2.4.9  Energetics of respiration

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(a)  Efficiency

Respiration represents a substantial loss of carbon from a plant, and under adverse conditions can be as high as two-thirds of the carbon fixed daily in photosynthesis. Both the rate and the efficiency of respiration will therefore affect plant growth significantly. The overall process of respiration results in the release of a substantial amount of energy which may be harnessed for metabolic work. In theory, the energy released from the complete oxidation of one molecule of glucose to CO2 and H2O in respiratory reactions leads to the synthesis of a net equivalent of 36 molecules of ATP. However, in plants, because there are alternative routes for respiration, this yield can be greatly reduced.

Mechanisms for regulating respiration rates in whole plants remain unclear. Convention has it that the rate of respiration is matched to the energy demands of the cell through feed-back regulation of glycolysis and electron transport by cytosolic ATP/ADP. However, since plants have non-phosphorylating bypasses in their respiratory chain that are insensitive to ATP levels, and since PEP carboxylase and PFP might be involved in sucrose degradation, the situation in vivo is not so simple. For example, the rotenone-insensitive bypass of complex I requires high concentrations of NADH in the matrix before it can operate and seems to be active only when substrate is plentiful and electron flow through complex I is restricted by lack of ADP. Alternative oxidase activity also depends on carbon and ADP availability. In other words, non-phosphorylating pathways act as carbon or reductant ‘overflows’ of the main respiratory pathway and will only be active in vivo when sugar levels are high and the glycolytic flux rapid, or when the cytochrome chain is inhibited during stress. In glycolysis, the interaction between environmental signals and key regulatory enzymes, as well as the role of PFP and its activator fructose-2,6-P2, will be important.

(b)  Allocation of respiratory energy to process physiology

One way of viewing respiratory cost for plant growth and survival is by subdividing measured respiration into three components associated with (1) nutrient acquisition, (2) growth and (3) maintenance. Such conceptual distinctions are somewhat arbitrary, and these categories of process physiology must not be regarded as three discrete sets of biochemical events. Such energy-dependent processes are all interconnected to some extent because ATP represents a universal energy currency for all three, while a common pool of substrates is drawn upon in sustaining production of that ATP (Amthor 1989). Nevertheless, cells do vary in their respiratory efficiency, while genotype × environment interactions are also evident in both generation and utilisation of products from oxidative metabolism. Such variation has implications for growth and resource use efficiency (Section 6.5).

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