INTERACTIONS OF CARBON DIOXIDE CONCENTRATION, LIGHT INTENSITY AND TEMPERATURE ON PLANT RESISTANCES TO WATER VAPOUR AND CARBON DIOXIDE DIFFUSION

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1 New Phytol. (1967) 66, INTERACTIONS OF CARBON DIOXIDE CONCENTRATION, LIGHT INTENSITY AND TEMPERATURE ON PLANT RESISTANCES TO WATER VAPOUR AND CARBON DIOXIDE DIFFUSION BY P. C. W H I T E M A N* AND D. ROLLER Botany Department, Hebrezv University, Jerusalem, Israel (Received 28 January 1967) Exchange of water vapour and carbon dioxide, between whole sunflower plants and an atmosphere controlled to required CO, and water vapour concentrations, was measured in a polythene chamber. Plant resistances to diffusion of CO2 and H2O were calculated. From the combined data of net flux of C02('P'), resistances to CO2 uptake, and atmospheric CO2 concentration [C02]a,m, the intercellular space COj concentration [C02]),,,, could be calculated. [C02]ini was linearly related to [C02]a,m and this relationship was modified by light intensity. Plant age, between 3 and 6 weeks, did not alter the relationship. The ratio of [C02] n, to [C02]atm was shown to be equivalent to the ratio of mesophyll resistance to CO2 diffusion (r^) to total resistance to CO2 diffusion (R^oj)- At a given [CO,]^,^, [CO,]j^, decreased with increasing light intensity between 500 and 5500 ft-candles but was little affected by leaf temperatures between 18 and 34 C. On the other hand, the minimum internal CO2 concentration T, measured as the equilibrium CO2 compensation point, increased linearly over the same temperature range but was little affected by light intensity above 500 ft-candles. The effects of temperature and light on [CO2]in, are in contrast to their effects on the F value. Therefore, it is less likely that temperature effects on [C02]in,, measured in the normal photosynthetic [002]^,^ range, are responsible for 'midday stomatal closure' as inferred from observations of temperature effects on T alone. By comparing the effects of temperature and light intensities on stomatal resistance over the same range of [C02]jnt values, direct effects of light and temperature, not mediated through changes in [C02]in, were demonstrated. The interactions ofthe factors involved are discussed. INTRODUCTION The CO2 concentration in the intercellular spaces of the mesophyll and, in particular, in the substomatal cavities has a profound effect on stomatal movement and therefore on epidermal resistance to gaseous diffusion, r^. The intercellular CO2 concentration, [C02]in,, is affected by environmental factors which influence the rates of photosynthesis, respiration, or both, such as light energy and its spectral distribution, temperature, and ambient CO2 concentration outside the leaf, [C02],,,^. Furthermore, epidermal resistance acts as a feed-back mechanism, by controlling the rate of gaseous exchange between the intercellular and external atmospheres of the leaf. Direct measurement of [C02]ini bas not been reported and studies involving forced mass flow of air with a predetermined [CO2] through the mesophyll were possibly complicated by modification of tbe [CO2] as it passed through tbe leaf and by possible drying effects (Heath and Russell, 19540, b). * Present address: C.S.I.R.O., Division of Tropical Pastures, Mill Road, St Lucia, Queensland. 463

