The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves

Abstract

Excised green leaves of mung bean (Phaseolus aureus L. var. Mungo) were used to determine the effect of light on the rate of endogenous respiration via the tricarboxylic acid cycle. ILlumination with white light at an intensity of 0.043 gram calories cm'minn (approximately 8600 lux) of visible radiation (400-700 nm) gave a rate of apparent photosynthesis, measured as net C02 uptake, of 21 mg CO2 dm-2hr-' which was about 11-fold greater than the rate of dark respiration. The feeding of '4CO2 or 14C-labeled acids of the tricarboxylic acid cycle in the dark for 2 hours was established as a suitable method for labeling mitochondrial pools of cycle intermediates.At a concentration of 0.1 mM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, apparent photosynthesis was inhibited 82%, and the refixation of 14CO2 derived internally from endogenous respiration was largely prevented. In the presence of this inhibitor endogenous respiration, measured as "CO2 evolution, continued in the light at a rate comparable to that in the dark. Consequently, under these conditions light-induced nonphotosynthetic processes have no significant effect on endogenous dark respiration. Inhibitors of the tricarboxylic acid cycle, malonate and fluoroacetate, were used to determine the relative rates of carbon flux through the cycle in the dark and in the light by measuring the rate of accumulation of 4(C in either succinate or citrate. Results were interpreted to indicate that the tricarboxylic acid cycle functions in the light at a rate similar to that in the dark except for a brief initial inhibition on transition from dark to light. Evidence was obtained that succinate dehydrogenase as well as aconitase, was inhibited in the presence of fluoroacetate.Controversy has surrounded the question of the effect of light on endogenous aerobic respiration via glycolysis and the tricarboxylic acid cycle in algae and green leaves of higher plants. Reports have included stimulation, inhibition, or no effect of light on respiration. For example, physiological studies, using the mass spectrometer to measure respiratory and photosynthetic gas exchange simultaneously, showed that relatively low light intensities had little or no effect on rate of respiratory oxygen uptake in barley leaves or in suspensions of the alga Chiorella (8). With the green alga Ankistrodesmus braunii (9) or the algal flagellate Ochromonas malhamensis (52) it was found that respiratory CO2 evolution was almost independent of light intensity, and that respiratory oxygen consumption was not affected by low light although it was enhanced at high light intensities. Hoch et al. (24) concluded from their results obtained with the mass spectrometer that low light intensities inhibited endogenous respiration in Anacystis, a blue-green alga, but not in Scenedesmus, a green alga, whereas higher light intensities promoted oxygen uptake in both organisms. The total oxygen consumption or CO2 evolution cannot be separated, however, into dark endogenous respiratory and light-induced photorespirator...

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves

“…In these, the sum of aspartate and malate is plotted for simplicity. Figure 2a shows that in A. hastata, little radioactivity was transferred to sucrose on illumination, a response consistent with that in mung beans (6). The net loss of label from the malate + aspartate in the light reappeared in glutamate.…”

“…In leaves of higher plants, this conclusion is supported by studies with labeled tricarboxylic acid cycle intermediates (6,7), and by the persistence in the light of 02 uptake at low concentrations of 0°known to inhibit glycolate synthesis and metabolism, or photorespiration (25,30). The role of continued tricarboxylic acid cycle respiration in the light in green cells is not clearly understood but it contributes to C02-generating reactions in these tissues (23,30).…”

“…The Chlbased specific activity of fumarase, citrate synthase, and NAD (9) it seems likely that they also contain more chloroplasts or at least more Chl (31). Few quantitative examinations of electron micrographs have been made but these suggest only minor differences in the ratio of chloroplasts to mitochondria between mesophyll and bundle sheath cells of Zea and Sorghum, and between cells of C3 and C, grasses (12) (6,7,14,15,23) and to algae (24). We Karpilov (20) Malate-2, 3-"C, on the other hand, yields only pyruvate-2, 3-"C. Because the carbon from the C-4 carboxyl of malate-1 ,4-"C enters the carbon reduction cycle more rapidly than that of the remaining C, skeleton (16), the transfer of carbon from succinate-I , 4-"C to photosynthesis should be initially more rapid than that from succinate-2, 3-`C.…”

“…The experiments emphasized the rapid exchange of malate and aspartate between mitochondria and chloroplasts in leaves of C4 plants. Leaf Experiments with intact leaves were conducted as described earlier (6,7). Leaves were placed in small vials containing labeled substrates and allowed to absorb '4C for 2 hr in the dark.…”

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves

“…In the previous paper (9) evidence was presented which indicated that a relatively high light intensity had only a temporary effect in depressing the rate of the tricarboxylic acid cycle in detached green leaves of mung bean (Phaseolus aureus L. var. Mungo).…”