THE LOSS OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE CAUSED BY24-HOUR RAIN TREATMENT FULLY EXPLAINS THE DECREASE IN THE PHOTOSYNTHETIC RATE IN BEAN-LEAVES/

Recently, we have found that simulated rainfall causes a chronic inhib ition of leaf photosynthesis in Phaseolus vulgaris (M. Ishibashi and I . Terashima [1995] Plant Cell Environ 18: 431-438). Mechanisms of this inhibition were examined in the present study. After the plants were treated with continuous mist for 24 h and then dried to unwet conditio ns, light-saturated photosynthetic rates of the leaves measured at 35 Pa ambient CO2 decreased to one-half of the control level. The extract able activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubi sco) also decreased to the same extent. Unexpectedly, this decline was due not to the lowered activation state but to the decrease in the am ount of Rubisco. Before or after the ''rain'' treatment, the relations hip between the net photosynthetic rate and the amount of Rubisco was expressed as a unique linear function with a small intercept (r(2) = 0 .84). From these it was inferred that the main cause of the rain-induc ed decline in photosynthetic rate was the loss in amount of Rubisco.