Effects of Root Temperatures on Leaf Gas Exchange and Growth at High Air Temperature in Phaseolus acutifolius and Phaseolus vulgaris

Abstract

Heat stress influences plant growth, development, and productivity. This research was initiated to determine whether differential root sensitivity to heat is important in determining differences in plant heat tolerance. Tepary bean (Phaseolus acutifolius Gray) and heat tolerant (GNUI 59) and heat sensitive (BBL 47) genotypes of common bean (P. vulgaris L.) were used to study leaf gas exchange and growth after exposure to high air temperature combined with either high or low root temperature. The effect of temperature on endogenous abscisic acid (ABA) levels in leaves and roots was determined using an enzyme‐immunoassay technique. High shoot/root temperature (45/45°C) for 5 h reduced carbon exchange rate (CER) and subsequent growth in all genotypes. Maintaining root temperature at 25°C restored CER to nearly the control level. High root temperature decreased endogenous ABA levels in the roots in all genotypes, but increased ABA in the leaves in the tepary bean and BBL 47 without stomatal closure. Water stress or exogenous ABA pretreatment prior to high temperature treatment induced stomatal closure at high air temperature. There were no differences in leaf temperatures between the two root temperatures, suggesting that the lower levels of CER, mesophyll conductance, and growth under high root temperature were due to root rather than to leaf temperature.