The effects of water stress on gas-exchange in Pinus brutia, var eldarica

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Authors Soumana, Diallo Amadou, 1957-

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The effects of water stress on gas-exchange in Pinus brutia, var. eldarica

Soumana, Diallo Amadou, M.S.

The University of Arizona, 1990

U MI 300 N. Zeeb R& Ann Arbor, MI 48106

THE EFFECTS OF WATER STRESS ON GAS-EXCHANGE

IN PINUS BRUTIA. VAR. ELDARICA

by

Diallo Amadou Soumana

A Thesis Submitted to the Faculty of the

THE DEPARTMENT OF SOIL AND WATER SCIENCE

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 9 0

STATEMENT BY AUTHOR

2

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: SoonftMlV

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

Qu*~ t \ . -71/ln-nMAa f o i l , g . mo Allan D. Matthias (Pat© Associate Professor of Soil and Water Science

3

ACKNOWLEDGEMENTS

The author wishes to specially thank Dr. Allan Matthias,

his thesis Advisor and Major Professor, for his continuous

advice, helpful suggestions and patience throughout the entire

study. I am very grateful to Dr. J. W. Moon and Dr. J. R.

Simpson for their technical contribution and assistance in the

study. I also thank Dr. Richard Jensen for the use of his

laboratory and instrumentation for the gas-exchange

measurements.

Finally, special gratitude is extended to my parents in

Niger for their patience, to my sponsor in Washington D.C. and

Carol Davis at the International Agricultural Program,

University of Arizona, for their encouragement and support,

and to all Americans.

4

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS 5

LIST OF TABLES 6

ABSTRACT 7

CHAPTER 1, INTRODUCTION 8

CHAPTER 2, LITERATURE REVIEW 11

CHAPTER 3, MATERIALS AND METHODS 13

Experimental Units, Treatments, and Design 13

Environmental Monitoring within Shade House .... 16

Gas-Exchange Measurement Procedures 17

Computational and Data Analysis Procedures 20

CHAPTER 4, RESULTS AND DISCUSSION 22

CHAPTER 5, SUMMARY AND CONCLUSION 29

APPENDIX A: Air Temperature within Shade House .... 31

APPENDIX B: Solar Radiation within Shade House .... 32

APPENDIX C: Photosynthetic Parameters for Four Conifer Species 33

REFERENCES 34

5

LIST OF ILLUSTRATIONS

Figure Page

1 Schematic diagram of open-chamber system used for gas-exchange measurements 18

2 C02 assimilation rate (A) versus intercellular C02 mole fraction (C,) for non-stressed (control) plants, stressed plants (medium ^»L < -2.0 MPa), and plants irrigated 7 days following 2 to 3 days medium stress 23

6

LIST OF TABLES

Table Page

1 Description of control and treatment designations 14

2 Gas-exchange parameter mean values measured for potted Pinus brutia plants under well watered (control), medium water stress, and water stress + 7 days recovery conditions .... 24

7

ABSTRACT

Pinus brutia var. eldarica is often considered to possess

photosynthetic characteristics that make it highly tolerant to

drought conditions. However, very little is known about it»s

photosynthetic response to water stress under either

laboratory or field imposed drought conditions. The purpose

of this study was to utilize laboratory gas-exchange

measurements to determine the effects of water stress and

recovery from stress on photosynthetic capacity of potted

Pinus brutia var. eldarica plants. Analysis of the rate

assimilation (A) versus intercellular C02 (C^) data indicates

that recovery of carboxylation efficiency (g'J (i.e. Rubisco

activity and amount) does occur after daily re-watering of

moderately stressed (^2 - -2.0 MPa) Pinus brutia. However,

these same data indicate that irreversible damage to cell

membranes results in an inability to re-generate RubP.

Although inferences on the effect of stress on A in field

grown Pinus brutia is tenuous, these laboratory results

indicate that Pinus brutia may not be as drought tolerant as

commonly believed.

8

CHAPTER 1

INTRODUCTION

Water stress due to drought is often the single most

important environmental factor affecting the survival and

growth of a plant species (Kramer,1983). Newly planted tree

seedlings are particularly sensitive to water stress because

their poorly developed root systems (Brix, 1979) are often

inadequate for preventing severe plant water deficits (Davies

et al.,1973; Kozlowski,1975) during drought. Tolerance of

seedlings to water stress is, therefore, an important factor

affecting their survival and growth.

