Retrospective Theses and Dissertations
1975
Photosynthesis, photorespiration, and dark
respiration in Populus x euramericana: effects of
genotype and leaf age
Dean Harold Gjerstad
Iowa State University
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GJERSTAD, Dean Harold, 1944»
HEfrOSÏNIHESIS, fflOTORESPIRATICN, AND DARK
RESPIRATION IN POPUIUS X EURAMERICANA:
EFFECTS OF GENOTYPE AND LEAF AGE.
Iowa State IMversity, Ph.D., 1975
Agriculture, forestiy and wildlife
Xerox University Micrcfiinris. An-.A-boMviichigan asios
THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RFCFIVFD
Photosynthesis, photorespiration, and dark respiration
in Populus X euramericana;
Effects of genotype and leaf age
by
Dean Harold Gjerstad
A Dissertation Submitted to the
Graduate Faculty in Partial Fulfillment of
The Requirements for the Degree of
DOCTOR OF PHILOSOPHY
Department:
Major:
Forestry
Forestry (Forest Biology)
Approved:
Signature was redacted for privacy.
In Charge of Major Work
Signature was redacted for privacy.
Signature was redacted for privacy.
For the Gracfalte College
Iowa State University
Ames, Iowa
1975
ii
TABLE OF CONTENTS
Page
INTRODUCTION
1
LITERATURE REVIEW
3
OBJECTIVES
10
MATERIALS AND METHODS
11
RESULTS
17
DISCUSSION
58
CONCLUSIONS
65a
LITERATURE CITED
66
ACKNOWLEDGMENTS
71
1
INTRODUCTION
The incipient shortages of wood fiber in the North Central region of
the U. S., combined with projected increases in demand for wood fiber prod­
ucts, has precipitated research on intensive, short rotation management
systems for hardwood tree crops.
In cooperation with the North Central
Forest Experiment Station of the U. S. Forest Service at Rhinelander, Wis­
consin, the Forestry Department at Iowa State University is investigating
genetic and physiological characteristics of a number of hybrid poplar
clones.
The objective of this research is to provide basic biological data
required for practical application of intensive culture.
To provide a framework for these basic investigations, Promnitz and
Rose (1974) have developed a mathematical model to study intensive silvicultural systems of poplar through simulation.
A physiological knowledge
of clonal differences has been gained through a number of studies recently
completed:
..
^jJUULLixgu J
physiological responses to temperature and soil moisture
1 r» 1 \
.• «_ « - ^
^
-
"»
f T>_ 1
Ci.xuj.Ccii. It J. uj-ugcu jLcvcj. ^
tion and peroxidase activity (Wray, 1974).
u 1. CI ^
1 r\ ^ A N
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J
^J
-1
_
But further model development
and verification is hampered by the lack of information on clonal differen­
ces in the effects of leaf age on physiological processes.
To add informa­
tion for the model's development, I examined the relationship of carbon
dioxide gas exchange, diffusive resistance, CO^ compensation concentration,
and glycolate oxidase activity to leaf ontogeny in rooted cuttings of three
Populus clones.
The question arises, however, whether or not developmental studies are
a logical direction to pursue in tree physiology research at this time.
2
But research on a specific biochemical reaction demands the selection and
use of leaf tissue.
Without knowledge of ontogenic variation in leaf phys­
iological rates, the validity of research results generalizing leaf proces­
ses from haphazardly selected leaf material is questionable.
Furthermore,
developmental studies are critical in the researcher's understanding of
physiological variation in leaf age and genetic variation between tree
sources.
The present study, in the strictest sense, is not a true developmental
study but a series of physiological observations on successive leaves on a
plant.
It is not legitimate to assume in such a "vertical" series that
each leaf has exactly the same physiological developmental history as every
other leaf (Richards, 1934),
But by growing trees in a nearly constant
environment of a growth chamber and through the use of clonal material, a
close approximation of leaf development can be assumed.
3
LITERATURE REVIEW
To more fully understand and predict fiber production, a better knowl­
edge of photosynthetic and respiratory activity throughout the tree crown
is necessary.
In poplar, little is known about the rate of photosynthate
production in relation to leaf ontogeny on the entire stem.
Individual
expanding cottonwood leaves have shown a rapid ontogenesis of the photosyn­
thetic system, particularly during early stages of growth with older leaves
declining in photosynthetic activity (Dickmann, 1971a, 1971b).
photosynthesis rates occurred in nearly mature leaves.
Maximum net
Similar results
have also been found in herbaceous species (Baker and Hardwick, 1973;
Fellows and Geiger, 1974; Sestak, 1963; Smillie, 1962; Steer, 1971;
Woolhouse, 1967).
In addition, Larson and Gordon (1969) found maximum
translocation of photosynthate from leaves to occur just prior to full leaf
expansion.
A number of recent studies on poplar have delineated the ana­
tomical changes and translocation of photosynthate within developing leaves
(Isebrands and Larson^ 1973; Larson and Dickson^ 1975; Larson. ïsebrands;
and Dickson, 1972).
Dark respiration rates in cottonwood were high in the developing leaf
zone but declined rapidly as the leaves matured (Dickmann, 1971b).
A small
but significant rise in dark respiration rates occurred in Perilla 2-3 days
prior to abscission (Woolhouse, 1967).
Respiration of Leaves in the Light
Two enzymatically distinct respiratory processes operate in leaves in
the light:
dark respiration and photorespiration.
They differ in that
dark (mitochondrial) respiration produces energy and reducing power during
4
the breaking of carbon bonds of single sugars, while the breakdown of fixed
carbon into CO^ during photorespiration is not coupled with an energy-con­
serving reaction (e.g., ATP formation) and is considered a wasteful proc­
ess,
Nonetheless, a number of useful functions for photorespiration have
been postulated.
For example, it has been suggested that photorespiration
operates to prevent the total depletion of COg within the chloroplasts
(Egle and Fock, 1967).
Tolbert (1971) discussed a number of other possible
functions of photorespiration;
it may dissipate excess reducing power pro­
duced during the light reaction of photosynthesis; the glycolate pathway of
photorespiration is a source of the amino acids glycine and serine: and it
may also act as a growth regulator by disposing of excess reducing power
(NADPHg).
Tolbert also hypothesized that photorespiration is an unavoid­
able or protective mechanism against excess photosynthetic capacity under
conditions of high light and low COg, in which 3/4 of the carbon is actu­
ally salvaged for further resynthesis.
The effect of leaf age on photorespiratory rates is presently unclear.
Salin and Homann (1971) found younger tobacco leaves exhibited lower photorespiratory rates and glycolate oxidase activity than older leaves,
Kisaki
et al. (1973), however, complicated the issue by finding maximum glycolate
oxidase activities and photorespiratory rates in young tobacco leaves.
Phctcrsspiration occurs at a rapid rate in the leaves of most species
(Jackson and Volk, 1970), but the extent of dark respiration in the light
is currently the subject of controversy issue.
Jackson and Volk (1970)
summarized data supporting an inhibition of dark respiration in the light,
whereas Zelitch (1971) marshalled evidence indicating no effect of light on
this process.
Recently, Mangat et al. (1974) analyzed CO^ and
14
CO, flux
5
in bean leaves and found a 75% inhibition of dark respiration in the light.
They theorized that photophosphorylation supplies ATP to the cytoplasm
which causes the reduction of dark respiration by a feedback mechanism.
Using respiratory and photosynthetic inhibitors. Chapman and Graham (1974),
on the other hand, found that dark respiration in bean leaves continued in
the light at a rate conçarable to that in the dark.
Measurement of Photorespiration
If photorespiration is a wasteful process, controlling or preventing
photorespiration could allow the shunting of more photosynthate into fiber
production.
mined.
But first the magnitude of photorespiration needs to be deter­
The direct, quantitative measurement of photorespiratory COg
release is difficult due to underestimations caused by recycling of COg
within the leaf.
