Temperature is a critical factor in plant growth. Consequently, the distribution of species throughout the world, both as natural populations and as crop plants, is influenced by temperature. It is a plant’s ability to respond to environmental change that helps determine its ecological niche and often affects its usefulness to humans. In the absence of other biotic and abiotic constraints, plant growth and subsequent yield is maximal when a crop is cultivated as close to its temperature optimum as possible. However, the physical environment is neither constant nor optimal for crop production; fluctuations, often extreme, occur both in time and space. Such changes occur over the different seasons of the year and often more rapidly over the course of an individual day. Longer-term environmental changes can also occur, such as appears to be occurring at present due to increased greenhouse gas emissions. Current climate models project that the mean annual global surface temperature will increase by 1.7 to 3.8°C by 2100 (Wigley and Raper, 1992). Such changes in temperature could have radical effects on agriculture. Relatively small changes in the mean temperature could have a marked effect on the frequency of extreme temperature events occurring. For example, the number of very hot days, i.e., above a particular threshold, which can cause damaging heat injury to plants, could increase markedly as a result of a small increase in mean temperature. The effect of global warming shows considerable regional variation and it is likely that developing countries will be affected to a much greater degree (Rosenzweig and Parry, 1994). As many agricultural environments in such countries are already marginal and vulnerable to interannual climate fluctuations, this will result in increased food insecurity in such regions. Developing countries are also highly vulnerable to climate change because many are located in arid and semiarid regions. In the tropics and subtropics, where some

crops are near their maximum temperature tolerance and where dryland, nonirrigated agriculture predominates, yields are likely to decrease. The livelihoods of subsistence farmers and pastoral peoples, who make up a large portion of rural populations in some regions, also could be negatively affected. This indicates the enhanced requirement to improve the adaptation of plants to their temperature environment and to understand the mechanisms by which such adaptation occurs.