PHOTOSYNTHESIS AND TRANSPIRATION IN

YOUNG LARIX LEPTOLEPIS AND TRANSLOCATION OF

14C PHOTOSYNTHATE IN MATURE LARIX DECIDUA

By
John LeRoy Crane Jr.

A DISSERTATION

Submitted to
Michigan State University
in partia] fquiTTment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Forestry

1981

 

 

ABSTRACT

PHOTOSYNTHESIS AND TRANSPIRATION IN
YOUNG LARIX LEPTOLEPIS AND TRANSLOCATION OF
14C PHOTOSYNTHATE IN MATURE LARIX DECIDUA

 

 

By
John LeRoy Crane Jr.

The effects of light intensity and temperature on net photosynthesis
(Pn) and transpiration rate for leaves of three-year-old Larix leptolepis
trees was determined with an infrared, differential open gas analysis
system. Immature to recently mature, and mature long shoot (LS) foliage
borne on the terminal leader or first major branch apex, and interior
branch, respectively, was used for these measurements. Similar foliage
was sealed inside Mylar bags and CO2 depletion of air inside bags was
measured by infrared gas analysis to determine C02 compensation concentra-
tion.

Net photosynthetic response to increasing levels of photosynthetic
photon flux densities (PPFD) was similar for each foliage position and
stage of maturity. Light compensation was between 25 and 50 uE m-2 3—].
Rates of Pn increased rapidly at PPFD above compensation intensity and
saturated at approximately 900 uE m'2 s']. Transpiration rates at con-
stant temperature likewise increased with increasing PPFD and leveled
between 800 and 1000 uE m'2 s‘1.

Photosynthetic response to temperature was determined at saturating

PPFD and was also similar for all foliage positions. Pn increased

steadily from low temperatures to an optimum range between 15 and 2l° C,

John LeRoy Crane Jr.

and at temperatures above 21° C Pn decreased rapidly. Transpiration
rate, however, increased continuously with rising temperature up to the
experimental maximum, suggesting that the rapid decline in Pn above 21° C-
was due to internal factors and not stomatal closure.

The lowest C02 compensation concentrations were 58 and 59 pl 1'1

for the mature foliage of low and mid-crown branches respectively. The

foliage borne at the apex of the terminal leader had the highest C02 com-

pensation concentration of 75 pl 1'}.

In a second series of experiments leaves of both long and short

shoots (LS and SS) borne on three-year-old first order branches of mature

l4 l4

L, decidua were exposed to C02 and the distribution of fixed C was

traced after exposure. This was done on July 1, August 10, and September
8, and followed the course of LS growth from a period of rapid expansion
to early bud set. Labeled foliage included SS borne in the middle of
3-year and 2-year branch increments (3YBI and 2YBI) or 1—year terminal LS
(lYTLS).

In July, the vigorous growth of LS was approximately 50% completed

and SS eXpansion had ceased approximately two months prior to this. The

14

majority of C fixed and transported by LS leaves was retained in 1YTLS,

but basipetal transport of 14C-photosynthate was detected in 2YBI, 3YBI,

and main stem as early as 5 h after exposing TLS leaves to 14CD The

2.
majority of 14C fixed by 2-year and 3-year SS was transported to the 2YBI

and 3YBI and was retained for local use. Movement of excess 14C out of

2YBI and 3YBI was predominantly translocated basipetally to the main stem

l4

and downward. Similar patterns of C distribution were observed in

August when LS growth was much slower and nearly complete, and in September

 

John LeRoy Crane Jr.

1 . .
4C, however, was retained in

after LS had set bud. Progressively more
branch increments or basipetally transported.

Results suggest that 1YTLS growth is essentially self supported by
July and contributes increasing amounts of photosynthate to older branch
increments and main stem as the season progresses. Recovered 14C in
partitioned 2YBI and 3YBI also suggests that SS during this time contrib—
ute photosynthate almost exclusively to branch portions supporting these

SS, older branch increments, and the main stem.

ACKNOWLEDGMENTS

I wish to thank Dr. D. I. Dickmann for his continued support, and
for the technical assistance he afforded me in preparing this disserta-
tion. Appreciation is also extended to Drs. J. u. Hanover, M. R.
Koelling and J. A. Flore for their critical review of the manuscript
and their suggestions. Dr. Flore also kindly provided access to an
infrared open gas analysis system utilized in much of this study.

Acknowledgment is also given to the faculty, staff and graduate

students of the Department of Forestry, Michigan State University.

J. L. Crane Jr.

ii

 

TABLE OF CONTENTS

CHAPTER

II.

III.

IV.

LIST OF TABLES ............................................
LIST OF FIGURES ...........................................

LITERATURE REVIEW .........................................

Taxonomy and Distribution ..............................
Genetics ...............................................
Provenance Studies .....................................
Injury Factors .........................................
Ecology ................................................
Growth and Reproduction ................................
Physiology .............................................

PHOTOSYNTHESIS AND TRANSPIRATION BY YOUNG LARIX LEPTOLEPIS

 

PLANTS: C3 RESPONSES TO LIGHT AND TEMPERATURE ............

Introduction ...........................................
Materials and Methods ..................................
Results and Discussion .................................

MID- AND LATE-SEASON REDISTRIBUTION 0F 14C-PHOTOSYNTHATE
FROM SHORT SHOOTS AND LONG SHOOTS OF MATURE LARIX DECIDUA
TREES .....................................................

 

Introduction ...........................................
Materials and Methods ..................................
Results and Discussion .................................

SUMMARY AND CONCLUSIONS ...................................

BIBLIOGRAPHY .....................................................

Page

iv

vi

comm—I

l2
18

22
22
27

45
45
49
71
76

 

LIST OF TABLES

TABLE Page

3.1. Distribution of 14C in 3-year first—order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Terminal long
shoot leaves were exposed to 14C02 for l h on July 1, and
branches were removed 5, 24, and 48 h after ................ 51

3.2. Distribution of 14C in 3-year first-order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Second-year
short shoot leaves were exposed to 14002 for l h on July l,
and branches removed 5, 24, and 48 h after ................. 53

3.3. Distribution of 14c in 3-year first-order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Third-year short
shoot leaves were exposed to 14CO for l h on July 1, and
branches removed 5, 24, and 48 h gfter ..................... 54

3.4. Distribution of 14C in 3-year first-order branches of
L, decidua expressed as specific activity (DPM/mg) and per-
cent total recovered translocate (%TRT). Terminal long
shoot leaves were exposed to 14C02 for l h on August l0,
and branches removed 5, 24, and 48 h after ................. 58

3.5. Distribution of 14C in 3-year first-order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Second-year
short shoot leaves were exposed to 14002 for l h on
August 10, and branches removed 5, 24, and 48 h after ...... 60

3.6. Distribution of 14C in 3-year first-order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Third-year short
shoot leaves were exposed to 14C02 for l h on August 10,
and branches removed 5, 24, and 48 h after ................. 61

3.7. Distribution of 14C in 3-year first-order branches of
L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Terminal long
shoot leaves were exposed to 14C02 for l h on September 8,
and branches removed 24, 48, and 72 h after ................ 63

iv

 

TABLE

3.8.

3.9.

3.10.

Distribution of 14C in 3-year first-order branches of

L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocafie (%TRT). Second-year short
shoot leaves were exposed to CO for l h on September 8,

and branches removed 24, 48, and 92 h after ................

Distribution of 14C in 3-year first-order branches of

L, decidua expressed as specific activity (dpm/mg) and per-
cent total recovered translocate (%TRT). Third-year short

shoot leaves were exposed to 14CD for l h on September 8,

and branches removed 24, 48, and 22 h after ................

Movement of 14C in the 2- and 3-year branch internodes of
3-year branches relative to the section of each internode
exposed to 14C02 for l h. Figures are specific (dpm/mg)
obtained in partitioned branch sections ....................

Page

64

66

69

 

LIST OF FIGURES

FIGURE Page

2.1. Photosynthetic response to PPFD for L. leptolepis leaves
borne on the terminal leader. Long shoot foliage included
newly initiated to recently mature leaves near the apex.
Different symbols represent replications .................. 28

 

2.2. Photosynthetic response to PPFD for L, leptolepis leaves
borne on the first major lateral branch. Long shoot
foliage included newly initiated and recently mature
leaves near the apex. Different symbols represent replica- 2
tions ..................................................... 9

 

2.3. Photosynthetic response to PPFD for mature L. leptolepis
leaves borne on the first major branch. Different symbols
represent replications .................................... 30

 

2.4. Light saturation of photosynthesis for L, leptolepis
leaves from three crown positions ......................... 32

2.5. Photosynthetic response to temperature for L. leptolepis
leaves borne on the terminal leader and measured at satur-
ating PPFD (ca. 900 pE m-2 5'1). Long shoot foliage
included newly initiated and recently mature leaves near
the apex. Different symbols represent replications ....... 33

2.6. Photosynthetic response to temperature for L. leptolepis
leaves borne on the first major lateral branch and
measured at saturating PPFD (ca. 900 uE m 25 ’ ). Long
shoot foliage included newly initiated and recently mature
leaves near the apex. Different symbols represent replica-
tions ..................................................... 34

 

2.7. Photosynthetic response to temperature for mature L.
leptol_pis leaves borne on the first major lateral branch
and measured at saturating PPFD (ca. 900 uE m 2 5'1).
Different symbols represent replications .................. 36

 

2.8. Optimum temperature for photosynthesis of L. leptolepis
leaves from three crown positions and measured at satura-
tion PPFD (ca. 900 uE m' ............................ 37

vi

 

FIGURE Page

2.9. Transpiration rate response to light for L. leptolepis
leaves borne on the first major lateral branch. Long
shoot foliage included newly initiated to recently
mature leaves near the apex. Different symbols represent
replications .............................................. 39

2.10. Transpiration rate response to light for mature L, lept -
lepis leaves borne on the first major lateral branch.
Different symbols represent replications .................. 40

2.11. Transpiration rate response to temperature for L. lepto-
lepis leaves borne on the first major lateral branch and
measured at saturating PPFD (ca. 900 uE m'2 5"). Long
shoot foliage included newly initiated to recently mature
leaves near the apex. Different symbols represent
replications .............................................. 41

2.12. Transpiration rate response to temperature for mature
L, leptolepis leaves borne on the first major lateral
branch and measured at saturating PPFD (ca. 900 uE n-Zs-l)
Different symbols represent replications .................. 42

 

3.1. Diagrammatic representation of a three-year-old first-

order branch of Larix decidua. Starred abbreviations are

leaves and shoots exposed to 14CO2 for l h ................ 47
3.2. Relative distribution of recovered 14C-translocate in

(A) 1YTLS, (B) 2YBI, and (C) 3YBI 48 h after exposing

different shoot foliage to 4002 on July 1 ................ 56

 

3.3. Relative distribution of recovered 14C-translocate in
(A) 1YTLS, (B) 2YBI, and (C)43YBI 48 h after exposing
different shoot foliage to 002 on August 10 ............. 62

3.4. Relative distribution of recovered 14C-translocate in

(A) 1YTLS, (B) 2YBI, and (C) 3YBI 72 h after exposing
different shoot foliage to 4CO2 on September 8 ........... 67

vii

 

CHAPTER I

LITERATURE REVIEW

Taxonomykand distribution. The genus Larix is one of six northern

 

polytypic genera in the order Coniferales with extensive natural ranges

 

(others are Cupressus, Juniperous, Abies, Picea, Pinus). Species in

 

 

these genera typically show considerable variation in habitats and
natural distribution, with some species restricted and local in occur-
rence, while others form extensive forests in northern glaciated regions
(Li 1953).