However, several indirect indicators of photorespiration
have been devised including (1) photosynthetic enhancement in 2% 0^ com­
pared to phytosynthetic rate in 21% 0^ (the Warburg effect), (2) rates of
i
^1^/3
*
.1 m
w
^ Wo 'ÎIW
^
^ *L . 01
\ y2)
^ \ w x v ^^
^ —«
w A . C * ^ ^^
l a
from a CO^ depletion curve, (4) CO^ compensation concentration, and
(5) glycolate oxidase activity (Ludlow and Jarvis. 1971),
The key to photorespiration measurements by method 1 is that dark res­
piration rates are not affected over a large range of oxygen concentrations,
whereas photorespiration rates are strongly affected by the level of oxy­
gen (Forrester et al., 1966; Tregunna et al., 1966; Poskuta, 1968).
High
concentrations of oxygen stimulate photorespiratory CO^ evolution and
inhibit photosynthetic CO^ absorption with the opposite effect occurring at
low 0^ levels (Tregunna et al., 1956).
Hence, the magnitude of the differ­
6
ence between the measurement of CO^ uptake in the light at a high concen­
tration of oxygen versus a low concentration of oxygen is an indicator of
the rate of photorespiration.
Increases in. photosynthetic rates ranging
from 30-60%, have been noted in various species (Bjorkman, 1966; Bulley et
al., 1969; Downton and Tregunna, 1968; Parkinson et al., 1974; Zelawski and
Kinelska, 1967).
Poskuta (1968) measured a 30% increase in photosynthesis
of spruce twigs when oxygen level was lowered from 21% to 1%, while
Kriedemann and Canterford (1971) found photosynthesis in pear leaves
increased 30% in a similar measurement.
Besides measuring a 40% increase
in photosynthetic rate in dwarf French beans. Parkinson et al. (1974) found
growth doubled in plants grown in 5% oxygen when compared with plants grown
in 21% oxygen.
Hypothetically, the increase in net photosynthesis at low oxygen can
be attributed to an increase in carboxylation efficiency, an inhibition of
photorespiration, or both.
At present, however, a clear explanation of the
exact nature of 0^ enhancement does not exist.
Various research suggests
glycolate metabolism and photorespiration as being accountable for oxygen
enhancement (Bowes and Ogren, 1972; Zelitch, 1971).
On the other hand,
Ludlow (1970) found that lower 0^ tensions decrease the inhibitory effect
of oxygen on CO^ diffusion and fixation, although Fair et al, (1973)
reported that lew 0^ levels reduce the activity of RuDP carboxylase.
D'Aoust and Canvin (1973) indicated that Og-enhancement is divided almost
equally between an increase in CO^ fixation and an inhibition of photorespiratory CO^ evolution.
An indirect measure of photorespiration and relative photosynthetic
efficiency is the CO^ compensation concentration; i.e., the CO^ level at
7
which photosynthetic uptake of CO^ and photorespiratory release of CO^ are
at an equilibrium.
Compensation concentration has been used as an indica­
tor of Calvin (C^) metabolism or Hatch-Slack (C^) metabolism (Canne11
et al., 1969; Downton and Tregunna, 1968).
Calvin plants synthesize C^
phosphorylated compounds as initial photosynthetic products, detectably
photorespire, and usually compensate between 50 and 60 ppm COg (Moss,
1971).
Hatch-Slack plants synthesize C^ dicarboxylic acids as initial
products of photosynthesis, do not detectably photorespire, and have com­
pensation concentrations near zero.
Dickmann and Gjerstad (1973), using
the Mylar bag method (Gcldswcrthy and Day, 1970), found high compensation
concentrations in the developing and senescing leaves of poplar, but mature
leaves had compensation concentrations similar to C^ plants.
Since compen­
sation concentration can be a quick and easy measurement, rapid screening
of Populus to determine the photosynthetically efficient genotypes could
expedite selection of better clones.
Oxygen competes with carbon dioxide to react with ribulose 1,5-diphos­
phate to produce glycolate - the initial step in the photorespiratory path­
way (Ogren and Bowes, 1971; Andrews et al., 1973).
The oxidation of gly­
colate is catalyzed by the enzyme glycolate oxidase.
(1967) compared the rate of release of
1-
Marker and Whittingham
from leaves supplied glycolate-
14
14
C or glycine-1- C and found both substrates equally effective precur­
sors of COg.
Their findings left unclear whether glycolate or glycine is
the immediate precursor of photorespiratory CO^, but they did find that the
CO2 produced in photorespiration is ultimately derived from the carboxylcarbon of glycolate.
In one of the few studies using woody plants,
Dietrich and Rose (1974) measured glycolate oxidase activity of extracts of
8
deciduous tree leaves.
They found that only Catalpa and Lirodendron leaves
showed the high rates of glycolate oxidase normally associated with
plants, while the other tree species exhibited rates comparable to
plants.
Further work on woody plants is needed to substantiate these find­
ings.
Resistance to Gas Diffusion
The rate of carbon dioxide movement from ambient air to sites of carboxy lation in the leaf is critical to the process of photosynthesis.
Resistances to the influx of carbon dioxide are theoretically useful in
explaining variations in photosynthetic rates.
Since portions of the path­
way for water vapor efflux and carbon dioxide influx are similar, the more
easily measured water vapor efflux is used as an estimate of the resistance
to carbon dioxide influx (Jarvis, 1971).
But transpiration is more depen­
dent on stomatal condition than is photosynthetic rate (Gaastra, 1959),
hence measurable resistances to water vapor efflux can only be considered
as estimates or resistafices to carbon dioxide influx.
Beginning with th£
youngest fully expanded leaf and measuring the same leaf for three weeks,
Holmgren et al. (1965) found that stomatal resistance doubled in maple
leaves.
They considered the pathways for water vapor efflux and carbon
dioxide influx and their associated resistances to be different, since
water can be assumed to evaporate from cell wall surfaces while carbon
dioxide must move in solution from the cell walls to the reaction sites in
the chloroplasts.
Resistance to carbon dioxide influx has been found to be a rate-limit­
ing process in photosynthesis at saturating irradiance and normal carbon
9
dioxide concentration (Chartier et al., 1970).
For example, Dykstra (1974)
showed an inverse relationship between carbon dioxide transfer resistance
and net photosynthesis of lodgepole pine seedlings.
Developing, mature,
and senescing tobacco leaves showed close correlations between transpira­
tion rates and leaf diffusion resistance, but net photosynthesis and leaf
diffusion resistance were not closely correlated (Vaclavik, 1973).
The
internal resistance to net photosynthesis appeared to increase with leaf
age more rapidly than that of leaf diffusion resistance.
Measurements of resistances to CO^ diffusion are hampered by technol­
ogy lagging behind theoretical considerations of possible resistances
involved in the pathway.
But measurements of water vapor efflux are still
the best estimates of resistance to carbon dioxide influx and could be use­
ful in explaining variations in photosynthetic rates.
10
OBJECTIVES
The objectives of the present study were:
1) to determine the extent
and variation of photorespiration in leaves of various ages on stems of
three Populus clones; 2) to determine if differences occur in CO^ uptake in
the light (NP), CO^ evolution in the dark (RD), respiratory COg flux in the
light (RL), compensation concentration, diffusive resistance, and glycolate
oxidase activity between leaf ages and between clones; 3) to determine if
variation in diffusive resistance between leaf ages and between clones is
related to NP, RD, RL, and compensation concentration, 4) to determine if
variation in compensation concentration between leaf ages and between
clones is related to NP, RD, RL, and diffusive resistance; 5) to determine
the relationship between glycolate oxidase activity and apparent photores­
piration at different leaf ages; and 6) to examine if the above relation­
ships provide a partial physiological explanation for growth rate differen­
ces between the three clones.
11
MATERIALS AND METHODS
Plant Material
Three clones of Populus x euramericana (jP. deltoides x
nigra"),
North Central Forest Experiment Station numbers 5321, 5326, and 5328, were
selected on the basis of wide diversities in growth rates (Wray, 1974).
Softwood tip cuttings taken from stock plants of each clone were planted in
Jiffy-7 peat pellets and placed under a mist system for two weeks.
After
rooting, the cuttings were planted in one-gallon pots containing a 1:2 mix­
ture of perlite and Jiffy Mix (sphagnum peat and vermiculite).
