Photosynthetic Assimilation of14CO2by Waterstressed Sunflower Leaves at Two O2Concentrations and the Specific Activity of Products

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Abstract
Attached leaves of sunflower (Helianthus annuus L. var. Mennonite) with water potentials of −5 to −18 × 105 Pa, were exposed for different times to 300 vpm CO2 containing 14CO2 and 21 or 1.5% O2. 14C accumulated linearly with time in both O2 concentrations and at all stresses. 3-Phosphoglyceric acid was saturated with 14C after 10 min in unstressed plants at both O2 concentrations but with increasing stress the rate of accumulation and the specific activity decreased. With decreasing leaf water potential there was accumulation of radioactivity in the glycolate pathway intermediates glycine and serine. Other amino acids contained a slightly larger proportion of assimilated carbon as water potential decreased. The specific activities of all compounds were smaller with stress. In contrast to the amino acids less radioactivity accumulated in sugars, organic acids, and sugar phosphates and their specific activities decreased with stress. The radioactive labelling patterns and specific activity measurements are interpreted as showing increased carbon flux in the glycolate pathway and inhibition of the metabolism of serine to sucrose. These changes are related to previous results showing that with stress photo respiration increases as a proportion of photosynthesis. Lowering the O2 concentration to 1.5% decreased the accumulation of radioactivity in glycine and stopped photorespiration. It increased the amount of radioactivity in serine and sucrose but did not greatly change specific activities. Oxygen effects were independent of water stress. Glycolate pathway metabolism is discussed in relation to photorespiration and the effects of water stress.