Plant water stress can cause large reductions in

photosynthetic activity. This response is mainly the result

of both increasing resistance to C02 diffusion and a reduction

in the efficiency of the photosynthetic mechanism (Boyer,

1976; Hinckley et al., 1981). Plants previously exposed to

water stress continue photosynthesis at lower water potentials

than do plants which have never experienced any water stress

(Ashton, 1956; Ackerson and Hebert, 1981; Matthew and Boyer,

1984) .

Attempts at introducing new tree species, such as Pinus

brutia, to an arid or semi-arid region often meet with

failure, because planted tree seedlings are frequently unable

to survive the stress imposed by drought conditions within the

9

region. The effects on photosynthesis due to prolonged periods

of water stress through lack of rain and/or irrigation is a

primary cause of failure to establish tree seedlings.

Pinus brutia is a species of pine commonly believed to

have characteristics favorable for growth in arid and semi-

arid regions, such as the southwestern United States. It is

relatively unique among species of pine in that it is readily

adaptable to growth in soils with high pH (Abido, 1986). It

is also commonly believed to be tolerant to drought

conditions, however, the response of the photosynthetic

capacity of Pinus brutia to water stress has not been studied

either in the laboratory (potted plants) or under field

conditions.

Techniques of gas-exchange have made possible the

detailed investigation of the effects of water stress on

photosynthesis in Pinus brutia. For almost all other plant

species previously studied, it is well documented that severe

water stress causes a decrease in net photosynthesis (Brix,

1969). Furthermore, there is evidence that the capacity of

the mesophyll to assimilate carbon is sometimes affected by

water stress, as is also stomatal conductance (Boyer, 1976).

Gas-exchange measurements have also shown a reduction in

quantum yield and in light-saturated rates of C02 assimilation

(Boyer, 1971; Mooney et al., 1977).

10

The effects of water stress on the so-called "mesophyll

conductance" (e.g. initial slopes ofA/C,. curve) of carbon have

been examined by several authors, but the results remain

equivocal (Troughton and Slatyer, 1969; Laisk and Oya, 1971;

Jones, 1973; Moldau, 1973; Coolatz, 1977; O'Toole et al.,

1976; Mooney et al.,1977; Radin and Ackerson, 1981). The

results are equivocal in respect to many different methods of

imposing water stress that have been used, thus, making

interspecies comparisons difficult.

Measurements of the rate of C02 assimilation (A) to

intercellular C02 mole fraction (C{) response functions for

several pine tree species (e.g. Pinus silvestris. Pinus

contorta. and Pinus radiatal have shown that rapidly induced,

short-term water stress leads to reductions in assimilation in

plants grown under high C)2 (C, 660 /xmol mol"1) but not in

plants grown under ambient C02 (C{ - 330 /mol mol'1) (Havrane

and Brenecke, 1978).

Although studies of photosynthesis have been made for

many species of pine (see Brix, 1969), there is, however,

little information available at present on photosynthesis in

Pinus brutia (Havrane and Benecke, 1978). The overall

objective of this study was, therefore, to determine the

effects of water stress, and recovery from water stress on

photosynthetic capacity in potted Pinus brutia var. eldarica

under laboratory conditions.

11

CHAPTER 2

LITERATURE REVIEW

The basic physiological parameters describing

photosynthesis are obtained from the response of the rate of

C02 assimilation (A) to the intercellular C02 concentration

(C{). Seedling physiological response to water stress has

been measured by leaf moisture content, transpiration,

diffusive resistance, photosynthesis and respiration. The

effect of moisture stress on the physiology of the needle has

generally been observed to decrease transpiration, close

stomates, and reduce photosynthesis. In addition, Abido

(1986) used electron microscopy to determine that chloroplasts

of mesophyll cells Pinus brutia were markedly disrupted when

relative water content decreased to 62 percent during water

stress conditions.