Six repli­
cations per clone were placed in growth chambers in a random design.
An
18-hour photoperiod was maintained with 25°C day and 15°C night tempera­
tures.
Illumination averaged 2,000 foot-candles at the apex of the rooted
cuttings.
Plants were fertilized once weekly with a commercial water-solu­
ble fertilizer (20-20-20, N-P-K), micronutrient solution (containing B, Mn,
Zn, Cu, Mo, and Mg) and Fe-EDTA and watered as necessary.
Pots were
flushed with wacer every two weeks to prevent salt accumulation.
Statistical models demonstrated that when plastochron index and leaf
plastochron age (L-PA) are applied to developmental studies of woody plants,
one can adjust plants of different developmental stages to a standardized
morphological time scale and predict developmental processes and events
from simple, nondestructive measurements (Larson and Isebrands, 1971).
Previous investigations indicated that a logical index leaf in Populus is
the first leaf at the plant apex 2 cm in length or greater (LPA 0).
This
standard was used in the present study with leaves on a plant numbered con­
secutively from the index leaf (LPA 0) to the tree's base.
LPA 4, 6, 10,
12
15, 20, and the three oldest leaves were measured when the trees were
approximately 100 cm tall.
Laboratory Measurements
Diffusive resistance determination
Resistance to water vapor diffusion was determined on each measured
leaf while in the growth chamber with a Lambda LI-60 diffusive resistance
meter fitted with a Kanemasu sensor.
The sensor, clamped to the leaf mid­
way between the leaf base and leaf tip, measured diffusive resistance by
the time lapse required for a given amount of water vapor to diffuse into
the sensor cup and be absorbed by the humidity sensing element.
Three
determinations were made at the same location on each measured leaf.
Infrared gas analysis
Carbon dioxide exchange was measured using both open- and closed-cir­
cuit gas system.
While still attached to the plant, a leaf was placed in
an appropriate size water-jacketed plexiglass leaf chamber.
The petiole
was sealed into an entry slot in the chamber with florist's clay.
Leaf
temperature was monitored by two copper-constantan thermocouples appressed
to the lower leaf surface and was controlled by circulating water from a
temperature-controlled water bath through the upper leaf chamber wall.
Relative humidity in the leaf chamber was maintained at a level similar to
that of the growth chamber by bubbling the air stream through two flasks of
water at 15°C in a Model FK Haake constant temperature circulator.
By
passing the resulting air stream through a copper coil in a temperaturecontrolled water bath (25 + 1°C), a 50% relative humidity was produced.
The air stream then passed in sequence from the leaf chamber through a
13
Drier!te drying column, a ceramic dust filter, a 1/60-hp rubber membrane
pump, Beckman Infrared Gas Analyzer (Model 215/a), a Brooks Sho-rate flow
meter controlled by a needle valve flow regulator, humidity control, and
back to the leaf chamber.
Analyzer output was recorded on a strip chart
with an Esterline-Angus model recorder.
The analyzer was calibrated at
1-LPM flow rate every two hours using a COg'free nitrogen zero standard and
a 370 ppm COg-in-nitrogen up-scale standard.
Five 200-watt water-cooled
reflector spot lights provided a light intensity of 760 ^Einsteins m
sec
-2
at the leaf surface.
After sealing a leaf in the leaf chamber. 15 minutes were allowed for
the leaf to equilibrate with the chamber's environment.
taken in the following order at 21% oxygen:
Measurements were
respiratory CO^ flux in the
light (RL), CO2 uptake in the light (NP), and CO^ evolution in the dark
(RD).
The same measurements were then taken at 2% oxygen after flushing
the system with low oxygen for 15 minutes.
RL, a measure of respiration in the light, was determined from the COg
efflux of an irradiated leaf into COg-free air.
A flow rate of 1-LPM was
maintained for this open-system determination.
Following the RL measure­
ment, the system was opened to the laboratory's atmosphere, and the COg
concentration was allowed to reach 400-500 ppm.
NP, a measure of net pho­
tosynthesis, was determined from the decrease in ambient CO^ concentration
caused by CO^ uptake of an irriated leaf in a closed system.
RD, a measure
of dark respiration, was determined from the increase in COg concentration
caused by COg efflux from a darkened leaf in the closed system.
tangents were measured at 370 ppm CO^ on the depletion curve.
calculated in terms of mg CO, exchanged per hour per dm
2
NP and RD
Rates were
leaf surface.
14
C0« compensation
concentration
determination
"
"
Goldsworthy and Day (1970) developed a technique for rapid determina­
tion of COg compensation concentration using CO^-impermeable Mylar bags.
This technique was adapted by Dickmann and Gjerstad (1973) to woody spe­
cies.
After COg-exchange measurements, leaves were excised from the
plants.
The petiole was wrapped with a strip of foam rubber and inserted
in a vial filled with deionized water.
The leaf was then placed, under-
surface upward, in a Mylar bag, 15 x 36 or 20 x 35 cm, depending on the
size of the leaf.
The bag was closed by placing a sampling probe consist­
ing of a 10 cm glass tube, a length of Tygon tubing, and a polyethylene
snap connector into the mouth of the bag, twisting the bag around the glass
tube, and winding with a Twist-em,
The bag was then inflated with air con­
taining approximately 400 ppm COg, and the sampling probe was closed with a
spring clamp.
The bag assemblies were placed in a growth chamber at 25°C and 760
^Einsteins m
-2
sec
-1
.
After one hour, the air was manually expelled from
each bag, while still illuminated, through a small desiccant column into
the sample cell of a Beckman infrared gas analyzer.
The analyzer was cali­
brated at 1-LPM flow before each run by using COg-free nitrogen as zero
standard and a 55 ppm CO^ in nitrogen upscale standard.
Area determination
The measured leaves were outlined on paper, and area was determined
planimetrically.
15
Glycolate oxidase determination
Two replications of leaves at LPA 4, 6, 10, 15, 20, and the three old­
est leaves on the plant for each clone were freeze-dried, ground to pass
the 20-mesh screen of a Wiley mill, and stored at -5°C until glycolate oxi­
dase analyses could be performed.
Fifty mg of freeze-dried leaf were ground for three minutes at 0°C in
a Duall homogenizer with 3 ml of extraction buffer (0,05M glyclyglycine pH
8.0 in O.IM sucrose).
The resulting homogenate was centrifuged at 3500 g
for 5 minutes.
A Braun Model V 166 Warburg apparatus was used for mancmetric determi­
nations of oxygen uptake.
A constant temperature of 25°C was maintained by
a thermostatically controlled water bath, while a constant volume was
obtained by resetting the top of the fluid column on the closed side of the
manometer back to 150 at the time of each reading.
The change in oxygen
pressure was determined from the change in height of the fluid column on
the open side of the manometer over 10-minute time periods.
The flask cen­
ter well contained a wick saturated with concentrated potassium hydroxide
to scrub COg from the flask.
Two ml of the leaf extract were placed in the
outer portion of the flask, and the substrate (0.05M sodium glycolate, pH
5.0) was placed in the flask's side arm.
W-WL-WW
s;.w UJ. A.
o. I.JL.WJ11.
JL&I
ULlC
WAUCt
UfCL Uti g
Ul&C
After 5-10 minutes for tempera\J 1.
Ultc
i it iitir* i tr i
WCld
set to the reference mark of 150, and the position of the open side fluid
column was recorded.
The substrate from the side arm was dumped into the
leaf extract, and the stopcock was closed to begin the 10-minute reaction
period.
Three replications with substrate and two replications with water
in place of substrate were run per leaf,
A thermobarometer was used to
16
correct for atmospheric pressure changes.
Quantitative oxygen uptake by
the leaf extracts was confuted according to Umbreit, Burris, and Stauffer
(1972).
Development of productivity models
Due to a lack of modeling in this area, an initial knowledge about the
functional form of the relationships involved was needed.
On this basis,
developmental trends were reviewed with respect to mathematical properties
of the models.
Models that were theoretically acceptable were fitted to
the data in a step-wise procedure.
