ANALYSIS OF INHIBITION OF PHOTOSYNTHESIS UNDER WATER-STRESS IN THE C-4 SPECIES AMARANTHUS-CRUENTUS AND ZEA-MAYS - ELECTRON-TRANSPORT, CO2 FIXATION AND CARBOXYLATION CAPACITY

Two C-4 species, Amaranthus cruentus and Zea mays, were studied to eva luate the mechanism of inhibition of photosynthesis due to water stres s. The net rate of carbon dioxide fixation (A) and transpiration (E) w ere measured by gas exchange, and stomatal conductance (g(s)) and inte rcellular CO2 (C-i) calculated, while the true rate of oxygen evolutio n (J(O2)) was calculated from chlorophyll fluorescence analysis. Follo wing the withholding of water there was a progressive decrease in g(s) and E during the stress cycle. The results clearly indicate that, ini tially, C-i decreased with little effect on A (indicating the CO2 pump is providing sufficient CO2 for carbon assimilation), and that the ev entual inhibition of photosynthesis by water stress was caused by a li mited supply of CO2 to Rubisco (ribulose-1,5-bisphosphate carboxylase/ oxygenase). As A decreased during water stress, the photosystem II act ivity per CO2 fixed increased, a phenomenon also observed when well wa tered plants were provided with very low atmospheric levels of CO2, wh ich 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 ra tio of initial extractable activity of Rubisco to A increased, which s uggests that neither RuBP regeneration nor Rubisco capacity is limitin g photosynthesis. When plants were rewatered after photosynthesis had dropped to 5-10% of the original rate, both species showed near full r ecovery in 2-4 days.