Second year loblolly pine seedlings were found to

decrease carbon dioxide uptake with increasing diffusion

pressure deficit (DPD) of the needles and photosynthesis was

found to be highly positively correlated with transpiration

rate (Brix, 1962). Bilan et al. (1977) and Millan and Allen

(1971) showed that stomatal aperture and mesophyll resistance

could both play a part in photosynthesis reductions. Most

frequently cited as the cause of the decline in photosynthesis

as a result of water stress, several authors have observed a

12

major effect of non stomatal factors on photosynthetic

inhibition (Boyer and Bowen, 1970; Cornic et al. 1983;

Hutmatcher and Krieg, 1983).

A photosynthesis response to lower water potentials would

convey an advantage to plants during periods of drought.

Photosynthesis and growth in such plants could continue longer

during the period of water deficit. This response, however,

has been examined in only a limited number of species and has

not been investigated in Pinus brutia var. eldarica.

13

CHAPTER 3

MATERIALS AND METHODS

Experimental Units. Treatments, and Design

Ninety potted Pinus brutia var. Eldarica seedlings formed

the experimental units of this study. The one-year old trees

were approximately 0.3 to 0.5 m tall at the time of the study,

which was done during April, May, and June 1989. Prior to the

study, the trees were transplanted to 4-L plastics pots in a

1 soil: 2 sphagnum peat: 2 perlite (by volume) medium with pH

of 6.2. The medium was amended with 0.744 kg of treble

superphosphate, 0.946 kg of potassium nitrate, 0.496 kg

magnesium sulfate, 4 kg ground calcite limestone, and 0.062 kg

Peters Frit Industries Trace Elements No. 555 (Peters

Fertilizers Products, Fogesville, Pa.) per cubic meter. During

plant establishment, the potted trees were fertilized at each

watering with 200 mg L"1 each of N and K supplied from 517 mg

L"1 KN03 and 367 mg L'1 NH4N03. The fertilizer solution was

maintained at pH 6.0 by injecting 75% (w/w) technical grade

phosphoric acid into the irrigation system.

On 1 April 1989 the 90 trees were placed at random within

an array of 18 rows (5 plants per row with 0.25 m spacing) on

a bench in a 50% shade house at the University of Arizona

Campus Agricultural Center in Tucson. Note that the trees

were acclimated to the reduced light intensity within the

14

Table 1. Description of control and treatment designations

Designation Description

Control

Low i|rL

Medium i|tL

Plants irrigated daily in shade house. VL did not become less than about - 0.5 MPa as determined at time of gas-exchange measurements.

Gas-exchange measurements made at end of a 3 to 5 day interval during which plants did not receive any irrigation.

- - 4.0 MPa at time of measurements.

Gas-exchange measurements made at end of a 2 to 3 day interval during which plants did not receive any irrigation. i|»L - - 1.0 to -1.5 MPa at time of measurements.

Medium \|»L + Recovery Gas-exchange measurements made at end of 7 day recovery period with daily irrigations following a 2 to 3 day interval with no irrigations. -- 2.0 MPa at time of measurements.

Low i(fL + Recovery Gas-exchange measurements made at end of 7 day recovery period with daily irrigations following a 3 to 5 day interval with no irrigations. i|»L -- 3.0 MPa at time of measurements.

15

shade house for a period of 6 weeks prior to the establishment

of water stress treatments in April 1989. Establishment of

treatments commenced on 2 April. The treatments were

designated (see Table 1) as high stress, medium stress, and

control. Also, recovery of the plants over about a seven day

period from the stress conditions was studied. Both the high

and medium stress treatments involved withholding irrigation

for several days. The high stress plants did not receive

irrigation water for intervals of 3 to 5 days, resulting in

needle water potentials that decreased to about -4.0 MPa. The

medium stress plants did not receive irrigation water for

intervals of 2 to 3 days, resulting in needle water potentials

of about -1.5 MPa. The control treatment involved daily

irrigation with tap water which maintained needle water

potentials at about -0.5 MPa. During selected periods of the

study, several of the high stress and medium stress plants

were irrigated daily for about 7 days following a stress cycle

in order to permit investigation of the recovery of the

photosynthetic process following stress conditions. Needle

water potentials (i|rL) at the end of the 7 day recovery period

were about -2.0 MPa and -3.0 MPa for the medium and high

stress plants, respectively. Needle water potentials were

measured with a Soil-Moisture Equipment Model 1000 pressure

chamber immediately before and after the gas-exchange

measurements. The needle water potential measurements were

16

facilitated by removal of three to four 0.05 to 0.1 m long

branches from a plant under study. Water potential values

reported here are averages of readings taken on all plants

within a treatment.

