then moved to ND or 75% shade (SHD) and/or water
stressed (V5) to ca. - 0.6 MPa. Day of anthesis was
54 to 55 days after the start of SD for all treatments.
indicated by —" . Expt. 2.

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68

Mean percent cyathia abscission 49 to 77 days after the
start of short days (SD) for poinsettia 'Annette Hegg
Dark Red' grown in a controlled environment chanber
(125 141101 s-1 m-2 from 0800 hr to 1600 hr) at 16°C or
21°C night tenperature (NT) or moved after 5 weeks of
SD from the 16° or 21° NT chamber to the 21° or 160

chamber respectively. Day of anthesis for each treatment
indicated by —> .

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70

Figure 8. Mean percent cyathia abscission 16 days before anthesis

to 16 days after anthesis for poinsettia 'Annette Hegg
Dark Red' grown in a controlled environment chanber
(125 111101 s-1 m-2 from 0800 hr to 1600 hr) at a

constant 21°C night teaperature (NT) or noved to a 16°

NT after 3 or 5 weeks of short days.

71

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Literature Cited

3.

4.

12.

72

Literature Cited

Addicott, F. T. 1982. Abscission. Univ. Calif. Press, Berkley.

Carlson, W. A. 1982. Professor of Horticulture, Michigan State
thiv” East Lansing. Personal commnication.

Kinet, J. M., D. Hurbedise, A. Parmentier, and R. Stainer. 1978.
Fraction of inflorescence development by growth substance
treatments to tcuato plants grown in insufficient light cmditions.
J. HIE! Soc. Hort. Sci. 103:724-729.

Mae, T., and C. R. Vonk. 1974. Effect of light and growth
substances on flowering of Iris x hollandica cv. Wedgewood. Acta

Dhrynick, M. C. 1976. Studies on abscission in cotton explants.
Ph.D. dissertation, Univ. Calif., Davis. Cited by 1.

Miller, S. 8., and R. D. Heins. 1984. Physiological factors
influencing prenature cyathia abscission in poinsettia 'Annette
Hegg Dark md'. [Inpublished (manuscript).

mt, Y., and A. H. Halevy. 1980. Pranotion of sink activity by
developing rose shoots by light. Plant Physiol. 66:990-995.

Noggle, G. R., and G. F. Fritz. 1976. Intoductory Plant Physiology.
Prentice-Hall, Ehglewood Cliffs, N. J.

Schneider, G. W. 1977. Studies on the mechanism of fruit abscission
in amle and peach. J. Mar. Soc. Hort. Sci. 102:179-l81.

Scott, L. F., T. M. Blessington, and J. A. Price. 1983. Postharvest
effects of tenperature, dark storage duration, and sleeving on
quality retention of 'Gutbier V-l4 Glory' poinsettia. HortScience
18 :749-750.

. and .l.984 Postharvest
effects on quality retention of poinsettia. HortScience 19:290-291.

Shanks, J. B. 1981. Poinsettia - the Christmas flower. mryland
Florist 231:1-51.

13.

14.

15.

16.

17.

18.

73

Staby, G. L., and A. M. Kofranek. 1979. Production conditions as
they affect harvest and postharvest characteristics of poinsettia.
J. Miler. Soc. Hort. Sci. 104:88-92.

Tse, A., W. P. Hackett, and R. M. Sachs. 1973. Pranotion of
flowering in mainvilla 'San Diego Red' by renoval of young
leaves and cytokinins. Plant Physiol. 51:5-29.

Wiatr, S. M. 1978. Physiology and ultrastructure of petal
abscission in western blue flax (Linum liis—ii). Ph.D. dissertation
Univ. Calif., Davis. Cited by 1.

Wittwer, S. H. 1954. Control of flowering and fruit setting by
plant regulators. p. 62-80. In: H. B. Tukey (ed.), Plant
mgulators in Agriculture. John Wiley and Sons, New York.

Woodhead, S. H., nd A. E. Einert. 1973. Influence of the hone
environment on poinsettia development. Florists' Review 152(1027) :

Yamaguchi, S. 1954. Some interrelations of oxygen, carbon dioxide,
sucrose, and ethylene in abscission. Ph.D. dissertation, Univ.
Calif., Los Angeles. Cited by 1.

Section II
Physiological Factors Influencing
Premature Cyathia Abscission in

Poinsettia 'Annette Hegg Dark Ibd'

Physiological Factors Influencing Premature Cyathia Absbission in
Poinsettia 'Annette Hegg Dark md'

Steven H. Miller and R. D. Heins]-

D_egr_tment of Horticulture, Michigan State Universig,

East Lansygg' , MI. 48824

Additional index words. Euphorbia pulcherrima, center drop, cyathium,

 

carbohydrate depletion, source/sink relationships

Abstract. As plant density increased, transmission of photosynthetically
active radiation (PAR) through the bracts to the leaf canopy decreased
significantly while cyathia abscission increased concomitantly. More
than 90% of the PAR above the bracts was absorbed or reflected 5 cm
below the bracts on 20 cm tall plants spaced at 65 or more plants m‘z.
Reducing natural irradiation 75% by shading leaves of poinsettia
promoted cyathia abscission while removing immature bracts decreased

abscission. Leaf removal on plants with intact bracts promoted

Received for publication . Michigan Agricultural
Experiment Station Journal Article No. . The authors appreciate
the discussions and ideas of Dr. William H. Carlson and Dr. Arthur
Cameron on this research project. This project was supported in part by
a grant from Paul Ecke Poinsettias, Encinitas, CA. The cost of
publishing this paper was defrayed in part by the payment of page
charges. Under postal regulation, this paper therefore must be hereby
marked advertisement soley to indicate this fact.

lResearch Assistant and Associate Professor respectively.

