The spectral properties of plant leaves and stems have been obtained for ultraviolet, visible, and infrared frequencies. The spectral reflectance, transmittance, and absorptance for certain plants is given. The mechanism by which radiant energy interacts with a leaf is discussed, including the presence of plant pigments. Examples are given concerning the amount of absorbed solar radiation for clear sky and overcast conditions. The spectral properties of desert plants are compared with those of more mesic plants. The evolution of the spectral properties of plant leaves during the early growing season is given as well as the colorimetric behavior during the autumn.

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These materials were designed to be used by life science students for instruction in the application of physical theory to ecosystem operation. Most modules contain computer programs which are built around a particular application of a physical process. This report introduces two models of the thermal energy budget of a leaf. Typical values for environment variables and leaf parameters are discussed and simple calculations are made to see how radiation, convection, and transpiration affect leaf temperature. A graphical method of analysis is used to present a more detailed energy budget model. A problem set and an accompanying computer program called TRANS permit the student to explore the consequences cf the models. Algebra and some knowledge of heat transfer physics are prerequisites. (Author/CS)

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The environmental factors affecting the flow of energy between a plant and its environment are described. These factors are solar and thermal radiation, air temperature, water vapor density of the air, and wind speed. The mechanisms of radiation, convection, and transpiration which transfer energy between the plant and the environment are expressed in analytical form. An example is given of a 24—hour cycle for a plant illustrating the daily variation of each of these factors and of the resulting plant temperature. Basic plant properties, such as absorptance to radiation, convection coefficient, and water vapor diffusion resistance, determine the extent to which the environment influences the energy content and temperature of the plant. Photosynthesis is light and temperature dependent, and other physiological processes are temperature dependent only. Maxima and minima in photosynthetic activity occur during a day as a consequence of changes in light intensity and leaf temperature produced by varying environmental conditions. The ecological significance of these environmentally influenced physiological processes is enormous in terms of productivity and competition.

A biochemical model of C 3photosynthesis has been developed by G.D. Farquhar et al. (1980, Planta 149, 78-90) based on Michaelis-Menten kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase, with a potential RuBP limitation imposed via the Calvin cycle and rates of electron transport. The model presented here is slightly modified so that parameters may be estimated from whole-leaf gas-exchange measurements. Carbon-dioxide response curves of net photosynthesis obtained using soybean plants (Glycine max (L.) Merr.) at four partial pressures of oxygen and five leaf temperatures are presented, and a method for estimating the kinetic parameters of RuBP carboxylase-oxygenase, as manifested in vivo, is discussed. The kinetic parameters so obtained compare well with kinetic parameters obtained in vitro, and the model fits to the measured data give r (2)values ranging from 0.87 to 0.98. In addition, equations developed by J.D. Tenhunen et al. (1976, Oecologia 26, 89-100, 101-109) to describe the light and temperature responses of measured CO2-saturated photosynthetic rates are applied to data collected on soybean. Combining these equations with those describing the kinetics of RuBP carboxylase-oxygenase allows one to model successfully the interactive effects of incident irradiance, leaf temperature, CO2 and O2 on whole-leaf photosynthesis. This analytical model may become a useful tool for plant ecologists interested in comparing photosynthetic responses of different C3 plants or of a single species grown in contrasting environments.

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