Photosynthetic and transpiration rates of soybean as affected by different irradiances during growth

The diurnal variation of net photosynthetic (PN) and transpiration (E) rates in soybean [Glycine max (L.) Men. cv. Fukuyutaka] plants grown under 100, 50, or 25 % of full sun irradiance (I-100, I-50, I-25 plants) were compared. In the morning, activities of the plants were measured at irradiances under which they grew. However, during the afternoon, all the plants were tested under full irradiance. The lower the growth irradiance, the lower PN, E, and mesophyll conductance values were found. Stomatal conductance was considerably lower in I-25 plants only. Both the increase in specific leaf area (SLA) and the decrease in nitrogen content per leaf area unit contributed to the P-N reduction of soybean plants grown under low irradiances. Though E of the plants grown under different irradiances differed less markedly than P-N, the water use efficiency declined from I-100 to I-25.