Final models were selected only after a
careful review of the adequacy of the fit with respect to the residual
plots.
The model search was conducted primarily on one clone, and for con­
sistency in presentation, the same model was used for all clones.
reason, the selected model may not be as adequate for all clones.
For this
17
RESULTS
Results of this study are presented in the following order;
carbon
dioxide exchange, productivity models, diffusive resistances, compensation
concentrations, and glycolate oxidase activity.
For each of these dependent
variables, results are presented as affected by leaf plastochron age (LPA)
and clone.
Leaf age had a profound effect on the measured dependent variables.
Clones 5321 and 5326 responded similarly in the various dependent variables
measured, whereas clone 5328 differed in response at the various leaf ages.
Diffusive resistance and compensation concentration were poorly correlated
with most dependent variables,
Glycolate oxidase results mirrored the car­
bon dioxide exchange measurements.
Photorespiratory rates as demonstrated
by enzymatic activity and productivity models displayed consistent trends.
Photorespiratory rates ranged from undiscernible in the youngest leaves to
maximum in the mature leaves to a decline in old leaves.
CO2 Uptake in the Light in 21% Oxygen
Leaf age had a profound effect on net photosynthesis (NP) as measured
by CO^ uptake in the light at normal (21%) oxygen concentrations.
In gen­
eral, NP increased rapidly until LPA 10 and then remained stable until LPA
20, when rates began to decline slowly.
In addition, clone interacted with
leaf age in its effect on NP, indicating that clones differ quantitatively
in the physiological processes that drive leaf maturation and aging (Table
1).
Clones 5321 and 5326 were similar in that NP was negligible at LPA 4
and increased rapidly during leaf expansion to LPA 10 (Figures 1 and 2).
Table 1,
Mean square:; for the measured dependent variables of three Populus x euramericana clones.
Superscript!! indicate the probability of a larger F
21% Og
2% O2
CO2 evolution
in the dark
RD
RD
CO2 hr
Uiu )
21% Og
2% Og
13.»*^^
8.2'^:^
8.2"^'°^
OO2 uptake
in the light
Nl?
NP
Source
Clone
Error A
LPA
(]lone*LPA
Error B
df
2
10
7
5.2
345.6^"01
14
105
3.0
16.1
0.4
15.0*^*°^
1.2
Respiratory CO2
flux in the light
RL
RL
23.4"-°^
21% Og
1.1
820.3^"^^
50.3^'°^
52.2^'°^
50.3^"°1
16.7"^*®^
1 j^<.01
2 o3 ^ ' ^ ^
2.1-09
0.3
0.6
1.3
7.5
2% Og
43.1<'°1
1.3
98.
10.3*^*°^
3.0
Compen­
sation
concen­
Diffusive
resistance
tration
(sec cm ) (ppm COg)
257
1565'^*
105
6493
332'^*°'
77846^"'°^
70<.01
3485*^^
29
2341
Clone 5321
O Means 21%0
• Means
2%0
CNJ
O
o
en
V,
x:
•M
1/1
•M
x:
o.
J
0
5
10
"15
20
25
30
35
40
LEAF AGE-PLASTOCHRONS
Figure 1,
CO2 uptake in the light (NI') of clone 5321 In relation
The uppei' dashed line connects the means of NP for the
the lower dashed line connects the means of NP for the
solid lines represent model responses (NP =
+ Pj^LPA
to leaf age in plastochrons (LPA),
measured leaves at 2% oxygen while
measured leaves at 21% oxygen. The
+ p^LPAlnLPA)
Clone 5326
25
O Means 21%0
A Means
2%0
CNJ
20
-a
t\j
o
u
O)
15
,:2 10
5
0
0
20
25
LEAF A6E-PLAST0CHR0NS
{figure 2,
CÛ2 intake, in. the light (NP). of clone 5326 in relation
The upper dashed line connects the nieans of NP for the
the lower dashed line connects the means of NP for the
solid line,3 represent model responses (NP =
+ p^LPA
to leaf age in pLastochroas.
measured leaves at 2% oxygen while
measured leaves at 21% oxygen. The
+ p^LPAlnLPA)
21
Maximum rates of NP were observed at LPA 15 in both clones.
NP in the old­
est leaves was approximately one-half the maximum observed rates.
LPA's 4 and 6 in clone 5328 had higher NP rates than the corresponding
leaves in clones 5321 and 5326 (Figure 3).
The highest NP was achieved at
LPA 10 in clone 5328, some five plastochrons earlier than 5321 or 5326, and
the oldest leaves in clone 5328 had lower NP in 21% oxygen than similar
leaves in the other clones.
Clone 5328 had 5 to 10 fewer leaves per tree,
however, than clones 5321 and 5326.
COg Uptake in the Light in 2% Oxygen
As in 21% oxygen, leaf age had a profound effect on NP in 2% oxygen.
Trends of NP in 2% oxygen mirrored NP in 21% oxygen with respect to LPA,
clone, and the interaction of clone and LPA (Table 1; Figures 1, 2, and 3).
However, NP in 2% oxygen differed from NP in 21% oxygen in that rates
increased more rapidly in the young leaves, reached a higher rate from LPA
10 to 20, and, although slowly declining, maintained a higher NP in the
UJ.UCi. J.CCLVCa ^£JL^Ui.Ca
4,, CllIU -J)•
Xlic lUagllXLUUC Ui.
XIA
Xi.&
21% oxygen and 2% oxygen ranged from an undiscernible difference at LPA 4
to the greatest difference at LPA 10 to 20.
CO2 Evolution in the Dark in 21% Oxygen and 2% Oxygen
tiSu S ITïSjOT cjlj.CC>.. Otl
21% and 2% oxygen concentrations.
^ £VOi.\SwI.Cn ill
IT tC ^
a
wO
In general, RD decreased rapidly until
LPA 10 and then declined slowly through the oldest leaves (Figures 4, 5,
and 6).
Clone interacted with leaf age in its effect on RD, again demon­
strating that the clones differed quantitatively in the physiological proc­
esses that drive leaf maturation and aging (Table 1).
Clone 5326 had the
Clone 5328
«f-
<0 2 5
(U
O Means 21%02
A Means
2%0o
CM
I
E
"O
20
*
OJ
s 15
en
E
I
</)
iA
<U
x: 1 0
•M
C
>1
(/)
O
•M
O
5 -
5
10
1!)
_L
_L
20
25
30
35
40
LEAF AGE-PLASTOCHRONS
Figure 3.
COg uptake in the light (NP) of clone 5328 in relation
The upper dashed line connects the means of NP for the
the lower dashed line connects the means of NP for the
solid lines represent model responses (NP =
+ p^LPA
to leaf age in plastochrons (LPA).
measured leaves at 2% oxygen while
measured leaves at 21% oxygen. "Hie
+ p^LPAlnLPA)
9r
4rtJ
O)
Clone 5321
CM
o Means
A Means
I
E
XJ
21% 0 ,
2%o!
x:
ff
(\j
O
o
O)
s:
I
c
lo
OJ
o
OJ
Q.
V)
OJ
Cd
_L
_L
10
15
20
25
30
35
40
45
LEAF AGE-PLASTOCHRONS
Figure 4.
CO^ evolution in the dark (110) of clone 5321 in relation to leaf age in plastochrons (LPA)
9 r
H"
QJ
Clone 5326
O Means
CM
I
21%0,
E
X)
L.
x:
CVJ
o
o
en
z:
I
c
N)
o
•r**
fO
t.
•r^
O.
(/>
Q)
ex
0
0
îi
10
15
JL
_L
20
25
X
30
35
40
45
LEAF AGE-PLASTOCHRONS
Figure 5.
CO^ evolution in the dark (RD) of clone 5326 in relation to leaf age in plastochrons (LPAX
Clone 5328
o Means
• Means
10
L
J.
_L
15
20
25
30
%0
2%0
35
40
45
LEAF AGE-PLASTOCHRONS
CO2 evolution in the dark (RD) of clone 5328 in relation to leaf age in plastochrons (LPA)
26
highest, and clone 5328 had the lowest, RD for each leaf age.
