Method of Water Potential Measurement and Some Results of Experiment with That Method

Abstract

Soil, plant and atmosphere form a physically unified system in which water flow takes place from higher to lower potential energy of water. To understand the water flow through the plant as a part of water flow in the system, the water potential based on energy concept, which was proposed by Slatyor and Taylor in 1960, is the most desirable unit. Therefore, we presented a method of measuring the water status by using a thermocouple psychometer, which can measure water potential comparatively easily. We also examined the effect of some environmental factors on the water potential of the plant, and showed the results. The time required for a thermodynamical equiliblation of vapor pressure within the sample chamber was different according to sample materials. We made a caliblation check on each of the three apparatus, and concluded all of them to be reliable. We measured the component potentials of organs of rape. Osmotic potential was measured with the freezing and thawing method. Relationship of osmotic potential and water potential was different between organs. By using corn stems, we examined estimating the osmotic potential from its sap water potential. As a result, this method was found to be usefull for speedy measurement. We examined the relation between the water content and the water potential of Kuriyagawa volcanic ash soil and Ishidoriya alluvial soil. There was found a clear difference between these two soils. Futhermore, we examined the effects of the water potential of soil, atmosphere and light intensity on the water potential of the plant. It was thought that soil water potential decided the highest value of water potential of plant, and water potential of atmosphere controlled the diurnal change of plant water potential. Light intensity also affected the water potential of plant through the effect of stomatal aperture, but it was not so great. The diurnal change of the water potential gradient in the soil-plant-atmosphere system was observed on a hot summer day. In the daytime, the water flow was controlled mainly by the water potential of atmosphere. In the evening, the water deficiency of soil around the roots came to restrict the water flow in the system. In the night, there appeared water potential gradient again with the recovery of water stress of soil around the roots, and this was regarded to indicate that water was flowing through the plant at a little rate.