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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 organisms react to systematic, predictable patterns in the supply of a resource reflects both their present physiology and their past evolution. At a very broad scale, the seasonal shedding of leaves by deciduous trees in temperate regions in part reflects the annual rhythm in the intensity of radiation – they are shed when they are least useful. Amongst terrestrial species, plants that are characteristic of shaded habitats generally use radiation at low intensities more efficiently than sun species, but the reverse is true at high intensities (Figure 3.4). Part of the difference between them lies in the physiology of the leaves, but the morphology of the plants also influences the efficiency with which radiation is captured. The leaves of sun plants are commonly exposed at acute angles to the midday sun, spreading an incident beam of radiation over a larger leaf area and effectively reducing its intensity (Poulson & DeLucia, 1993). The leaves of sun plants are also usually superimposed into a multilayered canopy. In bright sunshine even the shaded leaves in lower layers may have positive rates of net photosynthesis. Shade plants adopt a different strategy, commonly having leaves held near to the horizontal and in a single‐layered canopy.

Figure 3.4 The response of photosynthesis to radiation intensity in various plants at optimal temperatures and with a natural supply of CO₂. Note that corn and sorghum are C₄ plants and the remainder are C₃ (the terms are explained in Sections 3.3.1 and 3.3.2).

Source: After Larcher (1980), and other sources.

sun and shade leaves

Plants may also respond ‘tactically’ to the radiation environment in which they develop, producing ‘sun leaves’ and ‘shade leaves’ within the canopy of a single plant. Sun leaves (and indeed, leaves on sun plants) are typically smaller, thicker, have more cells per unit area, denser veins, more densely packed chloroplasts and a greater dry weight per unit area of leaf. They are said to have a smaller specific leaf area (leaf area per unit leaf mass). Acclimation to shade typically involves increasing chlorophyll concentration and decreasing investment in the rest of the photosynthetic apparatus. This allows the leaf to maximise capture of light, but does not waste resource on a high photosynthetic capacity, which is not needed under shade conditions. In turn, this releases nitrogen for use by the upper leaves. However, these tactical manoeuvres take time. It is impossible for the plant to change its form fast enough to track the changes in intensity of radiation between a cloudy and a clear day. It can, however, change its rate of photosynthesis extremely rapidly, reacting even to the passing of a fleck of sunlight.