Environmental Monitoring within Shade House

Air temperature (at 1.5 m height above ground) and solar

radiation conditions within the shade house were continuously

monitored between 3 April 1989 and 13 June 1989 with a

radiation-shielded thermistor (Campell Scientific Inc.(CSI;

Logan, Utah) model 107) and a Licor (Lincoln, Nebraska) LI

200S pyranometer, respectively. Output signals from both

sensors were sampled at 10 second intervals with a CSI 2IX

Micrologger and averaged over a 60 minute period to yield mean

air temperature (° C) and solar radiation flux density (MJ m'2

h"1). All data were processed by the Micrologger and were

stored on cassette tape for later analysis. As illustrated in

Appendix A, air temperatures were relatively invariant from

day-to-day throughout the study, with the exception of a brief

cooling trend during mid-April. Typically, daily maximum air

temperatures within the shade house were about 35 to 40 ° C,

and minimum temperatures were about 10 to 15 ° C. Incoming

solar radiation within the shade house was also relatively

invariant from day-to-day during the study, with total flux

densities typically in the range of 6 to 9 MJ m"2 d'1. Data

I

17

presented in Appendix B shows that the diurnal variations in

solar radiation flux density were similar from day-to-day.

The data presented in Appendices A and B, therefore, show

that the environmental conditions within the shade house were

relatively constant on a day-to-day basis. Thus, there

should not have been any marked confounding effect of changing

environmental conditions on the water stress treatments

imposed during the study. The data also show that the

relatively warm and sunny conditions were favorable for rapid

imposition of water stress in the potted plants studied.

Gas-Exchanae Measurement Procedures

Gas-exchange response to increasing C02 was measured in

a laboratory on the University of Arizona campus on well

watered (control) plants, water stressed plants, and water

stressed + recovery plants at intervals throughout the study.

At the completion of each stress cycle and each recovery

cycle, gas-exchange responses of approximately 4 stressed

plants and 4 control plants were measured.

18

A raj [mf>

-TMF

SPEN GAS EXCHANG!

SYSTEM

SAT V, B,

0"

(v)—! 3RIIR

LEAF CHAMBER

I R G A

D P

MF

zc LLI

0 £ zc LLI U I—i t-l Q£ ae. CS c

Fig. 1 Schematic diagram of open-chamber system used for gas-exchange measurements.

19

Gas-exchange measurements were made using an open system

and temperature controlled glass cuvette (see Fig. 1, leaf

chamber) as previously described by Perchorowicz et al.

(1981). Oxygen, nitrogen, and C02 were mixed in a manifold

using a multiple valve flow control system. The flow rates

into the mixing chamber were measured with mass flow meters

(Hastings Model ST 2263 and ST 2664). Response of C02

assimilation (A) to increasing C02 was monitored by measuring

C02 gas-exchange in step increments from low (<100 /xmol mol"1)

to high C02 (1000 nmol mol"1) mole fractions. The reference,

sample and the C02 differential concentrations between the

inlet and the outlet gas from the cuvette were measured with

an infrared gas analyzer (IRGA in Fig.l) (ADC Model LCA-2).

Flow rate into the glass cuvette was controlled at 3 L min"1

and the air temperature inside the water-cooled cuvette was

controlled throughout at about 25 ± 0.5 t. Air and needle

temperatures were monitored using thin-wire thermocouples.

Vapor pressure deficit (VPD) was maintained at 1.5 to 2.0 kPa

by saturating the gas stream in a temperature controlled water

bath, and then drying part of the air stream with silica gel.

