Light as a Source of Error in Estimates of Water Potential by Vapor Equilibration

A common method for estimating plant water potential is the vapor equilibration technique. This procedure, described originally by Archichovskij and Archichovskaja (1), and modified by Slatyer (4) and Kreeb (3), involves equilibration of plant tissue in the vapor phase above solutions of sucrose, mannitol, or salts of known concentration. Humidity jars or microdesiccators, containing the plant tissue and control solutions, are immersed in a temperaturecontrolled water bath, for maintenance of a constant water potential, during the equilibration period. Sensitive control of the water-bath temperature required in this procedure is normally accomplished by using one or more incandescent light bulbs as a source of intermittent heat. Although Kaufmann and Kramer (2) have reported on the importance of respiration and the heat of respiration during the equilibration period, no mention has been made in the literature regarding the influence of light from incandescent bulbs used to control the water-bath temperature. Since the microdesiceators are clear glass laboratory jars which permit incident light to be absorbed by the plant tissue inside, it would seem important to assess the influence of light in estimates of plant water potential using the vapor equilibration procedure. This paper reports the results of such a study.