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Gains of the feedback loops involving intercellular CO(2) concentration on one hand, and CO(2) assimilation and stomata on the other (= assimilation loop with gain [G(A)] and conductance loop with gain [G(g)]) were determined in detached leaves of Amaranthus powelli S. Wats., Avena sativa L., Gossypium hirsutum L., Xanthium strumarium L., and Zea mays in the absence and presence of 10(-5)m (+/-) abscisic acid (ABA) in the transpiration stream. Determinations were made for an ambient CO(2) concentration of 300 microliters per liter. In the absence of ABA, stomata were insensitive to CO(2) (G(g) between 0.00 and -0.02) in A. sativa, G. hirsutum, and X. strumarium, sensitive in A powelli (G(g) = -0.46), and very sensitive in Z. mays (G(g) = -3.6). Addition of ABA increased the absolute values of the gain of the conductance loop in A. powelli (G(g) = -2.0), G. hirsutum (G(g) = -0.31), and X. strumarium (G(g) = -1.14). Stomata closed completely in A. sativa. In Z. mays, G(g) decreased after application of ABA to a value of -0.86, but stomatal sensitivity to CO(2) increased for intercellular CO(2) concentrations &lt; 100 microliters per liter. The gain of the assimilation loop increased after application of ABA in all cases, from values between 0.0 (A. powelli) and -0.21 (Z. mays) in the absence of ABA to values between -0.19 (A. powelli) and -0.43 (Z. mays) in the presence of ABA. In none of the species examined did ABA affect the photosynthetic capacity of the leaves.The application of ABA caused stomatal narrowing which affected transpiration more than the assimilation of CO(2). In the case of A. powelli the transpiration ratio decreased without a concomitant reduction of the assimilation rate.

Effect of Abscisic Acid on the Gain of the Feedback Loop Involving Carbon Dioxide and Stomata