RD in 21%
oxygen and 2% oxygen did not differ.
Respiratory CO^ Flux in the Light in 21% Oxygen
Leaf age had a large effect on respiratory CO^ flux in the light (RL)
in 21% oxygen as measured by CO^ evolution into CO^'free air.
RL in 21%
oyxgen decreased rapidly until LPA 10 and then remained steady until LPA
20, when rates began to increase slowly (Figures 7, 8, and 9).
interacted with leaf age in its effect on RL (Table 1).
Clone
Clones 5321 and
5326 were similar in that RL in 21% oxygen was high at LPA 4, decreased
rapidly during leaf expansion to LPA 10, and then remained stable at LPA 15
and 20 (Figures 7 and 8).
RL in the oldest leaves measured of clone 5326,
however, was approximately twice the rate in corresponding leaves of clone
5321.
LPA's 4 and 5 in clone 5328 had lower rates of RL than corresponding
leaves in clones 5321 and 5326 (Figure 9).
Little variation occurred in RL
in 21% oxygen from LPA 10 co the oldest measured leaves.
Respiratory CO^ Flux in the Light in 2% Oxygen
As in 21% oxygen, leaf age had a significant effect on RL in 2% oxy­
gen.
Trends of RL in 2% oxygen mirrored RL in 21% oxygen with respect to
LPA, clone, and the interaction of clone and LPA (Table 1).
5L in 2% oxy­
gen differed from RL in 21% oxygen, however, in that rates decreased more
rapidly in the young leaves, reached a lower rate from LPA 10 to 20, and,
although slowly increasing, maintained a lower RL rate in older leaves
(Figures 7, 8, and 9).
The magnitude of difference in RL in 21% oxygen and
10
Clone 5321
OJ
O Means 21%0
A Means
2%0
8
_c:
CM
O
o
w
6
flj 4
CL
(/)
2
0
0
LEAF A6E-PLAST0CHR0NS
Figure 7,
Respiratory COg flux in the light (RL) of clone 5321 in relation to leaf age^in plastochrons (LIA)« ïhe dashed line represents model (log IlL • p + P],LFA +
) response
Z7, oxygen, while the solid line indicates the model rosponsS at 21% oxygen
«•-
Clone 5326
10
O Means 21%0.
(O
O)
CVJ
£=
•o
I
s>
j;:
A Means
8
2%0
2
A
O
«
CM
O
O
tïl
E
I
c
o
-s-»
<a
u
(X
i/)
(U
cc
A.
\]
10
15
20
25
30
35
40
LEAF AGE-PLASTOCHRONS
Figure 8.
Respiratory CO^ flu;ic in the light (RL) of clone 5326 in relation to leaf age.in plaatochrons (LIA). The dashed line represents model (log IIL - p + p^LPA + p_LPA ) respooae at
2% oxygen, while the solid line indicates the model rosponsi at 21% oxygen
Clone 5328
10
o Means 21%0,
• Means
2%0'
<0
<u
CM
I
B
X>
%
CVJ
o
o
CO
B
o
I
c
o
u
CL
(/)
Qi
a:
_L
10
15
20
25
30
35
40
45
LEAF A6E-PLAST0CHR0NS
Figure 9,
Respiratory COg flux in the light (RL) of clone 5328 in relation to leaf agegin plastochrons (Lï'A), The dashed line represents model (log IlL •
+ pj^LPA + p_LPA ) response at
2% oxygen, while the solid line indicates the model response at 21% oxygen.
30
TU oxygen ranged from an undiscemible difference at LPA 4 to the greatest
difference at LPA's 10 to 20.
Productivity Models
Since LPA had a pronounced effect on the dependent variables, linear
models were developed to describe the relationship between the dependent
variables NP and RL in whole leaves and the independent variables LEA.
Regression coefficients were generated from the study's data to model pro­
ductivity in terms of NP and RL.
The model demonstrated that the enhancement of NP in 2% oxygen
(Warburg effect) varied greatly with leaf age (Figures 10, 11, and 12).
In
general, the magnitude of enhancement of NP increased rapidly from LPA 4 to
10, was greatest from LPA's 15 to 25, and declined in the older leaves.
Enhancement of NP was larger and began at a younger leaf age in clone 5328
than in clones 5321 and 5326, again indicating that clones differ in the
physiological processes controlling leaf maturation and aging (Figures 13
and 14).
Clones 5321 and 5326 were similar in that NP on a whole leaf
basis was similar at all LPA's.
Leaves in clone 5328 had higher rates than
the corresponding leaves in clones 5321 and 5326.
Leaf productivity, as
measured by NP, was highest at LPA 17 in clone 5328 and approximately 30%
greater than the highest rate in the other clones.
The model for RL again demonstrated the enhancement of 2% oxygen
varied greatly with leaf age (Figures 15, 16, and 17).
In general, the
magnitude of the enhancement effect increased rapidly from LPA 4 to 10, was
greatest from LPA 12 to 20, and declined in the older leaves.
The enhance­
ment effect was larger in clone 5328 and began at a younger leaf age than
Clone 5321
N P - / Sq + U^ L P A + / 3 2 L P A I n L P A
LEAF AGE-PLASTOCHRONS
Figure 10,
Model renponse of CO» uptake in the light (NP) in l'L oxygen and 21% oxygen for whole
leaves of clone 5321 in relation to leaf age in plastochrons (LPA) NP " p + p.LPA +
pgLPAlnLPA
°
Clone 5326
NP" /ÎQ+^lLPA + zSgLPA In LPA
W
N)
LEAF AGE-PLASTOCHRONS
Figure 11,
Model refiponse of C0_ uptake in the light (NP) of 2% oxygen and 21% oxygen for whole
leaves of clone 5326 in relation to leaf age in plastochrons (LPA) NP •
+ p^LPA +
pgLPAlnLPA
LEAF AGE-PLASTOCHRONS
Figure 12»
Model reoponse of C0„ uptake in the light (NP) in 2% oxygen and 21% oxygen for whole
leaves of clone 5328 in relation to leaf age in plastochrons (LPA). NP =
+
PgLPAlnLPA
21%0.
N P - / 9 q+ 0 ^ L P A + 3 2 L P A I n L P A
w
U/
y/
LEAF A6E-PLAST0CHR0NS
Figure 13,
Model responses of CO2 uptake in the light (NP) in 21% oxygen for whole leaves of clones
5321, 5326, and 5328 in relation to leaf age in plastochrons (LPA). NP = 0^ + p^LPA +
pgLPAlnLPA
2%0,
75
N P - Pq + 0-1 L P A + P g L P A I n L P A
60
45
30
U)
Ln
15
LI:AF A6E-PLAST0CHR0NS
Figure 14»
Model responses of CO2 uptake in the light (NP) in 2% oxygen for whole leaves of clones
5321, 5326, and 5328 in relation to leaf age in plascochrons (LPA), NP + p^LPA +
pgLPAlnLPA
4
Clone 5321
LEAF AGE-PLASTOCHRONS
Figure 15,
Model response of respiratory CO2 flux in the light (RL) in 2% oxygen and 21% oxygen for
whole leaves of clone 5321 Ln relation to leaf age in plastochrons (LPA), Log RL 13^ + Pj^LPA + PgLPA?
Clone 5326
u.
«c
ij=
<Vi
o
o
E
7C.
W
O
»—1
>«C
DC
»—t
Ot/ï
Ul
oc
45
LEAF AGE-PLASTOCHRONS
Figure 16.
Model response of respiratory CO2 flux in the light (RL)- in 2% oxygen and 21/, oxygen for
whole leaves of clone 5326 :ln relation to leaf a«e in plastochrons (LPA). Log RL -
a
4- Pi LPA + P„LPA2
4
Clone 5328
li.
lU
«c
L.
C
(Si
o
(_)
CD
e
U)
x.
o
l-«
00
I—
2%0
QC
1^
a.
t/>
w
œ.
10
15
20
25
30
35
40
45
LEAF AGE-PLASTOCHRONS
Figure 17,
Model response of respiratory CO2 flux in the light (RL) in 2% oxygen and 21% oxygen for
whole leaves of clone 5328 in relation to leaf age in plastochrons (LPA), Log RL •
P + p LPA + p„LPA^-
in clones 5321 and 5326.