There are ten species of Lagix, with several varieties generally
recognized. All are distributed in the coolest regions of the northern
hemisphere (Ostenfeld and Larsen 1930; Rehder 1940). They occur spon-
taneously in mountain regions in the south and extend down to lowlands
in the north where they form extensive forests. Recently two new
species were described in central China (Chang and Fu 1978) and were
added to a growing list of reported species. Whether these species are
actually new remains to be substantiated, as much deviation from the
taxonomic characters used to distinguish larch species exists. This is
especially true where the natural range of a species is extensive, with
large environmental differences, or where the natural ranges of two
species overlap (Ostenfeld and Larsen 1930). Most new species described
in the past were based upon insufficient observation, e.g., Wright's

(1908) description of a new Larix species in Alaska.

   

According to international rules of nomenclature, five of the Lagix
species form a sub-genus. They are charaCterized by bracts of the cones
which are longer than and protruding out of the cone scales (Ostenfeld
and Larsen 1930). These species are all restricted in distribution to
the mountainous regions of western North America and southeastern Asia.

Western larch (L, occidentalis) attains the largest size of any larch

 

species and occurs on the slopes and high valleys of British Columbia,
Washington, Oregon, Idaho, and Montana. 0f the five species in the sub-
genus, it is the only one of commercial value. Alpine larch (L. 1yallii)
occurs in scattered stands in the higher elevations of the same mountain
systems of North America as western larch. It grows as an upright tree
at elevations higher than the krumholz forms of its associated species

(Arno and Habeck 1972). Larix_graffithii is restricted to the south

 

slopes of the Himalayas, while Masters larch (L. mastersiana) and Chinese

 

larch (L. potanini) occur in the mountains of Central China. Chinese
larch is reported to grow at 4800 m, an elevation higher than any other
species in the Pinaceae is found (Ostenfeld and Larsen 1930; Rehder
1940).

The other five species of Lagix vary in the extent and nature of

their natural distribution. Japanese larch (L. leptolepis) is a mountain

 

species, originally found in scattered stands on the island of Honshu,
Japan. European larch (L. decidua) occurs from 150 m to 800 m in the
mountains and high valleys of Austria, Switzerland, France, Italy,
Yugoslavia, Czechoslovakia, Poland, Ukraine and Rumania. Dahurian larch
(L, gmelini), Siberian larch (L, sibirica) and American larch or tamarack
(L. laricina) have, in contrast, extensive natural ranges. Tamarack

spans the North American continent from Newfoundland to Alaska.

 

Dahurian and Siberian larch essentially divide Eurasia. Larix sibirica

 

extends west and north from Lake Baikal to northeastern Europe and beyond

 

the Arctic Circle. Larix_gmelini occurs in Eastern Asia from Lake Baikal
to Korea in the south. It also occurs in Russia as far north as 73° N
latitude where no other tree species grows (Ostenfeld and Larsen 1930;
Rehder 1940; Polunin 1959) .

Genetics. Most of the larch species are allopatric. The natural
range of several species, however, overlap and spontaneous hybrids have
been reported for each. Hybrids between L_sibirica X L, gemelini occur
frequently on the west side of Lake Baikal. Hybrids also occur in

Central China where the natural ranges of L, potanini and L, mastersiana

 

overlap (Larsen 1937; Bobrov 1973). The natural range of L. occidentalis
and L. lyallii coincide geographically in many areas, but they are usually
separated altitudinally. Hybrids have been reported, however, in a few
areas of frequent disturbance where L. lyallii has extended its range
below normal elevation limits (Carlson 1969; Carlson and Blake 1969; Arno
1972).

Species hybrids are produced readily from open-pollinated trees
planted together or under controlled conditions. The two most famous
spontaneous hybrids produced from open-pollinated trees are L,)<pendula

and the Dunkeld larch from Scotland. Larix x pendula was originally

 

thought to be a native North American species that was introduced into
Europe around 1800 (Oppermann 1923). More recent evidence indicated
that L, decidua was first introduced into the United States and then
returned to Europe as L, x pendula, a L, decidua x L, laricina cross

(Ostenfeld and Larsen 1930; Larsen 1937).

 

The second hybrid, L, x eurolepis, originated from a spontaneous

cross between L, leptolepis and L, decidua planted on the Dunkeld and

 

Murthly estates in Perthshire, Scotland (Henry and Flood 1919). The
expression in the hybrid of the best traits from the parent trees gener-
ated much interest in it for future planting in England. European larch
had been present in England at that time since 1629 (MacDonald 1957) and
was highly regarded for its rapid growth, straight stem, light branches,
and durable wood. "In no other tree have such high hopes been placed,
hopes which too often have led to disappointment" (Hiley 1919). The high

susceptibility of European larch to the canker-causing fungus Dasyscypha

 

willkommii prompted this statement. Japanese larch grown in England,

 

on the other hand, was a lesser tree with coarse branches and not as
favored as European larch. It was, however, very resistant to the larch
canker. The Dunkeld hybrid combined the favored form and wood qualities
of the European larch parent with the canker resistance of Japanese larch.
When tested in England, Denmark, and other European countries, the
Dunkeld larch was found to grow faster than both parent types (MacDonald
1957; Larsen 1956; Langer 1957), while similar reports have been made
for the hybrid grown in the United States (Cook 1969; Holst 1974;
Littlefield and Eliason 1938, 1957; Paton 1944).

Larch breeding programs intensified after heterosis was observed in
the Dunkeld larch. The first controlled pollination of Japanese and
European larches took place in Scotland between 1924 and 1925 and in
Denmark in 1930 (Larsen 1937). The European larch used for this purpose
in Denmark was said to be the "finest" tree in the country and required
29 m of scaffolding. Much of the later breeding research was reviewed

by Larsen (1956) and general indications were that combination of any two

 

 

species was possible. Some species combinations that have produced

vigorous hybrids include: L, decidua x L, siberica, L, leptolepis

 

x L, gmelini, L. laricina x L, leptolepis, and L, occidentalis x

 

 

L._1eptolepis (Wright 1976; Larsen 1934; MacGillivray 1967; Wang 1971).

 

Wright (1976) states, however, that only about one-fourth of the forty-
five possible species combinations have been attempted, and at least

five of these included L, decidua as one parent. Triploid individuals
have been produced by pollinating L, decidua with pollen from L, gggi:
dentalis (Larsen 1956), but apparently no attempts at producing a tetra-
ploid from this triploid tree have been successful. One individual of

L, decidua growing in a private Danish arboretum was cytologically
identified as a tetraploid (Larsen 1956) but it was not produced by con-
trolled pollination. There is a possibility that this tree was introduced
from England as a variety of European larch (L, decidua var. pendula) and
thus may have been derived from a L, decidua x L, laricina.

Provenance studies. The use of larch (L. decidua in particular)

 

for reforestation increased during the 18th century when the original
forests of central Europe were being cut over. Interest in European
larch for this purpose was mainly based on its rapid growth and high
yield, strong valuable wood, and its ability to adapt to a variety of
sites, both inside and outside its natural range. This adaptability was
not uniform, however, and by the middle of the 19th century some plant-
ings had failed (Vincent 1958). It became apparent that European larch,
although of rather limited natural range, was quite variable.

The first comparative study of the growth of larch was initiated
in 1879 and several provenance studies were established shortly there-

after. Characteristics such as growth rate, wood density, time of spring

 

needle flush, and needle fall in autumn were found to be dependent upon
the geographic origin of the trees studied (Ciesler 1914). McComb (1955)
reviewed the European provenance studies and described the Alpen,
Sudeten, Tatra, and Polen races of European larch.

In the United States, European larch was planted sporadically in
Massachusetts, Connecticut, and Vermont after the Civil War. From 1860
to 1880 it was one of the most popular trees for planting in Illinois,
Iowa, Minnesota, and the Dakotas. Later it was planted extensively in
Pennsylvania and New York. Hunt (1932) reported on the performance and
silvical characters of European larch plantings scattered throughout
Massachusetts, Vermont, Connecticut and New York. The seed origin of
these early plantations, however, could not be stated with certainty.

In 1944 the International Union of Forest Research Organizations
(IUFRO) began an international study of geographic variation in European
larch. A few seedlots of other larch species were also included in this
early test. Henry Baldwin initiated the study in the United States and
plantations were established in New Hampshire and New York. The four-
year performance of the New Hampshire planting were reported by Baldwin
(1949) and twelve-year results for both New Hampshire and New York
plantings was published by Genys (1960). Reviews of all provenance
experiments on European larch and the 1944 IUFRO study have been made.
Early reports indicated that the best performance in Europe was obtained
with Japanese larch followed by Sudetan provenances of European larch
(Schober 1958). Later reviews, however, have indicated that early
resUlts were misleading and seven or more years are required for perform-

ance to stabilize. The most recent data indicate that Sudetan provenances

 

of European larch grew best, while Japanese larch demonstrated only
average performance (Giertych 1979).

A second international larch provenance test was initiated in the
1950's involving 68 provenances of larch tested in 14 European countries
and the United States (Lacaze and Birot 1974; Schober 1976). The
United States participation was organized by S. H. Spurr. Test planta-
tions were established in southeastern Michigan and included 22 European
larch provenances, one Japanese larch source and one F2 progeny of
Dunkeld hybrid larch. The 19-year performance of these plantations indi-
cated that the best height and diameter growth were obtained from European
sources having similar macroclimatic conditions as southeastern Michigan
(Barns 1977). Japanese larch was most severely damaged by ice, but it
and the Dunkeld hybrid had the greatest resistance to larch casebearer

(Coleophora laricella) and the larch wooly aphid (Chermes strobilobius).

 

Japanese larch is restricted to a ca. 200 square km natural range
between 900 to 2500 m on Honshu Island, Japan (Wright 1962). Despite
its limited range, the performance of this species when tested in Europe
has repeatedly been shown to depend on its geographic origin (Hattemer
1968, 1969; Langer 1961; Schonbach et a1. 1966). United States experi-
ments planted in New York (Stairs 1965), Maryland (Genys 1972, 1973), and
in the North Central States (Farnsworth et a1. 1972; Lee 1973) have also
demonstrated the seed source diversity of Japanese larch.

