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Inhibition of Photosynthesis by Carbohydrates in Wheat Leaves

1983, Plant Physiology

The rate of net CO2 assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% 02. 340 microbars C02) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO2 concentration (700425 microbars). In this latter case, the decline in the rate of net CO2 assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to C02, but this was not the primary cause of the decrease in CO2 assimilation because internal partial pressure of CO2 remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO2 assimilation of leaves in atmospheric and high CO2 concentrations. The decreased net CO2 assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO2 assimilation in high carbohydrate leaves as a function of intercellular CO2 partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO2 assimilation by 2% 02 (Warburg effect), and the apparent quantum yield, decreased after several hours of light.

Plant Physiol. (1983) 73, 681-686 0032-0889/83/73/0681/06/$00.50/0 Inhibition of Photosynthesis by Carbohydrates in Wheat Leaves Received for publication March 9, 1983 and in revised form May 25, 1983 JOAQUIN AZCON-BIETO' Department of Environmental Biology, Research School of Biological Sciences, The Australian National University, P.O. Box 475, Canberra City, A.C. T. 2601 Australia The rate of net CO2 assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% 02. 340 microbars C02) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO2 concentration (700425 microbars). In this latter case, the decline in the rate of net CO2 assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to C02, but this was not the primary cause of the decrease in CO2 assimilation because internal partial pressure of CO2 remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO2 assimilation of leaves in atmospheric and high CO2 concentrations. The decreased net CO2 assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO2 assimilation in high carbohydrate leaves as a function of intercellular CO2 partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO2 assimilation by 2% 02 (Warburg effect), and the apparent quantum yield, decreased after several hours of light. Accumulation of carbohydrates in leaves during photosynthesis is a common phenomenon which can be enhanced by several means including high photosynthesis rates, low translocation rates, or low sink demand (16). It has long been hypothesized that "the accumulation of assimilates in an illuminated leaf may be responsible for a reduction in the net photosynthesis rate of that leaf' (Boussingault, 1868; see 16). This hypothesis has been variously tested by manipulating (a) photosynthate source, (b) translocation of photosynthate, and (c) photosynthate sinks (see reviews by Guinn and Mauney [9], Herold [11], and Neales and Incoll [16]). The evidence of the occurrence of end product inhibition of photosynthesis from such experiments remains equivocal. The conclusions are often complicated by the fact that hormonal, nutritional, or other interactions may occur following defoliation and sink manipulation, especially in long-term experiments. In wheat leaves, Birecka and Daki&c Wlodkowska (3) and King et al. (12) were able to inhibit flag leaf photosynthesis by removing the sink (removal of the ear) and stimulate flag leaf photosynthesis by spraying the ear with DCMU (which inhibited photosynthesis in the ear, thereby increasing the sink). However, Austin and Edrich (1) were unable to confirm the ear removal response. Nevertheless, King et al. (12) showed that ear removal caused an accumulation of carbohydrates in the flag leaf. They were able to simulate this response by keeping plants in continuous light for a week. Leaf carbohydrates built up and photosynthesis was inhibited. Although these results suggest the occurrence of end product inhibition of photosynthesis in wheat, the treatments and the measurement methods used do not exclude other factors. Hormonal and stomatal responses could be involved. Thus, in this paper we describe experiments which are designed to avoid these difficulties and to determine if the photosynthetic rate is related to the carbohydrate status in wheat leaves. Carbohydrate status was increased by increasing the rate of photosynthesis