Summer crop plants grown in rain-fed fields often suffer from drought during a hot and relatively dry summer in Japan, even though there is much precipitation during their growing seasons compared with the semi-arid regions. Crop plants could withstand soil moisture deficiency by means of developing elongated root system under conditions where soil moisture decreases gradually. However, after they have grown under sufficient soil moisture conditions, they could not adapt under conditions of rapid moisture depletion due to shallow and poorly developed root systems and, therefore, would suffer from water deficits even when soil moisture does not deplete considerably. In many regions of Japan, summer field crop plants grow at their best during the vegetative phase in the rainy season called “Baiu” immediately before the hot and dry summer. This might be one of the reasons why they suffer from water deficits in the summer. This assumption has been confirmed by the experiment using the soybean plants ;(1)grown under moderate deficient soil moisture before flowering and (2) grown under sufficient soil moisture. Under severe deficient soil moisture conditions during ripening stage, high dry matter production and high grain yield were attained in the former plants due to (1)a well developed root system and, therefore, high capacity to absorb soil water, (2) maintenance of a high leaf water potential and, therefore, a high photosynthetic rate during daytime and (3) a delay in leaf senescence compared with the latter. The plants with well developed root system also attained higher dry matter production and higher grain yield under well irrigated conditions during ripening stage for the same reasons as in the experiment mentioned above. During the growing seasons of winter crop plants, a rainy season suppressing root system development also exist. Improved cultivation for developing root system, such as drainage in the rainy seasons, would be important in field summer and winter crop plants in Japan.