The geographic variation of native North American species of larch
is not well-documented. Tamarack has an extensive transcontinental
natural range but little was known about its genetic variation until 1961

when a cooperative rangewide seed source study was initiated by the

 

5‘

 

University of Minnesota. Seed for this test was obtained from more than
50 sources and early height growth of seedlings planted in Minnesota

and Wisconsin gave reasonable indication that there was geographic varia—
tion in the species (Pauley 1965). This was further indicated by the
survival and height growth of these plantations eight and nine years
after establishment (Jeffers 1975). More recent plantings of this range—
wide provenance test were established in 1969 in Michigan by Michigan
State University, but results have not yet been published.

Surpringly little information is available on the natural variability
in Lagix occidentalis, even though it is the largest of all larch species
and is of great commercial value in North America. When planted or seeded
outside its natural range, it is usually included for comparison with
other larch species (Genys 1968, 1976) or for ornamental purposes. No
significant efforts to determine inherent variability or adaptability to
other climatic or geographic regions have been attempted (Schmidt et a1.
1976). It is reasonable to expect, however, that a great deal of genetic
variation exists in western larch and that a potential for selection of
superior seed for plantation establishment within and outside its natural
range exists.

Injury factors. Larch is generally considered relatively resistant
to physical and biologic injury within the confines of its natural range.
It is also resilient and capable of good recovery from injury that would
kill or permanently disfigure other tree species. The greatest problems
have occurred when the natural range of a larch species was extended by
planting or exotic pests were introduced (McComb 1955; Hunt 1932).

The severity of injury, however, varies between and within species and is

 

dependent on age, time of year, and geographic location.

The effects of fire on larch vary with species and age. Seedlings,
saplings, and pole-sized larch are readily killed by fire (Sudworth 1918).
Mature tamarack and Dahurian larch are also very sensitive to fire, but
they seed in and occupy burned over areas readily (Roe 1957; Rozhkov 1966).
Mature and overmature western larch, on the other hand, are considered
the most fire-resistant tree in the northern Rocky mountains (Flint, 1925).
This is attributed to its thick bark with low resin content, high and
open branching, and low foliage flammability (Haig et a1. 1941). Siberian
larch also produces bark up to 25 cm thick on the lower trunk and is fire
resistant on sites that permit deep rooting (Rozhkov 1966).

Species such as European, Siberian and western larch develop deep,
wide-spreading root systems which make them moderately to highly resistant
to windthrow. This is decreased, however, with age and degree of root
rot or topography (Hunt 1932; Schmidt et a1. 1976; Rozhkov 1966).

Tamarack and Dahurian larch, conversely, produce shallow root systems and
are subject to windthrow (Roe 1957; Rozhkov 1966).

Larch is generally not susceptible to damage by snow and ice due to
its light branches and deciduous habit, but exceptions have been noted.
Ice storms have caused severe damage to European larch planted in the
northeast United States (Hunt 1932), while early wet snows cause bending
of western larch sapling and pole stands with full complements of
needles (Schmidt et a1. 1976). Young western larch, however, are
usually quite resistant to permanent snow damage and severely bent
trees recouperate within six years (Schmidt and Schmidt 1979).

Noxious fumes tend to have less effect on larch than other conifers

due to its deciduous habit. This is especially true when winter

 

10

air-inversions cause air pollutants to be particularly damaging (Hepting
1971). During the growing season gases such as fluorine and sulfur
dioxide can injure larch foliage and young needles are very sensitive to
these and other industrial pollutants, but susceptibility tends to
decrease with age (Leaphart and Denton 1961).

When the natural range of European larch was extended by planting
and seeding, particularly into the lowlands of Germany, the larch canker
became a serious disease of this species (McComb 1955). In the United
States this disease also is a tenacious problem with European larch
(Hunt 1932), but tamarack is not affected by this canker and is relatively
free of other stem diseases (Hepting 1971). The larch canker is also
known to be a problem with Siberian larch in moist areas of its range
(Rozhkov 1966), and has also proved parasitic to western larch when arti-
ficially infected (Hahn 1934; Hahn and Ayers 1943).

Larch dwarf mistletoe (Arceuthobium larcis) is the most serious

 

disease of western larch (Schmidt et a1. 1976; Weir 1916). A foliage
disease affecting eastern, European and western larches is the needle

blight caused by Hypodermella laricis (Cohen 1967; Hepting 1971).

 

Virtually all larch species are affected by 59mg; species causing various
root, stem and heart-rot diseases.

The insects causing the most serious damage to native or exotic
larch species in North America are of European origin (Hunt 1932; Graham
and Knight 1965). The larch sawfly (Pristiphora erichsonii) was first
observed in the United States in 1880 (Hunt 1932) and has subsequently
caused great losses in larch throughout the Northeastern and Lake States.

The larch species most seriously affected by this insect in North America

11

is tamarack and, although repeated infestations reduce growth, moderate
to severe defoliations for six to nine years are required to cause
mortality (Drooz 1960). Other larch species planted in Maryland have
shown different levels of susceptibility to larch sawfly attack. Among
five species of larch examined, Japanese larch was most susceptible and
western larch the most resistant, but some geographic strains of Japanese
and European larch were less susceptible than others (Harman and Genys
1970; Genys and Harman 1976).

The second insect of European origin causing serious problems is
the larch casebearer. This is the most serious insect pest of western
larch, but it is also found world-wide on nearly every other species of
larch (Denton 1979). It was first observed in 1886 on European larch
in Northhampton, Massachusetts, but quickly established itself on native
tamarack (Hagen 1886). It spread rapidly west through the southern part
of tamarack's natural range. The first outbreak of casebearer in western
larch forests was discovered in 1957 near St. Maries, Idaho (Denton,
1958). From there the population increased and spread unchecked until
by 1972 over one-half of the western larch type was infested. Prior to
this, western larch was relatively free of insect pests compared to most
of its coniferous associates. Loss of tree growth is the most serious
damage caused by casebearer defoliation of western larch, although tree
mortality has occurred. In severe infestations of 5-year duration,
radial increment can be reduced by 97 percent (Denton 1979).

Another insect pest jeopardizing western larch is the western

spruce budworm (Choristoneura occidentalis). This insect preferentially

 

feeds on the current-years foliage of many conifers, but since 1962 it

12

has been observed to sever the current-year terminal shoots of western
larch (Fellin and Schmidt 1967). This detracts from form and greatly
reduces net annual height growth. If continued for a period of time,
crooked holes and bushlike trees result (Schmidt and Fellin 1973; Fellin
and Schmidt 1973).

Ecology. Larches occur naturally in the cooler regions of the north-
ern hemisphere. They are well-adapted to northern continental or high
mountain climates and extreme temperature fluctuations. In southern
regions larch species also prefer cooler habitats and grow best on north
and east slopes or valley bottoms. Southwest exposures are usually not
favorable sites (Boe 1958). These species are also restricted to specific
altitudinal zones (Arno and Habeck 1972; Boe 1958; Larsen 1930; McComb

1955; Saki and Okado 1971). Larix_1yallii exemplifies the extreme of

 

species adaptation. This species has an affinity for cold rocky sites,
often at elevations above most of its associates. The growing season in
these areas may be less than ninety days and is marked by average daily

temperatures over 5.6° C. Larix lyallii east of the Continental Divide

 

in Montana and Alberta is also capable of enduring winter minimum tempera-
tures of -51 to -57° C. Likewise, larch species of the northern boreal
forests of North America and Siberia are capable of withstanding extremely
cold air and soil temperatures. In eastern Siberia, where mean annual
temperatures may be less than -10° C, the ground is permanently frozen
down to a depth of 250 to 400 m, and only 10 to 150 cm may thaw during

the summer. Yet species like L, gmelini grow well in these unfavorable
conditions and form extensive forests (Sakai and Okada 1971; Walter

1979).

13

Precipitation patterns throughout the entire range of larch are
varied. In relation to annual temperatures, however, precipitation
requirements may be rather exacting and delimit the natural distribution
of larch species. The range of L. laricina is one of the most extensive
in North America, and the variation in precipitation throughout this range
is considerable. Annual precipitation ranges from seven inches at
Fort Yukon, Alaska, to 55 inches in eastern Canada. The cooler tempera-
tures at Fort Yukon, however, reduce the precipitation requirement.

In the southern portion of its range L, laricina is characteristic of
bogs and swamps, but in the northwestern part, where annual temperatures
are reduced, it is found on drier upland sites (Fowells 1965). The dis-

tribution of L. occidentalis is also limited to areas receiving greater

 

than 18 inches annually (Schmidt et a1. 1976; Fowells 1965). Extended
periods of drought occur, but the effect of this is balanced by low wind
velocities and retention of snow cover in its preferred habitat, and
moderate precipitation during May and June (Haig et a1. 1941). European
larch likewise has a high moisture requirement, possibly greater than
that of any other European conifer (Burger 1945). Reflective of this are
transpiration rates for European larch which usually are higher than
those obtained for other associated coniferous species (Berger-Landefeldt
1936; Pisek and Cartellieri 1939).

The soil requirements of larch species are probably less exacting
as those of climate. European larch occurs naturally and grows well on
acidic soils derived from sandstone or clacarious soils derived from
limestone or dolimitic shales (McComb 1955). The results of planting

L, decidua in Europe and the United States indicate that deep, moist

 

l4

soils with physical properties conducive to adequate drainage are more
important than soil fertility for satisfactory growth (Aird and Stone

1955). Larix lyallii also occurs on extremely infertile soils and seems

 

preferentially distributed on acidic substrates derived from quartzite,
sandstone, and shale at timberline (Arno and Habeck 1972).

The best development of western larch is found on deep, moist, porous
soils on mountain slopes or valley bottoms (Larsen 1940). The most common
parent materials in the range of western larch are argillites and
quartzites, and these soils may be gravelly, sandy, or loamy in texture.
The one soil factor affecting western larch growth, however, is moisture-
holding capacity. Thus, soil variations affecting the distribution of
this species are also more likely to be physical than chemical (Schmidt
et a1. 1976).

The larch species with extensive boreal ranges grow on a wide variety

of soil types and moisture conditions. Larix laricina is found commonly

 

on moist peats and mucks of swamps or muskegs, and it can tolerate short
periods of high water. Best growth, however, is obtained on rich, moist
but well-drained, loamy soils bordering streams, lakes or swamps. It is
also found on soils ranging from heavy clays to coarse sand. Thus,
tamarack tolerates a wider variety of soil physical properties than any
species discussed previously. Like the other species, though, it is
sensitive to excess moisture and will not tolerate prolonged flooding
(Fowells 1965). Siberian larch is also found on soils and conditions
similar to tamarack. It is considered a poor competitor, however, and

is not found on the most favorable sites unless the "dark taiga” conifer-

ous species are set back by fire (Rozhkov 1966).