Clones 5321 and 5326 were alike in that KL on a
leaf basis was similar at all LPA's (Figures 18 and 19).
Leaves in clone
5328 again had higher rates than the corresponding leaves in clones 5321
and 5326.
RL was highest at LPA 12 in clone 5328, approximately 20 to 30%
greater than the highest rate in the other clones.
In terms of the whole plant, clones 5321 and 5326 had a similar
enhancement due to 2% oxygen for a whole plant, while clone 5328 had a
much greater increase in 2% oxygen (Table 2).
NP increased about 55% in
clones 5321 and 5326 for a whole plant, while clone 5328 increased 73%.
RL
increased about 46% in clones 5321 and 5326, while clone 5328 increased 51%.
Diffusive Resistance
Resistance to water vapor diffusion from leaves was also effected by
leaf age.
5326.
Leaves of LPA 4 were too small to measure in clones 5321 and
In general, diffusive resistance rates were low at LPA's 4 to 20 and
much greater in older leaves (Figures 20, 21, and 22).
Clone interacted
significantly with leaf age in its effect on diffusive resistance (Table 1).
Clone 5328 had lowest diffusive resistance rates at LPA 6 (Figure 22) while
clones 5321 and 5326 had the lowest rates at LPA 10 (Figures 20 and 21).
Diffusive resistances were highest in older leaves of all clones, with
older leaves of clone 5328 exhibiting the highest diffusive resistances and
clone 5321 the lowest.
Gas exchange measures and diffusive resistance were
not highly correlated in any of the clones (Table 3).
Photorespiration
(NP 27o - N? 21%) also was not highly correlated with diffusive resistance.
LU
_J
I
u.
x:
c\j
o
o
w
1=
X
o
»—I
»rc
Or:
•-«
a.
ê
i/>
IJLl
oc
LEAF AGE-PLASTOCHRONS
Figure 18,
Model responses of respiratory CO2 flux in the light (RL) in 21% oxygen for whole leaves
of clones 5321, 5326, and 5328 in relation to leaf age in plastochrons (LPA), Log RL + p^LPA + pgLPA^
4 r
2%0,
<c
LU
J-
jr
"CM
o
o
CJ>
B
z
o
<c
a:
I—I
CiL
(/)
UJ
«%
5321
5326
30
35
40
45
LEAF AGE-PLASTOCHRONS
Figure 19o
Model responses of respiratory CO flux in the light (RL) in 2% oxygen for whole leaves
of clones 5321, 5326, and 5328 in relation to leaf a#e in plastochrons (LPA), Log RL •
+ p^LPA + pgLPA^
15 r
Clone 5321
LEAF A6E-PLAST0CHR0NS
Figure 20,
Diffusive resistances of clone 5321 in relation to leaf age in plastochrons (LPA)
Clone 5326
o
5 10
5
10
1!)
_L
JL
20
25
30
35
40
LEAF AGE-PLASTOCHRONS
Figure 21,
Diffusive resistances of clone 5326 in relation to leaf age in plastochrons (LPA)
Clone 5328
LEAF AGE-PLASTOCHRONS
Figure 22,
Dlffuuive resistances of clone 5328 in relation to leaf age in plastochrono (LPA)
45
Table 2.
Simulated CO2 uptake in the light (NP) and respiratory CO2 flux
in the light (RL) in 21% O2 and 2% O2 for whole plants of three
Populus X euramericana clones. NP = p + p,LPA + p_LPA In LPA
log KL = pg + B^LPA + p^LPA^
o
i
Clone
21% 0^
5321
600.5
5326
NP
2% 0^
% increase
21% 0^
RL
2% Og
% decrease
948.0
57.9
65.4
35.6
45.6
536.5
830.9
54.9
59.8
32.2
46.2
5328
666.1
1152.0
73.0
73.2
35.6
51.4
Table 3.
Correlation coefficients (r) between various measures of COg
exchange and diffusive resistance for the same leaves of three
Populus X euramericana clones
Dependent variables
5321
-.04
CO2 uptake in the light (NP) 2% 0^
0
5328
1
Clones
5326
-.47
CO2 uptake in the light (NP) 21% Og
-.17
-.48
-.14
Respiratory CO^ flux in the light (RL) 2% Og
-.24
-.11
-.04
Respiratory CO2 flux in the light (RL) 21% Og
-.18
~ • 17
-.10
Compensation concentration
-.36
-.44
-.32
.02
-.36
.15
?N 2% - PN 21% (photorespiration)
46
Compensation Concentration
Leaf age had a marked effect on compensation concentration.
In gen­
eral, compensation concentration decreased rapidly until LPA 10 and then
remained stable until LPA 20 (Figures 23, 24, and 25).
In addition, clone
interacted with leaf age in its effect on compensation concentration (Table
1).
Compensation concentrations in older leaves varied considerably with
clone 5328 more erratic than clones 5321 and 5326, again indicating differ­
ent leaf development patterns in clone 5328.
CO^ exchange measures and compensation concentration were generally
poorly correlated (Table 4).
Clone 5328 displayed the best correlations in
most comparisons, while clone 5326 exhibited the poorest correlations.
Glycolate Oxidase
Different leaves were analyzed for glycolate oxidation than were used
in the CO^ exchange measurements, negating the possibility of direct leaf
comparisons.
The glycolate results, however, exhibited trends very similar
to the gas analysis results, with leaf age having a large effect on oxygen
uptake with either glycolate or water as a substrate (Figures 26, 27, and
28; Table 5).
Clone interacted with leaf age in its effect on oxygen
uptake, providing further evidence that clones differ quantitatively in the
physiological processes that drive leaf maturation and aging.
With glyco­
late as substrate, oxygen uptake decreased gradually with leaf age.
Mir­
roring the trends for CO2 evoluation in the dark (RD), oxidation with water
as substrate resulted in a rapid rate decrease from LPA 4 to 10 and then
remained stable through the oldest leaf.
Rates for all clones were similar
for both substrates at LPA 4, diverged to the greatest extent from LPA 10
Clone 5321
250
CM
O
U
E
a.
Q.
I
200
u
c
o
(_)
c 150
o
(O
10
c
(U
a.
E-
100
o
o
<NJ
O
O
50
10
..J.
jL
_L
JL
15
20
25
30
35
40
LEAF AGE-PLASTOCHRONS
l'igure 23,
Compensation concentrations of clone 3321 In relation to leaf age In plastochrons (LPA)
Clone 5326
LEAF A6E-PLAST0CHR0NS
Figure 24.
Compensation concentrations of clone 5326 in relation to leaf age in plastochrona (LPA)
Clone 5328
250
o
E
CL
O.
I 200
150
100
50
J..
5
10
15
20
jl
JL
25
30
35
40
LEAF AGE-PLASTOCHRONS
Figure 25,
Compensation concentrations of clone 5328 in relation to leaf age in plastoclirona (LPA)
50
Table 4.
Correlation coefficients (r) between other measures of CO2
exchange and compensation concentration for the same measured
leaves of three Populus x euramericana clones
Dependent variables
CO? uptake in the light
CO2 uptake in the light
Respiratory CO2 flux in
Respiratory CO2 flux in
Diffusive resistance
PN 2% - PN 21%
RL 21% - RL 2%
Table 5.
(NP) 2% O2
(NP) 21% O2
the light (RL) 2% O2
the light (RL) 21% O2
5321
Clones
5326
5328
-.62
-.60
.74
.76
-.36
-.52
-.58
-,59
-.57
.45
.46
-.44
-.51
-.15
-.72
-.64
.82
.76
-.31
-.70
-.26
Mean squares for oxygen uptake by leaf extracts of three Populus
X euramericana clones using the substrates glycolate and water.