Vapor pressure of the inlet and outlet gas was measured with

dew-point hygrometers (EG&G Model 911). Light supplied by a

1000 W metal halide lamp was filtered through 0.08 m of water

and neutral density screens. The irradiance level was measured

with a quantum sensor (Licor Model LI-180) positioned between

20

the screen and cuvette, and was maintained at a PPF between

1400 and 1600 jumol s"1 m'2. Assimilation rate (A) was

expressed on a per unit needle area basis. Total needle area

within the cuvette was determined by removing all needles from

the plant under study upon completion of gas-exchange

measurements. The needles were then dried at about 60°C

overnight. A regression equation developed for area versus

needle dry weight was then used to calculate total needle area

for each plant studied.

Computational and Data Analysis Procedures

Gas-exchange parameter values were calculated using

standard equations given by Moon and Flore (1986). Non-linear

regression equations were fitted to the calculated C02

assimilation (A) versus intercellular C02 concentration (cf)

data for use in determining A at light and C02 saturated

conditions (A^) and carboxylation efficiency (g'm). A^ for

each fitted curve was estimated as the value of A

corresponding to the "plateau" region of the curve where the

slope (dA/dCj) approaches zero. Carboxylation efficiency

(g'm) was determined from the slope of the linear portion of

the A/Cj curve.

Statistical analyses were made using Dunnett's one-sided

t-test (Steele and Torrie, 1960) to determine whether or not

photosynthetic parameter means for the stressed plants were

21

significantly different from the mean values determined for

the non-stressed control plants.

The relative stomatal limitation (I, expressed in units

of percent) to photosynthesis is the proportionate decrease in

C02 assimilation (A) that may be attributed to the stomata and

other gas phase limitations. This parameter is, thus, useful

for separating stomatal from mesophyll limitations to

photosynthesis (Long, 1985). Relative stomatal limitation is

calculated from Eq. 1 (Farquhar and Sharkey, 1982) given as:

1= A" A°ioo (1) A

where the rate of C02 assimilation (A) is calculated at the

normal atmospheric C02 mole fraction of about 350 /xmol mol"1,

and A0 is the rate which would occur if there was no stomatal

limitation. A0 is the value of A interpolated from the A

versus C, curve at C{ - 350 jtmol mol'1.

22

CHAPTER 4

RESULTS AND DISCUSSION

The characteristic changes in photosynthetic rate, (A)

with intercellular C02 concentration (Cf), are shown in Fig.

2 for the control plants, the medium stress plants, and the

medium stress + recovery plants.

Figure 2 shows that for the control plants the increase

in A with C. was linear over the low C02 mole fraction range.

At about 400 nmol mol"1 C02, the slope of the curve gradually

decreased. At this C02 mole fraction, referred to by Farquhar

and Sharkey (1982) as the inflection point, the rate of

assimilation (A) increases more slowly with increased c{. The

photosynthetic rate for the control plants reached a maximum

value (A^) of 27.7 fimol (C02) m"2 s"1 at a mole fraction of

about 650 jumol mol*1. Figure 2 also shows that medium water

stress (i.e. i|fL <-2.0 MPa) in Pinus brutia markedly decreased

C02 assimilation rates. In addition, the slope (g*m) in the

linear portion of the A/C. curve for the medium stressed

treatment is considerably lower than the slope for the control

plants. In contrast to the low g'm for the medium stressed

plants, the g'm for the recovery plants was nearly identical

to that for the control plants. However, the Amax values for

both stressed treatments shown in Fig. 1 were considerably

below the A(I)0X for the control plants.

23

</> 30-N I E

(N ° 20-^ O ~

O E 3. 1 0 -

• CONTROL ^ < -0.5 MPa

© STRESS + 7 DAYS RECOVERY

• STRESS < -2.0 MPa

C. /zmol mol"1

800

Fig. 2. CO, assimilation rate (A) versus intercellular C02 mole fraction (CJ for non-stressed (control) plants, stressed plants (medium ijr^ < -2.0 MPa), and plants irrigated 7 days following 2 to 3 days medium stress.

24

Table 2. Gas-exchange parameter mean values measured for potted Pinus brutia plants under well watered (control), medium water stress, and water stress + 7 days recovery conditions.

