Multiple Effects of CO 2 Concentration and Humidity on Leaf Gas Exchanges of Sweet Pepper in the Morning and Afternoon

In order to analyze the multiple effects of CO 2 enrichment and humidification on leaf gas exchanges (photosynthetic rate, transpiration rate, and stomatal conductance) in sweet pepper plant and their dependence on the time of day, we measured the steady-state leaf gas exchanges under different CO 2 concentrations (400 and 1000 μmol mol −1) and relative humidities (20, 40, and 60%) in the morning (0900–1130) and afternoon (1230–1500) periods. Increasing CO 2 concentration significantly increased the photosynthetic rate but decreased transpiration rate and stomatal conductance during the both periods. Increasing relative humidity also significantly increased the photosynthetic rate due to improved stomatal conductance at both CO 2 concentrations in the morning. In the afternoon, a similar pattern was observed only at the lower CO 2 concentration, but not at the higher CO 2 concentration because stomatal conductance was extremely low. These results suggest that both CO 2 enrichment and humidification in greenhouses would be effective in the morning; however, in the afternoon, humidification would have little effect under CO 2 enrichment condition, during which photosynthesis was improved but stomata were almost closed.