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‘7’

THE EFFECT OF SOIL TEMPERATURE ON THE GROWTH
OF GREENHOUSE ROSES

Thesis for the Degree of M. S.
MICHIGAN STATE UNWERSTTY'
DAVlD LEE SANFORD
1976

THESIS

 

 

ABSTRACT

The Effect Of Soil Temperature on

The Growth Of Greenhouse Roses

By

David Lee Sanford

A comparison was made of the root growth, shoot growth, and
flowering of the rose cv. Bridal Pink at soil temperatures ranging
from 5° to 35°C. Records included initial root and shoot growth of
dormant plants in Study I and in Study 11 data included number of
nodes per flowering stem, flower stem length, fresh weight of the
flower, days required for a return of stems to flowering, and internal
water stress on actively growing plants.

Results showed root regeneration increased as soil temperature
was increased from 5° to 25°C with shoot growth increasing in a similar
manner as soil temperature increased. Stem lengths of flowering shoots
were significantly shorter at a soil temperature of 35°C than at the
other soil temperatures. The number of nodes was significantly re-
duced at only the 5° and 10°C soil temperatures. Fresh weights did
not differ statistically among the treatments. A soil temperature
of 25°C produced 55.5% greater number of blooms than those grown at

l0°C. A fewer number of days were required for return to flowering

David Lee Sanford

as soil temperature increased from 5° to 30°C. Internal water stress
was reduced at soil temperatures of 20° and 25°C when compared to

higher and lower soil temperatures.

THE EFFECT OF SOIL TEMPERATURE ON
THE GROWTH OF GREENHOUSE ROSES

By

David Lee Sanford

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Horticulture

1976

To Trish

ACKNOWLEDGMENTS

I wish to express sincere appreciation to Dr. William J. Carpenter
for his guidance during the course of my research project and in the
preparation of this manuscript. Appreciation is given to Dr. William H.
Carlson for his assistance in the final stages of this manuscript and
my graduate studies.

Appreciation is expressed also to Dr. Roy Mecklenburg and
Dr. Irving Knobloch for their suggestions in editing the manuscript.

Special appreciation is expressed to my wife, Trish, for her

patience and encouragement throughout the course of my graduate program.

iii

TABLE OF CONTENTS

Page

LIST OF TABLES ......................... v
LIST OF FIGURES ................ 7 ...... '. . vi
INTRODUCTION ................. . ........ l
. REVIEW OF LITERATURE .......... ' ........ . . . . 2
Temperature and Root Growth ................ 2

. Temperature and Shoot Growth ............... 4

, Soil Temperature and Water Stress .......... . . . 6
MATERIALS AND METHODS ..................... 8
Study I .......................... 8
Study 11 ......................... l2
Pressure Bomb Data Collection ............... 13
RESULTS ............................ 15
Study I .......................... l5
Study 11 ......................... 23
DISCUSSION. . . 5.. . ..................... 4l
SUMMARY ............................ 45
BIBLIOGRAPHY .......................... 46

iv

LIST OF TABLES

Table ' Page

1. The effect of soil temperature on root growth 10 days
after planting dormant plants of rose cv. 'Bridal Pink'. . . 20

2. The effect of soil temperature on shoot growth To days
after planting dormant plants of rose cv. 'Bridal Pink'. . . 24

3. The effect of soil temperature on flower production of
rose cv. 'Bridal Pink' ................... 27

LIST OF FIGURES

Figure

l.

10.

Experimental equipment for maintaining medium tempera-
tures: (A) shows the systems for maintaining the warm

and cool medium temperatures, (B) shows the chamber top
with hole cut for insertion of container, (C) shows the
container inserted in the chamber. Note the perlite on
soil surface for insulation from air temperature and the
thermo-couple inserted into growing medium .........

Pressure Bomb Diagram ...................

The effect of soil temperature on the growth of roots

and shoots 10 days after planting dormant Triple X rose

cv. 'Bridal Pink'. Plants were grown at: (A) 5°C;

(B) 10°C; (C) 15°C; (0) 20°C soil temperatures .......

The effect of soil temperature on the growth of roots and
shoots 10 days after planting dormant Triple X rose cv.
'Bridal Pink'. Plants were grown at: (A) 25°C; (B) 30°C;
and (C) 35°C soil temperatures ...............

