Effect of Water Supply and Defoliation on Photosynthesis, Transpiration and Yield of Soybean*

Abstract

While increasing leaf area increases C02-fixing ability of plants, it also increases transpiration and may cause water deficit within a plant. This study was conducted in order to analyze the combined effects of leaf area and water-supply treatments on photosynthesis, water status, and yield of pot-grown soybeans (Glycine max [L.] Merr. cv. Tachinagaha). When plants reached the R4-R5 stage, the leaf area of a whole plant was altered by defoliating one, both, or none of the lateral leaflets of each trifoliate, and three levels of water-supply treatments (2100, 1000, 500 ml/day) were given until maturity. Transpiration per plant (Tr/P) was approximately equal to water supply regardless of leaf area under water-restricted conditions (1000, 500 ml/day). With sufficient water supply (2100 ml/day), Tr/P varied with leaf area. Across all the treatments, there was a clear linear relationship (r∼ 0.994) between apparent photosynthetic rate (AP) and stomatal conductance (gs) at one week after the initiation of water-supply treatment. Increasing water supply increased gs and A.P. In water-restricted plants, decreasing leaf area by defoliation raised leaf water potential and caused higher gs and AP of the remaining leaves. Successive water stress of water-restricted plants decreased leaf chlorophyll content and AP at the late ripening stage. Grain yield was reduced by defoliation only under well-watered conditions but was not affected by defoliation in water-restricted plants. The data suggest that water supply proportionate to leaf area is essential to maximizing soybean yield. © 1992, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.