74

75

abscission to a degree that 100% of the cyathia abscised prior to
anthesis while bract removal on plants with intact leaves decreased
abscission sufficiently that only 23% of the cyathia had abscised 25
days after anthesis. Measurements of nonsoluable carbohydrate showed a
significant increase in leaf carbohydrate on plants with bracts removed
while carbohydrate decreased in leaves of plants with bracts intact.
Carbohydrate depletion appears to be the primary factor rquonsible for

premature cyathia absc ission.

Introduction

The problem of premature poinsettia cyathia abscission and the
resulting economic losses have been reported (6). Premature cyathia
abscission, occuring either before or after anthesis is promoted by low
irradiance levels, high temperatures, or water stress in the greenhouse
or postharvest environment (6, 12, 13, 15, 19). However, low irradiance
appears to be the primary factor promoting abscission in either
environment (6). Although high temperatures or water stress alone will
promote abscission, they intensify the problem if plants are under low
irradiance levels in the greenhouse (6). low irradiance levels are often
found in greenhouses during fall production due to declining solar
radiation levels. Irradiance levels in the canopy are further reduced as
bracts expand, covering the leaves or if plant density is high. Under
these situations premature cyathia abscission can be observed when the
leaves or cyathia are shaded by the developing bracts, or by neighboring

leaves.

76

Under low irradiance levels, photosynthesis is reduced in source
leaves. The reduction in photosynthesis reduces the amount of
carbohydrate available for translocation to developing sinks (1).
Covering vegetative or reproductive sinks has been shown to decrease J-4C
translocation to these sinks (7, 10, 16). Covering flowers and pods of
soybean reduced their sink strength and promoted abscission (5)-

Competition between sinks can causes the younger, less developed
sinks to abscise (3, 4, 15). In species that bear a small number of
flowers or fruit, there is less abscission than in heavier bearing
species (2, 15, 1‘7, 19).

Even though it is known what environmental factors promote
premature cyathia abscission in poinsettia, there has been no
investigation into physiological factors such as source/sink
relationships in the plant that may also control abscission. Based on
previous source/sink work, the objective of this study were to: (1)
determine if abscission was promoted by covering the cyathia; (2)
determine if abscission was promoted by shading or removal of source
leaves; (3) determine if removing bracts reduced cyathia abscission by
eliminating competition between two sinks; (4) measure abscission at
different plant densities; and (5) measure carbohydrate content in
bracts and leaves over time under different environments to determine if
carbohydrate levels differed in plants grown under diferent irradiance

levels or night temperatures.

77

Materials and Methods

General conditions. Rooted cuttings of 'Annette Hegg Dark RedTM' (AHDR)
were received on Aug 25, 1983, (Expt. 142), Sept 22, 1983, (Expt. 3),
and Jan 25, 1984, (Ehrpt. 4-5) from Paul Ecke Poinsettias, Ehcinitas, CA.
Che cutting was planted per 10 cm plastic pot (an experimental unit) in
VSP medium (Michigan Peat Co., Houston, TX) composed of 2 peat : l
perlite : l vermiculite (v:v:v) ammended with dolomitic limestone,
superphosphate and trace elements. Plants were placed in a glass
greenhouse and grown single stem at a spacing of 33 plants m‘z. Plants
for Expt. 1-2 were grown under natural photoper iods (ND) for 1 week and
plants for Expt. 3-5 were grown for 2 weeks under ND plus a 4 hr night
interruption (2200 n: to 0200 hr) of s umol 3’1 n-2 photosynthetically
active radiation (PAR) from 60 W incandescent lamps. Short days (SD)
were initiated Sept 1 (Expt. 1-2), Oct. 1 (Expt. 3), and Feb 8 (Expt. 4-
5). Black sateen cloth was pulled over the plants from 1600 hr to 0800
hr daily under SD.

The initial night temperature (NT) setpoint was 18°C. Day
temperature (DT) and venting temperature setpoints were 3°C and 6°C
above NT respectively in all experiments irrespective of NT.
Temperatures were lowered during the first 2 weeks of SD to 16° NT.
Plants were then moved to different greenhouse sections depending on
experimental temperature. Plants were fertilized with 260 mg 1'1 N, 130
mg 1‘1 K, and 0.1 mg 1'1 Mo. Chlormequat was applied at 1500 mg 1‘1 as a
foliar spray for height control the day SD started (mpt. 1-2) , after 1

week of SD (Expt. 1-5), and/or after 2 weeks of SD (Expt. 4-5). A third

78

application of 750 mg 1‘1 chlormequat was applied after 3 weeks of SD
(Expt. 1-5).

Data collection. The date of anthesis and the number of abscised cyathia
were recorded on all plants daily for 25 days past anthesis (Expt. 1-
3). or every 2 days for 16 days (Expt. 4). The total number of cyathia
per plant was determined at the end of each experiment by counting the
number of cyathia greater than 1 mm in diameter still present plus the
number of stubs remaining from previously absc ised cyath is. The percent
abscission per plant was calculated by dividing the number of abscised '
cyathia by the total number of cyathia formed.

