STUDIES ON THE EFFECT OF THE RELATIVE-HUM IDITY OF THE ATMOSPHERE ON THE GROWTH AND PHYSIOLOGY OF RICE PLANTS - EFFECTS OF AMBIENT HUMIDITYIN THE DARK PERIOD ON THE GROWTH AND THE TRANSLOCATION OF C-13-LABELED PHOTOSYNTHETIC PRODUCTS

Rice seedlings (age in leaf number of 3) were grown for IO days at 75% relative humidity in the light period (12-hr) and at 60, 75 or 90% hu midity in the dark period (12-hr). Dry matter production of the plants grown under higher humidity in the dark period was higher than that o f the plants grown under lower humidity. High humidity in the dark per iod also increased the leaf area, heightened the plant length and acce lerated leaf emergence. Then, the effects of humidity in the dark peri od on the C-13-labelled CO2 assimilation, translocation of C-13-labell ed photosynthetic products, water content and respiration were examine d. The amount of C-13 assimilated by plants grown under high humidity in the dark period was greater than that by plants grown under low hum idity in the dark period. High humidity in the dark period brought abo ut a greater transfer of C-13-labelled photosynthetic product to the y oungest (late-emerging) leaf than low humidity. High humidity in the d ark period also induced an increase in water content, which resulted i n the promotion of photosynthesis. Respiration in the dark was not inf luenced by humidity. Therefore, the promotion of increased leaf area, increased plant height and accelerated leaf emergence, which were obse rved In plants grown under high humidity in the dark period, is due to increased photosynthesis and the translocation of photosynthetic prod uct to the late-emerging leaves in the plants. These results indicate that humidity in the dark period plays an important role in the growth and development of rice seedlings.