15

Growth and reproduction. Larch winter dormancy is broken early in
the spring and is signaled by mitotic activity in the leaf primordia of
vegetative buds, followed by apical divisions and leaf elongation (Owens
and Molder 1979a). The buds begin to swell when daily mean temperatures
reach 4 to 5° C and leaf emergence occurs about two weeks later (Arno and
Habeck 1972; Tranquillini 1979). Terminal elongation, however, does not
begin until average temperatures are 7 to 10° C and the foliage borne at
the base of terminal shoots and on short shoots have fully expanded (Cook
1941; Fowells 1965; Owens and Molder 1979a). Short-shoot leaf expansion
may take over 2 months but the foliage is able to photosynthesize early
and may contribute to larch's rapid height growth once terminal shoot
elongation begins (Tranquillini 1979).

Reproductive buds usually differentiate on young vigorous vegetative
short shoots one year prior to pollination (Owens and Molder 1979b).
These buds swell and the new reproductive strobilli emerge before leaf
flush and develop rapidly thereafter. The wingless pollen are shed in mid-
spring. The pollination mechanism in Lagix_species resembles that of

Pseudotsuga mensiesii (Doyle 1945; Owens and Molder 1979c). The ovules of

 

L, occidentalis cones are nearly fully enlarged prior to fertilization and

 

development may proceed in the absence of pollination. This results common-
ly in unfertile but normal-appearing cones (Owens and Molder 1979c) and may
contribute to the low viability of seed produced by this species and L,
lyallii (Arno and Habeck 1972; Shearer 1961). Ovulate cones ripen in late
summer with seed shed through the fall and winter months.

Larch species are known to reproduce vegetatively when branches

make contact with a soil substrate. Due to the light, deciduous nature

16

of their branches, however, this does not occur readily. Larix decidua
will layer occasionally when branches are weighted down by heavy snow
(Plesnik 1973) but this is much less common than with its spruce associ-
ates. The same is true for L. laricina when branches are covered by
fast-growing sphagnum moss or by drifting sand. This species is also
reported to produce root sprouts (Duncan 1954; Lewis et a1. 1928).

Larch are intolerant, pioneering species and are often the first to
occupy disturbed sites (Boe 1958; Roe 1957; Arno and Habeck 1972).

Some shade is tolerated by seedlings at higher elevations, or in the
southern part of a species range (Arno 1972; Boe 1958; McComb 1955; Roe
1957; Rozhkov 1966), but larches do not readily become established under
their own canopy. In mixed stands a dominant position must be maintained
for survival and some form of disturbance is required to regenerate a new
stand of larch.

Fire has been one of the most important forms of disturbance for
regeneration of many larch species and in its absence they are replaced
by more tolerant associates. Western larch is considered a fire climax
species (Kozlowski and Ahlgren 1974) in parts of its range and probably
has the closest ecological relation to fire of all the larches.

Natural seeding is the preferred method of regenerating western
larch (Schmidt and Shearer 1973). Some seedbed preparation is usually
required for successful regeneration and broadcast burning is commonly
used to accomplish this (Shearer 1975; Beaufait et a1. 1977). Prescribed
fire is often the preferred method due to reduced cost, limitation of
topography, and nutrient release back to the soil (Artley et a1. 1978).

Thirteen-year-old western larch have been shown to grow about one-third

17

faster on broadcast burned areas than on mechanically scarified sites
(Schmidt 1969). This increased growth on the burned site was attributed
to the added nutrients. Establishment of tamarack stands on burned
peatlands is also improved over establishment on adjacent unburned sites
(Johnston 1973).

In natural stands early growth rate of larch generally exceeds that
of its natural associates (Cunningham 1972; MacGillivray 1969; Sartz and
Harris 1972). This is true for the first 90 years growth of western
larch (Deitschman and Green 1965) which will reach five to six m in the
first 20 to 30 years (Cummings 1937). Planted larch in the United States
will also generally outgrow all native species for the first 25 years
(Hunt 1932). Growth rates exceeding 0.6 m annually are common for both
European larch and tamarack (Jeffers and Isebrands 1972; Lemmien and
Rudolph 1968; Littlefield and Eliason 1956; Zavitkovski and Dawson 1978),
while Japanese larch height growth may be over 1.0 m annually for 20
years (Isebrands and Hunt 1975; Turner and Myers 1972), The performance
of planted Japanese or European larch is only surpassed by that of the
Dunkeld hybrid (Holst 1974; Reck 1977).

The rapid height growth rate of larch is not always associated with
increased dry matter accumulation when compared to other coniferous
species grown under similar conditions. In a comparison of tamarack and

jack pine (Pinus banksiana) grown under intensive culture, the tamarack

 

produced two to three times the biomass of that measured in natural
stands. This production, however, was less than that obtained with jack
pine (Zavitkovski and Dawson 1978). Sweet and Wareing (1968) reported

that height and diameter growth of Japanese larch was greater than

l8

Pinus contortaeuuiPinus radiata planted in England, but the pines made

 

 

up 25% of the growth difference after the larch lost its needles in the

fall. At the end of the growing season Pinus radiata had produced more

 

dry matter than the Japanese larch. The relative growth rate (RGR) of
the larch exceeded the pines, but this advantage was reduced by the ex-
tended growing season experienced by the pine species. When compared to

the hardwood sycamore (Plantanus occidentalis), a species having similar

 

free growth and deciduous habits, larch displayed comparable patterns of

CO -uptake and growth (Ledig and Botkin 1974).

2
Physiology. The growth potential of a tree depends not only on its

 

photosynthetic rate but also on seasonal patterns of photosynthesis in
relation to respiration, the duration of growth, and the distribution of
photosynthate within the tree. Larch species do have patterns of growth
unique among other north temperate coniferous species. Frampton (1960)
described the seasonal periodicity of long shoot growth of L, decidua

and suggested that the early and late leaves of long shoots were dimorphic
in character. The seasonal growth of long and short shoots of

L, laricina have also been described (Kozlowski and Clausen 1966a;

Clausen and Kozlowski 1967), but leaf dimorphism of long shoots was not
found in this species (Clausen and Kozlowski 1970). Long shoot and short

shoot bud development was described for_L. leptolepis (Fijimoto 1978),

 

and more recently apical changes of long and short shoots of

‘L. occidentalis were reported by Owens and Molder (1979a).

 

Terminal long shoots (TLS) bear leaves similar to those produced by
seedlings. The terminal long shoot buds (TLSB) produced at the apex of

long shoots are not totally preformed and consist of bud scales, basal

 

leaves, and long shoot (LS) leaf primordia. After release from dormancy
basal leaves elongate and new LS leaf primordia are initiated up the
flanks of the apex. Elongation of the shoot progresses slowly at first
until the basal leaves have fully expanded. The shoot then elongates
vigorously and then slows again. New bud scales and basal leaves are
formed and the first set of long shoot leaf primordia are initiated after
shoot elongation ceases but before the TLSB becomes dormant (Clausen

and Kozlowski 1970; Owens and Molder 1979a).

Axillary buds are initiated in the TLSB about the time of flushing
but all leaves along the axis of long shoots do not bear axillary buds.
The apices of these new buds differentiate into axillary long shoot buds
(ALSB) or short shoot buds (SSB). The ALSB act as TLSB do and initiate
the first LS leaf primordia before becoming dormant. The differentiated
SSB initiate leaves at the base of the apex and all but the last primordia
are present. Axillary SSB are preformed before becoming donnant and
each season they pass through the same phases of bud—scale and leaf initia-
tion to form a dormant preformed bud. Annual short-shoot elongation is
about 1 mm and the apex may live many years (Clausen and Kozlowski 1967;
Owens and Molder 1979a).

The apex of short shoots may differentiate into long shoots and
Ligg_v§[§g (Cook 1969; Owens and Molder 1979a). Short shoot differentia-
tion into long shoots usually occurs in young vigorous shoots. The leaves
initiated in the developing bud include basal leaves borne at the base
and the leaf primordia borne part way up the flanks of the apex. The
new dormant bud is no longer totally preformed as it was as a SSB and

Will initiate new LS leaves the next season. The long shoot growth

20

will be the same as that from a TLSB.

The anatomical distinction between axillary SSB and ALSB in larch
is apparent in the first year of development and before dormancy. This
predetermination of SSB and LSB persists throughout the life of the short
or long shoot and occurs each season as the buds develop. In Ginkgg
bilgba anatomical differences between preformed SSB and LSB are not
apparent before dormancy but becomes distinguishable after dormancy due
to differential elongation (Gunkel and Wetmore 1946a, 1946b).

In addition to unique growth patterns, there are some aspects about
larch photosynthetic physiology that may be unique. The rate of C02—
uptake is often reported to exceed that of most coniferous species (Fry
and Phillips 1977; Growin et a1. 1980; Havrnaek and Benecke 1978;

Larcher 1969). Likewise, net assimilation rates based on leaf weight

and determined over extended periods or entire seasons exceed those of
other species (Sweet and Wareing 1968; Growin et al. 1980). Larch foliage
is less dense than other conifer leaves (Strong and Zavitkovski 1978),
which may raise questions about the validity of these comparisons.

Little doubt about larch's unique characters can be raised, however, when
their growth and performance compares to or exceeds that of other species
in a shorter period of time (Sweet and Wareing 1968; Tranquillini 1962).

Recently Fry and Phillips (1976) reported some intriguing results
which place L, leptolepis photosynthetic physiology intermediate between
C3 and C4 species. They based this conclusion on phosphoenol pyruvate
to ribulose bis phosphate carboxylase activities, chlorophyll a/b ratios,
initial products of 14C02 incorporation, C02 compensation concentration,

and saturation light intensity. These results have not been substantiated

21

but it is difficult to envision a tree species so well-adapted to the
coolest regions of the northern hemisphere possessing such characters.

Environmental effects on larch photosynthesis are in general not
well-documented. With the exception of the high saturation irradiance
reported for L. leptolepis (Fry and Phillips 1976, 1977), most environ-
mental research has been performed with L, decidua. Much of this work,
including the effects of air and soil temperature, humidity, wind
velocity, and elevation have recently been compiled by Walter Tranquillini
(1979) and assembled into a monograph on alpine timberline physiological
ecology. The productivity and gas exchange of L. decidua as influenced
by constant growth temperature has also been recently reported (Growin
et a1. 1980).

The anatomy and morphology of a few larch species have received
some examination. The distribution of stomata and cross-section leaf
anatomy has been described for L, decidua, L, leptolepis, and the hybrid
L, X eurolepis (Gathy 1954) as well as a comparison of these characters
between the races of L, decidua (Gathy 1959). Other studies have
included epidermal wax deposits (Grill 1973), leaf surface area-leaf
length relationships (Strong and Zavitkovski 1978), general structure
and chloroplast ultrastructural characteristics (Fry and Phillips 1976),
and ultrastructure of plastid development (Fry and Phillips 1977;

Medghini-Bonatti and Boneta-Conta 1976).

