EFFECTS OF ROOT TEMPERATURES ON LEAF GAS-EXCHANGE AND GROWTH AT HIGH AIR-TEMPERATURE IN PHASEOLUS-ACUTIFOLIUS AND PHASEOLUS-VULGARIS

Heat stress influences plant growth, development, and productivity. Th is research was initiated to determine whether differential Foot sensi tivity to heat is important in determining differences in plant heat t olerance. Tepary bean (Phaseolus acutifolius Gray) and heat tolerant ( GNUI 59) and heat sensitive (BBL 47) genotypes of common bean (P. vulg aris L.) were used to study leaf gas exchange and growth after exposur e to high air temperature combined with either high or low root temper ature. The effect of temperature on endogenous abscisic acid (ABA) lev els in leaves and roots was determined using an enzymeimmunoassay tech nique. High shoot/root temperature (45/45 degrees C) for 5 h reduced c arbon exchange rate (CER) and subsequent growth in all genotypes. Main taining root temperature at 25 degrees C restored CER to nearly the co ntrol level. High root temperature decreased endogenous ABA levels in the roots in all genotypes, but increased ABA in the leaves in the tep ary bean and BBL 47 without stomatal closure. Water stress or exogenou s ABA pretreatment prior to high temperature treatment induced stomata l closure at high air temperature. There were no differences in leaf t emperatures between the two root temperatures, suggesting that the low er levels of CER, mesophyll conductance, and growth under high root te mperature were dire to root rather than to leaf temperature.