Effect of soil temperature on root count and length 10 days
after planting dormant plants of rose cv. 'Bridal Pink' . .

Effect of soil temperature on shoot count and length 10
days after planting dormant plants of rose cv. 'Bridal
Pink' ...........................

Effect of soil temperature on flowering stem length of rose
cv. 'Bridal Pink' .....................

Effect of soil temperature on node number per flowering
stem of rose cv. 'Bridal Pink' ...............

Effect of soil temperature on fresh weight of flowering
stems of rose cv. 'Bridal Pink' ..............

Effect of soil temperature on flower production on rose
cv. 'Bridal Pink' from February 8, 1972 to May 9, 1972. . .

vi

Page

14

16

18

21

25

28

3O

32

35

Figure Page

11. Effect of soil temperature on days to flower of flowers
harvested from rose cv. 'Bridal Pink‘ ............ 37

12. Effect of soil temperature on internal water stress for
rose cv. 'Bridal Pink' ................... 39

vii

INTRODUCTION

Cut roses are one of the nation's leading commercially-grown
cut flowers. The production of roses has become highly competitive
in a changing business environment which requires the careful
attention of the producer. This places the responsibility on the
grower as to how resources will be used in the production of a crop.
The environment under which a plant is placed will determine how
much of the potential growth that can be achieved.

Comparatively little attention has been given to the impor-
tance of root temperature in the growth of the rose plant, as is
true with most other crops. This study was undertaken to investi-
gate the effect that various soil temperatures would have on the

growth of rose plants.

REVIEW OF LITERATURE

Temperature and Root Growth. The growth of roots is influenced by
soil temperature and carbohydrate supplies from the photosynthetic
processes of the plant. Each genus and species seems to have its
own optimum root temperature for growth. Darrow (23) found that

the greatest root growth (rhizome production) in Poa pratensis

 

(bluegrass) occurred at soil temperatures of 15° and 25°C and de-
creased at higher temperatures. Wort (72) working with Marquis
wheat, found total root growth decreased with increasing tempera-
tures from 22° to 42°C. Stuckey (65) and Brown (13) have also
found the greatest root growth in forage crops occurs in the early
spring and fall when soil temperatures are low.

Dickson (26) and Wort (72) working with wheat showed that
greatest root growth occurred when soil tempéetures were relatively
low. They reported that corn roots made their greatest growth at
higher soil temperatures than for most cereal crops studied.

Working with vegetable crops, Burkholder (l4), Richards (54),
and Bushnell (15) have reported growth responses to soil tempera-
ture also vary with species and environmental conditions. They
showed that bean top growth and yield increased as the soil tem-
perature was increased from 18° to 33°C, and that potato tops
grew best as a soil temperature of 21° but tuber formation was

optimum at 17°C.

Bates et.al. (7) compared root growth of woody plant species
and found that soil temperature is the most important factor in
controlling the growth of Englemann spruce. Barney (4) working
with loblolly pine seedlings reported increased root growth as
soil temperature was increased to 25°C then decreased at higher
soil temperatures. Larson (43) reported that no optimum soil
temperature was evident for root growth in Red Oak Seedlings, but
that low temperatures severely retarded root regeneration. Toy-
nette et.al. (67) studying five woody plant species noted differ-
ences in their tolerance to short periods of high soil temperatures.
Rose and peach were the least tolerant to high soil temperatures
of the five species studied.

Apple roots were found by Collison (20) to make some root
growth at temperatures near 0°C. Nightingale (48) noted that
less root growth occurred on dormant plants placed at 7°C than
on actively growing plants that were placed at this low tempera-
ture. He found the optimum soil temperature for apple and
peach roots at approximately 18° with poorer root growth at
extremes of 7° and 35°C. Batjer et.al. (8) obtained similar re-
sults using dormant apple trees. Rogers (56) and Proebsting (52)
reported apple trees develop more roots as the temperature was
raised from 7° to 21°C. Nelson and Tukey (47) found that new
roots of E. Malling rootstocks were initiated significantly
earlier at soil temperatures of 18.8° and 25.0°C than at 12.7°
and 6.6°C.

Woodruff and Woodruff (71) studied the effect of root

temperatures on pecans; Girton (29) and Hass (31) citrus;

Gray (30) and Roberts (55) strawberry; Woodham and Alexander (70)
and Skene and Kerridge (61) grapes, and Pearson et.a1. (49) cotton;
all reported a close relationship between temperature and root
growth.

