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30 L0 50 60 70 80 A/gH20(mmol mol"1 )

Fig. 4.2 Leaf-internal CO2-partial pressures (p1C0 ) and CO2-assimilation (A) to leaf conductance to water vapour (gh2o) ratios of five tropical rainforest tree species in artificial stands under common conditions in French Guiana. Closed circles: Pioneer species (Jacaranda copaia, Goupia glabra, Carapa guianensis); open circles: late successional species (Dicorynia guianensis, Eperna falcata). (After data of Huc et al. 1994)

that of the understory shrub C. glabellus in Fig. 4.1B shows the distinct differences between sun and shade plants with respect to all the features listed in Box 4.1.

Pioneer and late successional rainforest species also regulate their leaf gas exchange in different ways. Shade plants have lower leaf conductance to water vapour, gH2o, than sun plants which leads to lower gas exchange and growth (Bonal et al. 2000; Sack et al. 2005). Thus, pioneer species operate at lower A/gHlO ratios (CO2-assimilation A), i.e. with a greater stomatal aperture and higher internal CO2-partial pressures (^CO2) as compared to trees found in the later stages of succession (Huc et al. 1994; Fig2 4.2).

4.1.2 The Photosynthetic Apparatus: Pigments, Enzymes and Nitrogen

It is very often observed in the tropics, that individual plants of a given species growing in deep shade inside a forest and exposed to full sun-light in an open habitat respectively, form morphologically very different phenotypes, which are also strongly distinguished by pigmentation. For example, this is frequently found among rosettes of bromeliads, e.g. in the genera Bromelia and Ananas which belong to genetically identical clones propagating vegetatively by formation of ramets. In Bromelia humilis shade plants are much larger than sun plants; they have long and slender leaves, whereas sun plants overall have a more stunted appearance (Fig. 4.3). The most conspicuous difference is leaf colour, which is dark green in the shade plants, brightly yellow in the sun plants and light green in intermediate forms.

What is behind this pronounced difference in pigmentation? Addressing this question requires a reminder of the basic structure of the photosynthetic apparatus situated in the thylakoid membranes of the chloroplasts (Box 4.2). The major

Fig. 4.3 Phenotypes of Bromelia humilis, from left to right yellow/exposed form, intermediate light green/exposed form and dark green/shaded form

Box 4.2 Structure and function of the photosynthetic apparatus

A. Scheme of a chloroplast with outer membrane (envelope), inner membrane, the stroma (= plastoplasm or "cytoplasm" of the chloroplast), stromal and granal thylakoids and thylakoid interior.

Box 4.2 (Continued)

B. Scheme of the thylakoid membrane with the elements of photosynthetic electron transport.

Stroma pH 8

oppressed membranes: 3H'

Stroma pH 8

oppressed membranes: 3H'

There are four important integral proteins or protein complexes in the thylakoid membrane:

• Photosystem II (PS II) with antenna and reaction-centre pigments; it occurs only in the appressed regions of granal thylakoids.

• The cytochrome b6,f-complex.

• Photosystem I (PS I) with antenna and reaction-centre pigments; it occurs in the stroma thylakoids.

• The NADP-reductase (NADP = nicotinamide-adenine-dinucleotide-phosphate).

• The FoF1-ATPase or coupling factor.

When PS II and PS I are excited by the absorption of photons (hv), H2O is split into oxygen, protons and electrons (e-), electrons flow from PS II via membrane-bound plastoquinone (Q) and mobile plastoquinone (PQ) to the cytochrome-b6,f-complex and plastocyanine (PCy) at the lumen side of the membrane, and from PCy further to PS I, ferredoxin at the stroma side of the membrane and to the NADP-reductase, which finally generates the reducing equivalents needed for CO2 reduction. If they are not utilized in CO2 reduction, the systems of this electron-transport chain may become overre-duced, and damage may result if the excitation energy cannot be dissipated in other ways.

Electron transport is associated with charge separation and simultaneously leads to establishment of a proton-electrochemical gradient across the thylakoid membranes between the lumen (~pH 5) and the stroma (~pH 8). This is the driving force for the movement of protons through the ATPase which is coupled to ATP synthesis providing the energy needed for CO2 assimilation.

Box 4.2 (Continued)

C. Model of a light trap or light-harvesting complex (PS I or PS II) with antenna pigments and reaction-centre pigment.

Q Electron- Electron -

Donator Acceptor

Q Electron- Electron -

Donator Acceptor

It is sufficient that one of the antenna pigments absorbs a photon (hv) and is excited. Transfer of the excitation energy between antenna pigments always eventually leads to excitation of the reaction center. The larger the light trap the more probable is excitation of the reaction centre.

components are the pigments of the two photosystems (photosystem I and II), the thylakoid proteins embedded in the membrane lipid-bilayer and the cofactors of the photosynthetic electron-transport chain. Pigments are the chlorophyll a of the light harvesting complex (light trap) and accessory antenna pigments such as chlorophyll b, carotenoids and xanthophylls. Thylakoid-proteins are the various elements of the electron-transport chain (redox-chain), the ATP-generating coupling factor (F0F1-ATPase) and chlorophyll-protein light harvesting complexes.

Since proteins as well as chlorophyll and cytochrome molecules contain much nitrogen, this element plays a prominent role in constructing the photosynthetic apparatus in the thylakoids. Given that the components of the photosynthetic apparatus acclimate to changing light climate then nitrogen supply is important for these processes. Thus, it is appropriate to determine the N-costs of thylakoid membranes, which can be expressed in units of mol N:mol chlorophyll. For example, in Alocasia macrorrhiza, a shade tolerant species native to tropical rainforest understories in Australia, this was 45 molN/molchl under natural shade, and 56molN/molchl in Pisum sativum grown at high irradiance (Evans 1988). A general comparison of shade plants and sun plants reveals a number of characteristics, as follows:

Box 4.2 (Continued)

D. Photosynthetic Pigments

D. Photosynthetic Pigments

Box 4.2 (Continued)

The reaction-centre pigment chlorophyll a and antenna pigments chlorophyll b and carotenoids. The essential feature of the light absorbing pigments are systems of conjugated double bonds.