CHAPTER II

PHOTOSYNTHESIS AND TRANSPIRATION BY
YOUNG LARIX LEPTOLEPIS PLANTS: C RESPONSES TO
LIGHT AND TEMPERATURE

 

Introduction

 

From an economic standpoint, efficient growth of trees is that which
produces the maximum yield of desired products with a minimum amount of
silvicultural input. To the horticulturist this may mean the largest
possible yield of fruit. The forester, on the other hand, is more
interested in efficiently increasing the yield of wood. Yield is affected
by the interaction of various physiological, environmental, and genetic
factors controlling growth. Further, environment and genetic makeup
place limits on physiological capacity. Within these constraints the
growth potential of a tree depends on photosynthetic rate, seasonal pat-
terns of photosynthesis in relation to respiration, duration of growth,
and the distribution of photosynthate within the tree. Increasing or
altering any one of these physiological processes may increase tree
growth.

Trees in the genus Lagix are unique among north temperate trees.
They are the only deciduous conifers found in northern habitats. Their
growth in plantations or natural stands is usually more rapid than any
of their natural associates (Hunt 1932; MacGillivray 1969). Terminal
extension and height growth is free and continuous in contrast to the
fixed growth pattern of most other northern confiers (Kramer and

22

 

23

Kozlowski 1979). Larch displays distinctly different patterns of long
and short shoot growth (Clausen and Kozlowski 1967; Kozlowski and
Clausen 1966a; Owens and Molder 1979a). These two shoot types may have
different roles in the seasonal distribution of photosynthate. Short
shoots leaf-out early and may photosynthesize for over two months before
vigorous terminal and lateral long-shoot extension occurs (Tranquillini
1979). Larch photosynthetic rates and net assimilation rates (NAR) are
also characteristically higher than other conifers (Growin et a1. 1980;
Larcher 1969; Sweet and Wareing (1968). Recently L, leptolepis was
reported to have a photosynthetic physiology intermediate between C3
and C4 species, based on phosphoenol pyruvate (PEP) to ribulose bis
phosphate carboxylase activities, chlorophyll a/b ratios, initial products

of 14

CO2 incorporation, CO2 compensation concentration, and saturation
light intensity (Fry and Phillips (1976). These observations have not
been substantiated by independent workers nor have the environmental
effects on larch photosynthetic physiology been well—documented.

This study was performed to establish the photosynthetic response of

L, leptolepis to different levels of photosynthetic photon flux density

 

(PPFD) and temperature, and to establish the C02 compensation concentra-
tion of plants grown under near ideal conditions. It was felt that

the pattern of response to these environmental variables would reflect the
basic photosynthetic physiology of the species and provide a base for

future studies.

24

Materials and Methods

 

Japanese larch (L, leptolepis Sieb. and Zucc., Gord.) were
established and grown in 5 X 5 X 30 cm paper bands filled with a peat-
vermiculite (1:1) media in the winter of 1979. The seedlings were main-
tained under optimal conditions for continuous growth (Hanover et a1.
1976), with fertilizer and water added as needed. Seedlings were
repotted in May into 20 1 plastic pots using a loam—sand (2:1) soil mix
and moved outside to grow under natural conditions until mid-August.

The repotted trees were then returned to the greenhouse and maintained
again under optimal conditions through the winter of 1980. All C02—
exchange determinations were performed in March and April, 1980, on
terminal immature and mature foliage. Average tree height at this time
was approximately 1 m.

The rate of CO2 exchange in light or dark was determined in an open
gas analysis system (Sams and Flore 1979, 1981) by measurement of differ-
ential C02 concentrations with a Beckman 865 Infrared Gas Analyzer.
Steady state CO2 exchange was determined with intact or excised shoots
placed into controlled environment chambers (Series 500 chambers,

15.3 X 10 X 10 cm, Paige Instruments). Temperature was controlled by
circulating water from a refrigerated bath through a finned aluminum
heat sink at the bottom of each chamber. Chamber air was circulated by
a variable speed fan (Pameter Model 900) incorporated into the aluminum
bottom of the chambers; Sams and Flore (1979, 1981) determined that this

reduced the boundary layer resistance to less than 0.2 s cm'1.

11'

25

Humidity of the air stream entering the chambers was controlled by
saturating the air in water at a temperature equal to or lower than
chamber temperature. The temperature of the air stream was then raised
to chamber temperature before entering the chambers. The humidity of
the air entering and exiting the chambers was monitored by a flow-through
hygrometer (General Eastern Systems 1100 AP). Transpiration rates were
determined by calculating the difference between water content of the
air entering and exiting the chambers relative to the air stream flow

1 h-1.

rate and were expressed on a foliage dry weight basis as mg H20 9'
Chambers were illuminated by GE 400 W multivapor (metal halide)
lamps. Light intensity was controlled by adjusting the distance between

the light source and chambers or by inserting neutral-density filters
between the light source and chamber. The GE 400 W lamps produce a
spectral distribution of light high in the photosyntheticly active region
(400-700 nm) and this distribution was unaffected by the neutral density
filters (Sams and Flore 1979, 1981). Chamber PPFD was monitored by a
LI-COR Model LI 188 integrating meter connected to LI-COR Model LI 1905
quantum sensors incorporated into each chamber.

After photosynthetic and dark respiration rates were determined on
mature or immature long shoot (LS) foliage of Japanese larch shoots,
foliage was removed, weighed, dried at 75° C for 48h and re—weighed.
Steady state C02 exchange was then calculated per unit fresh and dry
leaf weight and the photosynthetic or dark respiration rate expressed as

1 h—]. Net photosynthetic (Pn) response to light was determined

by exposing each foliage type to PPFDs ranging from O to 2000 pE m-zs'1.

mg C02 9'

26

Chamber temperatures were maintained at 20 or 25° C for the light-
response determinations.

Photosynthetic response to temperature was determined by measuring
steady state CO2 exchange while varying chamber temperature from 5 to 35°C.
Foliage used to determine temperature response was exposed to saturating
PPFD as determined in initial light-response experiments.

Dark respiration was estimated by measuring the increase in C02
concentration of the air in darkened chambers. Foliage for these deter-
minations was allowed to equilibrate to chamber environment (20°C and

saturating PPFD) and a steady state CO exchange prior to darkening

2
chambers. Dark respiration rates were calculated from the stable CO2
exchange after chambers were darkened.

Carbon dioxide compensation concentrations for mature or immature
(terminal) foliage was determined by adopting a rapid method developed
by Dickmann and Gjerstad (1973). Terminal or proximal portions of
branches were sealed in Mylar bags (20 X 36 cm) and allowed to reduce
internal C02 concentration for l h. The seals were made by placing a
longitudinally sliced foam rubber cylinder (2 to 4 X 3 cm) over the
basal portion of the branch, wrapping the bag around the cylinder, and
fastening with a Twist-em. A clamped section of Tygon tubing placed
along side the branch was included in the seal to facilitate sampling
the internal air. Measurement of mature foliage required cutting the
closed end of a Mylar bag, sliding it over the branch and making two
seals. All CO2 compensation determinations were performed under green-
house conditions where PPFD ranged from 300 to 600 pE m-2 s'] and air

temperature inside bags was 25 to 25.9°C. After 1 h the air inside

27

the bags was manually expelled through the sample cell of a Beckman 864

infrared gas analyzer.

Results and Discussion

Light response. Pn response to different levels of PPFD was

 

determined at three crown positions with LS foliage at varying stages of
maturity. The terminal apex (Figure 2.1) and the first major lateral
apex (Figure 2.2) included foliage ranging from newly initiated to re-
cently mature leaves. The third position included mature foliage of the
first major branch (Figure 2.3) basipetal to the foliage used in Figure
2.2. Different symbols on each light response curve represent replica-
tions for each foliage position.

Light compensation for all three foliage positions was between 25
and 50 uE m'2 5']. Intolerant species such as larch are generally con-
sidered to have higher light requirements for Pn, and these compensation
intensities may at first seem low. The foliage of long shoots in ngix
species, however, resembles that produced by seedlings (Frampton 1960).
Photosynthetic capacity changes with the stage of development, and seed-
lings can generally make better use of low illumination than older plants
(Larcher 1969). Light saturation was reached at approximately 900 pE
m'2 s-1 at all three foliage positions. The range of Pn values obtained
beyond saturation in all three figures are higher than values obtained
for other conifer species (Larcher 1969), and are consistent with those
reported for Lgrjx_(Fry and Phillips 1977; Growin et a1. 1980; Neuwirth

1967; Polster and Wise (1962). The overall shape of these curves is

hyperbolic and typical of a response obtained with multiple leaves of a

28

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31

C3 species (Leopold and Kriedemann 1975). Dark respiration rates ob—
tained with replicates of all three leaf positions ranged from 0.9 to
1.8 mg C02 9—1 h_1. A post-illumination burst of C02, typical of plants
possessing a C3 photosynthetic pathway, was not detected.

Average Pn values (from Figures 2.1, 2.2, and 2.3) plotted as a
percent of maximum Pn clearly illustrate that light saturation was

2 s.1 (Figure 2.4). Previous work

reached between 800 and 1000 pE m—
with L. leptolepis (Fry and Phillips 1976, 1977) indicated that individ—
ual leaves had higher saturation light intensities resembling those
obtained with Zea mays L., a C4 species. This was not found in the
present study. Furthermore, the present measurements were made with
multiple leaves which normally have even higher light saturation intensi-
ties than individual leaves (Lakso and Seeley 1978).

Temperature response. The effect of temperature on Pn was deter—
mined at saturating levels of PPFD (ca. 900 pE m-2 5‘1). The location
of LS foliage used was similar to that used to determine light response
curves.

Pn response to temperature for the terminal foliage of the leader
and first major lateral branch is presented in Figures 2.5 and 2.6.
Response to the leader foliage (Figure 2.5) to increasing temperature
was similar for each replication up to an optimum range between 15 to
20°C. This steady increase in Pn as temperatures become more favorable
is indicative of young leaves at or near full expansion (Leopold and
Kriedemann 1975). Beyond the optimum range Pn dropped sharply and
replication values were much more variable. This rapid decrease in Pn

above optimum temperature probably reflects the adverse effects of high

32

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35

temperature on young foliage. Pn response to temperature curves for

the terminal foliage of the first major lateral branch (Figure 2.6) were
similar, except that maximum Pn values for all replications were higher
than those in Figure 2.5. Response curves to temperature for mature
leaves on the first major lateral branch displayed the greatest varia-
tion between replicates (Figure 2.7). The optimum temperatures for
maximum Pn was less pronounced than in Figures 2.5 and 2.6, and the
mature LS foliage appeared more tolerant to a broader range of tempera-
tures.

