The Physiology of Flowering Plants - KHAM PHA MOI

48 FLOW OF ENERGY AND CARBON THROUGH THE PLANTTable 2.2 Respiration rates of a variety of plant tissues at 25 8C. Where therate has been measured at some other temperature, the rate for 25 8C has beencalculated assuming a Q 10 of 2 (i.e. a 2-fold change per 10 8C). Rates are given asO 2 uptake per unit dry weight; the Arum spadix exhibits thermogenicrespiration. From Öpik (1980).TissueRespiration rate(mL O 2 h –1 (mg dry weight) –1 )Resting dry seeds 0.0005Resting tubers, storage roots 0.23–2.6Leaves 0.80–5.9Young growing root tips up to 15Flower spadix of Arum maculatum 600alternative oxidase is not coupled to ATP synthesis and the energy isliberated as heat. One ATP per NADH can be formed before theelectrons enter the alternative pathway but presumably this doesnot suffice to cover the cell’s energy needs. The physiological significanceof the alternative pathway is therefore much disputed. There isone situation in which a clear physiological role can be ascribed tothe alternative oxidase, namely in the floral organs of certain speciesof the Araceae, the arum family. In these plants, when the flowers areready for pollination, a sterile part (spadix) of the inflorescenceundergoes extremely rapid thermogenic respiration (Table 2.2)which causes the tissue to heat up by 10–20 8C above the ambient.This thermogenic respiration utilizes the alternative oxidase; it canproceed so fast because the alternative oxidase is not subject to thefeedback controls associated with ATP-producing electron transport(see below, Section 2.11.2). The heat volatilizes chemicals whichattract pollinating flies (although to the human nose the smell ishorrible!). In some Araceae, the inflorescence melts its way upthrough snow. But the aroid inflorescence is a special case; the alternativeoxidase is found in many species and in all types of plantorgan. Attempts have been made to explain the alternative oxidaseas an ‘overflow system’ enabling plants to dispose of surplus reducingpower when a large amount of C skeletons is withdrawn fromrespiratory pathways while there is a low demand for ATP. Theweakness of this argument is that a process requiring large amountsof metabolic intermediates (e.g. growth) is also likely to require alarge supply of ATP and/or reductant. Another suggestion is that thealternative oxidase prevents the build-up of damaging superoxideradicals during electron transport via the cytochrome system.2.9.3 The ATP balance sheet and energy-conversionefficiency of respirationThe theoretical balance sheet for respiratory ATP production is easilydrawn up. If glucose is completely oxidized via glycolysis and