under otherwise comparable conditions, by increasing CO2 concentration or reducing 02 concentration. In other experiments, a portion of the leaf below the region in which photosynthesis was monitored was chilled to reduce transport of photosynthate (8, 21). MATERIALS AND METHODS Plant Material. Triticum aestivum L. (cv Gabo) plants were grown from seed in a controlled environment cabinet in pots of soil. They were watered twice a day, and were fertilized every other day with nitrate-type Hewitt's solution containing: KNO3, 4 mM; Ca(NO3)2, 4 mM; MgSO4, 1.5 mM; NaH2PO4, 1.33 mM; EDTA FeNa, 60 gM; MnSO4, 10 uM; ZnSO4, 1 ,uM; CuSO4, 1 ALM; H3BO3, 50 uM; Na2MoO4, 0.5 jiM; NaCl, 0.1 mM; Co(NO3)2, 0.2 uM. The pH of the solution was 6.5. PPFD2 was about 600 to 700 gE _m 2 -s-'. The day/night temperature regime was 25/ 20°C with a daylength of 13 h. RH was between 60 and 80%. Gas Exchange Techniques. Wheat plants were selected from the growth cabinet near the end of the night period (10 AM). One or two attached recently mature leaves were enclosed in a photosynthetic chamber, which received an incident PPFD of 1000 iE -m2 - s-'. The rest of the plant, which was kept intact, was also illuminated. CO2 and water exchanges were measured in leaves using an open system gas analysis apparatus (2). Calculations of gas exchange parameters were made as before (20). Leaf to air water vapor pressure differences were maintained at about 8 to 12 mbar at 20 to 23°C and at 20 to 23 mbar at 30°C in most experiments. Light Responses of Photosynthesis. Net rate of CO2 assimilation (A) was measured in wheat leaves for 1 h at a PPFD of 1000 jiE *m-2.s-' to open stomata. Incident PPFD was measured with a quantum sensor (Lambda Instruments, model LI- 190 SR). PPFD was then decreased by interposing copper screens, and A was measured at every step. Finally, dark CO2 efflux was measured. Leaf temperature was kept constant during these procedures. These measurements were repeated after a period of 4 h 2 Abbreviations: PPFD, photosynthetic photon flux density; A, rate of CO2 assimilation; pi, intercellular partial pressure of C02; RuBP, ribulose bisphosphate. ' Present address: Department of Plant Physiology, Faculty of Biology, net University of Barcelona, Diagonal 637-647, Barcelona-28, Spain. 681 Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 ABSTRACT AZCON-BIETO 682 Plant Physiol. Vol. 73, 1983 from a bath through a sandwich-type brass jacket in close contact with both sides of the leaf. The surface of both sides of the jacket 3C formed a square of 4 cm2. Temperature of the leaf region in E contact with the jacket was 0 to 2C. These procedures did not E 0 la30C produce alterations in the temperature of the enclosed portion E 25 0.9 of the leaf, which was kept at 21 °C. No changes in the leaf 0 30C a A 2C _ 9 turgidity were apparent. Two experimental procedures were 0.7 C adopted. 22C (a) A leaf was allowed to photosynthesize for 4 h at external E 15 -0 CO2 pressures of either 350 or 700 ,ubar. Then the chilling V0 treatment was applied as described above and the gas exchanges 10 0 of the enclosed part of the leaf were monitored for a subsequent period of 4 h. At the end of this period, the cooling jacket was S removed, and the leaf gas exchanges were measured for about an z n 1 h. additional vI 6 3 4 2 (b) In other experiments, leaves were subjected to the chilling treatment from the start ofthe light period, and the gas exchanges Time in the light (Hours) were monitored for 5 h. CO2 pressures inside the chamber were FIG. 1. Time course of net CO2 assimilation (0, A) and total leaf either 310 or 800 ,abar. conductance to diffusion of water vapor (0, A) in wheat leaves at two All experiments were duplicated. temperatures. External CO2 pressure was 340 ,bar and 02 concentration Determination. Free glucose plus fructose, inCarbohydrate was 21%. vertase sugars (mostly sucrose), and starch fractions were determined as described by Azcon-Bieto and Osmond (2). at a PPFD of 1000 gE * m2 s'. The leaf temperature and CO2 pressure were varied as indicated in the text. Each experiment RESULTS was replicated three times. Effect of Prior Photosynthesis on Subsequent Rate of Net CO2 CO2 Responses of Photosynthesis. Net CO2 assimilation (A) and transpiration rates were first measured in flag wheat leaves Assimilation. Under certain