E_xpt. l: Qathia shadgg’ . Plants were initially grown for 5 weeks under
SD at 16° NT. After 5, 6, or 8 weeks of SD , the inflorescence on a
group of plants was covered with an aluminum foil ”cap“. One group of
plants was left as a control. The experiment was analyzed as a
completely randomized design with 4 treatments, 5 plants per treatment.
Mean percent abscission was determined for each treatment and
statistical significance determined by analysis of variance.

fit. 2: Bract removal and leaf shading. Plants were initially grown
under SD at 16° NT for 5 weeks. After 5, 6, or 8 weeks of SD, groups of
plants were treated by (l) removing bracts, (2) shading leaves with 75%
saran,(3) removing bracts and shading leaves, or(4) leaving them
untreated (control). Bracts were removed with a razor blade leaving no
petiole. The 4 treatments were arranged in a completely randomized
design, 5 plants per treatment. All plants remained at the same

temperature.

79

Mean percent abscission was calculated for each treatment and

statistical significance determined by analysis of variance.
Expt. 3: Bract and leaf removal. After 5, 6, 7, or 8 weeks of SD,
plants were placed at 180 NT or 210 NT. Plants were treated by removing
(l)bracts, (2) leaves, (3)bracts and leaves, or (4) no tissue (control).
Treatments were per formed all 4 dates at both temperatures. Bracts and
leaves were removed using a razor blade, leaving no petiole.

Data were analyzed as a completely randomized split, split plot:
with temperature as the main plot, removal treatment and removal times
as subplot factors, giving 32 treatments (2 temperatures x 4 removal
treatments x 4 removal times), 5 plants per treatment. Mean percent
abscission was calculated for each treatment and statistical

significance determined by analysis of variance.

mt. 4: Plant gacigg. Six weeks after the start of SD, plants were
either moved to a 21° NT greenhouse or remained at 16° NT and were
spaced at 11, 33, or 65 plants m‘z.

Photosynthetically active radiation (PAR) transmission was
measured in the plant canopy at 10, 15, 20, and 25 cm from the bench l,
2, and 3 weeks after spacing: using a Li-Cor LI-lBSB meter and LI-l9OS
quantum sensor (Li-Cor Instrument Co., Lincoln, NE.). Transmission was
expressed as a percent of irradiance above the canopy. For each date,
three measurements were made at each height and spacing.

The abscission data were analyzed as a split plot design with

temperature as the main plot and spacing as the subplot, giving 6

80

treatments (2 NT x 3 spacings), 4 plants per treatment. Mean percent
abscission was calculated every 2 days and statistical significance
determined by analysis of variance. Mean percent PAR transmission at
each spacing was determined by averaging all measurements over the 3
measurement dates.
ggpt. 5: Nonsoluable carbohydrate determination. Six weeks after the
start of SD, plants were placed under ND or SHD at both 160 and 210 NT.
Plants were treated by removing (l) bracts, (2) leaves, or (3)no tissue.
There were 12 treatments, 4 plants per treatment.

Six weeks after the start of SD and every week for the following
3 weeks at 0800 hr, three 18.8 mm2 leaf disks were removed from the
first 3 leaves below the lowest bract and 3 bract disks were removed
from the first fully expanded bracts in each control plant. Likewise, 3
leaf disks were removed from each plant with bracts removed and 3 bract
disks were taken from the first 3 fully expanded bracts on each plant
with leaves removed. For each plant, the same 3 leaves and bracts were
sampled each week. The three bract or leaf disks were boiled in 95%
ethanol. The tissue was then ground and diluted with 100 ml distilled
water (leaf disks) or 20 ml water (bract disks). A 2 ml aliquot was
pipeted out into an 18 x 150 mm test tube. Four mls of anthrone reagent
(2 g anthrone dissolved in l l of 100% sulfuric acidO was added to each
2 m1 aliquot. The two solutions were mixed thoroughly and the test tubes
were placed in a boiling water bath for 3 minutes. A marble was placed
on top of each tube to prevent loss of water. The tubes were allowed to
cool and the absorbance of the sample was read at 620 nm using a Beckman

81

325 spectrophotometer. The absorbance of each sample was compared to the
absorbance of standard glucose solutions of 15, 30, 60, 120 mg 1'1 of
glucose. .

Analysis of variance with orthogonal contrasts was conducted to

compare treatment means.

Results

m Covering the inflorescence with an aluminum foil "cap" at S, 6:
or 8 weeks of SD resulted in non significant increases in cyathia
abscission compared to the control (Table 1).

mt. 2. lbmoving bracts from plants both delayed and decreased cyathia
abscission compared to non-treated plants, while shading of leaves
promoted abscission (Table 2). Combined bract removal and leaf shading
further hastened and increased abscission. The earlier bracts and leaves
were removed or the earlier leaves were shaded, the earlier abscission
occured and the greater the total abscission.

Expt. 3. Removing leaves greatly promoted cyathia abscission whereas
bract removal delayed abscission (Table 3). Removing both bracts and
leaves gave an intermediate effect. Removing leaves caused complete
abscission on some plants as early as 1 week after anthesis, and the
earlier leaves were removed, the earlier abscission occured. Only 20% of
the plants reached anthes is prior to cyathia abscission when leaves were
removed after 5 or 6 weeks of SD, compared to 100% of the plants
reaching anthesis for all other treatments (Table 3). Removing bracts

significantly delayed abscission compared to the non-treated control

82

plants. Similar results were observed at 210 (data not presented).
E_xpt. 4. Spacing plants at 11 or 33 plants m‘2 resulted in less
abscission than spacing plants at 65 or more plants 10‘2 (Table 4).
Plants spaced in a 21° NT greenhouse had greater abscission than at a
16° NT.