Bateman and Dimock (6) observed that the rate of root develop-
ment of poinsettia was greatest at soil temperatures near 26°C.
with no root growth occurring below 13°C and a progressive re-
duction in growth at soil temperatures above 30°C.

Working with greenhouse roses, Shanks and Laurie (60)
reported a progressive decrease in the fresh weight of roots as
the temperature increased from 13° to 22°C.

Hansen (32) found while working with the propagation of
Yew, Poinsettia, and English Ivy that root initiation and
elongation were optimum at 25°C, while Geranium had the greatest
root initiation and elongation at 30°C than at higher or lower

medium temperatures.

Temperature and Shoot Growth. Cannon (17) found shoot growth
could be increased under unfavorable atmospheric conditions by
maintaining favorable root temperatures for growth. Barr and
Pellett (5) reported that the soil temperature for greatest
shoot growth was generally higher than the soil temperature
for greatest root growth.

The effects of root temperature on shoot growth of forage
' crops has been reported by Jones and Tisdale (34), Darrow (23),

Studkey (65), Earley and Catter (27), Maderski and Jones (46),

and Knoll et.a1. (38). Even though there were differences between
the species studied, top growth was generally reduced at excessively
low and excessively high root zone temperatures.

Dickson (26) and Wort (72) found that the optimum root tem-
perature for maximum shoot growth for wheat varied with age of the
plant. Higher temperatures were optimum during early stages of
growth while lower root temperatures were optimum for later stages.
Dickson (26) found the response of corn to be the opposite that
observed for wheat.

Working with peas, Brenckley and Singh (11), found that
root temperature appears to be of greater importance than air tem-
perature, since good growth can be produced under high air tempera-
' tures provided the roots are kept relatively cool. Lingle and
Davis (44) working with tomato seedlings, showed shoot growth
increased with root temperature from 10° to 25°C. When the root
temperature was at 10°C, the growth ratio of the shoots to roots
approached zero.: Tew, et.al. (66) reported that sunflower plants
grown at a 10°C soil temperature had only 10% the leaf area as
at 25° and 40°C.

Nightingale (48), Batjer et.al. (8), Proebsting (52),

Rogers (57), and Nelson and Tukey (47) found the top growth of
apple trees to be closely associated with root growth. Bailey
and Jones (3) reported that blueberry height and linear growth
increased with increasing root temperatures from 13° to 32°C.
Stephens (64) has reported similar results on Yellow-Poplar
seedlings. Larson (43) reported reduced shoot growth of Red Oak

seedlings at low soil temperatures and favorable air temperatures.

Jones (35) reported Gardenia veitchii size of leaf and rate
of growth are related to soil temperature. Leaf size increased
with increased temperature to 32°C. Woodham and Alexander (70),
observed increased grape shoot growth as the soil temperature
increased from 10° to 30°C. .

Allen (1) reported a reduction in the flowering of several
greenhouse crops grown at excessively low and excessively high
temperatures. Seeley (59) found heating snapdragon soil to a
minimum of 21°C produced more flowering stems and earlier flowering,
but the differences were non-significant. Seeley (59) also found
no advantage in heating carnation soils. Carpenter and Rasmussen
(19) reported Chrysanthemum plant heights and fresh weights
decreased as irrigation water temperatures decreased from 18° to
2°C.

Pfahl et.al. (50) and Kohl et.al. (39) observed a reduction
in the flowering of roses grown at excessively low and excessively
high soil temperatures.' Carpenter and Rasmussen (19) using cold
irrigation water on roses found no difference in numbers of
flowers, but stem lengths, and fresh weights were reduced signi-
ficantly. Shanks and Laurie (60) found the fresh and dry weight
of shoots of Better Times Roses increased directly with soil
temperatures. It was determined that a root temperature of
approximately 18°C was optimum for shoot growth where night air

temperatures were maintained at 16°C.