(Superscripts indicate the probability of a larger F)
-1
Substrate (Mg02 hr
Source
df
Glycolate
-1
mg
DW)
Water
0.12
Clone
2
0.53
0.0614
1.3394
Error A
2
0.0694
0.1783
1.6533'^*°^
3.1282
L,ri\
<.01
<.01
Clone*L?A
14
0.2050^"°^
0.2241
Error B
21
0.0518
0.0554
to 20 and converged again in the older leaves (Figures 26, 27, and 28).
These results again demonstrate the similar trends observed between glyco­
late and the results for CO^ flux in the light (NP) and respiratory CO^
flux in the light (RL).
4r
Clone 5321
it<0
<u
3E
O
a>
e
•
7
u
2
Glycolate
CM
0
o>
E
01
<d
•frJ
CL
HgO
_i.
0
X
JL.
10
l£i
_L
20
25
30
35
J.
J
40
45
LI:AF AGE-PLASTOCHRONS
l'igure 26.
Uptake of oxygen by leaf extracts using glycolate or water as substrate for clone 5321
relation Co leaf age in pladtochrons (LPA)
4r
Clone 5326
4-
m
O)
3
a
O)
B
A
Glycol ate
«
I
t-
\
CM
0
01
e
<u
-V
ra
4->
"Aft.
m
a.
ZD
OO
JL
_L
0
10
15
20
25
30
_L
35
40
45
LEAF AGE-PLASTOCHRONS
Figure 27,
Uptake of oxygen by leaf extracts using glycolate or water as substrate for clone 5326
relation to leaf age in plasiCochrons (LPA)
4r
Clone 5328
4rtJ
<U
3
o
cn
E
"^4
Glycolate
%.
JC.
CM
0
D)
E
01
v
OJ
A
m
+>
a.
%)
0
0
10
1!5
JL
_L
±
_L
20
25
30
35
J
40
45
LEAF AGE-PLASTOCHRONS
Figure 28,
Uptake of oxygen by leaf extracts using glycolate or water as substrate for clone 5328 in
relation to leaf age: in plastochrons (LPA)
54
Mirroring photorespiration (NP 2% 0^ - NP 21% 0^) trends, glycolate
oxidase activity (glycolate 0^ uptake - water 0^ uptake) was negligible at
LPA 4J increased rapidly during leaf expansion, and maximum rates were
observed in leaves with highest NP rates.
Glycolate oxidase activity and
photorespiration in the oldest leaves were approximately one-half the maxi­
mum observed rates (Figures 29, 30, and 31).
Clone 5321
«f(«
0)
3
O
en
E
S.
JZ
C\j
o
ai
E
0)
ira
•M
CL
LEAF AGE-PLASTOCHRONS
29,
Glycolate oxidase activity (glycolate O2 uptake-water O2 uptake) compared with photores­
piration (NP 2% 0„-NP 21% 0^) for clone 5321 in relation to leaf age in plastochrons
(LPA)
«4n>
0)
Clone
5326
cva
I
E
XI
&
jr.
CM
o
0
cm
Ln
01
.it:
CL
=)
LEAF AGE-PLASTOCHRONS
Figure 30.
Glycolate oxidase activity (glycolate O2 uptake-water O2 uptake) compared with photores­
piration (NP 2% 0„-NP 21% 0.) for clone 5326 in relation to leaf age in plastochrons
(LPA)
Clono
5328
4IT>
0)
«t-
«J
<u
C\J
E
O
C7>
E
I
i.
JC
Ln
cvi
O
u
en
:c
0)
esj
O
en
E
a>
(O
m
4-»
a.
•(->
Q.
:3
LEAF AGE-PLASTOCHRONS
Figure 31,
Glycolate oxidase activity (glycolate O2 uptake-water O2 uptake) compared with photores­
piration (NP 2% 0»-NP 21% 0,,') for clone 5328 in relation to leaf age in plastochrons
(LPA)
58
DISCUSSION
The ontogenesis of net photosynthesis in developing leaves of the
three hybrid Populus clones used in this study was similar to the pattern
reported by Dickmann (1971a) for Populus deltoides•
Maximum COg uptake in
the light (NP) rose from zero in very young leaves to a maximum in the
first fully-expanded leaves and then declined in older leaves.
These
trends in NP correlate well with the ontogenetic pattern of ribulose
diphosphate (RuDP) carboxylase synthesis and activity (Dickmann, 1971b;
Steer, 1971).
Photosynthesis in old poplar leaves declined, as would be
expected, because RuDP activity is directly related to photosynthetic rate
and because the carboxylating step of the Calvin cycle is rate-limiting in
photosynthesis at saturating light and normal CO^ levels (Ogrsn and Bowes,
1971).
Photorespiration in this study was estimated by the increase in NP of
leaves in a 27» oxygen atmosphere.
The enhancement of NP by low 0^ concen­
trations (Warburg effect) is a veil knoT-m phenomenon (Bjorkroan. 1966:
Bulley et al., 1969; Downton and Tregunna, 1968; Forrester et al., 1966),
but the exact physiological basis for enhancement is unclear.
Oxygen
enhancement is likely the result of a combination of factors (D'Aoust and
Canvin, 1973).
On the one hand, CO^ fixation is increased and CO^ diffu­
sive resistance decreases (Ludlow, 1970) while photorespiratory CO^ evolu­
tion and glycolate metabolism are inhibited under low oxygen (Bowes and
Ogren, 1972; Zelitch, 1971).
In support of findings by Salin and Homann (1971), I found that young
leaves did not exhibit as large an increase in NP as older leaves when
59
placed under low oxygen, indicating a reduced photorespiratory level.
Recently mature leaves had the greatest magnitude of 0^ enhancement.
This
estimate of photorespiration, amounting to approximately one-third of total
photosynthesis in mature leaves, compares closely with those observed for
plants (Samish et al., 1972; Zelitch, 1971) but is considerably higher
than the extrapolated values determined by Luukkanen and Kozlawski (1972)
for Populus leaves.
The decline of the enhancement effect in older leaves
indicates a decrease in photorespiratory activity as leaves age.
As Dickmann (1971b) found in cottonwood leaves, RD rates were high for
newly fcrrsed leaves but declined rapidly as the leaves matured.
Old leaves
were lowest in RD and did not exhibit the small rise in RD observed by
Woolhouse (1967) in Peri11a 2-3 days prior to abscission.
Dark (mitochon­
drial) respiration (RD) was unaffected by changes in 0^ concentrations,
allowing the use of O2 enhancement as an estimate of photorespiration
(Forrester et al., 1966; Poskuta, 1968; Tregunna et al., 1966).
Respiratory CO^ flux in the light (RL) in 21% O^. measured by COg
efflux into C02~free air, consists if photorespiration and dark (mitochon­
drial) respiration.
However, the extent of dark respiration in the light
is controversial (Chapman and Graham, 1974; Jackson and Volk, 1970; Mangat
et al., 1974; Zelitch, 1971).
In this study, the dark respiration compo-
ûerit of RL was esiiimaced by efflux of CO^ into COg-free air containing 2%
oxygen.
Under these conditions, both photosynthesis and photorespiration
are inhibited, hence the major CO^ flux is due to dark respiration.
Dark
respiration in mature leaves in the light was considerably less than respi­
ration in the dark, similar to the findings of Mangat et al. (1974).
How­
ever, the interpretation of the present data is complicated by the constant
60
recarboxylation of respired CO^ in photosynthesis that occurs even in low
CO2 and oxygen.
But some reduction of dark (mitochondrial) respiration in
the light seems likely, perhaps due to feedback inhibition by photophosphorylated ATP (Mangat et al., 1974).
Because there was no difference between RL at 2% 0^ and RL at 21% 0^
in young leaves, the high rates of CO^ evolution observed in these leaves
were probably almost entirely due to mitochondrial respiration.
This fits
well with the observation that young leaves do not carry on photosynthesis
and related processes at a high rate because they have not developed the
anatomical and physiological prerequisites.
The anatomical and physiologi­
cal development of leaves with aging can be observed by the increase in the
difference between RL at 2% 0^ and 21% 0^, the maximum difference occurring
in the mature leaf zone.
Since there was no difference between 2% 0^ and
21% 0^ in old leaves, mitochondrial respiration apparently was the major
respiratory pathway in the light in old leaves.
