Water is essential to life as we know it on earth. Water acts as a medium for carrying nutrients, ions, and suspended particles from the soil into a plant and as a conducting system within a plant. Most organic tissue is permeable to water and when the external water vapor pressure is less than that inside a plant, water will evaporate from it. Plant leaves have evolved stomates or pores through which to take in carbon dioxide from the surrounding air, but since the water molecule is of lighter molecular weight than the carbon dioxide molecule, it tends to escape readily from the leaf through the stomates. Normally the air in the intercellular spaces within a leaf is at or near saturation. This process of water evaporation from a plant is known as transpiration. Energy is required to convert liquid water to vapor. As a consequence of transpiration, energy is lost by a leaf and the temperature of the leaf is reduced.
Temperature is extremely important to the physiological processes within a leaf including photosynthesis, respiration, growth, cell enlargement and division, protoplasmic streaming, and translocation. Most chemical and biological processes are temperature-dependent and as a general rule, they proceed slowly at relatively low temperatures, speed up in reaction with increased temperature, and at high temperatures become limited or terminated by adverse reactions such as the denaturation of proteins and the breakdown or dissociation of molecules. A typical biological response with temperature is illustrated in Figure 11.1 for net photosynthesis. Many temperature zone plants have a maximum rate of photosynthesis at about 25 to 30°C while plants of arctic or alpine habitats have an optimum temperature of about 15°C and some plants of extremely hot regions have optima near 40 or 45°C (e.g., Lange et al. 1975, Bjorkman et al. 1972, Billings et al. 1971). When a plant of a cool habitat is functioning optimally at a particular temperature, a plant of a warm habitat will scarcely function at all at that same temperature. Many plant leaves undergo irreversible thermal damage when their temperatures exceed 45 to 48°C while some will not be damaged at 50 or even 55°C and others will be severely denatured at 42°C. Therefore, the temperature of a leaf is always of enormous significance to the viability of it. However, when one wishes to know the temperature of a plant or, in particular, the temperature of a leaf, it is important to ask why and with what accuracy. Is it necessary to know the leaf temperature to an accuracy of 0.1, 0.5, or 1°C, and why? The answer to this question will often determine the method of measurement and the precision necessary, or it will guide one with respect to the detail required in an energy budget calculation.
The temperature of a leaf is the result of energy flow between it and its surroundings. A leaf exchanges energy by the basic physical processes of radiation, conduction, convection, and evaporation, all of which go on simultaneously. If a leaf takes in more energy than it puts out, its temperature increases and if it loses more than it gains, the temperature will decrease. When energy in equals energy out, the leaf will maintain a constant temperature.
Once the processes by which a leaf exchanges energy are recognized, it is straightforward to identify the properties of the environment which must be known. In fact, there are only two ways by which an environment interacts with an organism, including a plant leaf, and that is by energy flow or mass flow. In this module, we are dealing with both since the exchange of heat is an energy flow process and the diffusion of water vapor is a gas exchange process which also involves the transport of energy.
The remainder of the module is devoted to developing and analyzing simple models of how a leaf exchanges thermal energy with the environment. Although our knowledge of leaf morphology and physiology is more detailed than what is included in the models, the level of description here is sufficiently general to provide valuable insights.
In the following pages, the reader is formally introduced to the leaf energy budget. Typical values for environment variables and leaf parameters are discussed. Some simple calculations are then made to see how radiation, convection and transpiration affect leaf temperature.
The graphical method of analysis used by Gates (1968) is then employed to understand a more detailed energy budget model. Problems with accompanying solutions conclude the chapter.