Mean control and treatment valuesy

Parameter z

Control

(*L <-0.5 MPa)

Medium

(1»L —2.0 (MPa)

Medium i|rL + Recovery

(^r, —1.0 to -1.5 MPa)

LOW i|»L + Recovery (l|lL <-3.0

MPa)

A (Mmol C02 m"2 s" 1) 17.1 5.1* 11.1* 5.4*

Anax (/mol co2 m"2 s' 1) 27.7 10.1* 17.6* 12.7*

g's (mmol C02 m"2 s" 1) 227.2 38.6* 128.7* 47.7*

ci (jumol mol • 1 ) 278.9 206.3NS 272.7NS 234.7NS

E (mmol H20 m"2 s" 1) 2.5 1.2* 1.2* 1.1*

g'm (mmol C02 m'2 s" 1) 85.6 36.8** 57.6NS 40.4**

ZC02 assimilation (A), C02 assimilation at saturation values of intercellular C02 > 500 /xmol mol"1 (Amax), stomatal conductance (g's) to C02, transpiration (E), and mesophyll conductance to C02 (g'_) were measured at needle temperatures ranging from 24 to 28 C, 350 /umol mol*1 CO,, vapor pressure deficits ranging from 1 to 1.5 kPa, and light intensities ranging from 1400 to 1600 /mol m"2 s"1 PPF.

yEach value is the mean of four replicates.

NS, *, **; Nonsignificant, and significantly different from control at P = 0.05 and P = 0.01, respectively, using Dunnett's one-sided t-test. For each parameter, the mean values for the three water-stressed treatments are compared to the mean value for non water-stressed control.

I

25

Photosynthetic and transpiratory parameter values

determined from gas-exchange measurements and from the A

versus C{ curves (Fig. 2) are summarized in Table 2. Note

that Table 2 does include values for the low i|rL + recovery

treatment. It does not, however, contain values for the low

t|il (see Table 1 for description of this treatment) because the

A response to C, for this treatment was virtually nonexistent

due to the severity of the 3- to 5-day water stress condition.

As shown in Table 2, there was measurable recovery in the A

versus C{ response in the highly stressed plants following 7

days watering.

Results summarized in Fig. 2 and in Table 2 give an

indication that Pinus brutia var. eldarica may not be as

drought tolerant as commonly believed. Analysis of the rate

of assimilation (A) versus intercellular C02 mole fraction (C.)

data indicates that recovery of the efficiency of

carboxylation (g'm) (i»e., ribulose l,5-biphosphate

carboxylase/oxygenase (Rubisco) activity and amount) does

occur after daily re-watering of moderately (but not highly)

stressed Pinus brutia plants. This is brought out clearly in

Table 2 which shows that the g• m value (57.6) for the medium

stress + recovery plants is not significantly different from

the g'm value (85.6) for the control plants. This indicates

that the Rubisco proteins in the medium stressed plants did

regenerate during the re-watering phase. However, these same

26

data (in Table 2) indicate that irreversible damage to cell

membranes may have resulted in an inability to re-generate

ribulose 1,5-biphosphate (RubP) by photosynthetic electron

transport. This is seen by the fact that the A^ value (17.6)

for the medium stress + recovery treatment was significantly

different from the A^ value (27.7) for the control plants.

Table 2 shows that all stress levels did significantly

reduce the transpiratory parameters (i.e. transpiration, E;

and stomatal conductance to water vapor, g's) relative to the

control. It is noteworthy that reduction in g*s due to water

stress did not, on the basis of C,- data, impose a severe

stomatal limitation to C02 diffusion to the intercellular

spaces. This is seen by the fact that the water stress

treatment C{ values in Table 2 were slightly lower but not

significantly different from the mean value (278.9) for the

non-stressed control. The measured slight decrease in C{ with

decreasing \|iL in Pinus brutia is, interestingly, opposite to

the increase in C. with decreased i|»L measured by Farquhar and

Sharkey (1982) in Encilia farinosa.

