Diurnal Course of Transpiration Rate, Stomatal Aperture, Stomatal Conductance, Xylem Water Potential and Leaf Water Potential in the Rice Plants under the Different Gorwth Conditions

Abstract

It was suggested from the previous investigation that the photosynthetic rate affected by gas exchange rate would be different due to the stomatal aperture in rice plants under different growth conditions even though the rice plants grew under submerged soil condition. The present study was conducted to investigate the difference in gas exchange rate in rice plants under different growth conditions through the measurement of diurnal course of transpiration rate, stomatal aperture and conductance, and xylem and leaf water potentials (Ψ_x and Ψ₁). There were four plots prepared. One was a control, the others were as follows; a plot with additional ammonium sulfate applied to soil several days before measurement to increase nitrogen content in leaves; a plot with soluble starch and additional ammonium sulfate applied to soil several days before measurement to make soil reductive and to decrease root activity without the decrease of nitrogen content in leaves; a plot grown under shading and high humidity for several weeks to decrease the root-top ratio and root-leaf area ratio. The stems of the rice plant in a pot were kept apart from each other by spreading outward with cotton thread in order to minimize mutual shading, so that all leaves on each stem were exposed to the same solar radiation and wind speed when transpiration rate was measured by weighing the pot. Therefore the difference of gas exchange rate through stomata among rice plants can be estimated by simultaneous measurements of transpiration rate. Transpiration rate was higher and stomatal aperture and conductance were larger almost all day long in rice plants with high nitrogen content in leaves induced by additional ammonium sulfate than those in the control. The difference in transpiration rate and stomatal aperture between the two plots was larger in the "Morning" than that in the "Afternoon". Transpiration rate was higher in the afternoon due to high solar radiation, atmospheric vapor pressure deficit and wind speed, so that Ψ₁ was lower in the "Afternoon" than that in the "Morning". It is clear from these results that gas exchange rate through stomata increases due to high nitrogen content in leaves, especially in the "Morning" when rice plants were undcr little water stress. In the rice plants with low root activity induced by the application of soluble starch to soil and with low root-top ratio and root-leaf area ratio induced by shading and high humidity, transpiration rate and stomatal aperture were much lower than those in the control in the "Afternoon." Ψ_x and Ψ₁ were lower in these rice plants than those in the control even though transpiration rate was lower in the former. These results indicate that water absorption decreases due to low root activity and poor development of root system, and it is considered that gas exchange rate through stomata decreases remarkably due to severe water stress under high radiation and atmospheric vapor pressure deficit in rice plants with low root activity and poor development of root system, even though they grew under submerged condition with enough water supply. From the results mentioned above and by the consideration of high correlation between gas exchange rate through stomata and photosynthetic rate, it is suggested that The lncrease of photosynthetic rate owing to higher nitrogen content appears remarkably In the case without or with little water stress, and the decrease of photosynthetic rate owing to the decrease of water absorption appears remarkably under the condition which brjngs on intense transpiration.