Principles underlying the prediction of temperature in plants, with special reference to desert succulents.

The thermal motion of molecules responsible for the property we know as temperature influences essentially every aspect of plant biology. Emphasis in this review is on principles of wide biological applicability, with specific examples being chosen from research on agaves and cacti. Plant temperatures can be predicted using energy budgets incorporating shortwave and longwave radiation, heat conduction and convection, latent heat, and heat storage. Energy budgets are used here to show the effect of plant size and shortwave absorptance on tissue temperature. The importance of air temperature or transcription rate for tissue temperature is calculated quantitatively. The influences of surface appendages, such as apical pubescence and spines, on minimal temperatures near the apical meristem of cacti are predicted, such minimal temperatures influencing the geographical distribution of various species. The Boltzmann energy distribution and Arrhenius plots are presented and used to analyse thermal responses in terms of energy barriers and activation energies. The many ways that temperature can influence CO2 diffusion into a leaf are also considered. Tolerances to low and high temperatures, such as the ability of agaves and cacti to tolerate extremely high tissue temperatures of 70 degrees C, are discussed from both cellular and ecological perspectives. Although the influences of temperature on plants are multitudinous, many can be predicted, or at least analysed, based on well-established physical principles.