2017 ASHS Annual Conference

There have been many reports that investigate effects of atmospheric humidity on plant growth in many species, but their interactions with other factors are less well known. In the present study, we grew cucumber seedlings hydroponically under two vapor-pressure deficit (VPD) levels, two nutrient-solution water potential (Ψ_s) levels, and two CO₂ concentration levels in controlled-environment chambers, and determined their growth and photosynthetic properties by means of growth analysis and gas-exchange measurements to reveal the potential interactions among these environmental factors. The increased VPD inhibited growth of the seedlings, but the causal factor that inhibited growth differed between Ψ_s and CO₂ concentration levels. Under ambient CO₂ concentration, increased VPD inhibited the seedlings’ growth at high Ψ_s due to decreased leaf expansion, versus decreased CO₂ assimilation per unit leaf area at low Ψ_s. The decrease in intercellular CO₂ concentration (C_i) with decreasing stomatal conductance (g_s) caused by high VPD did not significantly limit CO₂ assimilation at high Ψ_s, but at low Ψ_s the decrease in C_i occurred within the range in which CO₂ assimilation is more strongly limited by C_i because of smaller g­_s. This difference created a VPD × Ψ_s interaction that limited CO₂ assimilation. There was a significant VPD × Ψ_s interaction for leaf expansion; high VPD reduced leaf expansion only at high Ψ_s. At high CO₂ concentration, increased VPD inhibited leaf expansion, but did not affect CO₂ assimilation per unit leaf area. This is because the decrease in C_i with decreasing g_s occurred where CO₂ assimilation was only slightly limited by C_i because of the higher CO₂ concentration level. Such complex interactions resulted from the interactions among these environmental factors on CO₂ assimilation and expansion growth in complicated ways through the resulting changes in the gaseous diffusion and water balance, and through direct responses to water deficit conditions.