14.5 Plant heat budgets

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Figure 14.18 Solar radiation striking a leaf is either reflected, transmitted or absorbed. Photosynthetically active radiation (0.4 to 0.7 µm) is strongly absorbed, whereas near-infrared radiation which would heat leaves but is useless for photosynthesis (between about 0.8 and 2 µm) is only weakly absorbed. Far-infrared radiation is strongly absorbed, but energy per quantum is greatly diminished at these long wavelengths, so that consequences for leaf heating are greatly diminished (Based on Gates 1965)



Figure 14.19 A notional energy budget for a mesophytic leaf on a well-watered plant. Taking an incident irradiance of 400 J m-2 s-1 as representative of midday conditions, 30 units are reflected, leaving 370 J m-2 s-1 absorbed. If air temperature is taken to be 20°C, relative humidity 50%, wind speed 0.8 m s-1 and leaf area 0.01 m2 (i.e. 100 cm2), then transpiration will be about 4 mmol H20 cm-2 s-1, and latent heat exchange will account for about 180 J m-2 s-1. Sensible heat exchange will account for the remainder (about 190 J m-2 s-1) (Based on Nobel 1983)

Physical principles responsible for heat and mass exchange between plants and their aerial environment are well established (Gates 1965). Process-based models of energy fluxes have been extensively validated under both laboratory (wind tunnel) and field conditions, with much attention focused on canopy and leaf heat budgets.

When solar radiation strikes a leaf (Figure 14.18), some is reflected, some is absorbed and some is transmitted. Absorbed energy is subsequently dissipated via three main avenues: reradiation of long wavelengths to nearby surfaces and sky; conduction and convection of sensible heat (directly measurable or able to be ‘sensed’); and latent heat exchange (energy consumed to evaporate water and commonly referred to as transpirational cooling).

In Figure 14.19, conduction plus convection of sensible heat compared with latent heat exchange are about equally responsible for dissipation of absorbed energy. Photosynthetic utilisation of radiant energy is of course crucial in biological terms, but insignificant for leaf energy budgets and is ignored here.

Irrespective of species or location, any organism, over an extended period, will lose exactly as much energy as it gains otherwise that organism would grow either hotter and hotter, or cooler and cooler. In either circumstance it would die, and noting that vascular plants exist across a remarkable range of thermal environments, they must have evolved with adaptive features that make them expert at balancing their energy budgets. Note, however, that plants probably never achieve short-term (moment by moment) equilibrium with their environment. Energy inputs, energy utilisation and options for energy dissipation vary incessantly, and often abruptly. For that reason, plants in nature would only ever achieve a quasi-steady energy state and useful inferences about their heat budgets can only be drawn from detailed process models plus comprehensive sets of test data.