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

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Abstract
Injecting abscisic acid (ABA) into the petiole of attached soybean leaves simultaneously decreased quantum yield and carboxylation efficiency. These decreases were proportional to the accompanying decrease of light-saturated photosynthetic rate under normal atmosphere. ABA did not affect the CO2 compensation point at saturating irradiance, or the stimulation of net photosynthesis produced when the partial pressure of oxygen was reduced from 210 to 20 mPa Pa−1 at a rate-limiting irradiance. The decreased carboxylation efficiency could not be attributed to a deactivation of RuBP carboxylase since the activation status of the enzyme in leaves was insensitive to ABA. Although photosynthetic capacity was decreased, the content of RuBP present was increased in leaves treated with ABA at saturating irradiance and normal atmosphere. The increase in RuBP content was proportional to the corresponding inhibition of photosynthetic capacity. Following exposure to incident irradiances close to the light compensation point, the photosynthetic rate of ABA-treated leaves did not increase when exposed to a subsequent increase of irradiance in the light-limited range. This response was not due to stomatal closure.