Short-term (one hour) application (painting on surfaces of leaves) of 9 muM GA(3) increased net photosynthetic rate (Pn) in broad bean leaves at 31 Pa CO(2) and saturating light by more than 20% compared with that of control. The increased Pn was accompanied by an increase in stomatal conductance and a decrease in intercellular CO(2) partial pressure. Moreover, the GA(3)treatment increased the rate of photosynthetic oxygen evolution in isolated broad bean protoplasts to an extent similar to that of leaves. It had little effect on apparent photosynthetic quantum yield and photosynthetic electron transport rate, but could significantly increase carboxylation efficiency in leaves. In consonance with the increase in the carboxylation efficiency, RuBPCase activity and relative content of Rubisco large subunits were also increased by GA(3) treatment. Chloramphenicol, an inhibitor of chloroplast protein synthesis, could eliminate the enhancing effect of GA(3) on photosynthetic oxygen evolution and relative content of Rubisco large subunits in broad bean protoplasts. Nevertheless, actinomycin D and rifampicin, DNA transcription inhibitors, could not eliminate the enhancement effect of GA(3). Similar results were obtained with soybean leaves treated by 90 muM GA(3). These results suggest that the increase in leaf net photosynthetic rate caused by GA(3) short-term treatment is mainly due to the increases in content and activity of RuBPCase, and that GA(3) stimulates the synthesis of Rubisco subunits at translation rather than transcription level.