Variations in the intensity and quality of radiation

A major reason why plants seldom achieve their intrinsic photosynthetic capacity is that the intensity of radiation varies continually (Figure 3.3). Plant morphology and physiology that are optimal for photosynthesis at one intensity of radiation will usually be inappropriate at another. In terrestrial habitats, leaves live in a radiation regime that varies throughout the day and the year, and they live in an environment of other leaves that modifies the quantity and quality of radiation received. As with all resources, the supply of radiation can vary both systematically (diurnal, annual) and unsystematically. Moreover, it is not the case simply that the intensity of radiation is a greater or lesser proportion of a maximum value at which photosynthesis would be most productive. At high intensities, photoinhibition of photosynthesis may occur (Long et al., 1994), such that the rate of fixation of carbon decreases with increasing radiation intensity. High intensities of radiation may also lead to dangerous overheating of plants. Radiation is an essential resource for plants, but they can have too much as well as too little.

Annual and diurnal rhythms are systematic variations in solar radiation (Figure 3.3a, b). The green plant experiences periods of famine and glut in its radiation resource every 24 h (except near the poles) and seasons of famine and glut every year (except in the tropics). In aquatic habitats, an additional radiant energy must be captured or is lost forever photosynthetically active radiation photoinhibition at high intensities systematic variations in supply

Incident Radiation Plant Canopy

Figure 3.2 The reflection (R) and attenuation of solar radiation falling on various plant communities. The arrows show the percentage of incident radiation reaching various levels in the vegetation. (a) A boreal forest of mixed birch and spruce; (b) a pine forest; (c) a field of sunflowers; and (d) a field of corn (maize). These figures represent data obtained in particular communities and great variation will occur depending on the stage of growth of the forest or crop canopy, and on the time of day and season at which the measurements are taken. (After Larcher, 1980, and other sources.)

Figure 3.2 The reflection (R) and attenuation of solar radiation falling on various plant communities. The arrows show the percentage of incident radiation reaching various levels in the vegetation. (a) A boreal forest of mixed birch and spruce; (b) a pine forest; (c) a field of sunflowers; and (d) a field of corn (maize). These figures represent data obtained in particular communities and great variation will occur depending on the stage of growth of the forest or crop canopy, and on the time of day and season at which the measurements are taken. (After Larcher, 1980, and other sources.)

systematic and predictable source of variation in radiation intensity is the reduction in intensity with depth in the water column (Figure 3.3c), 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 also phytoplankton (see below).

The way in which an organism reacts to systematic, predictable variation in the supply of a resource reflects both its present physiology and its past evolution. The seasonal shedding of leaves by deciduous trees in temperate regions in part reflects the annual

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Figure 3.3 (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). ((a, b) after de Wit, 1965, and other sources.) (c) Exponential diminution of radiation intensity in a freshwater habitat (Burrinjuck Dam, Australia). (After Kirk, 1994.)

Figure 3.4 As population density (•) of the unicellular green alga, Chlorella vulgaris, increased in laboratory culture, this increased density reduced the penetration of light (o; its intensity at a set depth). Bars are standard deviations; they are omitted when they are smaller than the symbols. (After Huisman, 1999.)

Light Penetration Into Algal Culture

Figure 3.4 As population density (•) of the unicellular green alga, Chlorella vulgaris, increased in laboratory culture, this increased density reduced the penetration of light (o; its intensity at a set depth). Bars are standard deviations; they are omitted when they are smaller than the symbols. (After Huisman, 1999.)

rhythm in the intensity of radiation - they are shed when they are least useful. In consequence, an evergreen leaf of an under-story species may experience a further systematic change, because the seasonal cycle of leaf production of overstory species determines what radiation remains to penetrate to the understory. The daily movement of leaves in many species also reflects the changing intensity and direction of incident radiation.

Less systematic variations in the radiation environment of a leaf are caused by the nature and position of neighboring leaves. Each canopy, each plant and each leaf, by intercepting radiation, creates a resource-depletion zone (RDZ) - a moving band of shadow over other leaves of the same plant, or of others. Deep in a canopy, shadows become less well defined because much of the radiation loses its original direction by diffusion and reflection.