Soil Temperature and Water Stress. Plant growth is influenced

by the internal water stress of a plant. Limiting water restricts

plant growth by reducing the physiological processes within the
plant. Crafts (21) gives an excellent discussion on how water
deficits influence physiological processes. Kramer (40, 41)
has shown that root zone temperature affects the water absorption
capacity of roots which can cause plant water stress.
It has been shown by several workers (10,25,36,37,58,68)
that the pressure bomb could be used to evaluate the status of
water within a plant. The water column within a plant is generally
under tension which results from the demand of the leaves for
replacement of water lost through transpiration. When a stem
is cut, the water column withdraws into the stem. To measure the
original tension, the cut stem is placed, with the cut end pro-
truding through a seal, in a pressure chamber. Pressure is
applied to the leafy stem and forces the water column back to
the cut surface. When this water is observed the pressure is
assumed to be the tension on the water column before it was cut.
Waring and Cleary (68) have shown the importance of standar-
dizing the testing procedure. If the pressure is increased
very slowly or very rapidly higher readings may be recorded,
therefore, standardizing the rate of increase is essential
for comparing treatments. This rate will vary with plant species,
for example, Waring et.al. has shown for Douglas fir a rate of
increase at about 10 psi per second to be satisfactory whereas
2.5 psi per second pressure increase with more herbaceous materials

may be necessary.

MATERIALS AND METHODS

Study I. Dormant plants of rose cv. Bridal Pink were planted in
a medium consisting of a 1:1 mixture of peat and perlite in 17.5
cm plastic pots. The pots were placed in experimental chambers
for refrigeration or heating to maintain constant medium tem-
peratures of 5°, 10°, 15°, 20°, 25°, 30°, or 35°C (Figure l).

The refrigeration system consisted of four refrigerated
units 60 x 60 cm with a 30 cm depth with tubes containing cir—
culating freon around the inside perimeter to maintain the de-
sired air temperature. The top of each unit was 5 cm thick
styrafoam with holes 17.5 cm diameter to suspend four pots in
each unit. By controlling the temperature of the air surrounding
the pots, the desired medium temperature was maintained at 5°,
10°, 15°, and 20°C. Perlite was placed over the medium in the
pots to insulate it from the constant room temperature of 21°C.

To maintain the warm medium temperature of 25°, 30°, and
35°C, three styrafoam units each of similar size (70 x 52 x 30 cm)
were constructed. Each chamber was heated by two 60 watt in-
candescent light bulbs connected to individual thermostats. Four
pots were suspended in each chamber through the lid made of 5 cm
thick styrafoam.

Rose plants were lighted 16 hours daily beginning after
planting the dormant plant, using Wide Spectrum Gro Lux fluore-

scent tubes mounted 4 inches apart in 4-foot by 8-foot fixtures.
8

Figure 1. Experimental equipment for maintaining medium temperatures:
(A) shows the systems for maintaining the warm and cool medium tempera-
tures, (B) shows the chamber top with hole cut for insertion of con-
tainer, (C) shows the container inserted in the chamber. Note the per-
lite on soil surface for insulation from air temperature and the thermo-
couple inserted into growing medium.

 

TO

 

 

11

There were 12 tubes per fixture. Each tube emitted 73 lamp watts,
totaling 876 lamp watts per 32 square feet. The tubes when
lighted emitted 27.4 lamp watts per square foot.

The root initiation and shoot break data were collected from
plants growing in a constant temperature room to maintain a 21°C
air temperature for all 7 media temperature treatments.

Thermocouples of 24-gauge copper-constantan wire were
placed approximately in the center of each pot during all studies.
A 24-point recording potentiometer was used for recording soil
temperatures for one hour at 6 hour intervals during the day.

The plants were fertilized at each irrigation with a 200
ppm NPK solution throughout the duration of the study.

The root and shoot development from dormant rose plants was
determined after 10 days at each medium temperature. The rose
plants were removed from each treatment and root lengths mea-
sured and root counts determined.

The growing medium was removed from each plant by gently
agitating the roots soaking in a bucket of water. Individual
roots were removed with tweezers and measured in centimeters
from the base of the root to the tip. Shoots also were
counted and the length measured in centimeters.

The experiment was repeated three times from January 31, 1971
to March 12, 1971, using 4 dormant rose plants per treatment for
each trial. Dormant rose plants of Triple X grade were received
from the propagator and stored at 35°F prior to their use in the
study. Data were analyzed statistically using Tukey's test at

the 5% level.

