Changes in photosynthesis,rlbulose bisphosphate carboxylase (RuBPCase), and proteolytic activity were followed in the leaves of field-grown soybeans [Giycine max (L.) Merr. cv. Kent] from flowering through senescence. These parameters were followed in relation to changes in leaf resistance, chlorophyll, protein, starch, total N levels, and seed development. In addition, changes in leaf ultrastructure were observed. The initial symptoms of senescence (evident 3 to 4 weeks after flowering) were a decline in photosynthesis, chlorophyll, and total leaf N and an increase in proteolytic activity. Preceding these changes there was a swelling of the chloroplasts and a disorientation of the chloroplast lamellae, possibly resulting from the apparent increase in starch deposition. Also, large numbers of osmiophilic granules appeared within the chloroplasts.These changes were evident prior to the time the seed entered its most rapid period of growth which was 4 to 7 weeks after flowering, The initial decline in photosynthesis did not appear to be due to an increase in leaf resistance or a decline in RuBPCase activity or level. The decline in protein levels began between 5 and 6 weeks after flowering and was paralleled by the decline in carboxylase activity and level. Associated with these changes were an increase in the size of the osmiophilic granules within the chloroplasts, a decrease in the number of chloroplasts with a corresponding increase in the apparent cellular breakdown products, and a dissolution of the vacuoles. No large increase in leaf resistance or change in specific activity of carboxylase was observed until late in senescence.

The threshold leaf water potential required to initiate stomatal closure in cotton (Stoneville 213) became progressively more negative when plants were subjected to a series of water stress cycles. The shift in the threshold water potential required for induction of stomatal closure was dependent on the number of previous stress cycles and leaf age. The basal level of endogenous abscisic acid (ABA) in fully turgid leaves increased in response to the stress treatments, whereas the amount accumulated in response to a subsequent stress did not differ greatly among plants that had experienced different degrees of stress conditioning.Stomatal sensitivity to (±)-ABA fed through the transpiration stream was enhanced in detached leaves of plants which had experienced repetitive water stresses. The increased sensitivity was apparently the result of ABA synthesized during the stress periods since foliar applications of ABA sensitized stomata in an analogous manner. Furthermore, the amount of (+)-ABA required to initiate stomatal closure in leaves from the various stress treatments was not related to the amounts accumulated during wilting.

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