2 464 P- C. WHITEMAN AND D. KOLLER The present paper reports some analyses of effects of the environment and of plant age on gaseous exchange between the leaf and the ambient atmosphere using a [COjJin, value calculated from data of simultaneous measurement of transpiration and net photosynthesis under closely-controlled environmental conditions. MATERIALS AND METHODS Net photosynthesis and transpiration were determined by measuring net flux of COj ('P') and water vapour {'T') respectively, using the 'null-point compensating system' (Koller and Samish, 1964) and the general procedure previously described (Whiteman and Koller, 1964). In brief, measurement of leaf temperature and of air temperature and humidity allowed the calculation of the vapour pressure differential between the mesophyll evaporating surface, [HjOji^, (assuming the latter to be nearly saturated) and the atmosphere [HjO]^,^. Total resistance to exchange of water vapour, R^, was then calculated as in equation (i). ^w = ' e + ^=([H2O]i,-[H2O]3.JT) (1) where r^ and r^ represent the epidermal and boundary layer resistances to diffusion of water vapour, respectively. An estimate of r^ for the leaves of the test plant was obtained by measuring the 'transpiration' from simulated 'leaves' made from wetted blotting paper, according to Kuiper (1961). In the turbulence conditions maintained constant in all subsequent experiments, r^ was 0.8 sec/cm. Epidermal resistance, r^, was obtained by subtracting r^ from R,^. Resistance to gaseous diffusion of CO2 over the same pathway was obtained hy multiplying R^ by the ratio of the coefficients of diffusion of the two gases in air, Z = DH2O/DCO2 The effects of this combined resistance atm are such that P = (2) on exchange of CO2 at a given ij(r;-f r;) (3) where [C02]ini is the concentration in the mesophyll intercellular spaces. The latter can now be calculated according to Moss and Rawlins (1963) as follows: The 'minimal' [C02]in,, T, was determined by stopping CO2 compensation into the system thus allowing the plant to photosynthesize and deplete [C02]atm to a constant level. At this point, net exchange equals zero and [C02]a,m = [COjJin, = T (Heath and Orchards, 1957; Meidner and Heath, 1959; Heath and Meidner, 1961; Meidner, 1962). The initial [CO2]a,m at the beginning of a determination of T was always 115 ppm. The plants used were sunflowers {Helianthits anmius L.). grown from seed in a halfstrength Hoagland nutrient solution. The latter was replenished with water daily and replaced by a fresh solution twice each week. The plant was transferred to the laboratory from the glasshouse and the polythene chamber sealed around the base of its stem on the (4)

3 Resistances to diffusion of CO2 and water vapour 465 day prior to measurements. The chamber was continuously purged with fresh air Each value was obtamed by three consecutive lo-minute readings, made once the plant had reached a steady state at each new level of [CO^],,,. Two plants were used for each determmation m Experiment A, three in Experiment B, while Experiment C was repeated twice using three plants each time. The plant was equally illuminated on all sides, using light from three Philips Attralux spot lamps, filtered through 5 cm of water. Wire screens were used to adjust light intensity. The latter was measured near the plant, inside the chamber, by means of a Weston light meter equipped with a flat photocell. A fine thermocouple inserted in one of the leaves and a shaded thermocouple situated in the air outlet from the chamber were used for measuring leaf and air temperature. The data were subjected to analysis of variance for main and interaction effects. RESULTS Experiment A. Effects of plant age (3, 4, 5 and 6 weeks after sowing; 4000 ft-candles; C). Rates of photosynthesis, F, as well as total resistance to water vapour exchange R^, increased as [COj],,^ was increased from 115 to 400 ppm, but the effects of plant age on the former were neghgible (Eig. ic) and on the latter were not consistent (Fig. ib). (It must be remembered that as r^ was constant, changes in /? represent changes in r^ only.) The relationship between [C02]in, and [C02]atn, was totally unaffected by plant age, and was almost strictly linear within the [COj]^,^ range tested (Fig. ia). The values of T were highest in the youngest plants (84 ppm) and did not differ markedly in the older ones (61, 53, 65 ppm for 4-, 5- and 6-week-old plants, respectively). Experime?tt B. Interactions of light intensity ( ft-candles) and [COjJajm ( ppm), (23-26"" C). Photosynthesis increased with increasing light intensity (Fig. 2a), and [CO^ja,^ (Eig. 2b). The response to increasing [COj],,^ was linear only at the highest light intensity (5500 ft-candles), while at 2000 ft-candles and below the response was limited by light intensity. The data in Eig. 2(c) show that response to light intensity is similarly limited at low [CO^],,^ and that the light saturation value increases as [COjJatm increases. The ratio of photosynthesis to [COjJa,^ was used as a measure of efficiency of the plant in utilizing available COj. The data in Eig. 2(d) show that efficiency increased with increase in light intensity as well as with increasing [COjJatn,. However, the promotive effect of increasing [C02]atm levelled off after a given value, depending on light intensity, although photosynthesis itself continued to increase over the same range (Eig. 2b). The main effects of light and [COjJatm on [C02]in( show that [C02]in, increased linearly with increasing [C02]atm (Fig- 2f), while increasing light intensity reduces [C02]int (Fig. 2e). The interaction between the two factors (Fig. 3a) shows that the linear relationship between [C02]a,m and [C02]int was light dependent, the slope becoming steeper as intensity decreased. On the other hand, F was hardly affected by increasing light intensities above 1000 ft-candles (Eig. 3b). Epidermal resistance, r^, was reduced when light intensity was increased from 500 to 1000 ft-candles, but further increases caused an increase in r^ (Eig. 3c). Epidermal resistance also increased with increase in [CO2] external to the leaf, or internal (Eig. 3d), which was to be expected in view of the linear relationship between the two within that range (Eig. 2f). The curves describing the interactions of light intensity with [C02]int in the control of r^ (Eig. 3e) show that when both light and [C02]int were low, the latter

