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4. .\‘et photosynthesis during grain till in the maize described in Fig. 2 and 3.
Photosynthesis in the desiccated plants was virtually zero after the seventh day of
desiccation. Thus. the desiccated plants had a total of 16 days and the control
plants had a total of 58 days of photosynthesis alter maximum pollen shed.
concept. the reduced diffusion of carbon dioxide through the stomata would
limit photosynthesis because of the reduced amount of carbon dioxide available
for fixation by the chloroplasts. Although transpiration is affected by stomatal
closure. the effect on photosynthesis cannot be predicted from correlations between
transpiration and photosynthesis alone. There is little doubt that stomatal
closure restricts the entry of carbon dioxide into the leaf. but the supply may
or may not control the rate of photosynthesis depending on how severe stomatal
closure is. An additional test of the importance of stomatal closure is required
in this situation. This has recently been done by Boyer (197 la) who showed that
the rate of photosynthesis of sunflower leaves did not change when carbon
dioxide levels were increased around the plant. despite the increased availability
of carbon dioxide to the cells within the leaf when this treatment was carried
out. IIe concluded that photosynthesis could not be limited by stomatal closure
in this particular ease and suggested that changes at the chloroplast level probably
accounted for the changes in photosynthetic activity. In a related study.
Wardlaiv (1967) has shmvn that increased external carbon dioxide does not
diminish the inhibition of photosynthesis during drought in wheat. Whether
this is true for other species remains to be seen.
Several investigators have demonstrated changes in chloroplast activity when
chloroplasts are isolated from desiccated leaves (Nir and Poliakoff-Mayber.
1967'. Boyer and Bowen. 1970'. Fry. 1970: Potter and Boyer. 1973; Keck and
Boyer, 1974). These changes involve a decrease in electron transport and photophosphorylatitin.
and there are reports that carbon dioxide fixation by isolated
chloroplasts is reduced (Plant. 1971; Plaut and Bravdo, 1973). The changes in
electron transport have been demonstrated in viva (Boyer and Bowen. 1970;
Boyer. 1971a. 1971b). and they parallel thc inhibition of photosynthesis in