The Physiology of Flowering Plants - KHAM PHA MOI

54 FLOW OF ENERGY AND CARBON THROUGH THE PLANTThe relationship between respiration and growth has been thesubject of much study and speculation. As already noted, growingtissues are characterized by high rates of respiration. The quantitativerelationship between growth rate and respiration rate is, however,not a simple one. On the whole, plants with higher growth ratestend to have the higher respiration rates, although there are alsoreports to the contrary, e.g. of faster-growing genotypes of a grass(Lolium perenne) having lower respiration rates. When plant systemsare compared, the differences in their rates of respiration are typicallyless than the differences in their growth rates. In a study withnine species of grasses, for a 2–3-fold higher relative growth rate, RGR(for definition of RGR see Section 6.6.1), the increase in respirationwas only 1.4–1.7-fold (Scheurwater et al. 1998). Explanations must besought in differences in metabolism, resulting in a greater efficiencyof respiratory energy production (or utilization) in the faster-growingspecies. One such difference might be in the degree to which thealternative oxidase (Section 2.9.2) participates in the respiration ofdifferent tissues. In a study of root respiration and growth in fourinbred lines of the greater plantain (Plantago major) it was found thathigher RGR were associated with a higher proportion of respirationpassing through the more energy-efficient cytochrome pathway asopposed to the alternative oxidase.An understanding of relationships between growth and respirationis of great relevance to considerations of plant productivity.Respiration results in a loss of biomass, even while it isnecessarytosupportgrowth.Therehavebeenargumentsastowhether it is more favourable to breed a crop with a low respirationrate, for minimal loss of biomass, or with a high rate, associatedwith a high RGR. It appears now that it is not just overall respirationrate that is relevant to productivity. To maximize productivity, onewould need to maximize the efficiency with which the potentialenergy of the respiratory substrates is converted to usable form –ATP and reduced coenzymes. We are not yet certain wherein thisefficiency lies.2.11.2 Metabolic control of rates: feedback mechanismsRespiration rate is geared to the requirements of tissues by numerousmechanisms. One of these is the cellular concentration of ADP, andthe ATP : ADP ratio. Respiration produces ATP, growth and maintenanceprocesses consume it and produce ADP, the total cellularamount of [ATP + ADP] remaining constant for prolonged periods.ADP is an essential reactant in respiration. It is directly used insubstrate-level phosphorylations, and mitochondrial electron transportthrough cytochrome oxidase is normally coupled to oxidativephosphorylation, which requires ADP. When there is an increase inan ATP-utilizing cellular activity, more ATP per unit time is convertedto ADP. The higher ADP concentration stimulates a faster rate ofsubstrate breakdown and mitochondrial electron transport – with a