4 466 p. C. WHITEMAN AND D. ROLLER was more effective in determining r^, but also that was affected by light intensity even at equivalent [C02]ini- Experiment C. Interactions of temperature ( C) and [COjlatm ("5-400 ppm) on [COiJint and diffusion resistances (4000 ft-candles). In neither of the two experiments was there a significant effect of temperature on the rate of net photosynthesis. In one experiment highest rates were observed at 25 C, but differences were not significant (at 5%). This is refiected also in Fig. 4(a) which shows the interaction of temperature 400 r (b) Fig. I. The effect of plant age and atmospheric CO2 concentration, [CO^Jai^ on: (a) the relationship between [CO2]aim and internal CO2 concentration [CO2]iot; (b) epidermal resistance to water vapour diffusion, r^; and (c) net photosynthesis, P. Measurements were made at 4000 ft-candles light intensity, C and at 3 weeks ( x), 4 weeks (O), 5 weeks (A) and 6 weeks ( ) from planting. and [COjJatm on the efficiency of COj utilization. The effects of [C02]a,m surpass those of leaf temperature. The relationship between [COjJaim and [COjJint was again approximately linear (Fig. 4b), and was not affected significantly by leaf temperature within the range studied. Nevertheless, T increased linearly with increasing leaf temperature (Fig. 4c). Stomatal resistance increased with increasing [COjjatm above 115 ppm (Fig. 4d), but temperature (above 25 C) caused a significant increase only in one of the two experiments

5 Resistances to diffusion of CO2 and water vapour 467 (a) 5500 ft-candles 2000 E ' " O X Cl. E ro O 0-5 a. (c) x400 X t-j o> I'O E (e) Light intensity (f t-cand les x 10 ) L'^O^}^ Fig. 2. The interactions of light intensity and [COjlacm on photosynthesis and (a) Main effect of light intensity on net photosynthesis; (b) interactions of light intensity and [CO2]atm on net photosynthesis, to show the linear response to increasing [CO2]aim at each ligbt level; (c) interactions of light intensity and [CO2]a(m on net photosynthesis, to show light saturation curves at each [CO2]aim level; (d) interactions of Hght intensity and [CO2]atm on the efficiency of CO2 utilization; and (e) and (f) main efiects of light intensity and [CO2 ]aim respectively, on [CO2]ioi. Least significant differences at the i % level are indicated by vertical bar.

6 468 P. C. WHITEMAN AND D. ROLLER (a) (b) Light intensity (tt-candles x l 'Jght intensity ( tt-candles x lo' tt -candles 500/- A fcoo"! (ppm) 1- -* int Fig. 3. Interactions of light intensity and [CO2]aim on [CO2]ini and stomatal diffusion resistances, (a) The interaction of light intensity and [CO2]aim on [CO2]ini; (b) the effect of light intensity on the minimal internal CO 2 concentration, F; (c) and (d) main effects of light intensity and [CO2]aim. respectively, on stomatal diffusion resistance, r^, and (e) the effects of light intensity on stomatal resistance within the same range of [C02]ini values. Least significant differences at 1 % are indicated by vertical bar. (Fig. 4e). However, when both light intensity and temperature were at maximum values, technical difficulties arose due to vapour condensation in the system and somewhat higher (e^ e^) values had to be employed, so that transpiration rates were higher (Fig. 4f). This could have led to a reduction in turgor, which would contribute to a measured increase in r,.

