Abstract
Net photosynthetic rates and leaf conductances to water vapor were measured in Abutilon theophrasti, Amaranthus hypochondriacus, and Glycine max after injecting abscisic acid into petioles of attached leaves. Changes in substomatal carbon dioxide partial pressure were measured in air in each species for various degrees of inhibition of the photosynthetic rate. Changes in the initial slope of photosynthesis versus substomatal carbon dioxide partial pressure were also characterised in two of the species. Responses to abscisic acid were compared with responses to water stress. Abscisic acid reduced photosynthetic rate and conductance in air in proportion to the amount applied. In no case was the relative decrease in substomatal carbon dioxide partial pressure as large as the relative decrease in photosynthesis. This indicated that nonstomatal inhibition of photosynthesis occurred. The changes in substomatal carbon dioxide partial pressure as photosynthesis was progressively inhibited and the changes in initial slope of photosynthesis versus substomatal carbon dioxide partial pressure were distinct for each species and were the same for abscisic acid and for water stress. The data strongly suggest that the reductions of both photosynthetic capacity and conductance by water stress are mediated by abscisic acid in these species.
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