Ecophysiological studies on the drought resistance of soybean [Glycine max], 1: Changes in photosynthesis, transpiration and root respiration with soil moisture deficit

Two Korean cultivars (cv. Hwangkeumkong, Danyeobkong) and one Japanese cultivar (cv. Enrei) were grown in the same pot under glasshouse conditions to investigate the influence of water deficit on the photosynthetic rate, transpiration rate and root respiratory rate. The transpiration rate and stomatal conductance rapidly decreased with decreasing of soil moisture compared to the photosynthetic rate. Net photosynthesis remained at a high constant rate during 4 days after non-watering treatment (pF 1.9-3.5), but at further treatment soil moisture deficit (over pF 3.5), net photosynthesis seriously decreased. These observed results were similar for all three cultivars. Different quadratic equations were obtained between net-photosynthetic rate and stomatal conductance, and a linear equation was obtained between transpiration rate and stomatal conductance in both periods of non-watering treatment and re-watering treatment. The photosynthetic and transpiration rates recovered to less than 67% and 49% of the previous levels, respectively. At prolonged suspension of water supply (41-46 days after emergence), a positive correlation was found between the root respiratory rate (25 degrees C) and root nitrogen content. We also recognized that the rate of photosynthesis was significantly correlated with root respiration rate under water stress condition