7 Resistances to diffusion of CO^ and water vapour 469 (b) o O ^zj C p p m ) (c) (d) Leaf temperature ( C ) (e) (f) Leaf temperature ( C ) (Ss-fa) (mmhq) Fig. 4. Interactions of temperature and [COaJaim on [CO2]ini and stomatal diffusion resisstances. (a) The effect of three temperatures, ig'"" C ( x), 25 C (C) and 33 C ( ), on the efficiency of CO 2 utilization over the [CO2]aim range ppm:, first experiment;, second experiment; p ; (b)) Main effect of [ijoim [COiJoim on [CO2]in,: 2 ] i n X,, first experiment; p O, second experiment; (c) Effect of leaf f temperature on the minimal internal [CO 2], T; (d) and (e) Main effects of [CO2]aim and leaf temperature, respectively, on stomatal resistance: second experiment only; (f) Transpiration rates (T), meaned over the [CO2]atm range, as a function of the vapour pressure deficit {e^ e^ at each light intensity, 5500 ( x), 2000 (A), 1000 ( :) and 500 ( ) ft-candles. DISCUSSION The response curves of photosynthesis to increasing [COjJatm were similar in all experiments, and similar also to those found for cotton (Bierhuizen and Slatyer, 1964) and sunflowers (Hesketh, 1963). The plants in Experiment A reached a higher maximal rate (at 4000 ft-candles and 400 ppm [COiJatm). of 2.0 x 10"^ mg C02/min/cm^ than K N.P.

8 470 P. C. WHITEMAN AND D. KOLLER the X io ^ mg COj/min/cm^ reached in Experiments B and C. The plants in E.xperiment A were grown outdoors under natural high light intensities and were thus 'sun-adapted', while the plants in Experiments B and C were raised in the glasshouse. Bjorkman and Holmgren (1963) also found a marked effect on rates of photosynthesis at light saturation caused by growing sun-adapted species at reduced light intensities. It is unlikely that the limited effect of temperature on net photosynthesis was due to insensitivity since F was strongly affected (Fig. 4c). This apparent lack of response could arise from compensation of temperature effects on gross photosynthesis by opposite effects on respiration. However, it is also possible that [COjJatm was sufficiently low to allow the rate of CO2 diffusion to limit the photosynthetic process, so that effects of IOO 500 ft-candles IOO Fig. 5. The interactions of light intensity: 5500 ( ), 2000 (A), IOOO ( ;) and 500 ( ) ftcandles and [CO2]a(m ( ppm), on total resistance to COi diffusion, i^co2 ( ). and apparent mesophyll resistance, ;, ( ). temperature may be slight (Gaastra, 1959; Bierhuizen and Slatyer, 1964). When is high, temperature effects are more readily demonstrated even when light is saturating since it is the capacities of the dark reactions (carboxylation) which are limiting and the rates of these have a temperature dependence (Gaastra, 1962). The ratio, P\\^Of\.,^^, which was used to express the efficiency of conversion of available COj to photosynthetic product, is actually a measure of the reciprocal of the apparent total resistance to CO2 uptake, if it is assumed that the [CO2] at the chloroplast is zero. When this assutnption was used to calculate actual values of RCO2, instead of reciprocals, over the [C02]atm range used at each light intensity, it appears that increases rapidly as light intensity or [C02]a,n, are reduced towards their compensation

9 Resistances to diffusion of CO2 and water vapoitr 471 points (Fig. 5). Moreover, this increase appears to be largely due to changes in r^, when it is calculated as the difference between i?co2 and (>-', + r'j according to Gaastra"'(i959). Such values of r^ obviously represent not only the resistance to CO2 diffusion in the mesophyll in the same sense as boundary layer or stomatal diffusion resistances but include biochemical or photochemical limitations on the photosynthetic mechanism by external factors. This analysis supports Gaastra's statement (1959, 1963), that comparative values of r^ should be measured only under conditions of saturating light intensities and limiting [C02]atm> where rate of photosynthesis is limited only by the rate of CO2 diffusion. The relationship between [002];^ and [C02]atm was in all cases linear (Figs, ia and 2f). This relationship may be analysed as follows: The flow of CO2 between the intercellular spaces and the chloroplasts is P, the concentration difference is [C02]in,, and the resistance is r^, so that by analogy with equation (3) we can put: Eliminating P from equations (3) and (5) gives: P 1 - = - (5) - = ^ (6) Thus the slope represents the ratio of resistance within the mesophyll cells to the total resistance to CO2 uptake. This ratio was constant over the [C02]aim range ppm, but was a function of light intensity as shown by Figs. 3(a) and 5. However, T values were little aitected by increasing light intensity in the range ft-candles. Below 1000 ft-candles, Y increased apparently due to insufficient light energy at the chloroplasts to reduce the respiratory CO2 output (Fig. 3b). This effect was also shown by Orchard (quoted by Heath and Meidner, 1961). Even though Tregunna, Krotkov and Nelson (1961) reported that photo-respiration was increased by increasing light intensity, our results indicate that the ratio between photosynthetic rate and photo-respiration, as expressed by the equilibrium CO2 concentration, Y, was almost constant above 1000 ft-candles. The Y values in the present study with sunflowers were ppm, compared with ppm reported by Forrester, Krotkov and Nelson (1966), for soybean (at 2i%02). Apart from the species difference, the higher values may result from measuring on a whole plant basis rather than on single leaves. Increasing the [C02]atm above the equiubrium Y concentration allowed differences in photosynthetic rate at different hght intensities to be expressed. Thus [C02]int values were a function of both light intensity and [C02]atm (Fig- 3^)- Temperature had a much greater effect on Y than on [C02]int in the normal [C02]ext range (Fig. 4b and c). Heath and Orchard (1957) also showed that in onions, coffee and Pelargoniitm spp. Y increased linearly with temperature over the range C. The increase in Y with increasing leaf temperature is usually ascribed to increased respiratory CO2 output (Heath and Orchard, 1957). In some species a marked increase in Y occurs above some critical temperature (Heath and Orchard, 1957; Meidner and Heath, 1959; Heath and Meidner, 1961), which may indicate the onset of temperature inhibition of the photosynthetic mechanism. On the other hand, [C02]in,, measured at [C02]3,m greater than the CO2-compensation point, was little affected by temperature in the range