12

Study II. In a second study, four dormant plants of rose cv. Bridal
Pink were planted in a soil medium consisting of a 1:1:1 mixture

of soil, peat, and perlite at each of seven soil-medium temperatures
used in Study 1. The chambers were moved into a greenhouse at re-
commended day and night air temperatures and at the seasonal solar
radiation for the duration of this experiment (April 1971 through
May 1972). Similar air temperatures were maintained at 21° to 23°C
days and 18°C nights for all treatments during the period when
flowering data was being collected (February, March, April, and

May 1972), while treatment soil medium temperatures were 5°, 10°,
15°, 20°, 25°, 30°, and 35°C. After flower buds had developed in
the initial stems; each stem was pinched once removing the bud

and the stem to below the first 5-leaflet rose leaf. The shoots
developing from the auxillary buds were allowed to flower.
Commercially recommended environmental and cultural recommendations
were followed during the study (45).

After February 8,.1972 all canes were tagged and dated after
each flower was cut. The.flowers were cut at their usual commer-
cially recommended stage, leaving one 5-leaflet on the new rose
canes. When canes flowered again, a new tag was dated and attached
to the cane.

Data collected immediately after cutting included date cut,
number of nodes per flowering stem, length of stem in centimeters
from the base of the bud to the point where the stem originated, the
fresh weight of the flower in grams.

Growth and flowering data collections were terminated May 9, 1972.

13

Pressure Bomb Data Collection. The pressure bomb equipment consisted

 

of a pressure chamber, a source of pressure (compressed nitrogen),

and a method of measuring the applied pressures (pressure gauge).

Figure 2. The procedure followed was as described by Scholander et.a1.(58)
and Waring and Cleary (68).

Data were collected on cloudy as well as sunny days with air
temperatures ranging from 25° to 32°C. Samples were cut beginning
at 2:00 pm. each day and immediately (less than 60 seconds) placed
into the pressure chamber and a reading taken before proceeding to
the next sample.

The sample, consisting of a leafy shoot 10-12 cm long, was in-
serted through a rubber stopper with a hole which allowed the cut end
of the sample to protrude 0.5 cm through the stopper. The stopper
was then firmly pressed into the chamber cover and the cover installed
into the chamber body. Pressure was then applied at 2.5 psi per
second by adjusting the rate control valve. Close observation of the
cut stem surface showed a film of water appearing, at which time the
chamber shut-off valve was closed. The plant moisture stress was then
read from the pressure gauge. This procedure required less than 60

seconds per sample.

14

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2.-—Pressure Bomb

a. Compressed gas pressure source
b. Pressure gauge

c. Valve

d. Stainless steel pressure chamber
e. Screw cap

f. Rubber stopper

g. Leafy shoot

h. Sample cut end

RESULTS

The growth and flowering of rose cv. Bridal Pink was significantly
altered when grown at low and high soil temperatures. Variables com-l
pared in Study I were initial root and shoot growth of dormant plants
and in Study II vegetative stem growth and flower development, which
included the number of nodes per flowering stem, flower stem length,
fresh weight of the flower, timing in days from cut-to-cut, and

internal water stress on actively growing plants.

Study I. The temperature of the growing medium influenced the
initial growth of dormant Triple x rose plants of cv. Bridal Pink.
In general, both root growth and shoot growth were affected in a
similar manner. The photographs (Figures 3 and 4) clearly illustrate
the differences observed in both shoot growth and root growth at the
various medium temperatures.

Root growth increased as medium temperature was increased from
5° to 25°C (Figure 5). The plants growing at a medium temperature
of 5°C produced the fewest number of roots; however, this treatment
did not differ significantly statistically from the 10°, 15°, and
35°C medium temperatures (Table l). The greatest root growth occurred
at a medium temperature of 25°C but this did not differ significantly
from the 30°C media temperature.

Root length was significantly reduced at 5°C when compared to

all other treatments (Table l). A 25°C media temperature produced
15

16

Figure 3. The effect of soil temperature on the growth of roots and
shoots 10 days after planting dormant Triple X rose cv. 'Bridal Pink'.
Plants were grown at: (A) 5°C; (8) 10°C; (C) 15°C; (0) 20°C soil
temperatures. - .

 

18

Figure 4. The effect of soil temperature on the growth of roots and
shoots 10 days after planting dormant Triple X rose cv. 'Bridal Pink'.
Plants were grown at: (A) 25°C; (8) 30°C; and (C) 35°C soil temperatures.

