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Fig. 10.2A-C Semiquantitative phenograms (relative units on the ordinates) of two tropical grasses with C4-photosynthesis (A, B) and a tropical savanna tree (C). (Sarmiento 1984; reprinted by permission of Harvard University Press)

Fig. 10.2A-C Semiquantitative phenograms (relative units on the ordinates) of two tropical grasses with C4-photosynthesis (A, B) and a tropical savanna tree (C). (Sarmiento 1984; reprinted by permission of Harvard University Press)

for L. lanatum (Fig. 10.2). The phenological diagram of L. lanatum also shows the stimulation by fire of shoot production at the end of the dry season (see Sect. 10.3.3 for more details). Such phenological diagrams, as discussed by Solbrig (1993; for many phenological diagrams of cerrado plants see Gottsberger and Silberbauer-Gottsberger 2006) allow the separation of functional groups of savanna grasses. Thus, there are grasses that grow early and reproduce quickly in the rainy season (more like L. lanatum in Fig. 10.2) as opposed to grasses that grow gradually and develop shoots slowly and reproduce in the middle or towards the end of the rainy season (more like T. plumosus in Fig. 10.2). In general terms, the former (i.e. early growers and reproducers):

• are more drought resistant,

• have higher turgor pressures during the dry season,

• have higher water use efficiencies (WUE),

• partition more of their photosynthetic products to roots and below-ground organs,

• are more competitive under dry conditions and have increased importance along wet to dry gradients.

Grasses need less water than savanna trees, but the water must be available during the growth period.

10.1.1.2 Metabolic Adaptation: C4-Photosynthesis

A major ecophysiological aspect of savanna grasses is the dominance of C4-photosynthesis (Box 10.2). C4-photosynthesis is occurring among a few shrubs and woody plants but it is absent in real trees. Although it is frequent in some dicotyledonous families, such as the Chenopodiaceae, C4-photosynthesis is a typical biochemical trait of sub-tropical and tropical grasses. Somewhat similar to CAM (see Sect. 5.2.2.2 and Box 5.1), during C4-photosynthesis CO2 is first cycled through malate, before it is assimilated in the Calvin cycle. However, during C4-photosynthesis there is simultaneously CO2-fixation via PEP-carboxylase (PEPC), together with CO2-remobilisation, refixation via RuBISCO and reduction via the Calvin cycle. The PEPC- and RuBISCO-functions are localized in different cell types and hence separated spatially, in contrast to CAM, where they occur in the same cells but are separated in time. Leaves of most C4-plants have two distinct photosynthetic tissues, first the outer spongy mesophyll where CO2 is fixed to produce malate and in many cases also aspartate via oxaloacetate, and second, the inner bundle sheath where malate or aspartate are decarboxylated and the CO2 is refixed. Depending on the enzymic mechanism of decarboxylation, three different types of C4-photosynthesis can be distinguished (Box 10.2). Because the affinity of PEPC for CO2 is about 60 times higher than that of RuBISCO, fixation of atmospheric CO2 in the mesophyll, which tightly surrounds the bundle sheath, is highly effective. The malate and aspartate so produced are transported symplastically to the bundle sheath, via plasmodesmata connecting the two tissues. Frequently, there is suberization of the cell walls between the two tissues to prevent leakage to the

Box 10.2 Biochemical pathways of C4 plants

Box 10.2 (Continued)

Primary CO2 fixation via phosphoenolpyruvate carboxylase (PEPC) and refixation of CO2 via ribulose-bis-phosphate carboxylase (RuBISCO) occur simultaneously in time and are separated in space.

The biochemical reactions in the mesophyll are basically similar in all types of C4 plants. The first CO2 fixation product is the C4 acid anion oxaloacetate, which is subsequently transformed to malate and/or aspartate. Three types of C4 plants are distinguished by the mode of decarboxylation of these C4 acids after their transport to the bundle-sheath cells:

• the PEP-carboxykinase type [reaction (10)].

Enzymatic reactions

(1) PEP-carboxylase (PEPC),

(2) NADP-dependent malate dehydrogenase,

(3) NADP-dependent malic enzyme,

(4) Pyruvate, Pi dikinase,

(5) 3-PGA kinase, NADP-dependent glyceraldehyde-3-P dehydrogenase and triose-P isomerase,

(6) Aspartate aminotransferase,

(7) NAD-dependent malate dehydrogenase,

(8) NAD-dependent malic enzyme,

(9) Alanine aminotransferase,

(10) PEP-carboxykinase,

(11) Mitochondrial NADH oxidation systems.

Metabolites and cofactors

AMP, ADP, ATP: adenosine mono-, di- and tri-phosphate;

DHAP: dihydroxyacetone phosphate;

NAD: nicotine-adenine-dinucleotide;

NADP: nicotine-adenine-dinucleotide phosphate;

OAA: oxaloacetic acid;

P: phosphate;

PCR: photosynthetic carbon reduction;

Pi: inorganic phosphate;

PPi: inorganic pyrophosphate;

PEP: phosphoenolpyruvate;

PGA: phosphoglyceric acid;

RubP: ribulose-bis-phosphate.

(Hatch and Osmond 1976; Hatch 1987)

apoplast, and decarboxylation in the bundle sheath leads to a 6- to 10-fold increase of CO2-concentration as compared to atmospheric CO2. This has several ecophys-iological advantages which are important for savanna grasses in dry open habitats with high irradiation:

• Under water stress the high CO2-affinity of the first step of CO2-fixation (PEPC) draws down CO2-concentration inside the leaf, providing a steeper gradient for inward diffusion of CO2, and allows operation of photosynthesis with partially closed stomata, which reduces transpiratory loss of H2O. Hence, the water use efficiency of C4-plants is much higher than that of C3-plants, although still lower than that of nocturnal CO2-fixation in CAM plants (Box 10.3).

• The high CO2-affinity of PEPC, together with the simultaneous use of light in refixation of CO2 via RuBISCO, also allows high maximum rates of photosynthesis and high productivity, which in C4-plants are the highest of the three modes of photosynthesis (Box 10.3).

• The high CO2-concentration in the bundle sheath cells reduces photorespiration and the plants are less susceptible to the danger of photoinhibition and photodamage.

Box 10.3

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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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