(Schemes after Luttge et al. 2005)

• Shade plants contain more chlorophyll b or have smaller chlorophyll a:b ratios.

The larger relative amount of antenna pigments assures that low photosyntheti-cally active photon flux densities (PPFDs) or light intensities are used efficiently, i.e. at low flux densities photons are absorbed effectively and the excitation energy can be transferred to the light trap reaction center chlorophyll (Box 4.2C). This explains the higher quantum yield of shade plants (Sect. 4.1.1, Fig. 4.1, Box 4.1). At a given nitrogen availability chlorophyll a : b ratios increase with increasing irradiance (Ki-tajima and Hogan 2003).

• Shade plants have lower rates of electron flow along the redox-chain in the thylakoids related to chlorophyll.

This is a consequence of the larger chlorophyll content of the photosystems.

• Shade plants have less soluble protein in relation to chlorophyll but shade plants have larger total N-contents in their biomass.

The soluble proteins of leaves include ribulose-bis-phosphate carboxylase/oxygenase (RuBISCO, see also below or RuBPC in Sect. 2.5). This enzyme-protein is responsible for photosynthetic CO2-fixation. It is the single major protein and hence N-containing compound in plant leaves. The lower protein/chlorophyll ratio in shade plants is due to the higher chlorophyll content and lower content of RuBISCO. However, generally shade plants have larger total N-contents in their biomass, and chlorophyll a : b ratios increase with decreasing N-availability especially under high irradiance conditions (Kitajima and Hogan 2003). An over production of RuBISCO may occur for an N-reserve and then there is a weak correlation of N-contents and RuBISCO and maximal rate of photosynthesis (Warren et al. 2000).

• Shade plants have larger photosystem II/photosystem I ratios.

This is related to the change of the spectral composition of light passing through the canopy, where the shorter-wave length red-light is filtered out to a larger extent than the longer-wave length red-light (see Sect. 4.2). The chlorophyll of photosystem II (PS II) is excited by somewhat shorter wavelengths (P-680 for absorption at X = 680 nm) than that of photosystem I (PS I; P-700). Since both photosystems must co-operate in photosynthesis, shade plants need more PS II in relation to PS I.

• Shade plants have larger chloroplasts and more grana formation.

Experiments to elucidate these relationships have often been made with tropical plants, since the contrast between deep shade in the dark rainforests and full sun exposure in clearings and open habitats with small solar inclination throughout the year is more pronounced in the tropical environment. Figure 4.4 shows the results of a study, where Alocasia macrorrhiza, which has a large capacity for photosynthetic acclimation to different light environments, was adapted to various light intensities during growth, i.e. from very low PPFDs up to 800 pmolm-2s-1. The photosynthetic capacity, the activity of RuBISCO, the content of cytochrome f - an important element of the photosynthetic electron-transport chain (Box 4.2) - and the chlorophyll a :b ratio increased considerably with light acclimation. The amount of trap chlorophyll of PS I showed no change, but PS II increased slightly at high growth irradiance up to 800 pmolm-2s-1. Thus, in this particular experiment a considerably larger PS II/PS I ratio was not seen in the shade grown plants. Total chlorophyll content decreased. There was a small decrease of quantum yield. It may be noted additionally, however, that a cost-benefit study which includes modelling and simulation suggests that shade leaves not necessarily have a lower photosynthetic capacity than sun leaves when leaf mass rather than area (as in Fig. 4.4) is used as a basis, although the higher investment in CO2-fixation-cycle enzymes and electron-transport carriers per unit of leaf surface in shade plants remains evident (Sims and Pearcy 1994; Sims et al. 1994). Thus, by and large the differences between low- and highlight grown Alocasia plants are in conformity with the general distinctions between shade and sun plants made above, despite the general observation that A. macror-rhiza is a typical understory rainforest plant in Australia, and still more pronounced effects may be observed with plants showing a greater phenotypic plasticity at still higher irradiance.

A study with the low and high-light grown phenotypes of Bromelia humilis (see Fig. 4.3) also corroborates these basic relationships, and emphasizes the modulation by nitrogen nutrition (Fetene et al. 1990; Table 4.3). In the low-light plants,

Fig. 4.4 Components and photosynthetic functions of leaves of the tropical understory plant

Alocasia macrorrhiza grown under varying light intensities up to 800|imolm 2s 1 photons (X = 400 - 700 nm). (After Chow et al. 1988; see Anderson and Thomson 1989)

Fig. 4.4 Components and photosynthetic functions of leaves of the tropical understory plant

Alocasia macrorrhiza grown under varying light intensities up to 800|imolm 2s 1 photons (X = 400 - 700 nm). (After Chow et al. 1988; see Anderson and Thomson 1989)

Table 4.3 Effects of light intensity and nitrogen on photosynthesis and leaf parameters of Bromelia humilis (Fetene et al. 1990)

Irradiance during growth mol photons m 2 s 1 ] High (700 - 800) Low (20 - 30)

Table 4.3 Effects of light intensity and nitrogen on photosynthesis and leaf parameters of Bromelia humilis (Fetene et al. 1990)

Irradiance during growth mol photons m 2 s 1 ] High (700 - 800) Low (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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