Nonstomatal Inhibition of Net CO2 Uptake by (±) Abscisic Acid in Pharbitis nil

Abstract

(.+-.) Abscisic acid (ABA) injected into petioles of attached transpiring leaves of P. nil Chois, cv. violet reduced the photosynthetic capacity of the mesophyll of these leaves as well as the stomatal conductance to CO2 diffusion. Greater than 75% of the injected ABA was recovered as ABA, suggesting that ABA rather than some metabolic thereof was the active compound. The nonstomatal effect of ABA increased from 30% reduction in photosynthesis at 0.25 .mu.M ABA in the leaf blade to 90% reduction at 18 .mu.M. Despite the effect of ABA on the nonstomatal component of leaf net CO2 uptake, it was calculated that a substantial part of the reduction in leaf net CO2 uptake (50-80%) could be accounted for by the effect of ABA on stomatal conductance.