EFFECTS OF WATER-TABLE ON PHYSIOLOGICAL TRAITS AND YIELD OF SOYBEAN .1. EFFECTS OF WATER-TABLE AND RAINFALL ON LEAF CHLOROPHYLL CONTENT, ROOT-GROWTH AND YIELD

The objective of this study was to obtain physiological information for the increasing yield of soybean at drained paddy fields by controlling the water table level. We studied the effects of various water table levels using lysimeter (15cm similar to 100cm constant, fluctuation, no-irrigation) on leaf chlorophyll content, root growth and yield of soybeans grown on alluvial soil, which is a major soil type in the soybean producing area in Japan. Experiments were carried out in 1991 (wet year) and 1992 (dry year) at Tsukuba (1991, 1992) and Fukuyama (1992). Effects of water table on chlorophyll contents varied with leaf position on the main stem. The lower leaves contained more chlorophyll when the plants were grown at lower water table. The upper leaves responded in the same way as lower leaves in wet year, whereas the chlorophyll content was highest at a 40cm depth in dry year. The root length densities in each soil layer were also affected by water table:two peaks of root length density (upper layer and just above the water table) were observed in the 70cm-depth treatment, whereas the peak was observed only at the uppermost layer in the 20cm or 40cm depth treated plots. The effects of water table on yield seem to be affected by the amount of rainfall. The highest yield was brought in by 70cm-depth water table treatment in wet year, and by 40cm in dry year. Fluctuation of water table reduced yield. The results indicate that for getting a higher yield of soybeans at drained paddy field, it is important to maintain water table at a suitable level, which should be adjusted according to rainfall.