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

Abstract

Carbon dioxide (CO2) concentration and humidity of the air are two extremely important environmental elements that influence photosynthesis in crops (i.e., growth). The difference in CO2 concentration between air and leaf plays a significant role in the photosynthetic rate (Wall et al., 2011), whereas humidity affects photosynthesis through stomatal conductance (Yabuki and Miyagawa, 1970; Nonami et al., 1990). These elements could become serious limiting factors for crop production in greenhouses. This occurs when the CO2 concentration frequently drops below 400 mol mol , mainly because of continuous uptake by crops (photosynthesis) and insufficient inflow of CO2 from outside to the greenhouse with ventilation (Yabuki and Imazu, 1965; Yasutake et al., 2014a), and when greenhouse air dries during daytime because of high air temperature inside the greenhouse (Yasutake et al., 2014b). Therefore, increasing not only CO2 concentration but also humidity (i.e. CO2 enrichment and humidification) are desired for increasing crop production with improved photosynthetic rate and water use efficiency. In particular, recent researches in relation to high-tech greenhouses and/or plant factories have focused on such environmental control technology with reference to multiple environmental elements (e.g., Suzuki et al., 2015; Hidaka et al., 2016). However, crop responses to controlled multiple environmental elements and their mechanisms could be more complicated. For example, in the case of CO2 enrichment and humidification, opposite effects on stomatal movement occurs, where the former induces stomatal closing and the latter induces stomatal opening (Kramer and Boyer, 1995). Furthermore, stomatal movement generally depends on the time of day (e.g., morning and afternoon) (Yoshimoto et al., 2005; Morandi et al., 2014), and therefore, the multiple effects of CO2 enrichment and humidification would also change with the time. However, detailed information on these phenomena is not fully understood yet. In this study, we conducted a fundamental experiment to measure steady-state leaf gas exchanges (photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency, etc.) in sweet pepper under different conditions of CO2 concentration and relative humidity by using a leaf chamber system in the morning and afternoon in a greenhouse. The goals of this study were 1) to analyze the multiple effects of CO2 concentration and humidity on leaf gas exchanges, 2) to analyze the difference in the multiple effects in the morning and afternoon, and 3) to show a strategy to control CO2 concentration and humidity in greenhouses on the basis of the results.