EFFECTS OF PRE-FLOWERING SOIL-MOISTURE DEFICITS ON DRY-MATTER PRODUCTION AND ECOPHYSIOLOGICAL CHARACTERISTICS IN SOYBEAN PLANTS UNDER DROUGHT CONDITIONS DURING GRAIN FILLING

Summer crop plants grown in rain-fed fields suffer from drought during a hot and relatively dry summer after the rainy season called ''Baiu' ' in Japan. Since crop plants might develop vigorous shoots with poorl y developed root systems during the rainy season, they might suffer fr om water deficits in the summer, even when soil moisture depletes on t he surface of the soil. In order to confirm this, the effects of pre-f lowering soil moisture deficits on dry matter production and ecophysio logical characteristics thereafter were investigated in soybean plants . Under deficient soil moisture conditions after flowering, higher dry matter production and higher grain yield were attained in the plants grown under deficient soil moisture before flowering (D-plot) than in the plants grown under sufficient soil moisture (W-plot). Large net as similation rate (NAR) was responsible for high dry matter production i n the plants of the D-plot. High NAR in the plants of the D-plot resul ted from (1) a root system that was well developed in deep soil layers , (2) their capacity to absorb much soil water, especially in deep soi l layers, (3) maintenance of a high leaf water potential and, therefor e, a high photosynthetic rate during daytime and (4) a delay in the de crease in photosynthetic rate due to senescence. It was concluded that improved cultivation for developing root systems, such as drainage in the rainy season, as well as irrigation in midsummer, will be importa nt in the cultivation of field summer crop plants in Japan.