The inverse relationship between leaf diffusion resistance and NP
reported by Dykstra (1974) for Pinus contorta was not observed in hybrid
poplar.
Diffusive resistance is more dependent on the stomata1 condition
than is NP (Gaastra, 1959; Vaclavik, 1973), which somewhat explains the
lack of correlation between CO^ exchange and diffusive resistance measurevx *•L. C *
VUV. &&
C
W*.*o ^W^
^^
m W
^C
V» i-y h
V
«
m
y» WVy 2
&
A
Wxs W ^WW
W
^ W ^ •-L* ^ tiXVL Ck O
^ -C
JLJ.WUA Ct
different pathway (namely, resistance to water diffusion) may have resulted
in the poor correlation between gas exchange measures and diffusive resis­
tance,
Low diffusive resistances were measured at a younger leaf age than
the highest CO^ gas exchange rates, likely indicating the high permeability
61
of the cuticle and the more advanced state of stomatal morphology in succu­
lent young leaves (Isebrands and Larson, 1973).
Compensation concentration in this study was a poor indicator of rela­
tive photosynthetic efficiency, as shown by the poor correlation between
compensation concentration and CO^ gas exchange measures.
Compensation
concentration is a closed system measure of the equilibrium between photo­
synthesis and respiration in the light.
Given enough time, most Populus
leaves will equilibrate in the C^-range of 50-60 ppm COg.
High compensa­
tion concentrations in young leaves can be attributed to high mitochondrial
respiration rates and low photosynthesis rates.
High compensation concen­
tration in old slowly-metabolizing leaves could result from a lack of time
to reach equilibrium or from higher mitochondrial respiration rates in the
light.
Since the substrates H^O and glycolate gave similar results in young
Populus leaves, the measured activity was probably due to mitochondrial
respiration, demonstrating the lack of photorespiratory enzymatic develop­
ment in young leaves.
Populus leaves had high rates of glycolate oxidate
activity which were similar to rates in herbaceous C^ plants (Kisaki
et al., 1973; Salin and Homann, 1971), as opposed to Dietrich and Rose
(1974) who reported that several tree species exhibited glycolate oxidase
activity comparable to C^ plants.
It is unlikely, however, that the latter
conclusion is valid since. Dickmann and Gjerstad (1973) showed that trees
over the whole taxonomic spectrum had CO^ compensation concentrations well
within the range of C^ plants.
Glycolate oxidase activity and photorespi­
ration (NP 2% Og - NP 21% Og) responded in a similar fashion as NP with
leaf age, supporting the supposition that oxygen competes with carbon
62
dioxide for ribulose 1,5-diphosphate (Ogren and Bowes, 1971; Andrews et al.,
1973).
Since anatomical maturation and physiological maturation are directly
interrelated (Isebrands and Larson, 1973), the NP responses of leaves in
clone 5328 indicate more advanced anatomical development at each leaf age
than found in comparable leaves of clones 5321 and 5326.
For example,
greater CO^ uptake in the light in young leaves of clone 5328 than in
clones 5321 and 5326 probably can be attributed to the earlier development
of intracellular spaces and more mature stomates.
Although leaves of clone
5328 developed more rapidly at earlier leaf ages, the leaves also matured
and senesced at younger leaf ages.
Leaves of clone 5328 developed very
rapidly initially, but an equal number of plastochrons occurred from leaf
maturation to leaf senescence in all three clones.
NP of clone 5328 exhib­
ited a larger enhancement in 2% oxygen in younger leaves, again demonstrat­
ing developmental differences between comparable leaf ages of clone 5328
and clones 5321 and 5326.
Clonal differences in RL again indicated that leaves of 5328 were
physiologically more mature at younger leaf ages than the other clones.
Clone 5328 again demonstrated a clonal difference in physiological
development from clones 5321 and 5326 by having lower resistances in
younger leaves and greater resistances in older leaves.
When clones were compared on a simulated whole leaf basis, NP in both
21% Og and 2% 0, was much greater in clone 5328 at all but the oldest leaf
ages, even though NP on an area basis was similar in all clones.
The large
difference between clones in NP on a whole leaf basis can be attributed to
clone 5328's much larger leaves.
The more rapid increase in NP in younger
63
Leaves and the more rapid decrease in NP in older leaves in clone 5328 than
in clones 5321 and 5326 is probably due to earlier anatomical and physio­
logical development and the more rapid onset of senescence in leaves of
clone 5328.
Modeling RL on a whole leaf basis substantiated the trends involving
leaf development and clonal differences noted in the NP model.
Trends in
models of NP and RL differed only in that clone 5328 maintained a higher RL
rate per leaf in old leaves than clones 5321 and 5326.
Higher RL rates in
old leaves of 5328 may again be caused by higher mitochondrial respiration
rates in old leaves.
When modeling productivity on a whole plant basis, response of the
clones to oxygen enhancement was similar to a number of studies (Downton
and Tregunna, 1968; Parkinson et al., 1974; Poskuta, 1968).
The 50-75%
increase of NP and the 46-51% decrease of RL in 2% oxygen as found in this
study, however, does not completely explain the total dry weight accumula­
tions over an extended time period for plants grown under low oxygen=
Quebedeaux and Hardy (1975) continuously exposed soybeans to a 5% 0^ atmos­
phere from an early seedling stage to senescence and measured a vegetative
dry weight increase of 107% over plants grown in 21% oxygen.
ence is likely due to a feedback effect:
The differ­
greater NP and less photorespira­
tion produce more photosynthate, resulting in more phocosynthetic surface
which in turn produces photosynthate at an even faster rate.
A wide diversity in ontogenetic variation of leaf physiological rates
was found in this study.
A developmental study, such as this, should aid
the scientist in a more exact selection of the correct leaf tissue needed
to examine a specific biochemical reaction.
Hence, developmental studies
64
should play a vital role in the researcher's understanding of physiological
variation in leaf age and genetic variation between tree sources.
To aid in further development of the tree productivity model (Promnitz
and Rose, 1974), future studies are needed to expand the physiological pro­
files of leaves in the expanding, mature, and senescent zones of poplar
plants.
Specifically, more research is required in the mature and senesc-
ing zones in order to more fully explain the aging processes of leaves.
As
shown by Quebedeaux and Hardy (1975), the potential increase in vegetative
productivity by controlling photorespiration could revolutionize forestry
practices as known today.
Therefore, investigations involving chemical and
environmental controls of photorespiration may provide techniques on how to
manipulate photorespiration in order to greatly increase productivity.
65a
CONCLUSIONS
In general, CO^ flux in the light (NP) in 21% oxygen increased rapidly
until LPA 10 and then remained stable until LPA 20, when rates began to
decline slowly.
NP in 2% oxygen differed from NP in 21% oxygen in that
rates increased more rapidly in young leaves, reached a higher rate
from LPA 10 to 20, and, although slowly declining, maintained a higher
NP in older leaves.
Glycolate oxidase activity was closely related to photorespiration (NP
2% Og - NP 21% Og), both of which exhibited the same trends as NP with
leaf age.
COg evolution in the dark (RD) was highest in young leaves, decreased
rapidly to the first mature leaf, and then decreased slowly through the
oldest leaves.
Respiration in the light in young leaves was almost entirely due to
dark (mitochondrial) respiration in contrast to mature and old leaves
in which photorespiration predominated.
CO2 compensation concentration and diffusive resistance were poorly
correlated with COg uptake in the light (NP), respiratory CO^ flux in
the light (RL), and photorespiration suggesting that these measures
would be poor indicators of photosynthetic productivity.
Physiological differences among the clones indicated that clone 5328
differed markedly in genetic control of plant development from clones
5321 and 5326.
65b
7.
Based on model simulation, suppression of photorespiration could
increase productivity by at least 50-75% on a whole tree basis in
PoDulus clones.
66
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71
ACKNOWLEDGMENTS
I wish to extend ray sincere gratefulness to Dr. D, I, Dickmann for his
encouragement and guidance throughout the duration of this study.
Equal
appreciation is extended to Dr. J. C. Gordon for his inspiration and advice
in my research.