Effects of Pre-Flowering Soil Moisture Deficits on Dry Matter Production and Ecophysiological Characteristics in Soybean Plants under Well Irrigated Conditions during Grain Filling

Summer field crop plants in Japan would develop large shoots but poor root systems during the rainy season called “Baiu”. This might have adverse effects on dry matter production in summer thereafter even when they grow under sufficient soil moisture conditions. The effects of pre-flowering soil moisture conditions on dry matter production and ecophysiological characteristics were investigated. Soybean plants were grown under sufficient (W plot) and deficient (D plot) soil moisture before flowering in the field. Under sufficient soil moisture conditions after flowering, the plants in the D plot produced higher dry matter due to higher net assimilation rate (NAR) and higher grain yield due to higher pod-flower set ratio and a heavier seed than in the W plot. The higher NAR in the D plot was attributed to (1) a lower resistance to water transport in plants, which is necessary to maintain a high leaf water potential and high photosynthetic rate during the daytime and (2) delayed senescence. The plants in the D plot had a well developed root system, and had roots with high physiological activity represented by a large amount of exudation from the basal cut end of the stem. The development of physiological activity of the root system maybe reflected in higher capacity of root functions, the higher pod-flower set ratio and the delay in the senescence. Improved cultivation practices such as drainage during the rainy season and breeding of the plants with well-developed root system during the rainy season may be necessary to increase the yield of summer crops in Japan. Irrigation during the summer may not be so effective for the plants with a poorly developed root system