10 472 P. C. WHITEMAN AND D. ROLLER 18-34" C, and this was reflected also in the small effect of temperature on net photosynthesis. The balance between rates of CO2 fixation and respiration appeared to he relatively constant over this temperature range. Therefore, extrapolation of measured effects of light intensity and temperature on the COj-compensation point may not give a true indication of their effects over the normal photosynthetic [COjJacm range. In fact, the effects of temperature and light intensity on T appear to be almost opposite to their effects on [C02]ini measured at higher [C02]atm concentrations. The inference made hy other workers that temperature effects on F may be responsible for midday stomatal closure by inducing higher [C02]int under field conditions (i.e. in normal [C02]atm) does not appear to hold for sunflowers. A direct effect of light intensity and temperature on stomatal resistance, not mediated through changes in [CO2]int, was demonstrated; r^ values were higher at the highest light intensity (Fig. 3e) and temperature (Fig. 4e), over the same [C02]int range. Stalfelt's (1962) explanation that increased r^ in Vicia faba above 30"" C in the light was due to increased [C02]int does not hold in the present case. However, because of the technical difficulties encountered in measurement at high temperature and light intensities, the possibility of their direct effect was complicated by possible water stress effects, even though the roots were held in aerated water. On the other hand, low intensity light (qoo ft-candles) did inhibit stomatal opening. Qualitatively these results are similar to those reported by Heath and Russell (1954^). The inhibitory effect of low light intensity increased with increasing [C02]atm> with little difference between light treaments at [C02]atm around 100 ppm (Fig. 3e and Fig. 3 of Heath and Russell, 19546). The latter investigated a lower range of light intensities, ft-candles, so it is interesting that the light effect is also evident at the higher intensities used here. In our experiments possible effects of transmission of stimuli between illuminated and non-illuminated areas are precluded, since the entire plant was exposed to, and measured in, the same experimental conditions. The fact that the effect of low light intensity becomes more pronounced with increasing [C02]ini appears to involve a direct action of CO2 concentration within the guard cells. Kuiper (1964) has shown that guard cell movement is dependent upon photosynthesis within the guard cells. Zelitch and Walker (1964) have suggested that the CO2 concentration may affect stomatal movement by controlling glycolic acid metabolism. The CO2 concentration within the guard cells will be affected firstly by the rate of photosynthesis within the guard cells themselves, which would be reduced at low light intensities, especially on the whole plant scale. The CO2 concentration in effective contact with the guard cells appears to be the [CO2],n, (Ketellapper, 1963). This, in turn, is modified hy the overall rate of photosynthesis in the mesophyll, dependent also on light intensity, and by the [C02]a,m- Thus the CO2 concentration effect on guard cell movement will depend upon both 'direct' and 'indirect' factors. ACKNOWLEDGMENTS This work forms part of a Ph.D. thesis submitted by P. C. Whiteman to the Senate of The Hebrew University, Jerusalem, 1965, and was supported by an Australian Services Canteens Trust Fund Post-Graduate Scholarship. The measuring system was developed through a grant from the Ford Foundation to Dov Koller. We thank Dr J. V. Lake, C.S.I.R.O., Canberra, for suggesting an improved analysis of slope in equations (5) and (6).

