Abscisic Acid Simultaneously Decreases Carboxylation Efficiency and Quantum Yield in Attached Soybean Leaves

Injecting abscisic acid (ABA) into the petiole of attached soybeanleaves simultaneously decreased quantum yield and carboxylationefficiency. These decreases were proportional to the accompanyingdecrease of light-saturated photosynthetic rate under normalatmosphere. ABA did not affect the CO² compensation point atsaturating irradiance, or the stimulation of net photosynthesisproduced when the partial pressure of oxygen was reduced from210 to 20 mPa Pa^–1 at a rate-limiting irradiance. Thedecreased carboxylation efficiency could not be attributed toa deactivation of RuBP carboxylase since the activation statusof the enzyme in leaves was insensitive to ABA. Although photosyntheticcapacity was decreased, the content of RuBP present was increasedin leaves treated with ABA at saturating irradiance and normalatmosphere. The increase in RuBP content was proportional tothe corresponding inhibition of photosynthetic capacity. Followingexposure to incident irradiances close to the light compensationpoint, the photosynthetic rate of ABA-treated leaves did notincrease when exposed to a subsequent increase of irradiancein the light-limited range. This response was not due to stomatalclosure. Key words: Abscisic acid, quantum yield, RuBP, RuBP carboxylase, carboxylation efficiency