In order to analyze the multiple effects of CO2 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 CO2 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 CO2 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 CO2 concentrations in the morning. In the afternoon, a similar pattern was observed only at the lower CO2 concentration, but not at the higher CO2 concentration because stomatal conductance was extremely low. These results suggest that both CO2 enrichment and humidification in greenhouses would be effective in the morning; however, in the afternoon, humidification would have little effect under CO2 enrichment condition, during which photosynthesis was improved but stomata were almost closed.
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