conditions, in ambient air (340 jbar at ambient CO2 and 02 pressures for about 1 h until steady state C02, 21 %02) A reached a maximum within the first few hours rates were reached. Measurements were then repeated at several of illumination and subsequently declined. This pattern was most CO2 concentrations below ambient, and finally at CO2 pressures frequently observed in leaves illuminated in air at temperatures higher than ambient. After the determination of the first curve below 25°C (especially in leaves with high rates of photosynof A versus Pi which took 2.5 to 3 h from the start of the light thesis), and was normally absent at higher temperatures (Fig. 1). period, leaves were allowed to photosynthesize for 5 h at external If wheat leaves were allowed to photosynthesize at 22°C in partial pressures of CO2 of either 800 ubar (Exp. A) or 50 to 60 atmospheres with elevated, ambient or depleted CO2 partial gbar (Exp. B). The curve of A versus pi was then determined pressure, the decline in A was more dramatic at 740 ,ubar than again over the next 1.5 to 2 h. At the end of this time, a piece of at 350 ubar and was absent at 125 pbar (Fig. 2A). This response the enclosed leaf was taken for carbohydrate analysis (see below) was further exaggerated by increasing CO2 partial pressure to 740 by carefully sectioning the leaf with a sharp razor blade. In the ,ubar and reducing the 02 concentration to 2%, resulting in a experiments where the rate of photosynthesis declined with time 35% decrease in A after 9 h photosynthesis (29-19 Umol CO2. (Exp. A), the remaining leaf fragment was kept in darkness for 3 m 2 s'). The decline in A also occurred at 30°C when the CO2 h. At the end of this period, the curve of A versus pi was concentration in the air was high (Table I). The decline in A was not due to stomatal closure. In ambient determined again to study recovery. The rest of the leaf was then used for carbohydrate analysis. Experiment A was performed air, the declining CO2 assimilation was accompanied by no three times and experiment B two times. Leaf temperatures change, or even a slight increase, in stomatal conductance (Fig. during the light and dark periods were 20 and 1 8C, respectively. 1) and corresponding small increases (from 260 to 280 ,bar) in Leaf Base Chilling Methods. A small portion (about 2 cm2) pi. Although larger decreases in stomatal conductance were obof the base of a wheat leaf, whose upper part was enclosed in the served following prolonged exposure to elevated atmospheric photosynthetic chamber, was chilled by circulating icy water CO2 partial pressure, this only resulted in small changes in pi A _ 22C -* - ---- -- E c A a 0 c 0 - ~ ____ _____ 0.5 0 0.3 0 0 - Time Course of Net CO2 Assimilation (A), Stomatal Conductance to Water Vapor (gj), Intercellular Partial Pressure ofCO2 (p,) and Carbohydrate Levels in Wheat Leaves at High External CO2 Pressures (825 ,ubar) Carbohydrate concentration was measured in leaf fragments sampled from the leaf enclosed in the photosynthetic chamber after 7 h in the light, and after an additional period of 2 h in darkness. The values shown are means ± SE of three experiments (except for the values shown in section C, which correspond to a single experiment. Table I. Period of Temperature A g5 Pi Carbohydrate °c OMOC2MmolC2O2.m2-sI' bar CO2 713 ± 24 C.m2 Notmeasured Not measured Experiment A. I h in the light B. 7 h in the light C. 7 h in the light plus 2 h in the dark Concn. 20 43.0 ± 1.1 mol.mM2 S-r I 0.67 ± 0.1 30 51.0 ± 0.5 0.45 ± 0.01 588 20 30 34.6 0.45 43.3± 1.1 0.28±0.02 690 ± 21 524± 15 463±23 30 49.4 0.30 495 325 ± 0.2 ± 0.06 ± 2 mmol 526 ± 39.5 Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 5 683 END PRODUCT INHIBITION OF PHOTOSYNTHESIS IN WHEAT 35 E 0 - A 0 c 0 0"~ 30F 0----~ E 740 pbor CO2 E %. a 25f .-_ E 350 pbar CO2 201 0 .2_ E =L . 0 15 0 U 125 pbar CO2 10 z 100 L II 600_ B - 0---- O ~ ~ -1 O-.......... O 100 200 0 300 FIG. 4. Relationship between the depression of net CO2 assimilation and several carbohydrate fractions in wheat leaves. C 0 -o -a B 40)o -* *--- * In *," X 'E 2C _o40S *'1 *2 * 3 6* Z 6 Z. E 4 3 2 1 5 4 Time in the light (Hours) of net FIG. 2. Time course CO2 assimilation (A) and intercellular partial pressure of CO2 (B) in wheat leaves at several external CO2 pressures. Leaf temperature was 22°C, and 02 concentration was 21 %. 