Submerged vegetation in aquatic habitats is likely to have a much less systematic shading effect, simply because it is moved around by the flow of the water in which it lives, though vegetation floating on the surface, especially of ponds or lake, inevitably has a profound and largely unvarying effect on the radiation regime beneath it. Phytoplankton cells nearer the surface, too, shade the cells beneath them, such that the reduction of intensity with depth is greater, the greater the phytoplankton density. Figure 3.4, for example, shows the decline in light penetration, measured at a set depth in a laboratory system, as a population of the unicellular green alga, Chlorella vulgaris, built up over a 12-day period (Huisman, 1999).

Figure 3.5 Changing spectral distribution of radiation with depth in Lake Burley Griffin, Australia. Note that photosynthetically active radiation lies broadly within the range 400-700 nm. (After Kirk, 1994.)

The composition of radiation that has passed through leaves in a canopy, or through a body of water, is also altered. It may be less useful photosynthetically because the PAR component has been reduced -though such reductions may also, of course, prevent photoinhibition and overheating. Figure 3.5 shows an example for the variation with depth in a freshwater habitat.

The major differences amongst terrestrial species in their reaction to systematic variations in the intensity of radiation are those that have evolved between 'sun species' and 'shade species'. In general, plant species that are characteristic of shaded habitats use radiation at low intensities more efficiently than sun species, but the reverse is true at high intensities (Figure 3.6). 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 (Poulson & DeLucia, 1993). This spreads an incident beam of radiation over a larger leaf area, and effectively reduces its intensity. An intensity of radiation that is superoptimal for photosynthesis when it strikes a leaf at 90° may therefore be optimal for a leaf inclined at an acute angle. The leaves of sun plants are often superimposed into shade: a resource-depletion zone attenuation with depth, and plankton density, in aquatic habitats variations in quality as well as quantity sun and shade species

Distribution And Plants

Figure 3.6 The response of photosynthesis to light 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). (After Larcher, 1980, and other sources.)

pigment variation in variation between species is accounted aquatic species for by differences in photosynthetic pigments, which contribute significantly to the precise wavelengths of radiation that can be utilized (Kirk, 1994). Of the three types of pigment - chlorophylls, carotenoids and biliproteins - all photosynthetic plants contain the first two, but many algae also contain biliproteins; and within the chlorophylls, all higher plants have chlorophyll a and b, but many algae have only chlorophyll a and some have chlorophyll a and c. Examples of the absorption spectra of a number of pigments, the related contrasting absorption spectra of a number of groups of aquatic plants, and the related distributional differences (with depth) between a number of groups of aquatic plants are illustrated in Figure 3.7. A detailed assessment of the evidence for direct links between pigments, performance and distribution is given by Kirk (1994).

Figure 3.6 The response of photosynthesis to light 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). (After Larcher, 1980, and other sources.)

a multilayered canopy. In bright sunshine even the shaded leaves in lower layers may have positive rates of net photosynthesis. Shade plants commonly have leaves held near to the horizontal and in a single-layered canopy.

In contrast to these 'strategic' differences, it may also happen that as a plant grows, its leaves develop differently as a 'tactical' response to the radiation environment in which it developed. This often leads to the formation of 'sun leaves' and 'shade leaves' within the canopy of a single plant. Sun leaves 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. These tactical maneuvers, then, tend to occur not at the level of the whole plant, but at the level of the individual leaf or even its parts. Nevertheless, they take time. To form sun or shade leaves as a tactical response, the plant, its bud or the developing leaf must sense the leaf's environment and respond by growing a leaf with an appropriate structure. For example, 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. The rate at which a leaf photosynthesizes also depends on the demands that are made on it by other vigorously growing parts. Photosynthesis may be reduced, even though conditions are otherwise ideal, if there is no demanding call on its products.

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  • Eric
    Why radiation intensity varies?
    3 months ago

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