19

 

20

Table l. The effect of soil temperature on root growth 10 days after
planting dormant plants of rose cv. 'Bridal Pink'.

 

 

Treatment Root Number Root Length
(°C) (cm)
5° 0.111 az 0.056 a
10° 10.556 a 0.818 b
15° 64.333 ab 1.032 be
20° 121.111 be 1.478 cd
25° 222.667 d 1.674 d
30° 178.444 Cd 1.674 d
35° 25.667 a 1.543 d

 

Treatment means within the same column with the same letter are not
significantly different by Tukey's test at the 5% level.

21

Figure 5. Effect of soil temperature on root count and length 10 days
after planting dormant plants of rose cv. 'Bridal Pink'.

22

«ca... \ 23:5: .8:

 

:5. 52.... .8:

Soil Temperature ('c)

23

plants with the greatest root length, although this was not different
significantly from the 20°, 30°, and 35°C treatments.

Shoot numbers developing from dormant rose plants were largest
for plants growing in a medium of 30°C, but did not significantly
differ from plants grown at 15°, 20°, and 25°C soil temperatures
(Talbe 2). At a 5°C media temperature, significantly fewer shoots
developed than for the other treatments. An increase in medium tem-
peratures to 30°C resulted in an increase in shoot numbers (Figure 6).

Shoot length was significantly reduced at 5°C media temperature
when compared to the 20°, 25°, 30°, and 35°C medium temperatures,
but the 10° and 15°C medium temperatures were not significantly dif-
ferent (Table 2). Similar to the regeneration of new roots, shoot
length increased as the medium temperature was increased from 5°

to 25°C (Figure 6).'

Study II. Plant growth and flowering of the rose cv. Bridal Pink
was significantly influenced by various soil medium temperatures.

Stem lengths of flowering shoots were significantly shorter
at a soil medium temperature of 35°C than at the other soil medium
temperatures (Table 3 and Figure 7). Stem length was not signifi-
cantly different among the other soil medium temperature treatments.

The number of nodes per flowering stem were significantly
reduced at the 30°C soil media temperature, but the node numbers
did not statistically differ among treatments at 15°, 20°, 25°.
and 35°C (Table 3 and Figure 8).

Fresh weights of the cut flowers were similar statistically

among the soil-medium temperature treatments (Table 3 and Figure 9).

24

Table 2. The effect of soil temperature on shoot growth 10 days after
planting dormant plants of rose cv. 'Bridal Pink'.

 

 

Treatment Shoot Number Shoot Length

(°C) (cm)

5° 6.583 az 1.90 a
10° 8.750 b 2.67 ab
15° 9.500 be 3.00 bc
20° 9.500 bc 3.98 c
25° 9.833 c 4.34 c
30° 10.167 c 3.56 c
35° 8.917 b 3.09 c

 

2 Treatment means within the same column with the same letter are not
significantly different by Tukey's test at the 5% level.

25

Figure 6. Effect of soil temperature on shoot count and length 10 days
after planting dormant plants of rose cv. 'Bridal Pink'.

26

“:0..— \ 23:52 . 1005. «00.5

 

 

 

« m a a 1 a a a a a 1
A
r
m M
9 m
o o n
\ . h m
.. o O
0... o
_/
o
a
a .,
. ...
/
,,..
r
/
z. x.
a. a a a
4 3 I 1

:5: 593.. .095

as
1°C)

15

Soll Tornperaturo

27

Table 3. The effect of soil temperature on flower production of rose
cv. 'Bridal Pink'.

 

 

Stem Node Fresh Pressure
Treatment Length Number Weight Bomb
(cm) (gm) (ATM)
5° 30.8516Z 7.013 b 16.004 a_ 6.43 bc
10° 29.254 b 6.841 b 16.016 a 6.16 bc
15° 30.172 b 6.784 ab 15.093 a 5.80 ab
20° 29.819 b 6.638 ab 15.218 a 5.26 a
25° 30.137 b 6.663 ab 15.155 a 5.26 a
30° 27.985 b 6.219 a 14.875 a 6.04 be
35° a24.893 a 6.677 ab 15.557 8 6.64 c

 

2 Treatment means within the same column with the same letter are

not significantly different by Tukey's test at the 5% level.