In contrast to the C,. data, however, it is interesting

that evaluation of the relative stomatal limitation (I, Eq. 1)

for each of the A/C{ curves given in Fig. 2 indicates that

stomatal limitation in the C02 pathway did reduce A in the

stressed plants. Calculations with Eq. 1 show that I values

were about 22, 37, and 49 % in the control, medium stress +

27

recovery, and medium stress (no recovery) plants,

respectively. The change from 22 to 49 % represents a

proportionate increase in stomatal limitation of 123 % when i|rL

decreased from about -0.5 MPa to -2.0 MPa. In contrast, Long

(1985) reported that for Helianthus annuus a lowering of i|iL

from -0.33 MPa to -1.63 MPa increased I from 0.27 to 0.31,

which was a proportionate increase of only 13 %. Stomatal

limitation data analysis for other species of pine have not,

to my knowledge, been reported in the literature.

Appendix C presents a tabulation of A, g's, and Cj values

taken from the literature (Wong et al., 1979, Brix, 1979) for

Pinus species: radiata, contorta, and silvestris. It also

presents the values (taken from Table 2) for non-water-

stressed brutia determined from this study. The data were

collected under somewhat different environmental and

silvicultural conditions, thus, making direct comparisons

difficult. The data presented, however, qualitatively

indicate that A values were similar among the four species

with the exception of contorta, which reportedly has a very

low carbon dioxide assimilation rate of 6.4 /zmol C02 m"2 s"1.

Interestingly, brutia has the largest stomatal conductance

(227.2 mmol m"2 s"1) of the four species considered, which may

be an advantage for carbon assimilation under non-water-

stressed conditions, but a disadvantage for conserving water.

The stomatal conductance for brutia is also markedly higher

28

than the value of 1.5 mmol m"2 s"1 reported for one-year old

needles of Pinus resinosa (see Long, 1985) Intercellular C02

concentrations (C,) among the species listed in Appendix C are

similar. Of Pinus contorta Doual.. Pinus radiata. and Pinus

svlvestris. Pinus contorta was determined by Brix (1969) to be

the most sensitive to water stress. However, results from

this study indicate that brutia may be equally sensitive to

water stress.

CHAPTER 5

29

SUMMARY AND CONCLUSIONS

This study was done to determine the effects of water

stress and recovery from water stress on the photosynthetic

capacity of Pinus brutia var. eldarica. Measurements of gas-

exchange were made using an open chamber system in a

laboratory. Water stress imposition was done on potted, one-

year old, eldarica plants that were grown under shade house

environmental conditions. Stress treatments included

withholding irrigation of plants to allow \|rL to decrease to

values ranging from about -2 MPa (medium stress) and to about

-4 MPa (high stress). Recovery from stress conditions was

facilitated by daily irrigation of plants for a period of

about 7 days. Control plants were irrigated daily throughout

the study.

Results showed that:

1) Assimilation (A) decreased with decreasing tJil .

2) Water stress decreased transpiration (E) and stomatal

conductance (g*8).

3) Intercellular C02 mole fraction (C{) decreased

slightly, but not significantly, with increased stress.

4) Relative stomatal limitation to gas phase C02

diffusion to the site of carboxylation apparently increased

with increasing stress.

30

5) Full recovery of Rubisco enzyme activity and amount

from medium stress did occur following 7 days irrigation.

Full recovery did not occur from high stress.

6) Full regeneration of RubP did not occur following 7

days irrigation of stressed plants, thus indicating

irreversible damage to cell membranes and photosynthetic

capacity.

7) Although inferences on the effects of water stress on

photosynthesis in field grown Pinus brutia is tenuous, these

laboratory results indicate that Pinus brutia may not be as

drought tolerant as commonly believed. And that other

physiological characteristics, such as its adaptability to

growth in high pH soil, make it a suitable plant for wood and

biomass production in arid and semi-arid regions.

APPENDIX A: Air temperature within shade house

31

0.2' 0.4 0.6 o.e

3 April to 13 June (hours x 1000)

APPENDIX B: Solar radiation within shade house

32

1.2

1.1

I 0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0.2 0.4 0.6 0.8

3 April to 13 June (Hours z 1000)

33

APPENDIX C: Photosynthetic parameters for four conifer species

- Species Parameter/units radiata contorta silvestris brutia

A (/xmol C02 m"z s"1) 20.6 £74 17.0 17.1 g»s (mmol C02 m"2 s"1) 133.0 100.0 96.0 227.2

(Mmol mol'1) 291.0 284.9 289.7 278.9

34

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