0 300 -2 6- I-o-o II**0i4 200 l, 2 00 10 S c * 0 0 - -8 - 0 00 ~~~~~~~0 0E E 0 * 0 -54 05.0 4 6 8 10 0 2 4 Time in the light (hours) FIG. 5. Effect of chilling the base of a wheat leaf on the rate of net CO2 assimilation and pi in the rest of that leaf. The chilling treatment was applied (as indicated by the arrows) either after 4 h of photosynthesis (A), or at the beginning of the photosynthetic period (B). In A, an unchilled control is also shown (0). External partial pressures of CO2 were 350 ubar (A) and 310 ubar (B). Temperature was 2 1C. 0 0 0 0 QA 4A 2 70 0 100 A 200 300 I 1 1 400 500 600 Carbohydrate concentration (mmol Cm-2) FIG. 3. Relationship between the depression of net CO2 assimilation and total carbohydrate concentration in wheat leaves. This relationship includes data obtained in experiments performed at different temperatures from 20°C to 30°C. which did not affect A (Table I; Fig. 2B). For example, in the 30°C treatment at 825 Abar (Table I), stomatal conductance declined by about 38%, A by about 15%, and pi by about 11% after 7 h photosynthesis. After 2 h recovery in the dark, stomatal conductance had not changed, A had returned to within 3% of the starting value, and pi declined further. Clearly, A was not responsive to stomatally controlled pi over the range 495 to 588 ,gbar CO2 in this experiment. Carbohydrates accumulate in wheat leaves under some conditions used in the above experiments and respiratory rates in the light increase (2). However, the increase in respiration is 1.5 Amol CO2 m 2 .s' at most, and cannot account for the large decrease in photosynthesis observed in Figures 1 and 2 and in Table I. The correlation between the decline in A and the total carbohydrate content (except fructosans) of wheat leaves after a period of photosynthesis under a variety of conditions (e.g. different CO2 concentrations and temperatures) is shown in Figure 3. There seems to be no effect on A below 100 mmol carbohydrate carbon m2 (about 3.1 g glucose eq m-2). Carbohydrates accumulated significantly less in leaves photosynthesizing at 30°C than at 20°C, in spite of the much higher assimilation rates observed at 30C (Table I). This suggests that translocation of recently accumulated assimilates is more effective at higher temperatures. Similar effects of temperature on translocation have been reported (8). This increased translocation efficiency may have been the reason for the low carbohydrate levcls measured in leaves photosynthesizing at high temperatures and ambient CO2 concentrations, which were less than 100 mmol carbon m2, which is the critical level for photosynthesis (see above). All carbohydrate fractions increased in about the same proportions. Although the concentrations of free glucose plus Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 VI 0 Carbohydrate concentration (mmol C mrn 5 684 .2 v AZCON-BIETO 5 I 30. E 25 ,;N - 6 °=;*_.C ~~~20~~~~~ 700 2 -) 0 I'-o 0O 0 400 2 00 0~~~~~~~~ 4 6 8 0 Time in the light ( hours) FIG. 6. Effect of chilling the base of a wheat leaf on the rate of net CO2 assimilation and p, in the rest of that leaf. External CO2 pressures were 700 to 770 \bar (A) and 800 Mbar (B). Other conditions and symbols are as in Figure 5. fructose and of starch were lower than those of invertase sugars (mostly sucrose) at maximum inhibition of A (Fig. 4), there is no evidence that any one component of the carbohydrate fractions was specifically more inhibitory than another. Effect of Chilling the Leaf Base on Net CO2 Assimilation. Chilling of the basal portion of wheat leaves is known to inhibit translocation, and in the present experiments it accelerated the rate of decline in photosynthesis. In wheat leaves allowed to photosynthesize in ambient air, cooling the leaf base to near 0°C accelerated the decline in A more than 2-fold, but had no effect on pi (Fig. 5). That is, the inhibition of photosynthesis was not due to stomatal closure. In leaves kept in the light at elevated CO2 partial pressures, the response to chilling the leaf base was even more dramatic (Fig. 6). It was immaterial whether the chilling was applied after 4 h photosynthesis or almost immediately after illumination; in both cases the rate of decline in A, compared to an unchilled control, accelerated from about less 'E 4 - A E 2 .2 2 320 ObarCO2 X B 200C Before 