Photosynthetically active radiation transmission into the plant
canopy for spacings of 11 to 97 plants m‘2 is shown in Figure l. The
greatest transmission occured amongst plants spaced at 11 plants m‘z. A
similar transmission curve was found through the top 10 cm of the
canopy for plants spaced at 33 plants m'z, but, PAR transmission dropped
off significantly as measurements were made deeper in the canopy.
Spacing at 65 or 97 plants m"2 had almost identical PAR transmission
curves with transmission decreasing to less than 10% of the above canopy
irradiance 5 cm into the canopy.
apt. 5. leaves. There was a significant increase in glucose equivalents
from 6 to 9 weeks of SD in leaves of plants with bracts removed compared
to a decrease in leaves of whole plants (bracts present) (Table 5).

Averaged over all treatments, leaves grown under ND had

significantly greater glucose equivalents than if grown under shade
however, glucose equivalents were similar at 16° and 21° NT.
21359 There was significantly greater glucose equivalents in bracts
of plants with leaves removed than in whole plants (leaves present)
averaged over both NT (Table 5). Bract glucose equivalents decreased in
all treatments from 7 to 9 weeks of SD.

Bracts grown under ND had greater glucose equivalents than if

83

grown under shade, and glucose equivalents were higher at 160 compared
to 210 (Table 5). At 160, there was no difference in glucose equivalents
under ND or shade, although glucose equivalents were higher in bracts

grown under ND than shade at 21°.

Discussion

Cyathia abscission in poinsettia is influenced by the
environment under which the plant is grown which in turn affects the
ability of the cyathia to mobilize carbohydrates. Carbohydrates
translocated from the source leaves are partitioned to the bracts and
cyathia. The bracts appear to be stronger sinks than cyathia, because
when leaves are removed, the bracts remain intact while the cyathia
never reached anthesis and abscised (Table 3). Removing the bracts had
an opposite effect in that little or no cyathia abscission occured and
the cyathia become larger than normal and develop far past anthesis
(Tables 2, 3). The removal of bracts apparently allowed more
carbohydrate to translocate to the cyathia, which accounted for their
larger size and abscence of abscission.

Shading leaves or spacing plants at high densities promoted
abscission. Both shading and high plant density decreased the irradiance
reaching the leaves, decreased photosynthesis and therefore the
carbohydrate available to the sinks resulting in cyathia abscission (1).
Measurements of PAR in the plant canopy showed that as plant density
increased, there was less available PAR penetrating to the leaves

(Figure 1). In commercial greenhouses, plant densities can be

84

sufficiently high that as bracts expand, the leaves become completely
shaded by the bracts, decreasing the PAR in the leaf canopy.
Further in 'Annette Hegg Dark Red', 3 to 6 inflorescences can develop
from a pinch (Miller and Heins, unpublished results) causing many of
these inflorescences to shade each other when plants are densely spaced.
The shaded inflorescences often have all of their cyathia abscised
(personal observation). While covering the inflorescence with an
aluminum cap did not significantly promote cyathia abscission in Expt. 1
(Table l) , plants in Expt. 1 were spaced at 33 plants m-2 which did not
result in excessive crowding. Spacing at a higher density might interact
with covering the inflorescence to promote abscission. Spacing plants
ata higher density would have increased interplant shading, decreasing
carbohydrates, and possibly resulting in a greater abscission response
to covering the inflorescence as occurs in plants where bracts shade
neighboring inflorescences. Similarly, covering the flowers and pods in
soybean decreased sink strength and reduced 14C translocation to the
flowers and pods, promoting abscission (5).

Measurements of glucose equivalents confirm that shade
significantly reduced nonsoluable carbohydrates in the bracts and
leaves (Table 5). Removing bracts (removal of sink competition)
increased glucose equivalents in leaves which could explain why there
was little or no cyathia abscission (Tables 2,3). Glucose equivalent
levels were higher in bracts or leaves at 160 compared to 21° probably
due to decreased respiration rates at higher temperatures (8). cyathia

abscission increased as temperature increased (6) probably due to higher

85

respiration rates consquently reducing carbohydrate in the plant.

Cyathia abscission appears to be promoted by environmental
changes which affect competition for carbohydrates. Environmental
changes can cause hormonal imbalances are often the initial stimulus for
abscission of a plant organ (1). The principle environmental change
promoting cyathia abscission was exposing plants to low irradiance
levels by shading (6). Under low irradiance levels, an organ does not
have the ability to synthesize sufficient abscission inhibiting hormones
such as auxins, giberellins or cytokinins (1). It has been suggested
that translocation of carbohydrates to sinks is directed by the hormone
level in the organ (1, 9, 13). However, if the level of auxin,
gibberellin, or cytokinin in the organ is low, the ability of the organ
to function as a sink decreases (1). This would explain why weaker sinks
are not able to attract large amounts of carbohydrate, causing the organ
to abscise. The decreased production of abscission-inhibiting tormones
namuxs hmalhonmumfl.imbahuoe«ammfing haxflizedenmesanoe<MEcelhsin
the abscission zone of the organ: this potentially leads to an increase
in ethylene synthesis and abscission (1).

