On the Relationship Between Electron Transport Rate and Photosynthesis in Leaves of the C4 Plant Sorghum bicolor Exposed to Water Stress, Temperature Changes and Carbon Metabolism Inhibition

We examined the effect of carbon metabolism inhibition, temperature, and water stress on the relationship between the linear electron transport and photosynthetic CO2 assimilation in sweet sorghum, Sorghum bicolor (L.) Moench. Carbon metabolism was inhibited either by removing CO2 from the air or by feeding glyceraldehyde to the leaves. Irrespective of the method used, the linear electron transport and photosynthesis were coordinately inhibited. However, when photosynthesis was totally inhibited, a residual electron transport between 20 and 35 μmol m-2 s-1 could be measured. The residual electron transport increased with increasing leaf temperature up to 38ºC and was higher in water-stressed leaves than in control leaves. Temperature affected photosynthesis in intact leaves. The optimal temperature for photosynthesis in control leaves was between 30 and 35ºC. The ratio between linear electron transport and photosynthesis showed a temperature dependency similar to that of photosynthesis. As a consequence, the electrons required to fix one mole of CO2 were 5.5 at suboptimal temperatures but were 6.5 at 30ºC. Our results indicate that the relationship between linear electron transport and photosynthesis is not perfectly steady in nature but is subject to transient changes. The observed changes in the linear electron transport were mostly related to changes in the efficiency of light trapping by open photosystem II (PSII) reaction centres, while the fractions of open PSII reaction centres were relatively constant during the experiment. Water stress severely reduced the photosynthetic CO2 assimilation of sweet sorghum leaves. The greater the water stress, the lower the temperature at which optimal photosynthesis was reached. The linear electron transport was coordinately inhibited by water stress but a residual electron transport was again found when photosynthesis was extremely reduced by water stress. Under water-stress conditions the fraction of PSII reaction centres in an open state was very low but constant, and the temperature dependent reduction of linear electron transport was caused by the reduction of the efficiency of energy capture of PSII reaction centres.