28

Figure 7. Effect of soil temperature on flowering stem length of rose
cv. 'Bridal Pink'.

29

2:3

  

 
 

I 7 B
B a I

59.0.. .095 @5332“.

80
Soil Temperature (°C )

15

1O

     
 

Figure 8.

30

Effect of soil temperature on node number per flowering stem
of rose cv. 'Bridal Pink'.

31

 
 

 
 

I 7 B

.86 8:26: \ .3532 682

    

ED 25
8011 Temperature 1°C)

15

1O

Figure 9.

32

Effect of soil temperature on fresh weight of flowering stems
of rose cv. 'Bridal Pink'.

4—— -—

33

.005 05332.... \ 8529 .5! :82“.

 

Soil Temperature 1°C)

34

The largest number of cut rose flowers from February 8, 1972
to May 9, 1972 were from plants grown at 25°C soil-media temperatures,
55.5% greater than those grown at a 10°C soil~media temperature
(Figure 10). The plants grown at 10°C soil-media temperature pro-
duced the fewest blooms. Plants growing at a soil-media temperature
of 35°C produced 52.4% more blooms than those at a 10° soil-media
temperature. Considerable treatment variability occurred in the
number of cut flowers produced at the various soil-medium temperatures
(Figure 10).

The number of days required from cutting a flower to flowering
of the stem again was altered by the soil-medium temperature
(Figure 11). A fewer number of days were required for return of
stems of rose cv. Bridal Pink to flowering as the soil-medium tem-
perature increased from 5° to 30°C. At a soil-media temperature of
5°C, 53.50 days were required for return of stems to flowering com-
pared to 47.68 days at 30°C. This represents a 10.9% reduction in
the number of days required from cutting a flower to flowering of
the stem again. A soil-media temperature of 35°C resulted in an
increase in the number of days required for the return of a flower
when compared to the 30°C soil-media temperature.

The least amount of internal water stress occurred at soil-medium
temperatures of 20° and 25°C when compared to higher and lower soil-
medium temperatures (Figure 12). An increase in soil-media tempera-
ture of 10°C resulted in a significant increase in internal water
stress while no significant difference occurred at the lower soil-

medium temperatures.

35

Figure 10. Effect of soil temperature on flower production on rose
cv. 'Bridal Pink' from February 8, 1972 to May 9, 1972.

36

100

8026 9:326.“— 3955 .30...

 

Soil Temperature 1%)

Figure 11.

37

Effect of soil temperature on days to flower of flowers

harvested from rose cv. 'Bridal Pink'.

._4 ._ _._ —.__ _ _‘ - - —I _ — _a __ .——- _— .___._._ .—

38

330.“. 0. «>3

 

5011 Temperature 1°01

39

Figure 12. Effect of soil temperature on internal water stress for
rose cv. 'Bridal Pink'.

4O

 

e. :30... 2.5

Temperature 1°01

Soil

DISCUSSION

The results indicate that soil temperature influences both root
growth, vegetative growth, and flowering of rose 'Bridal Pink'. Low
root temperature inhibited both root and shoot growth. This is prob-
ably due to several factors such as reduced water uptake, decrease
in translocation of growth substances, and the direct effect of tem-
perature on root metabolism which subsequently reduced shoot growth.

Brower (12) attributed the reduction of both root and shoot
growth at low and high soil temperatures to water stress caused by
reduced absorption of water. Low temperature decreases the absorp-
tion of water in a number of ways. The viscosity of water itself
is twice as great at 0° as at 25°C. The viscosity of the protoplasm
must also be greatly increased, which decreases the permeability of
the root tissue)‘ Kramer (42) has shown that low temperature reduces
the rate of water movement through a root at 5°C to be about 25% of
the rate of 25°C under the same pressure gradient.

In this study internal water stress was measured using the
pressure bomb. The results obtained in Figure 11 support the
hypothesis that low and high soil temperatures increase the moisture
stress within a plant.