Application of compounds associated with the delay or prevention
of abscission (silver thiosulphate, aminooxyacetic acid, gibberellic
acid, or 6-benzylamiro purine) were rot effective in delaying cyathia
abscission (6), therefore, preventative measures appear to be the only
means of controlling cyathia abscission. Preventative measures would
include flower initiation in late September which would allow the

cyathia to develop under higher irradiance levels. Proper night

86

temperature control in October and early November would allow lower
temperatures to be used in late November and early December. Spacing
plants as they were marketed would increase canopy irradiance levels
resulting in potentially greater carbohydrate levels available to the

cyathia.

87

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Table 5. Mean glucose equivalents per 3 leaf or bract disks sampled at 6. 8. or 9

a from bracts and leaves of poinsettia 'Annette
Hegg Dark Red' grown at 16 C or 21 C night temperatures under normal daylight or

weeks after the start of sBort day

75% shade. Exp. 5.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LEAVES
Heeks 0
after 15° 21
‘3" Normal da li ht 75 a: shade Normal da li ht 75 a: shade
short Hhole Bracts Hhole Bracts Hhole Bracts Nhole Bracts
days plant removed plant removed plant removed plant removed
6 29 -- 87 -- 34 -- 44 --
8 57 75 18 24 20 69 lo 18
9 39 89 14 31 24 83 9 34
Contrastsz
Hhole plant vs Bract removal ..,y
Normal daylight vs 75% shade ***
16°C vs 21° 0 NS
Hhole plant vs Bract removal (16 ) **
Normal daylight vs 75: shade (16° m
Hhole plant vs Bract removal 21: ***
Normal daylight vs 75% shade 21 **
weeks 0 0
after 16 21
sgart Normal daylight 75% shade Normal daylight 75 z shade
short whole Leaves Hhole Leaves Hhole Leaves Hhole Leaves
days plant removed plant removed plant removed plant removed
6 -- -- -- -- C- O- O- --
29 31 22 32 17 31 12 21
9 17 25 14 21 10 22 9 8
Contrasts

whole plant vs Leaf removal
Nosmal daylight vs 75% shade
16 C vs 21

**
i
W.

whole plant vs leaf removal (16:) *
Normal daylight vs 75% shade 160) NS
whole plant vs Leaf removal 210) NS
Normal daylight vs 75% shade (21 ) *

 

zContrasts on data at 63 days after the start of short days.

Nonsignificant (NS) or significant at 5% (*). 1% (**) or <.l% (***) level.

92

Figure 1. Percent photosynthetically active radiation transmission
into poinsettia 'Annette Hegg Dark Red' canopies at plant
spacings of 11, 33, 6S, and 97 plants m-Z. Expt. 4.

93

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Literature Cited

l.
2.

3.

6.

10.

94

Literature Cited

Addicott, F. T. 1982. Abscission. Univ. Calif. Press, Berkley.

Crane, J. C., I. Al-Shalan, and R. M. Carlson. 1973. Abscission of
pistachio inflorescence buds as affected by leaf area and nunber of
ants. Jo MEI. We Ibrto $1... 98:591-592.

Durieux, A. J. 8., G. A. Kamerbeek, and U. Van Meeteren. 1983. The
existance of a critical period for the abscission and a non-
critical period for blasting of flower-buds of Lilium 'Phchantment'
3 influence of light and ethylene. Scientia Hortic. 18:287-297.

El-Zik, K. M., H. Yamada, and V. T. Walhood. 1980. Effect of
management on blooming, boll retention, and productivity of upland
cotton. m hirsutum L. Pp. 1-4. In: Beltwide Cotton Prod.
Res. Conf. Proc.

Heindl, J. C., ind W. A. Brun.1983. Light and shade effects on
abscission and 4C-photoassinalate partitioning ammg reproductive
structures in soybean. Plant Physiol. 73: 434-439.

Miller, S. E., and R. D. Heins. 1984. Envirormental factors
influencing premature cyathia abscission in poinsettia 'Annette Hegg
Dark Red'. Unpublished (manuscript).

Mar, Y., and A. H. Halevy. 1980. Pramtion of sink activity of
developing rose shoots by light. Plant Physiol. 66 :990-995.

Noggle, G. R., and G. J. Fritz. 1976. Introductory Plant Physiology.
Prentice-Hall, Englewood Cliffs, N. J.

Patrick, J. W. 1976. norm-directed transport of netabolites. p.
433-436. In: J. F. Wardlaw and J. B. Passioura (eds.). Transport
and Transfer Processes in Plants. Academic Press, New York.

Schneider, G. W. 1977. Studies on the mechanism of fruit abscission
in apple and peach. J. MEI. Soc. Hort. Sci. 102:179-181.

12.

13.

14.

15.

16.

17.

18.

19.

95

Scott, L. F., T. M. Blessington, and J. A. Price. 1983. Postharvest
effects of tenperature, dark storage duration, and sleeving on
quality retention of "Gutbier V—l4 Glory' poinsettia. HortScience
18: 749-750.

. and ..l984 Postharvest
effects of storage method and duration on quality retention of
poinsettia. HortScience 19:290-291.

 

Seth, A. R., and P. F. Wareing. 1967. Hormone-directed transport
of metabolites and its possible role in plant senescence. J. Exp.

