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Solar radiation is the only source of energy that can be used in metabolic activities by green plants and algae. It comes to the plant as a flux of radiation from the sun, either directly, or having been diffused to a greater or lesser extent by the atmosphere, or after being reflected or transmitted by other objects. The direct fraction is highest at tropical latitudes north and south of the equator, since cloud cover is typically high at the equator itself (Figure 3.1). Moreover, for much of the year in temperate climates, and for the whole of the year in arid climates, the leaf canopy in terrestrial communities does not cover the land surface, so that most of the incident radiation falls on bare branches or bare ground.

Figure 3.1 Global map of the solar radiation absorbed annually in the earth–atmosphere system: from data obtained with a radiometer on the Nimbus 3 meteorological satellite.

Source: After Laing & Evans (2011).

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.