Reduction of photosynthesis in sun leaves of Gossypium hirsutum L under conditions of high light intensities and suboptimal leaf temperatures

Abstract

Attached sun leaves of cotton (Gossypium hirsutum L var Delta Pine 61) were enclosed in a gas exchange cuvette and exposed for 4 d to either 1 000 or 1 800 μmol photons m-2s-1 and a series of air temperatures (35, 30, 25 and 20 °C) during the 12-h light periods. The temperature during the 12-h dark periods was maintained at 25 °C. The highest CO2 assimilation rates (Amax: = 30 μmol CO2 m-2s-1) were reached at 1 800 μmol photons m-2 s-1 and air temperatures of 30 and 35 °C. At 30 °C and 1 000 μmol photons m-2s-1, Amax was reduced by 20%. At both light intensities Amax and stomatal conductance declined with decreasing temperatures. The reduction in Amax was most severe at 1 800 μmol photons m-2s-1 and 20 °C. The leaves exposed to these conditions also exhibited marked decreases in rates of CO2-saturated photosynthetic O2 evolution under light saturation (PSmax) and light limitation (Ø) and in FV FM. These 3 parameters were less severely affected by treatment with medium light intensity (1 000 μmol photons m-2s-1). The data show that when sun leaves of cotton are exposed for short periods of time to temperatures only slightly suboptimal for photosynthesis, high light intensities can cause marked photoinhibition. © 1993.