The influence of temperature on growth, productivity and gas
exchange of trees is dependent upon species and geographic origin
(Hellmers l963; Kramer 1957). The magnitude of temperature effects may
also be modified by environmental variables such as light intensity,
available C02, water, and preconditioning effects of environmental
factors. In the present experiments, C02 exchange in response to temper—
ature was determined in an open IRGA system at ambient C02 concentrations
and saturation levels of PPFD. The trees were also greenhouse-grown and
not allowed to become severely water stressed. The relatively constant
temperature of the greenhouse may have had some preconditioning effects
but results of previous work with larch indicate that this would be
minimal. Growin et al. (l980) grew L, decidua at four constant tempera-
tures ranging from 12 to 27°C. Optimal temperature for Pn, however, was
12°C for all growth temperatures.

The rate of photosynthesis in most temperate zone species increases
from near 0°C to a maximum reached between l5 and 25°C (Kramer and

Kozlowski l979). Figure 2.8 illustrates the optimum range of temperature

36

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38

for the terminal and mature foliage obtained in this study. Average Pn
values are plotted as percent of maximum, which occurred between 17 and
21°C. This is consistent with temperature optimums established for C3
species where maximum photosynthesis and quantum yield occur at tempera-
tures below 30°C (Bjorkman et al. 1975; Black 1973; Ehleringer and
Bjdrkman 1977; Ehleringer 1979). The apparent sensitivity of terminal
leaves of the leader to temperature is also clearly shown in Figure 2.8,
but why the same degree of sensitivity was not shown by the terminal
leaves of the lateral branch is unexplained.

Transpiration rate. Changes in transpiration rate with increasing

 

light intensity for mature and newly initiated to recently mature leaves
borne on the first major lateral branch (same as Figures 2.2 and 2.3)

are presented in Figure 2.9 and 2.10, respectively. The curves resemble
those for Pn, although much more variation between replication values is
evident. Transpiration increased up to 800 to 1000 HE m-2 s-1 then
leveled off, indicating that stomata were fully open at that light inten—
sity.

Transpiration rate response to temperature was determined on the
terminal leaves (Figure 2.11) and mature leaves proximal (Figure 2.12)
to the terminal leaves on a first major lateral branch. The rates of
transpiration below 20°C are consistent to those obtained for L, decidua
(Berger-Landefeldt 1936). Above 20°C transpiration rate increased
rapidly with temperature. Rates were determined at saturation PPFD
and the trees were well-watered so stomata apparently remained open up

to the maximum experimental temperature (31 to 33°C). These results

indicate that the rapid decrease in Pn above 20°C (Figures 2.5 to 2.8)

39

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43

is due to internal factors and not stomatal closure. These internal
factors would include antagonistic processes that increase with temper-
ature and eventual inactivation of the C02-fixation apparatus at high
temperature (Kramer and Kozlowski 1979). Investigations by Pisek

et al. (1968, 1969) with different European conifers, including

L, decidua, showed that this high cardinal, or compensation temperature,
was passed at 36 to 38°C. Based on the rapid decline in the Pn above
optimum (Figures 2.5 to 2.8), compensation temperature in the present
study would probably have been close to those reported by Pisek et a1.
(1968, 1969).

£99 compensation concentration. The C02 compensation concentration
L

 

of LS foliage was determined at various crown positions. Measurements
were made at constant temperature and PPFD ranged from 300 to 600

uE In-2 5-1. The lowest CO2 compensation concentrations, 58 and 59

pl 1'1 were obtained with mature foliage borne on low and mid-crown
branches. Foliage of the terminal leader had the highest C02 compensa-
tion concentration of 75 pl 1'1. These results agree with those obtained
by Dickmann and Gjerstad (1973) who determined C02 compensation concen-
trations for numerous conifer and hardwood species, including_L. decidua

and L, laricina. The values they obtained for the larch species were

mid-way between those obtained here with L, leptolepis and within the

 

range of C02 compensation values for plant species with a C3 photosyn-
thetic pathway (Moss 1971).

Plant species that are intermediate between C3 and C4 have been
reported (Kennedy and Laetsch 1974), and a large group of plants possess-

ing crassulacean acid metabolism (CAM, a modified form of C4 physiology)

44

are capable of shifting to C3 photosynthesis (Ehleringer 1979; Hatsock
and Nobel 1976). Woody arborescent plants, however, are of the C3 type
(Dickmann and Gjerstad 1973; Schaedle 1975). The only identified excep-
tions are the mangrove Aegiceras majus (Joshi et a1. 1974) and several
C4 Euphorbia species of Hawaii (Pearcy and Troughton 1975).

Larch may have certain enzymatic characters similar to C4 plants
(Fry and Phillips 1976), but the photosynthetic responses to light and
temperature determined in the present experiments were not suggestive of
C4 photosynthetic physiology. Light saturation occurred at a level below
that characteristic of C4 plants (Ehleringer and Bjorkman 1977; Bjorkman
1971; Gifford 1971; Leopold and Kriedemann 1975) while C02 compensation
concentrations were within the range of C3 species (Dickmann and Gjerstad
1973; Moss 1971). The optimum range of temperatures for maximum Pn was
also well below that associated with species having C4 physiology
(Downton 1971). This low temperature optimum for L, leptolepis is prob-
ably indicative of its cool native habitat in the mountains of the Japanese
island of Honshu. For larch species in general, their northern distribu-
tion and adaptation to some of the coolest regions of temperate zone
forests is also not characteristic of the hot, dry habitats of most C4

plants (Downton 1971; Stowe and Teeri 1978; Teeri and Stowe 1976).

CHAPTER III

MID- AND LATE-SEASON REDISTRIBUTION OF
C-PHOTOSYNTHATE FROM SHORT SHOOTS AND LONG SHOOTS OF
MATURE LARIX DECIDUA TREES

14

Introduction

The contrasting growth of short shoots and long shoots of larch
(Larix) raise questions about the role each plays in the growth of the
whole tree. Long shoots increase tree height and lateral branch length,
with shoot extension continuing late into the growing season. They also
produce both early (preformed) and late (current season) leaves. This
seasonal pattern of leaf production and growth by long shoots resembles
that of “indeterminate” species such as Populus (Critchfield 1960) and
figLuLa (Kozlowski and Clausen 1966b). Short shoots, on the other hand,
arise from preformed primordia and complete expansion early in the
season. This resembles the determinant or fixed growth of species like
Linus, Aggr, or fagus. The contribution, however, that one shoot type
makes to the growth of the other and of both to the tree as a whole is
not well understood.

According to Clausen and Kozlowski (1967), early long shoot growth
of_Laer laricina was dependent upon stored reserves. Latter expansion
appeared to utilize current photosynthate produced by early leaves and
maturing late leaves. Short shoot growth was characterized by an early
flush of leaves followed by rapid leaf and shoot expansion which pre—

ceded long shoot growth. A similar pattern of growth has also been

45

 

 

46

described for L. decidua (Tranquillini 1979) and L. occidentalis (Owens
and Molder 1979a) and appears consistent in the genus.

The present study was performed to investigate the role of each
shoot type during and after the period of vigorous long shoot growth.
Leaves of long and short shoots borne on three-year-old branches of
mature trees in the field were exposed to 14C02 and the translocation of

fixed 14

C was traced at various times after exposure. This was done to
further elucidate the photosynthetic contribution of each shoot type

during this period of rapid growth.

Materials and Methods

 

IIEEE- This study was carried out at the Michigan State University,
Department of Forestry Tree Research Center at East Lansing during the
summer of 1980. Sample trees were Lgrjx_decidua Mill. planted in east—
west windbreak rows and were 10 to 12 m in height. Experiments were
performed on three—year-old first—order lateral branches located within
1 m of the top of the trees. Branch selection was based on age and simi-
larity in length and vigor. Treatments were performed on foliage of
three year short shoots (3YSS), two year short shoots (2YSS), and one
year long shoots (1YTLS) borne on the third, second, and current seasons
branch growth increments, respectively (Figure 3.1).

Translocation. On July 1 and August 10, short-shoot leaves borne
on the middle portion of third-and second-year branch growth increments
were exposed to 14C02 and allowed to photosynthetize for 1 h. Both distal

and proximal portions of each branch increment relative to exposed

47

 

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48

foliage remained unlabeled. The same labeling procedure was applied

to all long shoot leaves of the current season's terminal long shoot.

A 4 ml beaker containing 0.1 mg BaMCO3 (specific activity 277 uC mg_])
was attached to each treated branch section and then enclosed in a

20 X 36 cm Mylar bag. Bag ends were sealed around the branch using foam

strips and wire ties. 14

CO2 was generated inside the bags by inserting
the needle of a 5 cc syringe through the bag and dispensing 2.0 ml
lactic acid (20% v/v) into the beaker. The syringe was withdrawn and
the hole sealed with cellophane tape. The Mylar bags were periodically
agitated to mix the gas inside and then removed after 1 h. At 5, 24,
and 48 h after labeling branches were excised and a bark sample was taken
from the main stem, directly above and below each treated branch, with
a 6 mm cork borer. Bark samples, branches, and foliage were then frozen
at -25° C. After freezing, branches were partitioned into leaf and
shoot types and third- and second-year branch increments were further
divided relative to labeled foliage position. These were dried at 75° C
for 48 h and then weighed.

The same experimental procedure was repeated on September 8, with
the following exceptions. 14CO2 was generated inside Mylar bags by

14

adding 2.0 ml lactic acid (20% v/v) to 1.0 ml Na C03 (specific activ-

2
ity 25 pC ml_1), and a 72 h sample was taken in place of the one at 5 h.

Radioactive determination. Dried samples were ground in an aspirated

 

Wiley mill to pass a 40 mesh sieve. Subsamples of 7.0 to 8.0 mg were
taken from ground samples and placed in low potassium glass scintillation
vials. Subsamples were rehydrated for 2 hours by first adding 2 drops

of 80% (v/v) ethanol followed by 5.0 ml PCS (Phase Combining Systems,

49

Amersham Corp.) and then allowed to dark equilibrate for 24 h. The
amount of subsample radioactivity was determined by liquid scintillation
spectrometry with a TriCarb 2002 liquid scintillation spectrometer
(Packard Instrument Co. Inc.). Sample radioactivity was, background
subtracted, quench corrected by channels ratio method and expressed as

distintegrations per minute (dpm).

Results and Discussion

 

In ngix species the first leaves to flush in spring emerge from
preformed buds borne on short shoots (SS). The stem of short shoots
elongates only about 1 mm per year so they resemble clusters of needles
along lateral branches and the main stem. The leaves of short shoots
develop rapidly, and are capable of net photosynthesis (Pn) soon after
flushing (Tranquillini 1979). This usually precedes terminal extension
growth by l to 2 months. Increases in tree height and branch length
occur by extension of long shoots (LS) produced by buds borne at the tree
apex and distal portions of branches. Long shoot buds contain preformed
leaves which flush shortly after short shoot leaves and are fully expanded
before vigorous LS growth occurs. Little internode extension occurs
between these leaves and they appear as clusters at the base of LS.

