Effects of water table on physiological traits and yield of soybean .2. Effects of water table and rainfall on leaf water potential and photosynthesis

The effects of constant and fluctuating water tables at 15 similar to 100 cm below the soil surface on the leaf water potential, stomatal conductance and photosynthesis of lysimeter-grown soybean (Glycine mau cv. Tachinagaha) were examined during 1991 (wet year) and 1992 (dry year) in Tsukuba, and during 1992 in Fukuyama. The effects varied greatly with the amount of rainfall. During the wet year in Tsukuba, the apparent photosynthetic rate did not change while the stomatal conductance and leaf water potential decreased with the increasing depth of the water table in dry soil. In contrast, the apparent photosynthetic rate and water potential increased with the increasing depth of the water table in wet soil During the dry year, the photosynthetic rate and stomatal conductance were highest when the water table was 40 cm below the soil surface. The leaf water potential decreased with the increasing depth of the water table both in Tsukuba and Fukuyama. These results suggest that the water table level affects the photosynthesis and water status of soybean leaves, the magnitude of the effect being dependent on the amount of rainfall.