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In Situ Measurement of Soil and Plant Leaf Water Potential¹

ABSTRACT

For a pepper plant (Capsicum frutescens L.) growing in a greenhouse, soil water potential was measured with tensiometers and a thermocouple psychrometer, plant water potential was inferred from a calibrated beta gauge, and transpiration was measured by weighing the soil-plant system. Water moved through the transpiration stream in the direction of the gradient of the water potential. Regardless of the distribution of water in the soil, transpiration was unaffected by soil water content until it dropped to a value corresponding to –6 to –8 bars water potential. Below this water content, transpiration decreased linearly with soil water content to a soil water potential of –37 bars. At this time the plant water potential was below –50 bars and the transpiration rate was near zero. After irrigation the plant regained full turgor.

¹ Contribution from the US Salinity Laboratory, Soil & Water Conservation Research Division, ARS, USDA, Riverside, Calif. 92502. This work was supported in part by the Atmospheric Sciences Laboratory, Research Division, US Army Electronics Command, Fort Huachuca, Ariz.

² Soil Scientist, US Salinity Laboratory.

³ Present Address: Department of Soils, University of Wisconsin, Madison, Wis.

Received for publication January 5, 1968. Accepted for publication February 29, 1968.