New leaves are initiated up the flanks of the LS apex and their inter-
node extension is greater than that of basal leaves.

By the July 1 exposure of foliage to 14C02, LS in the upper crown
had completed approximately l/3 to l/2 of their final growth increment.

Short shoots had flushed several months previously, but their contribution

50

to LS growth before this time is undetermined. Short shoot foliage,
however, was capable of high rates of Pn by late April and early May.

1 h”1 at

On May 5, for example, rates of SS leaves averaged 12 mg C02 g-
19° C and saturating Photosynthetic Photon Flux Density (PPFD) (Chapter
II).

14Cephotosynthate. Recovery of 14C-photosyn-

July distribution of
thatein July, 5, 24, and 48 h after exposure of the three foliage posi-
tions is presented in Tables 3.1-3.3. Specific activity (dpm/mg sample
dry weight) and percent of total recovered translocate (TRT, excludes

14C02) are tabulated

activity not transported from leaves exposed to
according to major branch component and bark samples. Total activity
(dpm/sample) is greatly affected by sample size, whereas specific activ-
ity is indicative of the metabolic activity and requirement for photo-
synthate regardless of size or weight. Percent TRT is also affected by
sample size, but reflects the relative mobility and requirement for
photosynthate by eliminating the activity retained in exposed leaves.

14C fixed and transported from LS

Terminal LS retained most of the
foliage (Table 3.1). Over 85% of the 14C transported out of LS leaves
after 48 h was retained in LS, reflecting their vigorous growth and
photosynthate requirement. The specific activity of LS leaves over the
same period dropped by nearly one-half and can be attributed to transport
to the L5 as well as high respiratory loss associated with young leaves
(Kozlowski and Keller 1966). Long shoot net photosynthetic capacity,
however, exceeded the needs of their elongating shoot. This is seen in

14C activity found in 2YBI, 3YBI, and the stem bark samples

14 14

the early

5 h after exposure of LS leaves to C0 Much of this C-photosynthate

2.

 

51

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52

appears to be moving toward the main stem as indicated by the steady
decrease in specific activity recovered from 2YBI with time of sample,
while that in 3YBI and bark samples increased over the same period.

The high specific activity in the 2YBI after 5 h may represent a transi-
ent pulse of early mobile photosynthetic products moving toward stronger
sinks in the tree. The pathway in the main stem also appears to be
directed toward the base of the tree, as higher specific activities were
found in the bark samples taken below each branch.

Labeled 2YSS leaves actively transported 14C-photosynthate to the

 

2YBI (Table 3.2). This accounted for over 99% of the TRT in the branch
sampled at 5 h, and about 63% at 24 h. Approximately the same proportion
was found in 2YBI at 48 h and appears to have been retained there.

Short shoot elongation was completed previous to this time so the re-
tained photosynthate may have been required for radial growth. Some

14C activity was found in the 1YTLS at all sample times, but the 104
dpm/mg at 48 h accounted for less than 1% of the TRT for the branch.
The majority of 14C-photosynthate not retained in the 2YBI was found in
the 3YBI and bark samples. Again, transport in the main stem was pre-
dominantly downward.

The distribution pattern of 14

C02 fixed by 3YSS leaves is similar
to that of 2YSS (Table 3.3). The direction of transport is mainly
basipetal, with the majority of 14C-photosynthate retained in the 3YBI.
Again, some specific activity is found in the 1YTLS as well as 2YBI
indicating bidirectional translocation of photosynthate. Considerably
more 14C-photosynthate entered the main stem from 3YSS, however, than

from younger branch sections. The specific activity of the bark sample

53

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1 comvp wwmo 1 mommp 1mo>oop mm
m.po mnmm mmpm m.mm op¢Om Hm>m
pm. ¢ m¢. mo om4<m
pm. mp pm we. Nm mm>owp mm
m.mm mpp mop N. mm Hm>m
papp mE\Eae oe\sae papp mE\Eae
.Loppo 4 we
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pLO4m Lomz1ocooom .wpmhpy mpmoopmcmpp OOLO>OOOL pmpop pcoosoo oco AmE\EOOv zpp>ppom
OpppOOOm mo OOmmogqxo manpomo am we monocmea LOOLO1mepp Loo>1m :p 9 4O copponpepmpo .N.m opnop

#—

54

 

 

 

.m4<m :O oceon .m4<m .m4pN .oomppop ppm monopocpm
moo Op OOmooxm Omoppom
.<.
co. mom m.N mnmm po.v Op gocopa zopon 4Lom
po.v O NO. RN po.v om cocmpn o>oao 44mm
mo. m pv. m po.v n mo>oop mpp
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mN. O Ow. Op mo. m Hm>m
m. w N.p mm mm. mm mm4<m
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pgogm Loo>1ugpsp .wpmppy OpOOO—mcopp OOLO>OOOL popop pcooeoo Oco AOE\EOOV app>ppoo
opppooom mo OOmmogoxo manpoou a4 mo mmgocopn LOOLO1mepp Loo>1m cw O mo copponpepmpo .m.m m—OOH

#—

 

55

below the treated branch represented over 2% of the TRT, and as indi-
cated by the previous treatments (Tables 3.1 and 3.2), the majority of
14C translocated to the main stem was directed toward the base of the
tree. Rangnekar et al. (1969) reported similar downward transport of
14C fixed by third-whorl lateral branches of Pinus resinosa trees,
whereas transport from the second whorl was predominantly upward. The
position of third—whorl branches in the Rangnekar et al. study would be
in approximately the same nodal position as the 3-year branches in the
experiments described here.

The primary function of leaves on 2YSS and 3YSS in July appears
to be providing photosynthate for maintenance of the branch section bear—
ing them. Some excess photosynthate is translocated basipetally out of
these branch sections, with the majority moving toward the main stem
and the rest acropetally to the 1YTLS. That bidirectional translocation
in these branch sections occurred is indicated by some basipetal movement

14

of C-photosynthate from the 1YTLS (Table 3.1). The reason for trans-

port to the 1YTLS from 2YSS and 3YSS is unclear in view of the excess
14C fixed by 1YTLS leaves and transported to the main stem. Young expand-
ing leaves of Populus deltoides, however, have been shown to simultane—
ously import and export photosynthate (Gordon and Larson 1968), and a
similar pattern may have been present in the LS leaves. Larson and
Dickson (1973) also reported that the direction of import or export by
leaves was dependent on the phyllotactic vascular connection between,
and the distance from, other importing or exporting leaves.

The relative contribution of the foliage on various shoot types to

the three branch increments is presented in Figure 3.2. By July the

 

 

56

._ xpgq

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.wmpw Amp .m4ppp A<V ep apeuopmeeep1o

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57
translocation pattern appeared almost exclusively basipetal. The
foliage of each shoot type was capable of producing photosynthate in
excess of that required locally. Further, the TLS appeared autonomous
and able to support its own vigorous growth. This is similar to find-
ings reported for other species that produce distinctly different short
shoots and long shoots, e.g., apple (Hansen 1967).

14

August distribution of Cephotgsynthate. Long shoots displayed

a gradient of growth activity from upper to lower crown positions by the

August 10 treatment of shoots with 14

C2. The terminal leaders of trees
were still actively growing and apical buds were not observed on long
shoots of upper crown branches. The frequency of bud set increased
toward the base of the tree, and long shoots without apical buds were
not evident on branches above mid—crown.

The distribution of 14C obtained after the August 10 treatment was
similar to that of July 1 and is presented in Tables 3.4 to 3.6. Leaves
on 1YTLS were exporters of 14C-photosynthate as suggested by the
specific activity found in older branch sections and the stem bark
samples (Table 3.4). Again, the specific activity of 2YBI was high
after 5 h and then decreased in 24 and 48 h samples. Conversely, the
specific activity in 3YBI increased with time of sampling while trans-

l4C

port to the main stem from 1YTLS was highest at 24 h, with more
heading downward. These results further suggest that LS leaves on
August 10 are capable of producing more photosynthate than required to
maintain the growth of LS.

Some acropetal translocation of 14C-photosynthate to the 1YTLS

from 2YSS and 3YSS leaves was evident on August 10, but the majority

58

 

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OF

 

59

of movement appeared to be basipetal (Tables 3.5 and 3.6). Foliage on
both the 3YBI and 1YTLS contributed substantial photosynthate to the
main stem as the high specific activity recovered from the bark samples
indicate.

The contribution of foliage on each shoot type to the three branch
sections in August is seen in Figure 3.3. The pattern of distribution was

14C-photosynthate

14

similar to that obtained in July, but the proportion of

translocated to older branch sections increased slightly. C trans-

located from 1YTLS and recovered in 2YBI and 3YBI was about 2% higher

and that from 2YBI to 3YBI about 7% higher than in July.

14

September distribution of C-photosynthate. Recovery of 14C-photo-

 

synthate 24, 48, and 72 h after exposure of the three foliage positions
in September is presented in Tables 3.7 to 3.9. By September 8, apical
buds had formed on the LS of upper crown branches, but the last LS leaves
initiated were still immature. There was no visual evidence of bud set,
however, in the terminal leaders at this time.

Compared to July and August, the specific activity of labeled
leaves borne on all shoots decreased appreciably with time after treat-

ment. Mobilization of fixed 14

C and basipetal transport was still very
active, but increased respiratory loss could also have added to the

14C in labeled foliage. Ursino et al. (1968) found that over
14

decreased

50% of the C02 incorporated by Pinus strobus seedlings late in the

 

season was lost through respiration.
The predominant direction of 14C transport in September was still
basipetal, as was found in July and August, but greater proportion was

translocated to 1YTLS from 2YSS at each sample time (Table 3.8).

4...- _.,,_.

60

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61

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62

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63

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upppuonm we nemmoeoxe ennpuon .4 po moguneen eoneO1pmepp eeu>11m np u po noppsnpewmpo .p.m wpnep

np

 

64

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peOgm eeux1nnoumm .ppmppv Opeuopmneep nuem>ouwe pepOp pnmuemn nne pmspsnnv zpp>ppue
upppumom np nmmmmeoxm ennpumn .4.po mmguneen euneO1pmepp eeoz1m np onp 4O noppnnpepmpo .m.m mpnep

14c fixed by 3YSS to 1YTLS and

This is also seen in the movement of
2ALS after 24 h but not later (Table 3.9). It appears that the contribu-
tion of older SS to newer LS was greater late in the season than in
July when vigorous LS growth occurred. Higher specific activities were
also found in the main stem bark samples above the treated branch than
earlier in the season. After 72 h, however, 3YBI still retained a large
proportion of assimilated 14C. This suggests that photosynthate is
required for cambial growth or storage at this time in the growing
season.

