Effects of day-to-day changes in root temperature on leaf conductance to water vapour and CO2 assimilation rates of Vigna unguiculata L. Walp.

Leaf gas exchange of Vigna unguiculata was influenced by short-term (day-to-day) changes in soil temperature and the response depended upon the aerial environment. When aerial conditions were constant at 30° C leaf temperature, high air humidity and moderate quantum flux, CO2 assimilation rate and leaf conductance increased with increases in soil temperature from 20 to 35° C, and this response was reversible. Decreases in CO2 assimilation rate and leaf conductance were observed at root temperatures above 30° C when root temperatures were increased from 20° C to 40° C and when air humidity was decreased in steps during the day. In contrast, varying soil temperatures between 20 to 35° C had no influence on gas exchange when shoots were subjected to a wide range of temperatures during each day.The gain ratio ∂A/τE remained constant at different air humidities when root temperature was less than or equal to 30° C indicating optimal gas exchange regulation, but changed with humidity at higher root temperatures. Leaf conductance responded independently from leaf water potential which remained relatively constant during individual experiments.The results indicate that plant responses to high root temperatures may have relevance to plant performance in semi-arid environments. They also illustrate the importance of controlling soil temperatures when studying the responses of potted plants in controlled aerial environments.