Radiation

Solar radiation is the only source of energy that can be used in metabolic activities by green plants. It comes to the plant as a flux of radiation from the sun, either directly 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 low latitudes (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 on bare ground.

When a plant intercepts radiant the fate of radiation 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 what are resources?

organisms may compete for resources

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. The units are J cm-2 min-1. (After Raushke et al., 1973.)

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. The units are J cm-2 min-1. (After Raushke et al., 1973.)

Imagines Ecology Center

stream. A small part may reach the chloroplasts and drive the process of photosynthesis (Figure 3.2).

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.

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 there are pigments, for example bacterio-chlorophyll in bacteria, that operate in photosynthesis outside the PAR range of green plants.

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