Energy, Radiation and Temperature Regulation in Plants

Abstract

Abstract The temperature of any plant organ depends on the balance between incoming energy and energy loss. The energy exchanges involved include radiative transfer, sensible heat transfer by convection processes, latent heat transfer as a result of evaporation and transfer to and from storage (by conduction). The radiative transfer is frequently classified into either short‐wave (or solar) radiation and long‐wave (or thermal) radiation; these contribute differently to the overall energy balance. Various biophysical mechanisms are available to plants, through manipulation of the energy balance terms, for temperature regulation. For example, enhanced evaporation can help cool leaves in hot environments, whereas maximising absorption of sunlight can raise the temperature in cold environments. This article outlines the relative importance of these different processes in both leaf and canopy temperature regulation, highlighting differences between treatment of the energy balance at leaf and canopy scales. Examples of applications, such as the measurement of canopy temperature to determine plant water stress or to aid in the selection of drought tolerant varieties of plants, are also discussed. Key Concepts: A critical requirement of plants is that they maintain their leaf temperature as close as possible to the optimal temperature for growth; plants have a range of mechanisms that help them to optimise their temperature. Leaf and canopy temperature are dependent on the leaf energy balance, which itself is dominated by radiant exchanges. Radiation in natural environments relevant to leaf energy balance is conventionally separated into the ultraviolet (0.2–0.38 μm), the visible (0.38–0.7 μm) and the infra‐red (0.7–100 μm). Radiation coming from the sun is often termed short‐wave radiation, about half of which is in the visible region, a small amount in the ultraviolet, and the remainder in the near infra‐red (0.7–3 μm). Long‐wave, or thermal radiation is conventionally taken as the radiation between 3 (or sometimes 4) μm and 100 μm. The radiation emitted by natural surfaces as a function of their temperature is primarily in this wave‐band. The thermal radiation emitted by any body increases according to the fourth power of the temperature (Stefan–Boltzmann's law). Leaf and canopy temperatures depend on the exchanges of energy between plants and the atmosphere both through radiation exchange and by means of sensible heat and latent heat fluxes. Leaf temperature therefore depends on the radiation climate (time of day, cloud cover and type, etc.), atmospheric conditions (wind speed, temperature, etc.), the soil conditions (soil type, water content, etc.) and the canopy properties (such as plant morphology, density, height, etc.) that together affect the size and ratios of the radiant, sensible and latent heat fluxes. Measurement of leaf temperature using infra‐red thermometry or thermography can be used to detect plant water‐deficit stress and to screen plants for their drought tolerance.