Staby, G. L., and A. M. Kofranek. 1979. Production conditions as
they affect harvest and postharvest characteristics of poinsettias.
J. Alter. Soc. Hort. Sci. 104:88-92.

Subhadrabandu, S.,M. W. Adams, and D.A.Reiocsky.l978.
Abmissicn of‘flcwers and fruits in Phaseolus vulgaris L. I.
Cultivar differences in flowering pattern and abscission. Crw.
Sci. 18:893-896.

'Ihaine, R., S. L. Ovender, and J. S. Turner. 1962. Translocation
of labelled assimilates in the soybean. Aust. J. Biol. Sci. 12:

Van Steveninck, R. F. M. 1957. Factors affecting the abscission of
reproductive organs in yellow lupins (M ;u_t;e_u_§ L.) I- The
effect of different patterns of flower renoval. J. Exp. Bot. 8:
373-381.

Woodhead, S. 8., and A. E. Einert. 1973. Influence of the hate
environment on poinsettia development. Florists' Review 152(1027) :
34-35, 77-80.

Yager, R. E. 1959. Effect of remval of leaves at various
developmental stages upon floral abscission in tobacco. Phyton 13 :
125-1310

VARIATION] IN POINSETI'IA CULTIVAR SEI‘BITIVITY TO CYATHIA ABSCISSICN

STEVEN n. MILLER and R. o. HEINSl
1mm of HorticulturekMichigan State University,

Michigan Agricultural Experiment Station Journal Article No. _.
(Accepted for publication )
ABSTRACT

Miller, S. H. and R. D. Heins. 1984. Variation in poinsettia cultivar

sensitivity to cyathia abscission. Scientia Hortic.,

Differences in sensitivity to cyathia abscission on poinsettia
cultivars 'Annette Hegg Dark Red' (Dark Red), 'Annette Hegg Lady'
(Lady), 'Annette Hegg Brilliant Diamond' (Brilliant), 'Gutbier V-l4
Glory' (V-l4), and 'Mikkel Triumph' were evaluated in the greenhouse and
in a postharvest environment. Abscission was evaluated chronologically
based both on the number of days after the start of short days (SD) and
on the number of days after anthesis. Seventy-seven days after the start
of SD, V—14 had the least abscission of the tested cultivars in the
greenhouse or postharvest environment, while Lady had the greatest
abscission. In contrast, 7 days after anthesis, V-14 had the greatest
abscission in the postharvest environment while Brilliant and Dark Ebd
had the least abscission. The differences in abscission on V-l4 based on

keywords: wrbia galcherring, carbohydrates, cyathium
96

Section III
Variation in Poinsettia
Cultivar Sensitivity to

cyathia Absc ission

97

evaluation method was due to it's reaching anthesis 7 to 10 days later
than the other cultivars. Abscission was greater in all cultivars in the
postharvest environment than in the greenhouse probably due to the lower
photosynthetically active radiation (PAR) levels in the postharvest
environment (5.1 mol d"1 in the greenhouse compared to 0.29 mol d“1 in

the postharvest environment).

MICE

Much of the potential variation in poinsettia postproduction
quality is related to genetic background (Rogers, 1981). Many of the
early cultivars had poor bract and leaf retention (Ecke and Matkin,
1976), however later breeding efforts introduced cultivars that
decreased this problem (Shanks, 1981).However, cyathia abscission is
still a problem in greenhouses and marketing which results in plants
looking prematurely aged (Shanks, 1981).

Flower or petal abscission has been attributed to low light
(Fortanier and Zevenbergen, 1973: Halevy, 1975; Kinet, 1977), high
temperatures (Fitting, 1911: Wittwer, 1954; Armitage et al., 1980), and
carbohydrate depletion in the flower (Kofranek, 1951; Subhadrabandu et
al., 1978; Durieux, et al., 1983). Information on cyathia abscission is
limited and the mechanism which causes it is unknown (Staby and
Kofranek, 1979; Miller and Heins, 1984a). Cyathia abscission occurs in
plants grown under low light (Staby and Kofranek, 1979) or in plants

placed in a postharvest environment with low light and warm temperatures

98

(Woodhead and Einert, 1973; Staby and Kofranek, 1979). 'Ihe longer plants
were held in dark storage the greater the cyathia abscission (Scott et
al., 1983; Scott et al., 1984).

Sprays of gibberellin or gibberellin plus 6-benzylamino purine
late in the crop delayed cyathia abscission (Shanks, 1981), but these
hormones can cause excessive stem elongation, smaller bracts, and
delayed flowering (Miller and Heins, unpublished results).

Information on cultivar sensitivity to cyathia abscission is
limited. Scott et a1 (1984) observed that 'V-l4' had less cyathia
abscission than 'Dark Red Begg' or 'Mikkel Improved Rochford' after
being held in either a dark or lighted postharvest environment. This was
attributed to 'V-l4' being more tollerant of stress than other
cultivars (Hammer et al., 1981: Shanks, 1981; Scott et al., 1983). The
objective of this research was to determine if there were differences in
cultivar sensitivity to cyathia abscission both in the greenhouse and in

a simulated postharvest environment.
MATERIALS AND NEH-IDS

garimental conditions. Rooted cuttings of 'Annette Hegg Dark md'm'
(Dark Red), 'Annette Hegg LadyTM' (Lady), 'Annette Hegg Brilliant
DiamondTM' (Brilliant), and 'Qitbier V-l4 GloryTM' (1714) from Paul Ecke
Poinsettias, Encinitas, CA. and 'Mikkel TriumphTM' from California-
Florida Plant Corp., Fremont, CA. were received 25 Aug 1983, (Expt. 1)
and 22 Sept 1983, (Expt. 2). (be cutting was planted per 10 cm plastic

