MODERATE‐TERM WATER STRESSES AND ASSOCIATED CHANGES IN SOME PHOTOSYNTHETIC PARAMETERS IN COTTON

Abstract

SUMMARYCotton plants (Gossypium hirsutum L., var Deltapine smoothleaf) were subjected to 1‐ or 2‐week periods of mild water stress using an osmotic soil cell. The treatments were severe enough to affect seriously several photosynthetic parameters, while good night‐time recovery of plant water‐potential minimized effects on development and leaf expansion.Most measurements were made soon after rewatering, owing to very high stomatal resistances in the stressed plants. The stressed plants had lower rates of potential photosynthesis and lower rates of actual photosynthesis (at normal ambient CO2 concentrations) than the controls. Both stomatal and intracellular resistances and the corresponding ‘photosynthetic limitations’ were greater in the stressed plants than the controls. Most of these parameters showed fairly complete recovery 24 hours after rewatering. Parallel changes were observed in the activities of carbonic anhydrase, ribulosediphosphate carboxylase and 14C fixation by leaf slices, all of which were lower in the stressed plants.Although activities of all the processes studied showed parallel changes, there was evidence that the major factor causing reduced photosynthesis in the stressed plants was stomatal closure. There was some evidence that the photosynthetic system adjusted itself to water stress over the periods studied.