4.1 Light Responses of Photosynthesis
Although gradients of several different environmental factors are noticeable in tropical forests (Sect. 3.4), intensity of irradiance is most highly variable and appears to play the most prominent role in determining the ecophysiological comportment of forest plants. At the top of the canopy and in larger clearings in full sun-light intensity of photosynthetically active radiation (PAR) at 400 - 700 nm wavelength may range from well above 1,000 up to 2,500 pmolm-2s-1 photons. On the forest floor there may be less 5 pmolm-2s-1 photons (see Figs. 3.26 and 3.28). Thus, light can become a stress factor from both too much (when it causes overenergization of the photosynthetic apparatus and hence photoinhibition or even photodestruction), or too little (when it becomes limiting as an energy source of photosynthesis). Focussing of light by leaf epidermal cells may increase irradiance intensity in the mesophyll of under story plants of tropical forests (Vogelmann et al. 1996). However, the major responses are those of the photosynthetic machinery.
4.1.1 Light-Response Characteristics of Sun and Shade Plants
The photosynthetic utilization of light by plants is described quantitatively by light-response curves (Fig. 4.1), which are distinguished by several cardinal points as follows (see also Box 4.1). In darkness (zero PAR), there is net-CO2 release due to respiration. As light intensity increases, net-CO2 release is gradually reduced until the light-compensation point is attained, where net-CO2 exchange is zero because photosynthetic CO2-uptake just balances respiratory CO2-release. Above this point, net-CO2 uptake increases until light saturation is reached. The light-saturation point often is hard to determine precisely, because light saturation is approached gradually. Hence, half saturation of photosynthesis is often quoted alternatively or additionally. The slope of the nearly linear part of the curve below saturation gives the apparent quantum yield (mol CO2 per mol photons) of photosynthesis.
Fig. 4.1 A Light-response curves of a sun plant Ploiar-ium alternifolium and a shade plant Lycopodium cernum measured in the field in a tropical secondary forest in Singapore. (After Luttge et al. 2005). B Light-response curves of four species in the gradient of understory, climax and pioneer systems in a secondary lower montane rainforest in Costa Rica (although only old units for light intensity are available this figure is quite illustrative; Stephens and Waggoner 1970)
Plants may be genetically determined for growth at low or high light intensity. In this case we can distinguish genuine shade and sun species. However, there are also ontogenetic and developmental modifications, where light exerts a signalling function rather than being only the energy source of photosynthesis, and plants may acclimate or adapt ecophysiologically to low and high irradiance, respectively (see Sect. 4.3.2). The potential for light acclimation is species specific and may involve major structural and functional changes in the photosynthetic apparatus (Bailey et al. 2001). Thus, there may be shade and sun forms of given species. Plants surviving in
Box 4.1 Light-response characteristics
maximum rate of photosynthesis
light saturation point
-half saturation point
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