11.8 Conclusion

In this module, two simple models of the thermal energy budget of a leaf have been introduced. Radiation, convection and evaporation, the most important heat transfer processes, are included in the models (note that conduction of heat along the leaf stem is not included). Because of the multidimensionality of the models, a graphical analysis was used. The models show that

1. leaf temperature can be above or below air temperature,
2. the larger the difference between absorbed radiation and the black body level of radiation, the larger the difference between leaf temperature and air temperature,
3. the greater the wind speed, the smaller is the difference between leaf temperature and wind speed, and
4. increasing evaporative water loss via transpiration always decreases leaf temperature.

In this formulation, the plant “can control” leaf temperature or transpiration by its size ($$D$$ and $$W$$) and by its physiology ($$k_2$$ and $$r_l$$). There are, however, a number of leaf characteristics which have not been explicitly or implicitly included in the leaf energy budget. These include stomata size and shape, wax cuticle properties, leaf orientation and leaf absorptivity to shortwave radiation.

Finally this work has help lead to research in related areas. Scientists have developed equations to model (1) photosynthesis which couples the interchange of water vapor and CO2 (Lommen et al. 1975; Tenhunen, Yocum and Gates 1976; Tenhunen et al. 1976), (2) whole-plant water transport (Farnum 1977), and (3) optimal leaf form (Taylor 1975; Parkhurst and Loucks 1972; Givnish and Vermeij 1976).