Distribution of translocated 14C 72 h after foliage on various
shoot types were labeled is presented in Figure 3.4. The recovered por-

14C fixed by TLS leaves and transported to 1YTLS was less than

tion of
found in July or August, but the proportion transported to 2YBI and 3YBI
increased. This reflects the termination of long shoot extension and
increased excess photosynthate available for other areas of growth.
Approximately equal amounts of 14C-photosynthate were translocated from
2YSS to 2YBI and 3YBI, and a small amount was recovered in 1YTLS. Only

14C fixed by 3YSS was recovered in the 2YBI with the

a trace of the
majority retained by 3YBI for local use or transported to the main stem.
The growth and development of LS from early July through the rest
of the season is apparently largely autonomous and self-sustained.
Long shoot extension was only 50% completed by July 1, but very little
14C fixed by SS leaves on older branch sections was translocated to
growing LS. Similar results in July were observed with young Eiflflé
strobus where small amounts of photosynthate were translocated to current

seasons needles from old needles (Shiroya et al. 1966). New needle and

 

 

66

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67

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68

shoot expansion of pine is at or near completion by July and new needle
maturity is reached shortly thereafter. The role of new leaves of coni—
fers has been shown to shift from net importers to net exporters of
photosynthate about this time (Gordon and Larsen 1968; Loach and Little
1973). Larch LS, however, were exporting 14C—photosynthate when shoot
and leaf expansion was only 50% complete.

The role of SS leaves in the current-season growth of LS is appar—
ently very minimal by July. Short shoot leaves flush, expand rapidly,
and photosynthesize for over two months before vigorous LS expansion
occurs. Their contribution prior to July, when they function as previous-
season's needles of other confiers, is probably considerable. 01d
needles in Pinus resinosa have been shown by tracer studies to be very
important in early season growth of new needles and shoots (Dickmann and
Kozlowski 1970a, 1970b; Rangnekar and Forward 1969; Schier 1970). The
role of old needles as exporters of photosynthate diminishes, however;
when new shoot maturation is completed, these new needles then become
the major producers (Gordon and Larson 1968; Loach and Little 1973;
Ursino and Paul 1973). The phenologically older leaves of larch SS do
not seem to fit this pattern.

Table 3.10 shows the specific activity of sections of 2— and 3-year
branch internodes bearing SS leaves when leaves on the middle section
Were exposed to 14C02. These data indicate that SS export considerable
photosynthate to their supporting branches and elsewhere in the tree
late into the growing season. The low activity found in sectiOns distal
to the treated leaves probably reflects the autonomous LS growth. This

uneven distribution of specific activity also suggests that each short

 

69

 

ppm NpO mwO nmp pmmm MOpN mmw mmom pmpN peprpwem
OpN omm NOm mmmp OMpN MONM nepp Onom pmpm nmpone4
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70

shoot provides photosynthate for the growth of the portion of branch
bearing it. The surplus, represented by the specific activity in sec-
tions proximal to the treated leaves, would be available for other grow-
ing tissues such as roots or stem cambium.

Long shoots of larch increase tree height and extend lateral growth
of branches. Their vigorous growth lags behind that of SS and initially
is at the expense of the phenologically older SS leaves. This would be
similar to the pattern shown in non-deciduous conifer species. The
results of these experiments, however, suggest that expanding LS of

larch differ from other conifer species by becoming net exporters of

 

photosynthate long before shoot and leaf expansion is completed. Short
shoots resemble the prior-season needles of other conifers in that they
are borne on older branch sections and produce leaves capable of net
photosynthesis early in the season. Unlike previous season needles,
leaves and SS are actually current-season growth and function as new

shoots late in the season when they export large amounts of photosynthate.

 

 

CHAPTER IV

SUMMARY AND CONCLUSIONS

In the first series of experiments an infrared differential open
gas analysis system was utilized to determine the effect of light intens-
ity and temperature on net photosynthetic (Pn) and transpiration rate for

leaves of three-year-old Larix leptolepis trees. Immature to recently

 

mature, and mature long shoot (LS) foliage was used for these measure—
ments. This foliage was borne on the terminal leader or first major
branch apex, and interior branch, respectively. Similar foliage was
sealed inside Mylar bags and C02 depletion of the air inside bags was
measured by infrared gas analysis to determine C02 compensation concentra-
tion.

The net photosynthetic response to increasing levels of photosyn-
thetic photon flux densities (PPFD) was similar for each foliage position
and stage of leaf maturity. Light compensation was between 25 and 50

uE M'2 5']. Rates of Pn increased rapidly at PPFD above compensation

intensity until saturation was reached at approximately 900 uE m"2 s ‘1.

This hyperbolic pattern of Pn response to light is characteristic for

multiple leaves of plant species possessing a C3 photosynthetic pathway.
Dark respiration rates for all foliage positions ranged from 0.9

-1

-1
to 1.8 mg CO2 g h A post illumination burst of CO2 was not

detected.

71

 

 

72

Photosynthetic response to temperature was determined at saturating
PPFD. Net photosynthetic rates of terminal leader foliage increased
steadily from low temperatures to an optimum between 15 and 20° C.

At temperatures above 20° C Pn decreased rapidly reflecting the sensi-
tivity of the young foliage to adverse high temperature. Similar photo-
synthetic response to increasing temperature was obtained for the branch
terminal foliage. Maximum Pn, however, was higher for all replications
of this foliage position than observed with the leader foliage. Mature
leaves of the first major lateral branches had a slightly broader range
of optimum temperature than found for the terminal foliage positions.

The adverse effect of temperature on Pn above optimum range also appeared
less pronounced for mature leaves. Maximum Pn for all foliage positions
occurred between 17 and 21° C, comparable to other north temperate trees.
This was also consistent with the optimum temperatures for Pn established
for C3 plants.

Transpiration rates obtained for terminal and mature foliage of
branches at increasing levels of PPFD resembled Pn response to light.
Temperature was maintained at 20° C so rising transpiration rates
appeared to be due to stomatal response to increasing PPFD. Transpira—

'1 and remained fairly

tion rate leveled between 800 and 1000 uE m'2 s
steady at higher PPFD.

Results of the transpiration rate response to increasing temperature
indicated that the rapid decline in Pn above optimum range of temperature
was due to internal factors and not stomatal closure. Transpiration

rates increased continuously with rising temperature up to the experi-

mental maximum. Rates of Pn, however, declined rapidly when temperature

 

 

73

exceeded the optimum range. If this decrease in Pn had been due to
stomatal closure a similar decrease in transpiration rate would have been
expected.

The 002 compensation concentration was determined for LS foliage
at various crown positions. PPFD ranged between 300 and 600 uE m_2 s—1
and temperature inside bags remained constant. The lowest C02 compensa-
tion concentrations were 58 and 59 ul l"I for the mature foliage of low
and mid-crown branches respectively. The foliage borne at the apex of
the terminal leader had the highest C02 compensation concentration or
75 ul 1‘]. These values are similar to those obtained for other trees
and plant species possessing a C3 photosynthetic pathway.

In a second series of experiments leaves of both long and short
shoots borne on three-year-old first order branches of mature L, decidua

14C was traced at

were exposed to 14C02 and the distribution of fixed
various times after exposure. This was done on July 1, August 10, and
September 8, and followed the course of long shoot growth from a period
of rapid expansion to early bud set. Labelled foliage included short
shoots (SS) borne in the middle of 3-year and 2-year branch increments
(3YBI and 2YBI respectively) or l-year terminal long shoots (1YTLS;
current season increment).

In July, the vigorous growth of long shoots (LS) was approximately
50% completed and SS expansion had ceased approximately two months prior
to this. At this time the majority of 14C fixed and transported by LS
leaves was retained in 1YTLS. Basipetal transport of 14C—photosynthate,
however, was detected in 2YBI, 3YBI and main stem as early as 5 h after

expc>sing TLS leaves to 14C02. This indicated the 1YTLS leaves were

 

74

capable of producing photosynthate in excess of that required to support

14

the rapid LS growth. The majority of C fixed by 2-year short shoots

(2YSS) was transported to the 2YBI where over 60% of the total recovered
translocate (TRT) was retained 48 h after labelling 2YSS leaves. The
predominant direction of movement of 14C out of the 2YBI was basipetal

14C was moved acropetally

14

to the 3YBI and main stem, but a small amount of

to the 1YTLS. This was less than 1% of the TRT. C fixed by 3YSS was

primarily transported to the 3YBI and retained for local use. Again a

14

small portion of C was transported to the 2YBI and 1YTLS, indicating

 

bidirectional translocation, but most of that not retained in 3YBI was

14

moved to the main stem. Movement of C in the main stem from all branch

increments was predominantly downward.

The distribution of 14

C-photosynthate transported out of labelled
foliage in August was similar to that observed in July. Basipetal trans-
location from all shoots, however, was slightly increased in August.
Terminal long shoot growth at this time was near completion and 14C
exported basipetally from 1YTLS after 48 h was approximately 2% higher

14

than observed in July. The amount of basipetal transport of C from

2YBI and 3YBI in August was also approximately 7% higher than in July.
Terminal long shoots of 3-year-old branches had recently set bud by
September 8, but the last leaves initiated near the apex were still
immature. Tree terminal leaders, however, were still actively expanding.
The proportion of acropetal transport from 2YBI was higher than observed

14

in July or August, but the majority of C translocated from all foliage

positions was still basipetal. 1YTLS actively exported to older branch

14

increments and retained proportionally less C-photosynthate reflecting

 

75

14C fixed and transported by 2YSS leaves

termination of LS expansion.
was about equally distributed between 2YBI and 3YBI 72 h after labelling
foliage. The majority of labelled photosynthate from 3YSS leaves was
also retained in the 3YBI for local use. Even though the proportion of
14C retained in 2YBI and 3YBI for local growth and storage was increased
over that in July or August, the specific activity in bark samples of the
main stem also increased. The proportion of this activity detected

above the branch was higher than observed previously and may be attributed
to the continued growth of the terminal leader.

The results of these experiments suggest that LS growth and develop-

 

ment is largely self-supported by early July when expansion is less than
50% completed. Active export of 14C-photosynthate to older branch incre-
ments and main stem was observed in July and then increased in later
months. The current-season shoots of other conifers also become active
exporters of photosynthate, but this usually occurs after shoots have
expanded and needle maturation is completed. Shoot growth prior to this
is largely at the expense of older needles.

The contribution of short shoots to the growth of LS prior to July
was undetermined. Like the older foliage of other conifers they must
contribute to the early growth of new LS, as their leaves flush early and
are capable of Pn several weeks before vigorous LS growth begins.

Unlike the old foliage of other conifers, however, SS are current season
growth and as such are capable of major export of photosynthate through-
out the growing season. This, in conjunction with the early autonomy of
LS growth, may partially explain the rapid growth rate generally associ-

ated with larch species.

 

 

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