0.064 * 0.051 C4 than 1 to about 1.5 to 3 umol CO2. m2. s-' each hour. About 2 h after the commencement of the chilling treatment, pi began to decline (Fig. 6), but again, the major effect of chilling on A occurred before the Pi response. Very high carbohydrate levels were measured in the leaves after the chilling treatments (not shown). When the chilling was halted, the inhibition of A and the closure of stomata persisted for at least 1 h. There was no evidence of recovery. Effect of Prior Photosynthesis on the Properties of Photosynthesis. It was routinely found that the apparent quantum yield (the initial slope of the curve of A versus PPFD, excluding the point in darkness) was lower in leaves after a period of photosynthesis at 20C (Fig. 7, A and C). If leaves were allowed to photosynthesize at 30°C at ambient C02, the apparent quantum yield was similar before and after the photosynthetic period (Fig. 7B). Apparent quantum yield was lower at higher temperature and higher at high C02, as expected in C3 plants (5). These comparisons indicate that light itself was not responsible for the decline in apparent quantum yield, i.e. photoinhibition was probably not a contributory factor. No changes in the light transmission properties of the leaves (6-7% transmission) were detected following these periods of photosynthesis; leaf absorbance could not be measured, but it is unlikely that changes in absorbance were large enough to account for the large variations observed in the apparent quantum yield. Carbohydrates were not measured in this experiment, but it is probable that they are also involved in these responses. The relationship between A and pi in flag wheat leaves before and after a period of photosynthesis which caused accumulation of carbohydrates is shown in Figure 8A. The initial slope was unaffected, but the saturated region of the curve was depressed. This depression was relieved by 3 h in darkness which also resulted in a decline in leaf carbohydrate status. If leaves were treated under the same illumination conditions, but at low CO2 (60-70 gbar), carbohydrates did not accumulate to the same extent and there were no changes in the shape of the curve of A versus pi (Fig. 8B). The 02 sensitivity of CO2 assimilation in wheat leaves also declined following an extended period of photosynthesis (Fig. 9), and the reduction in the 02 sensitivity of photosynthesis was less in leaves with lower photosynthetic rates (Table II). Stomatal conductance to water vapor was rather insensitive to pi changes at the beginning of the light period, but it decreased and showed sensitivity to pi after a high CO2 light period (not 1. PPFD (pEEm2s1) FIG. 7. Light response curves of net CO2 assimilation in wheat leaves determined before (@) and after (0) a period of photosynthesis of 4 h. Three typical experiments are shown. External partial pressures of CO2 and temperatures during the experiments are shown. 02 concentration was 21%. The apparent quantum yield values (mol C02E-'; which were calculated excluding the point in darkness) are also shown,in the figures. External CO2 pressure during quantum yield measurement was the same as during the photosynthetic period. Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 700- Plant Physiol. Vol. 73, 1983 685 END PRODUCT INHIBITION OF PHOTOSYNTHESIS IN WHEAT Table II. Effect of Decreasing 02 Concentration from 21 to 2% on the Rate ofNet CO2 Assimilation (A) at Different Times during the Light Period Temperature was 21C. For other details, see legend to Figure 9. Time in the Light (h) Experiment 1.5-2 A2 A21 molCO2 m-2.sr' 19.3 25.2 21.9 29.0 12.5 19.2 1 2 3 30 B A A2, A2 Osmol CO2 m-2.s' 21 18.8 21 19.9 18 13.5 Increase % 31 32 54 Increase % 12 6 33 21%02 21%02 21%02 E 0 0 ------- --O /, E C E /, E 20[ 0 OS / 1.) 0 zn 10 07l1 f0 v~~~~~~ o/ 0 j~~~~~~ 25 E 0 VI /0 u n 1 I 200 I1 400 200 600 400 600 Intercellular CO2 partial pressure (pbar) FIG. 8. Effect of a period of photosynthesis on the curve of A versus pi in flag wheat leaves. A, The curve was determined at the beginning of the light period (0), after 5 h in the light at 800 sbar CO2 (0), and after a further 3 h in the dark (A). The carbohydrate concentration of these leaves increased from 12 to 260 (±40) mmol C m-2 after 5 h in the light, and declined to 197 (± 11) after 3 h in the dark. B, The curve of A versus pi was determined at the beginning of the light period (0) and after 5 h in the light at 50 to 60 ,bar CO2 (0). The carbohydrate concentration increased to only 99 (± 10) mmol C. m2 after 5 h in the light. The arrows indicate the direction in which the pi level was changed (i.e. increased or decreased). 