Davis and Lingle (24) claimed the reduction in shoot growth of
tomatoes at low soil temperatures was not caused by reduced absorption

of water or mineral nutrients. They suggested that low temperatures

41

42

decreased translocation from shoots to the roots. The result is
accumulation of substances in the shoots which inhibit metabolic
activity. Esau and co-workers (28) have reported that low tempera-
tures may retard the rate at which materials are transported in the
phloem. Therefore, low temperatures might reduce the root's effec-
tiveness as a sink for phloem transported materials. Davis and
Lingle (24) reasoned that a large number of growth substances that
are produced in shoots have been shown to accumulate at girdles,
should the shoot experience an alteration in the concentration of
such compounds, due to an interruption or depression of phloem
transport, a partial inhibition of shoot cell metabolism might re-
sult. The results in Figure 5 support these previous findings and
correspond precisely with Hansen's (32) work on rooting.

It seems possible that the reduced growth of shoots as shown
in Figures 5,6, and 7 which occur when roots are subjected to un-
favorable environmental conditions may result from interference
with their metabolic activities as well as from interference with
absorption. Exudates from root systems of grapes grown at 30°C,
where greater root and shoot elongation occurred, were shown by
Skene and Kerridge (61) to contain more of certain cytokinins than
exudates from those grown at 20°C. In addition to their action in
stimulating cell divisions, cytokinins interact with auxin to in-
duce organ initiation (62). They induced callus tissue to form
roots, buds, or continue producing callus, or stop growing entirely
depending on the amount of IAA and kinetin provided. Atkin et.al.

(2) found increasing cytokinin in xylem exudate of corn as the

43

soil temperature was increased from 8° to 28°C. It was their obser—
vation that with more branched root system at higher temperatures

the increase in cytokinin level may be related to an increased number
of sites of production, such as root apices. If cytokinin - auxin
balance was influenced by the soil temperature to, in effect, cause
an increase in the number of flowering shoots produced, this could
possibly explain why, in this experiment, a high number of flowers
were produced at the higher soil temperature of 35°C as shown in
Figure 10.

The practice of covering a newly planted rose plant with
polyetheyne has been used for years by commercial growers in increase
the number of breaks (45). This microclimate creates an environment
of high humidity while also increasing the temperature surrounding
the plants adding further support to the findings in this study which
showed an increased shoot number at the higher temperatures (Figure 10).

Other substances having shoot regulatory activity are claimed
to be produced in roots (69,33). Although it has not been proved
that the differences in growth are caused by differences in production
of growth regulatory substances in roots, the possibility would
justify further investigation.

Figure 7 shows a significant decrease in stem length as the
temperature of the growing medium increases above 25°C to 35°C.

This corresponds with the evidence previously sited in that roots,
in addition to being organs of support and absorption, have the
further role of supplying some growth substances to the shoot. There

is evidence that gibberellins may be synthesized in roots and trans-

44

ported to the shoot (2,16,18,22,51,53). This export of gibberellins
and other growth substances to the shoot are influenced by environ-
mental factors. Thus, when the roots are subjected to stress, a
decrease in translocation to the shoot would be expected to occur.
A decrease in stem length could be expected to result with a decrease
in gibberellin supplied to the shoots, as occurred in this experiment.
Relatively little data is available concerning the effect on
growth and water absorption at high temperatures. Root temperatures
above 30°C reduced water absorption in lemons, grapefruit, and
oranges (9,31). Kramer (41) reported that exudation from detopped
tomato root systems attained its maximum at 24°C and decreased at
higher temperatures. The small number of roots in the top 30 cm
of soil in warm climates is attributed to high summer soil tempera-
tures (52). Several workers (4,48,56) have shown that root struc-
ture is modified at high soil temperatures by increased rate of
maturation. The efficiency of roots as absorbing surfaces is

greatly reduced because they become suberized almost to the tips.

SUMMARY

These studies show that prolonged root temperature treatments
influence the growth of rose cv. 'Bridal Pink'. Temperature has
a diverse and complicated impact on plant growth, for it influences
the plant's water status and this must affect endogenous plant
hormone production. One might also conclude that the growth and
survival of the root system may depend on factors originating in
the shoot, while the shoot may depend on factors originating in
the root.

The data from these studies would tend to suggest that a
higher soil temperature than commonly found in commercial rose
ranges would be of some value. However, the effect of a higher soil
temperature of 25°C was not significantly different from those of
10° to 15°C when comparing length, node number, and fresh weight of
the flowering stems. It appears from the data that during the
early stages of building the young plants, a soil temperature of 25°C
might be most effective for both root and shoot growth. The vigor
with which the crop starts is considered to be the most important

and critical time in the life of a greenhouse rose.

45

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