Interactive Effects of Soil Water Regime with Above-Ground Conditions on Photosynthesis in Wheat Plants : II. Combined effects of soil water deficit with low air humidity

Effects of soil water deficit, low air humidity and in combination were examined at the milky stage under controlled conditions. Low air humidity which plants experienced prior to measurement affected leaf photosynthesis in a manner different from effects of soil water deficit, in regard of the order of depression extent among leaves, i.e., photosynthetic depression by soil water deficit was in the order of flag leaf 2nd leaf < 3rd leaf. Stomatal (gs) and mesophyll (gm) conductances for CO2 diffusion, and Rubisco content varied in parallel with photosynthesis, though there was one exception. It was suggested that the above-mentioned responses of photosynthesis were caused by the corresponding depression in stomatal aperture and photosynthetic activity of mesophyll. Among the three leaves examined, little difference was observed in decrease of leaf water potential (ψ) caused by soil water deficit, while the decrease in ψ by precedent low air humidity was especially large in the flag leaf. The lower the leaf position, the larger the sensitivity of gs, gm, and Ribisco content to decrease in ψ was. Thus, difference in the extent of photosynthetic depression by soil water deficit among the uppermost three leaves was attributed mainly to different sensitivity to decrease in ψ, while that by dry air was attributed to the difference in the extent of decrease of ψ in addition to sensitivity differences. The effect of low soil water and low air humidity on photosynthesis seemed to be additive when combined.