2%02 2%02 . 0 I-- 20 z 1 2%02 1~~ I 1) A art Time in the light (Hours) FIG. 9. Changes in the 02 sensitivity of photosynthesis in wheat leaves with time. Net CO2 assimilation was initially measured in air (330 Mbar CO2; 21% 02); but, at intervals, the 02 concentration was temporarily reduced to 2%. The arrows indicate transfer to 2% 02 or 21% 02, and the dotted lines represent the periods at 2% 02 during which measurements were not taken. Temperature was 21 C _ observed. However, assimilation was very low in this study (about 2 Amol C02 m 2 s-') and a large carbohydrate build-up is not expected under these conditions. The inhibition of photosynthesis in wheat is correlated with accumulation of soluble sugars. Starch also accumulated in wheat leaves, but to a much less extent than soluble sugars. Accumulation of starch in leaves of other species has also been negatively correlated with the rate shown). In contrast, the stomatal responses topi were not affected of photosynthesis (9, 18). The changes in photosynthetic properties following carbohyby a period of photosynthesis at low CO2 pressures. drate accumulation in wheat leaves can be interpreted in terms of major biochemical processes in leaves of C3 plants, as outlined DISCUSSION by Farquhar and von Caemmerer (6). Reduced quantum yield These results and those obtained by Birecka and Dakic-WIod- suggests that carbohydrate accumulation impairs the production kowska (3) and King et al. (12) are consistent with the occurrence or consumption of ATP/NADPH in photosynthesis itself. Reof end product inhibition of photosynthesis in wheat leaves. The duction of the CO2-saturated rate of photosynthesis (Fig. 8A) decline in the rate of net CO2 assimilation with time of illumi- implies that carbohydrate accumulation leads to impaired regennation was greatest under conditions of reduced export (leaf base eration of the carboxylation substrate RuBP. It has been sugchilling, low temperatures) or higher rates of photosynthesis (high gested that soluble sugar accumulation may reduce the rate of CO2 and low 02 pressures in the air). The decline in CO2 RuBP regeneration by decreasing available stromal Pi (I 1). Very assimilation was related to carbohydrate concentration above a low phosphate levels inside the chloroplasts can restrict the rates certain critical level. The increase in respiration was not large of photophosphorylation and electron transport, probably via a enough to account for this decline (2). On the other hand, CO2 decreased ATP/ADP ratio and depression of 3-phosphoglycerate assimilation recovered substantially after a short period of dark- reduction (17). The use of mannose and glucosamine, which ness in which leaf carbohydrate levels declined. Factors such as sequester cytosolic phosphate, produces a decline in assimilation stomatal closure, photoinhibition and endogenous rhythms do and ATP levels which is consistent with this hypothesis of not seem to be involved in these responses. Similar observations phosphate limitation (1l, 14, 17). have been reported in other species (4, 13, 15, 18). In contrast, This interpretation receives some support from the observation Geiger (7) chilled the primary leaf petiole and node of a bean that the 02 response of C3 photosynthesis is also impaired when plant but no effect on the rate of net photosynthesis of that leaf carbohydrate accumulate (Fig. 9). A similar desensitivity of CO2 was Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 30 4-5 686 Acknowledgments-I am very grateful to Professor C. B. Osmond for his help and encouragement. I also thank Dr. S. C. Wong for his assistance in technical and experimental matters, and Drs. D. A. Walker, G. D. Farquhar, and D. A. Day for useful comments. LITERATURE CITED 1. AUSTIN RB, J EDRICH 1975 Effects of ear removal on photosynthesis, carbohydrate accumulation and on the distribution of assimilated "4C in wheat. Ann Bot 39: 141-152 2. Azc6N-BIETo J, CB OSMOND 1983 Relationship between photosynthesis and respiration. The effect of carbohydrate status