Analysis of Inhibition of Photosynthesis Under Water Stress in the C4 Species Amaranthus cruentus and Zea mays: Electron Transport, CO2 Fixation and Carboxylation Capacity
Two C4 species, Amaranthus cruentus and Zea mays, were studied to evaluate the mechanism of inhibition of photosynthesis due to water stress. The net rate of carbon dioxide fixation (A) and transpiration (E) were measured by gas exchange, and stomatal conductance (gs) and intercellular CO2 (Ci) calculated, while the true rate of oxygen evolution (JO2) was calculated from chlorophyll fluorescence analysis. Following the withholding of water there was a progressive decrease in gs and E during the stress cycle. The results clearly indicate that, initially, Ci decreased with little effect on A (indicating the CO2 pump is providing sufficient CO2 for carbon assimilation), and that the eventual inhibition of photosynthesis by water stress was caused by a limited supply of CO2 to Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase). As A decreased during water stress, the photosystem II activity per CO2 fixed increased, a phenomenon also observed when well watered plants were provided with very low atmospheric levels of CO2, which is indicative of a decreased supply of CO2 to Rubisco. At the same time the RuBP pool/RuBP binding site on Rubisco increased, and the ratio of initial extractable activity of Rubisco to A increased, which suggests that neither RuBP regeneration nor Rubisco capacity is limiting photosynthesis. When plants were rewatered after photosynthesis had dropped to 5-10% of the original rate, both species showed near full recovery in 2-4 days.