pot (an experimental unit) in VSP medium (Michigan Peat Co., Houston,

99

Tx.) composed of 2 peat: l perlite: 1 vermiculite (v:v:v) ammended with
dolomitic limestone, superphosphate, and trace elements. Plants were
placed in a single layer glass greenhouse and grown single stem, at a
spacing of 33 plants m-Z. Plants for Expt. 1 were initially grown under
natural daylight (ND) and plants for Expt. 2 were initially grown,
under ND plus 4 hr (2200 hr to 0200 hr) of 5 pmol s"1 m‘2 of
photosynthetically active radiation (PAR) from 60 W incandescent lamps
to prevent flower initiation. Short days (SD) were initiated 1 Sept
(Expt. 1) and 1 Oct (Expt. 2). Black sateen cloth was pulled from 1600
hr to 0800 hr daily throughout both experiments.

The night temperature (NT) setpoint during vegetative growth was
18°C. Day temperature (or) and venting temperature setpoints were 3°C
and 6°C above the NT respectively in all experiments. The NT was set at
160 when so started. Plants were fertilized with 260 mg 1-1 n, 130 mg 1"
1 K, and 0.1 mg 1"]- Mo. Chlormequat was applied as a foliar spray for
height control at 1500 mg 1‘“JL at the start of SD and 1 weeks later, and
at 750 mg 1'1 after 3 weeks of SD. When 50% of each cultivar reached
visible, bud half of the plants were moved to a 210 NT greenhouse.
Plants remained at the two NT until anthesis. When 50% of the plants in
each cultivar reached anthesis, half of the plants were moved and
evaluated in a simulated postharvest environment at 18° Nl‘ with an 8 hr
photoperiod (0800 hr to 1600 hr) under 10 umol 3'1 m‘2 PAR from 40 W
cool white fluorescent lamps measured at the top of the bract canopy.

The remaining plants were evaluated in the greenhouse.

100

Data collection. The date of anthesis and the number of abscised cyathia
were recorded daily for 25 days past anthesis for each plant. The total
number of cyathia per plant was determined at the end of the experiment
by counting the number of cyathia greater than 1 mm still present plus
the number of stubs remaining from previously abscised cyathia. Percent
abscission per plant was calculated by dividing the number of abscised
cyathia by the total number of cyathia formed.

Statistical analysis. The experiments were analyzed as a completely
randomized design, split, split plot, with NT as the main plot,
evaluation environment and cultivars as subplot factors. There were 20
treatments (2 NT x 2 environments x 5 cultivars), 5 plants per
treatment. Statistical significance was determined by analysis of
variance and Duncans multiple range test was used to compare cultivars
within each treatment (Steel and Torrie, 1980).

REULTS AN) DISQBSICN

Through 77 days from the start of SD, V14 had less abcission than
the other cultivars, in both the greenhouse and in the postharvest
environment (Table 1, Figures 1, 2). Lady consistantly had greater
abscission than the other cultivars throughout the evaluation period in
both environments. In the greenhouse, Brilliant had less abscission than

101

Dark Red, Lady, or Mikkel Triumph (Figure 1) while in the postharvest
environment, Brilliant and Dark Red had less abscission than Lady or
Mikkel Triumph (Figure 2).

When evaluated after a common number of days after anthesis,
Brilliant had less abscission than the other cultivars with one
exception in the postharvest environment where Dark Red had less
abscission (Table 1, Figures 3, 4). Abscission in V14 was equal to or
greater than the other cultivars at the same stage of physiological
development. In the postharvest environment, V14 had the greatest
abscission (Figure 4), while Dark Red had the least abscission.
Abmission was greater for all cultivars in the postharvest environment
than in the greenhouse (Figures 3, 4).

In Expt. 1, Dark Red, Lady, Brilliant, and Mikkel Triumph
reached anthesis in approximately 55 days when plants were grown at 16°
while V14 reached anthesis 7 to 9 days later (Table 2). When plants
were grown at 21°, all cultivars flowered 3 to 4 days earlier except
for Mikkel Triumph which flowered 2 days later. Compared to the "Hegg”
cultivars, V14 reached anthesis 7 to 10 days later.

In Expt. 2, plants flowered 4 to 11 days later than in Expt. 1
when grown at 16° and 3 to 5 days later when grown at 21°. Flowering in
Mikkel Triumph and Dark Red was delayed more than in the other
cultivars. There was more variation in time to flower between cultivars
in Expt. 2 than in Expt. 1. However, variation was less at 210 than at
16°.

While V-l4 had less cyathia abscission through 77 days after the

102

start of SD, on a chronological basis, when comparing all cultivars at
anthesis, V-l4 had equal to or greater abscission than the other
cultivars. The reason V-l4 had less abscission through 77 days of SD
was because it reached anthesis later than the other cultivars,
therefore the cyathia were not as physiologically old. In V-14 the
cyathia that were typically first to abscise were the younger, outer
cyathia (personal observation). Therefore, even though abscission was
greater for V-l4, it was not as visibly apparent as in the other
. cultivars, which normally abscised the older inner cyathia first leaving
a distinct "open center” in the inflorescence.