11 Resistances to diffusion of CO2 and water vapour 473 REFERENCES BIERHUIZEN, J. F. & SLATYER, R. O. (1964). Photosynthesis of cotton leaves under a range of environmental conditions in relation to internal and external diffusive resistances. Aiist. J. biol. Sci., 17, 348. B]ORKM.'\N, O. & HOLMGREN, P. (1963). Adaptability of the photosynthetic apparatus to light intensity in ecotypes from exposed and shaded habitats. Phvsiologia PL, 16', 889. FORRESTER, N. L., KROTKOV, G. & NELSON, C. D" (1966). Effect of oxygen on photosynthesis, photorespiration and respiration in detached leaves. I. Soybean. PI. Physiol., Lancaster, 41, 422. GA.^STRA, P. (1959). Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature and stomatal diffusion resistance. Meded. LandbHoogesch. IVageningen, 59, i. GA.\STRA, P. (1962). Photosynthesis of leaves and tield crops. Neth.J. agric. Sci., 10, 311. GA.«TRA, P. (1963). Climatic control of photosynthesis and respiration. Environmental Control of Plant Growth (Ed. by L. T. Evans), pp Academic Press, New York. HE.'iTH, O. V. S. & MEIDNER, H. (1961). The influence of water strain on the intercellular space CO2 concentration (T) and stomatal movement in wheat leaves. J. exp. Bot., 12, 226. HE.ATH, O. V. S. & ORCHARD, B. (1957). Midday closure of stomata. Temperature effects on the minimum intercellular space COi concentration (F). Nature, Lond., 180, 180. HE.^TH, O. V. S. & RUSSELL, J. (1954^). Studies in stomatal behaviour. VI. An in\'estigation of the light response of wheat stomata with the attempted elimination of control by the mesophyll (Part i). J. exp. Bot., 5, I. HEATH, O. V. S. & RUSSELL, J. (19546). Studies in stomatal behaviour. VI. An investigation of the light responses of wheat stomata with the attempted elimination of control by the mesophyll (Part 2). J. exp. Bot., 5, 269. HESKETH, J. D. (1963). Limitations to photosynthesis responsible for differences among species. Crop Sci., 3, 107. KETELLAPPER, H. J. (1963). Stomatal physiology. A. Rev. PI. Physiol., 14, 249. ROLLER, D. & SAMISH, Y. (1964). A null-point compensating system for simultaneous and continuous measurement of net photosynthesis and transpiration by controlled gas stream analysis. Bot. Gaz., 125, 81. KuiPER, P. J. C. (1961). The effects of environmental factors on the transpiration of leaves, with special reference to the stomatal light response. Meded. LandbHoogesch. IVageningen, 61, i. KuiPER, P. J. C. (1964). Dependence upon wavelength of stomatal movement in epidermal tissue of Senecio adorus. PI. Physiol., Lancaster, 39, 952. MEIDNER, H. (1962). The minimum intercellular space COT concentration (F) of maize leaves and its influence on stomatal movements. X exp. Bot., 13, 284. MEIDNER, H. & HE.\TH, O. V. S. (1959). Studies in stomatal behaviour. VIIl. Stomatal responses to temperature and CO2 concentration in Allium cepa L., and their relevance to midday closure. J. exp. Bot., 10, 206. Moss, D. N. & RAWLINS, S. L. (1963). Concentration of CO, inside leaves. Nature, Lond., 197, ST.4LEELT, M. G. (1962). The effect of temperature on opening of the stomatal cells. Physiologia PL, 15, 772. TREGUNNA, E. B., KROTKOV, G. & NELSON, C. D. (1961). Evolution of CO, by tobacco leaves during the dark period following illumination with light of different intensities. Can. j. Bot., 39, WHITEMAN, P. C. & KoLLER, D. (1964). Environmental control of photosynthesis and transpiration in Pinus halepensis. Israel J. Bot., 13, 166. ZELITCH, I. & WALKER, D. A. (1964). The role of glycolic acid metabolism in opening leaf stomata. PL Physiol., Lancaster, 39, 856.

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