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saturation of photosynthesis (mol CO2 mol-1 leafN)

saturation of photosynthesis (mol CO2 mol-1 leafN)

chlorophyll levels were high as compared to the high-light plants and increased by additional N-nutrition. The chlorophyll a : b ratio was higher in the high-light plants. The different construction of the photosynthetic apparatus was reflected in a development of the typical cytological structure of sun- and shade-plant chloro-plasts, respectively (Fig. 4.5). Low-light grown plants developed characteristically large, globular, shade-acclimated chloroplasts, with extensive grana-formation and hence appressed thylakoid membranes (Box 4.2). The number of thylakoid membranes per granum was threefold larger in low-light than in high-light grown plants, and the ratio of appressed to non-appressed thylakoid membranes was 3.5-5.0 and 1.0-1.5 respectively. Chloroplasts of high-light plants grown without N had only poorly developed thylakoids. Light compensation points were higher in the highlight plants of B. humilis (Table 4.3). It was most noteworthy, however, that with extra N-nutrition the low-light plants could attain similar light saturated rates of photosynthesis as the high-light plants and that the high-light plants almost reached the apparent quantum yield of the low light plants. Expressed on a leaf-nitrogen basis net photosynthetic CO2-fixation was similar in low and high-light grown plants independent of whether additional N as supplied. This demonstrates the optimisation of nitrogen use, but it also suggests an interaction with other factors, since the ratio of net-CO2-fixation to nitrogen levels was not constant but rather low at low N-levels and higher at high N-levels.

Maximum assimilation rates related to nitrogen levels of leaves give the nitrogen-use-efficiency (NUE), which is an important parameter relating the functioning of the photosynthetic apparatus to mineral nutrition. In general, assimilation vs leaf-nitrogen curves are linear over certain ranges of N-levels (Fig. 4.6; Nielsen et al. 1997). Often they do not appear to extrapolate to the origin, and thus, show reduced NUE at low N-levels. At higher N-levels there may be N-saturation of assimilation (see Diplacus in Fig. 4.6B). Therefore, Evans (1988) has evaluated the general

N-relationships of sun and shade plants somewhat critically. While a correlation between assimilation and N-levels is clearly given, other factors like characteristics of individual species (Fig. 4.6) and growth conditions must also be involved. This may include irradiance during growth since with spinach and peas (unlike in the experiment of Table 4.3), there was an effect of light intensity on NUE.

Fig. 4.5A-D Chloroplast ultrastructure of Bromelia humilis grown in low light with N (A) or without N (B) and in high light with N (C) or without N (D). (Fetene et al. 1990)

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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