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

Abstract

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 mol 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.

A Preliminary Experiment on the Effects of Leaf Wetting on Gas Exchange in Tomato Leaves

“…In fact, GS was decreased by 28% at 14:00 compared to that at 11:00. The depression of GS in the afternoon could be due to high leaf-to-air water vapor pressure deficit (Nonami et al, 1990), as this could induce plant water deficit through excessive transpirational water loss (Kitano and Eguchi, 1993;Yasutake et al, 2016). As a result, A was decreased by 21% at 14:00 compared to that at 11:00.…”

“…This means that midday depression of photosynthesis may often occur in crop production, and could be a limiting factor for crop yield. Midday depression would be induced by stomatal closure, because excessive transpirational water loss by high leaf-to-air vapor pressure deficit leads to stomatal closure on sunny days even though plants were well watered (Kitano and Eguchi, 1993;Hu et al, 2009;Yasutake et al, 2016). Although other factors are reported as causes of midday depression, such as photoinhibition (Long et al, 1994;He et al, 2007), and carbohydrate accumulation in leaves (Araya et al, 2006), stomatal closure is one of the major causes of midday depression, or at least partially involved in midday depression.…”