Plant responses to high temperatures.

Abstract

Research has shown that there are wide diversities in heat tolerance among crops, and that differences occur not only between crop species, but also among genotypes within the species. Sufficient variability occurs to select for genotypes with high heat tolerance. Under field conditions, drought stress often accompanies heat stress, and there are usually interactions of plant response to these two stresses. Consequently, heat and drought resistance are usually considered together in field response, but each mechanism must be considered separately in order fully to understand the response. An example is given in which sorghum is regarded as having greater stress resistance than corn from field performance. However, results showed that cellular tolerance to high temperatures was higher in corn than in sorghum and higher in pearl millet than in corn. This was true not only of cellular membrane stability, but also of isolated chloroplast activity and photosynthesis of intact leaves under controlled conditions. Results also showed that the drought-avoidance mechanisms were inadequate in corn as compared to sorghum; therefore, corn-leaf tissue may be more frequently exposed to high temperatures owing to decreased evaporative cooling, and the critical limits of high temperature and desiccation tolerance exceeded, resulting in leaf “firing,” even though cellular tolerance is greater than that of sorghum. In pearl millet, both heat tolerance and drought-avoidance mechanisms appear well developed. Tests have shown that heat tolerance frequently correlates positively with desiccation tolerance, but because they do not always correlate, different mechanisms must be involved in each kind of tolerance. Heat hardening occurs under natural conditions and may contribute significantly to total plant performance. Once plants are heat hardened there may be long-lasting effects. The age or stage of development of the plants at which high-temperature exposure occurs may also have a marked influence on the response. The relative resistance to high-temperature injury may change among genotypes as they age. Leaf temperatures in the range of 43°–45°C may have marked effects on photosynthesis of sorghum, with distinct differences shown in genotype response. The response of hybrids to high temperatures may be determined by one or both parents. Photosynthesis by plants selected for high heat tolerance by a leaf-disc technique was shown to be more stable at high temperatures than that of plants with low heat tolerance. The leaf-disc technique may be used for rapid field screening.