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• Distinctive characters of sun and shade plants

Sun plants High High High High High Low

Shade plants

Respiration Low

Light compensation point Low

Half saturation point Low

Light saturation point Low

Maximum rate of photosynthesis Low

Quantum yield High the understory or below canopy of tropical rainforests may experience opening and closure of the canopy several times during their lifetime. Understory species of Miconia (Melastomataceae) responded to canopy openings by production of new sun-leaves rather than acclimation of old shade-leaves, which could significantly increase maximum assimilation rates (Newell et al. 1993). Individual plants may also have both shade- and sun-leaves when part of the foliage is shaded and exposed respectively. Then we may define such plants as shade or light tolerant, but not shade or light demanding.

All of these various aspects are very important for plant life in tropical forests with their highly variable light climates. Comparisons of light response curves and their cardinal points provide distinctive characters for shade and sun species or phe-notypes (Box 4.1). Shade plants usually have lower rates of respiration but the activity of the alternative cyanide resistant pathway of mitochondrial electron transport not producing ATP is low and therefore the ATP/O2 efficiency of respiration is higher than in sun plants as required due the lower overall energy input at low irra-

diance in the shade (Noguchi et al. 2001b). Shade plants have lower rates of photosynthesis at light saturation and together with the lower rates of respiration this leads to lower light compensation and light saturation points, but higher quantum yields than in high light or sun plants. In Fig. 4.1 this is illustrated by the comparison of sun and shade plants (Fig. 4.1A) and pioneer, climax and understory trees and shrubs (Fig. 4.1B).

The different light-use characteristics of sun and shade plants are very important for understanding the distinct stages in the dynamics of tropical forests (Sects. 3.3.3 and 4.3.2). They distinguish pioneer species from climax species, and from plants of the under-growth. During growth (Sect. 4.3.2), pioneer species show the characteristics of sun plants, understory species that of shade plants (Eschenbach et al. 1998), and the dominant trees of later successions (climax species) show an intermediate behaviour as they may be extremely sun exposed in the upper canopy and shaded in the lower canopy layer (Fig. 4.1B, Tables 4.1 and 4.2). Thus, the pioneer species Cecropia peltata needs much higher light intensities for light saturation of photosynthesis and has much higher rates of maximum photosynthesis than the shade plant Croton glabellas, and a comparison of the light-response curves of dominant climax trees in the upper canopy, i.e. Cordia alliodora and Goethalsia meiantha, with

Table 4.1 Values for cardinal-points of light-response curves of sun and shade plants in general and of plants in tropical forests. (After Luttge 1985)

Plant type Light-compensation Light saturation Rate of CO2-uptake point of CO2-uptake at light saturation

(|imol photons m-2 s-1) (|imol photons m-2 s-1) (|imolm-2s-1)

Table 4.1 Values for cardinal-points of light-response curves of sun and shade plants in general and of plants in tropical forests. (After Luttge 1985)

Plant type Light-compensation Light saturation Rate of CO2-uptake point of CO2-uptake at light saturation

(|imol photons m-2 s-1) (|imol photons m-2 s-1) (|imolm-2s-1)

Sun plants 20 - 30

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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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