The most comprehensive book on whole plant ecology was by Lange, Nobel, Osmond and Ziegler in the Encyclopaedia of Plant Physiology, Part Physiological Plant Ecology, vols. 12A-D (Springer, 1982). Despite being published 20 years ago it is still a landmark in providing understanding of whole plant ecology. The textbook by W. Larcher Ecophysiology of Plants (Springer 4th ed. 2003) and Lambers and Chapin: Plant Physiological Ecology (Springer, 1998) are recommended for further reading.
The thermal relations of plants deal with the balance of radiation energy. Only slightly more than 1% of the incident solar energy is used for photosynthetic metabolism. The remainder of the energy (about 700-1000 Wm-2 at full sun light near the ground) has to be released again because the plant is fixed at the site and absorbs short-wave radiation dependent on its albedo (reflectivity). The energy balance can only be regulated by release of heat to the surrounding air (sensible heat) or by evaporation of water (latent heat). The flow of heat into the soil is too slow to regulate the thermal balance in a leaf or plant organ, with rapidly changing incident radiation. The temperature of organs (particu larly of leaves and flowers) or the temperature of surfaces (stems, flowers) results from the energy balance. The temperature must be kept within certain physiological limits, to avoid damage, and energy balance must be regulated by the plant in such a way that damaging temperatures do not occur, even for short periods. Moreover, the temperature should rather be within the range of the physiological optimum of metabolic processes, which could be above or below the general temperature of the habitat. Plants are able to influence their organ temperatures over a wide range. For example, in the inflorescences of the Araceae, temperatures of ca. 17 K greater than the ambient air are produced via cyanide-resistant electron transport in respiration in order to entice pollinators. In arid climates leaf temperatures may be ca. 17 K below the ambient temperature because of evaporation, thus avoiding heat damage. The temperature of the plant thus is a result of the energy balance and is connected within a certain range to edaphic and climatic conditions.
In the following, the atmosphere and air layers near the ground will be considered as part of the habitat followed by the analysis of the energy balance of a leaf and its effects on plant responses. Ecophysiological responses of plants at extreme temperatures were discussed in Chapter 1.3.
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