the fate of radiation
When a plant intercepts radiant energy it may be reflected (with its wavelength unchanged), transmitted (after some wavebands have been filtered out) or absorbed. Part of the fraction that is absorbed may raise the plant’s temperature and be reradiated at much longer wavelengths. In terrestrial plants, part may contribute latent heat of evaporation of water and so power the transpiration stream. Something like 80% may reach the chloroplasts and drive the process of photosynthesis, but of this, only a small proportion may end up in the plant’s organic molecules, because there is insufficient capacity in carbon metabolism to use all the energy absorbed. Again, the remainder is dissipated as heat.
radiant energy must be captured or is lost forever
Radiant energy is converted during photosynthesis into energy‐rich chemical compounds of carbon, which will subsequently be broken down in respiration, either by the plant itself or by organisms that consume it. But unless the radiation is captured and chemically fixed at the instant it falls on the leaf, it is irretrievably lost for photosynthesis. Radiant energy that has been fixed in photosynthesis passes just once through the world. This is in complete contrast to an atom of nitrogen or carbon or a molecule of water that may cycle repeatedly through endless generations of organisms.
photosynthetically active radiation
Solar radiation is a resource continuum: a spectrum of different wavelengths. But the photosynthetic apparatus is able to gain access to energy in only a restricted band of this spectrum. All green plants depend on chlorophyll and other pigments for the photosynthetic fixation of carbon, and these pigments fix radiation in a waveband between roughly 400 and 700 nm. This is the band of photosynthetically active radiation (PAR). It corresponds broadly with the range of the spectrum visible to the human eye that we call ‘light’. About 56% of the radiation incident on the earth’s surface lies outside the PAR range and is thus unavailable as a resource for green plants. In other organisms, though, there are pigments, for example bacteriochlorophyll in bacteria, that operate in photosynthesis outside the PAR range of green plants. Our understanding of the breadth and importance of prokaryotic photosynthesis is increasing rapidly (Bryant & Frigaard, 2006).
Note that it is not the case simply that the rate of photosynthesis increases with the intensity of radiation. At high intensities, excess light can increase the production of potentially damaging intermediates in the photosynthetic process and photoinhibition of photosynthesis may occur (Li et al., 2009), though what constitutes excess light varies considerably with the state of the plant. Under conditions of excess light, rapid changes in the photosynthetic membrane result in the excess absorbed light energy being harmlessly dissipated as heat, but the highest intensities of radiation may also lead to dangerous overheating. Radiation is an essential resource for plants, but they can have too much as well as too little.
Nonetheless, the highest efficiency of utilisation of radiation by green plants is 3–4.5%, obtained from cultured microalgae at low intensities of PAR. In tropical forests values fall within the range 1–3%, and in temperate forests 0.6–1.2%. The approximate efficiency of temperate crops is only about 0.6%. These can themselves be viewed in the context of a theoretical maximum efficiency of photosynthesis of 4.5–6% (Zhu et al., 2010). It is on such paltry levels of efficiency that the energetics of all communities depend.
3.2.1 Variations in the intensity and quality of radiation
systematic variations in supply
One important reason why plants seldom achieve their full photosynthetic capacity is that the intensity of radiation varies continually (Figure 3.2), and the plant morphology and physiology that are optimal for photosynthesis at one intensity will be suboptimal at another. As with all resources, this supply of radiation can vary both systematically and unsystematically. Annual and diurnal rhythms are systematic variations in solar radiation (Figure 3.2a, b). The green plant experiences periods of famine and glut in its radiation resource every 24 hours (except near the poles) and seasons of famine and glut every year (except in the tropics). In aquatic habitats, an additional systematic and predictable source of variation in radiation intensity is the reduction in intensity with depth in the water column, though the extent of this may vary greatly. For example, differences in water clarity mean that seagrasses may grow on solid substrates as much as 90 m below the surface in the relatively unproductive open ocean, whereas macrophytes in fresh waters rarely grow at depths below 10 m (Sorrell et al., 2001), and often only at considerably shallower locations, in large part because of differences in concentrations of suspended particles and phytoplankton (Figure 3.2c).
Figure 3.2 Levels of solar radiation vary over time and space and with depth in water. (a) The daily totals of solar radiation received throughout the year at Wageningen (the Netherlands) and Kabanyolo (Uganda). (b) The monthly average of daily radiation recorded at Poona (India), Coimbra (Portugal) and Bergen (Norway). (c) The vertical distribution of algal abundance (measured as fluorescence in units of mg chlorophyll a m–3) and of irradiance as a percentage of that at the surface, for two stations off the Arctic island of Svarlbard. The decline in irradiance with water depth is apparent at both stations, but at Station 1, higher algal densities in the surface waters led to that decline being more rapid: 10% of surface irradiance at around 7 m compared with 12 m at Station 2.
Source: (a, b) After de Wit (1965) and other sources. (c) After Meshram et al. (2017).
shade: resource‐depletion zones and spectral changes
Less systematic variations in the radiation environment of a leaf are caused by the nature and position of neighbouring leaves. Leaves in a canopy, by intercepting radiation, create a resource‐depletion zone (RDZ) – in this case, a moving band of shadow over other leaves of the same plant, or of others. The composition of radiation that has passed through leaves in a canopy, or through a body of water, is also altered. Typically, it is depleted in the blue and (especially through water) the red parts of the spectrum – the most effective wavelengths for photosynthesis. Figure 3.3 shows an example for the variation with depth in a freshwater habitat.
Figure 3.3 The spectral distribution of radiation changes with depth as shown here for Lake Burley Griffin, Australia. Note that photosynthetically active radiation lies broadly within the range 400–700 nm.
Source: After Kirk (1994).
sun and shade species
The way in which
