The Temperature Response of Gas Exchange in Sorghum Leaves and the Effect of Heterosis

Abstract

ABSTRACT The aim of this research was to describe the temperature response of gas exchange in grain sorghum (Sorghum bicolor L.Moench) leaves as temperature was increased in a fashion similar to its daily increase in a hot environment. The hypothesis thatsorghum hybrids manifested heterosis for carbon dioxide exchange rate on a wider temperature range as compared with theirparental lines was also evaluated. Gas exchange of detached turgid leaves was measured in four sorghum hybrids and theirparents as leaf temperature rose steadily from 32 °C to 43 °C in 4 h. CER was maximal at about 37—40 C C depending on thegenotype. In one genotype (Tx378), CER was maximal at 42 °C. In three out of four hybrids significant heterosis was found forCER at the lower, the higher or at the full range of temperatures tested depending on the hybrid. Consequently, all threeheterotic hybrids displayed a greater temperature range for high CER as compared with their parents. Heterosis in CER waslargely explained by heterosis in stomatal conductance, though the effects of non-stomatal components could not be elucidatedhere. Heterosis in transpiration was revealed in the same three hybrids, especially at moderate temperatures. Since heterosis inCER was relatively greater than heterosis in transpiration, significant heterosis in transpiration ratio at moderate temperatureswas seen in two of the four hybrids. An effect of heat hardening on CER was observed in parental line Tx430 and its two hybridsas seen in the reduction of CER at 36-37 °C and its subsequent recovery as temperatures rose slowly to 43 °C. The effect of heathardening on photosynthesis was also observed in some of the genotypes as a hysteresis in the association between stomatalconductance and CER as temperatures increased. This hysteresis indicated that, for the same stomatal conductance, CER wasgreater after than before leaves were exposed to high temperature.Key words: Photosynthesis, hybrid vigour, heat stress.