Mild Water Stress of Phaseolus vulgaris Plants Leads to Reduced Starch Synthesis and Extractable Sucrose Phosphate Synthase Activity

Abstract

Mild water stress, on the order of -1.0 megapascals xylem water potential, can reduce the rate of photosynthesis and eliminate the inhibition of photosynthesis caused by 02 in waterstress-sensitive plants such as Phaseolus vulgaris. To investigate the lack of 02 inhibition of photosynthesis, we measured stromal and cytosolic fructose-1,6-bisphosphatase, sucrose phosphate synthase, and partitioning of newly fixed carbon between starch and sucrose before, during, and after mild water stress. The extractable activity of the fructose bisphosphatases was unaffected by mild water stress. The extractable activity of SPS was inhibited by more than 60% in plants stressed to water potentials of -0.9 megapascals. Water stress caused a decline in the starch/sucrose partitioning ratio indicating that starch synthesis was inhibited more than sucrose synthesis. We conclude that the reduced rate of photosynthesis during water stress is caused by stomatal closure, and that the restriction of CO2 supply caused by stomatal closure leads to a reduction in the capacity for both starch and sucrose synthesis. This causes the reduced 02 inhibition and abrupt CO2 saturation of photosynthesis.Mild water stress, on the order of -1.0 MPa, can reduce the rate of photosynthesis in water-stress-sensitive plants such as Phaseolus vulgaris. The reduced rate of photosynthesis can be caused by stomatal closure or by direct effects of water stress on the biochemical reactions of photosynthesis. Two possible sites of water stress damage to the biochemistry of photosynthesis are (a) reactions associated with the thylakoid membranes (2), and (b) stromal reactions associated with the photosynthetic carbon reduction cycle especially the carboxylation reactions (8). Farquhar and Sharkey (5) argued that since the response of photosynthesis to intercellular CO2 partial pressure was reduced, there must be some effect of mild water stress on the biochemical reactions of photosynthesis. This argument is now suspect because it has been found that stomata can close more over some sections of the leaf than over other sections (patchy closure). The patchy stomatal closure was first seen in response to feeding abscisic acid (4,26) and has been demonstrated in water-stressed Phaseolus leaves (21).Kaiser (7) isolated chloroplasts and that the loss of photosynthesis in leaves resulting from water stress is probably caused by stomatal closure and direct effects on the partial pressure of CO2 within the leaf. In addition, Sharkey and Seeman (21) showed that carbamylation of RuBPCase, kca of RuBPCase, and RuBP pool size was not affected by a mild water stress (-0.7 MPa) even though the rate of photosynthesis was reduced by 75%. Similar results have been obtained with abscisic acid fed leaves (17). Only small effects on these parameters were seen in leaves with a water potential of -1.1 MPa, even though photosynthesis was almost completely eliminated by this degree of water stress. Sharkey and Seeman (21) concluded that the reduction in photosynthesis caused by...

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