on the rate of CO2 production by respiration in darkened and illuminated wheat leaves. Plant Physiol 71: 574-581 3. BIRECKA H, L DAKIC-WLODKOWSKA 1963 Photosynthesis, translocation, and accumulation of assimilates in cereals during grain development. III. Spring wheat-photosynthesis and the daily accumulation of photosynthates in the grain. Acta Soc Bot Pol 32: 631-650 4. CHATTERTON NJ 1973 Product inhibition of photosynthesis in alfalfa leaves as related to specific leaf weight. Crop Sci 13: 284-285 5. EHLERINGER J, 0 BjORKMAN 1977 Quantum yields for CO2 uptake in C3 and C4 plants. Dependence on temperature, CO2 and 02 concentrations. Plant Physiol 59: 86-90 6. FARQUHAR GD, S VON CAEMMERER 1982 Modelling of photosynthetic response 13. Ku SB, GE EDWARDS, D SMITH 1978 Photosynthesis and nonstructural carbohydrate concentration in leaf blades of Panicum virgatum as affected by night temperature. Can J Bot 56: 63-68 14. MIGINIAc-MASLOW M, A HOARAU 1982 Variations in the adenylate levels during phosphate depletion in isolated soybean cells and wheat leaf fragments. Z Pflanzenphysiol 107: 427-436 15. MILNER HW, WM HIESEY 1964 Photosynthesis in climatic races of Mimulults. II. Effect of time and CO2 concentration on rate. Plant Physiol 39: 746-750 16. NEALES TF, LD INCOLL 1968 The control of leaf photosynthesis rate by the level of assimilate concentration in the leaf: a review of the hypothesis. Bot Rev 34: 107-125 17. ROBINSON SP, DA WALKER 1981 Photosynthetic carbon reduction cycle. In MD Hatch, NK Boardman, eds, The Biochemistry of Plants. A Comprehensive Treatise, Vol 8, Photosynthesis. Academic Press, New York, pp 193236 18. UPMEYER DJ, HR KOLLER 1973 Diurnal trends in net photosynthetic rate and carbohydrate levels of soybean leaves. Plant Physiol 51: 871-874 19. USUDA H, GE EDWARDS 1982 Influence of varying CO2 and orthophosphate concentrations on rates of photosynthesis, and synthesis of glycolate and dihydroxyacetone phosphate by wheat chloroplasts. Plant Physiol 69: 469473 20. VON CAEMMERER S, GD FARQUHAR 1981 Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta 153: 376-387 21. WARDLAW IF 1968 The control and pattern of movement of carbohydrates in plants. Bot Rev 34: 79-105 Downloaded from https://academic.oup.com/plphys/article/73/3/681/6079207 by guest on 09 October 2023 AZCON-BIETO Plant Physiol. Vol. 73, 1983 to environmental conditions. In OL Lange, PS Nobel, CB Osmond, H fixation to 02 has been observed in spinach leaves infiltrated Ziegler, eds, Physiological Plant Ecology, Vol B, Water Relations and Phowith mannose (10), and Usuda and Edwards (19) have discussed tosynthetic Productivity. Springer-Verlag, Heidelberg, pp 549-587 the sparing effect of RuBP oxgenation on phosphate deficiency 7. GEIGER DR 1976 Effects of translocation and assimilate demand on photosynin experiments with isolated chloroplasts. It is possible that the thesis. Can J Bot 54: 2337-2345 rate of photosynthesis becomes insensitive to the ratio of carbox- 8. GEIGER DR, SA SOVONIK 1975 Effects of temperature, anoxia and other metabolic inhibitors on translocation. In M Zimmermann, JA Milburn, eds, yation to oxygenation when phosphate is sequestered in the Encyclopedia of Plant Physiology, New Series Vol 1, Phloem Transport. cytoplasm by mannose (10) or by the accumulation of carbohySpringer-Verlag, Berlin, pp 256-286 drates in the leaf cells, because under these conditions the rate 9. GUINN G, JR MAUNEY 1980 Analysis of CO2 exchange assumptions: feedback control. In JD Hesketh, JW Jones, eds, Predicting Photosynthesis for Ecoof CO2 assimilation may be controlled by the rate of release of system Models, Vol 2. CRC Press, Boca Raton, FL, pp 1-16 phosphate in the cytosol (6, 10, 11). These data establish a 10. HARRIS GC, JK CHEESBROUGH, DA WALKER 1983 Effect of mannose on correlation between inhibition of photosynthesis and carbohyphotosynthetic gas exchange in spinach leafdiscs. Plant Physiol 71: 108-1 1 1 drate build-up, and suggest control of the regeneration of RuBP. 11. HEROLD A 1980 Regulation of photosynthesis by sink activity-the missing link. New Phytol 86: 131-144 The phosphate limitation hypothesis provides one framework 12. KING RW, IF WARDLAW, LT EVANS 1967 Effect of assimilate utilization on for further investigation of the mechanisms involved. photosynthetic rate in wheat. Planta 77: 261-276