Effect of High-Temperature Stress on the Photosynthetic Apparatus

Abstract

The action of physiological stresses on plants has been a subject of numerous studies. Plants are able to survive and grow even under unfavorable environmental conditions. Living organisms can hardly survive below 3°C, because most of the biochemical reactions are hampered owing to the high content of water in the living tissues. Some plant species can grow under desert conditions and relatively high temperatures, but most species are sensitive to a variation of a few degrees below or above their normal growth temperature. In the higher temperature zone (37–47°C), tolerance mechanisms may allow some extent of adaptation. Hence, photosynthetic organisms can tolerate temperatures above their normal growth temperature. The latter may be very high for desert plants but relatively lower for other species. Each species has its own optimal temperature and its lower and higher temperature limits. When exposed to changing temperatures, various structural modifications are known to happen at the molecular level. Such modifications include changes in the rate of metabolic reactions as well as modifications of subcellular structures. A heat shock is induced when a plant is brought near to its higher temperature limit for growth. Of course, even a relatively weak temperature variation can alter the normal cellular biochemical processes which could be considered as a heat stress. However, an increase of 10–15°C above normal growth temperature will cause a deeper modification of growth without being necessarily lethal. Those changes involve protein denaturation, enzyme inactivation, and more specifically a reduction in the chloroplast’s photosynthetic activity. In this chapter, the action of high-temperature stress at the level of the photosynthetic apparatus will be examined. Heat stress induces significant modifications in the composition of the chloroplast membrane lipids and proteins together with structural changes of the thylakoid membranes. Those changes greatly affect the activity of the photosystems. However, some adaptation mechanisms can prevent excessive damage if plants are preexposed to high temperature. Acclimation