There was greater cyathia abscission in the postharvest
environment than in the greenhouse probably due to the lower irradiance
level in the postharvest room. Actual total irradiance received by
plants in the greenhouse during the 25 days past anthesis averaged 5.89
mol d‘1 in Expt. 1 and 4.36 mol d'1 in Expt. 2. This compares with 0.29'
mol d‘1 in the postharvest room. Low irradiance levels have been shown
to promote cyathia abscission (Woodhead and Einert, 1973; Staby and
Kofranek, 1979: Miller and Heins, 1984a: Scott et al., 1984).

Temperatures higher than the greenhouse are often found in
postharvest environments and also promote cyathia abscission (Woodhead
and Einert, 1973: Staby and Kofranek, 1979). In contrast plants finished
at 210 were evaluated under lower temperatures in the postharvest room
(ca. 19°) and still abscised more cyathia than when evaluated in the
greenhouse. When cultivars were evaluated in the greenhouse, plants

grown at 16° NT had less abscission than plants grown at 21° (Table l)

103

however, there was little difference in abscission between plants grown
at 160 or 210 and evaluated in the postharvest environment, probably due
to the lower irradiance levels. Therefore irradiance appears to be the
primary environmental factor controlling absc ission.

The cyathia abscission under low irradiance levels in the
postharvest environment was probably due to declining carbohydrates in
the plant (Addicott, 1982). Bracts and leaves show a steady decline in
carbohydrate content as plants approach anthesis, declining faster under
reduced irradiance and/or higher temperatures (Miller and Heins, 1984b).
Carbohydrates are probably also declining in the cyathia at the same
time.

In the greenhouse, declining natural irradiance levels in the
fall and expanding bracts shade the leaves and decrease photosynthesis.
reducing carbohydrate supply. High temperatures will also increase
respiration rates which will also reduce carbohydrates in the plant
(Noggle and Fritz, 1976). During expansion, bracts compete with the
cyathia for carbohydrates. The bracts appear to be stronger sinks as
they do not abscise (Miller and Heins, 1984b). Similarly in _I_._i_l_igm
(Durieux et al., 1983), the youngest flower buds, being the weakest
sinks prematurely abscise as a result of competition for carbohydrates
(Fawazi and El Fouly, 1979).

These results show that through 77 days of SD, V14 was superior
to the other cultivars. The delayed abscission in V14 is due to delayed
anthesis. 0f the 'Heggs", cyathia abscission in Brilliant is delayed

relative to the other cultivars. mce anthesis is reached, abscission in

104

V14 progresses as rapidly as in the Hegg cultivars. Under all
circumstances, Lady appears to abscise cyathia faster than the other
tested cultivars. Warm finishing temperatures and/or low irradiance
levels late in crop development or in the postharvest environment
promotes abscission.

To decrease the occurance of cyathia abscission, early flower
initiation in late September is important to maximize reproductive
growth under the higher irradiance levels in early fall. Proper night
temperature control in October and early November will permit lower
night temperatures in late November and Decenber and hence should help
reduce abscission. Spacingplants to maximize light penetration as
plants are marketed is another method to help maintain carbohydrate
levels in the plant and delay abscission.

W

The authors appreciate the helpful discussions and ideas of Dr.
William H. Carlson and Dr. Arthur Cameron on this research project. This
project was supported in part by a grant and plant material from Paul
Ecke Poinsettias, Encinitas, CA. Plant material was also provided by

California-Flor ida Plant Corp., Fremont, CA.

105

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106

Table 2

Number of days to anthesis for five poinsettia cultivars grown at 160
until visible bud (ca. 28 days), then at 160 C or 21° C night temperature
to anthesis

 

 

 

 

 

 

Temperature
Cultivar 16°C 21°C
Expt. 12 Expt. 2 Expt. 1 Expt. 2

'Annette Hegg Dark Red' 54 a’ 65 b 51 a 56 a
‘Annette Hegg Lady' 56 a 63 b 52 a 57 a
'Annette Hegg Brilliant Diamond' 54 a 58 a 50 a 55 a
'Gutbier V-l4 Glory' 63 b 70 c 60 b 64 b
'Mikkel Triumph' . 54 a 65 b 56 ab 59 a

 

zShort days started on Sept. 1 - Expt. 1; Oct. 1 - Expt. 2.

yMean separation within columns using Duncan's multiple range test
(5% level).

107

Figure. 1. Mean percent cyathia abscission 56 to 98 days after the
start of short days of five poinsettia cultivars
finished at 21°C night tenperature and evaluated in
the greenhouse. Day of anthesis indicated by -> .
Ebtpt. 2.

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109

Figure. 2. Mean percent cyathia abscission 56 to 98 days after
the start of short days of five poinsettia cultivars

finished at 21°C night tenperature and evaluated in a

postharvest environment. Day of anthesis indicated by
# O apt. 2.

110

 

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111

Figure. 3. than percent cyathia abscission 7 days before anthesis
to 35 days after anthesis of five poinsettia cultivars
finished at 21°C night tenperature and evaluated in
the greenhouse. Day of anthesis indicated by —>
Expt. 2.

112

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NOISSIUSSU lNBUBEd

113

Figure. 4. Mean percent cyathia abscission 7 days before anthesis
to 35 days after anthesis of five poinsettia cultivars
finished at 21°C night tenperature and evaluated in
a postharvest environment. Day of anthesis indicated
by ... . Expt. 2.

114

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lbferences

115

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