Physiological effects of heat stress

A principal effect of heat is to accelerate growth and development, shortening the window of opportunity to intercept radiation. As a result of accelerated growth rate, total leaf area available for photosynthesis is frequently reduced also, further reducing yield potential. For example, wheat has been shown to lose 3-4% of yield/°C above the optimum daytime temperature of 15°C (Wardlaw et al., 1989). However, the actual degree of heat stress experienced by a crop depends on the interaction of many environmental and genetic effects (Table 5.1), including evaporative cooling which may vary considerably throughout the crop's life cycle and at a local level. When evaporative cooling is insufficient to maintain plant organs at close to optimal temperatures, the plants will experience metabolic inefficiencies associated with functioning outside optimal temperature ranges (Burke et al., 1988). For example, starch synthase may be rate limiting to grain filling at warmer temperatures (Hurkman et al., 2003) and elevated temperatures also increase wasteful photorespiration in C3 species (Parry and Hawksworth, Chapter 8, this volume). Increased rates of dark respiration are another source of lost productivity at high temperature and remain an important challenge to stabilizing crop productivity in the advent of climate change; even a tropical crop such as rice loses yield potential at warmer night temperatures (Mohammed and Tarpley, 2009). As under water deficit, high temperature stress can also lead directly to sterility by impairing meiosis, gametogen-esis and fertilization (Barnabas et al., 2008; Hedhly et al., 2009). In an agronomic context, heat stress can lead to macronutri-ent deficiency associated with the inability of transport processes to match accelerated growth rates (Rawson, 1986).

Short-term extreme increases in temperature of 5-10°C can have quite catastrophic effects on yield especially if they occur at critical stages of development. This sensitivity is not exclusive to cool season crops but is also observed in relatively heat-adapted crops, such as rice (Wassmann et al., 2009).

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