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a Department of Chemistry and Biochemistry Brigham Young University, Provo, UT 84602
b Department of Botany and Range Science Brigham Young University, Provo, UT 84602
c Department of Botany University of Western Australia Nedlands, Western Australia 6907
* Corresponding author (lee_hansen{at}byu.edu)
Component models of plant respiration (Amthor, 2000; Cannell and Thornley, 2000; Thornley and Cannell, 2000; Loomis and Amthor, 1999) have the general form
 | [1] |
Amthor (2000) states "both gR and mR can be estimated empirically by simultaneously measuring R and other variables, or calculated mechanistically from underlying process data." Regarding mechanistic calculation of mR, Thornley and Cannell (2000) state, "even when the quantifiable components of maintenance respiration are accounted for, a large ‘residual maintenance’ term remains." Evaluation of mR and gR by regressing G against R (Chiariello et al., 1989) assumes mR is a constant with tissue age and that the concentration of respiratory organelles is constant, both invalid assumptions.
The growth coefficient has been evaluated through "calculation of the amount of substrate and respiration involved in its (biomass) synthesis if all goes well in the plant and least-cost pathways are actually used" (Loomis and Amthor, 1999). This "true growth yield," YG, (Loomis and Amthor, 1999) "can also be estimated from complete ... or partial ... elemental analyses of biomass" (Loomis and Amthor, 1999) or from heat of combustion of the tissue (Gary et al., 1995). But, this calculated YG (or gR) is unrealistically high for two reasons. First, the goal of the calculation is "to determine maximum potential efficiency of growth" (Amthor, 2000), and second, the calculation includes only first law energy costs, second law costs, which are about three times greater, are not included.
The maintenance rate is said to "increase(s) with temperature with Q10 of about 2" (Loomis and Amthor, 1999), but proof for this statement is lacking because mR cannot be measured except at zero growth rate. The growth coefficient is assumed to be "temperature independent to the extent that substrates, pathways and biomass composition are temperature independent" (Amthor, 2000). However, even with these conditions, the energy cost of constructing a unit of new biomass changes with temperature (Jou and Llebot, 1990; p. 90–95, 108–118). Calculation of this cost (i.e., the entropy increase in the surroundings) for oat seedling growth from data recently collected in our lab show it to double from 15 to 25°C, i.e., from 8 to 16 J g-1 K-1.
The coefficients in component models cannot be measured and are not directly related to underlying biochemistry. As Amthor (2000) says, "Defining maintenance is tricky." And quoting Cannell and Thornley (2000), "Since a rigorous definition of maintenance respiration is elusive, it is hardly surprising that it is difficult to measure a maintenance coefficient unambiguously." Adding more components only adds complexity, which makes the problem worse. A change in direction is needed if progress is to be made toward a better understanding of the respiration–growth relation.
Not separating respiration into components reduces complexity and leads to more useful results via Eq. [2], where Y is the substrate carbon conversion efficiency.
 | [2] |
Y can be calculated from total carbon balances or from measurements of respiratory heat and CO2 rates (e.g., see van Iesel and Seymour, 2000; Criddle and Hansen, 1999). As an example of the superiority of this approach, component models cannot explain the physiological function of the alternative oxidase, but pathways that reduce efficiency are a natural outcome of models that treat respiration as an energy-coupled system of reactions with condition-dependent variable coupling (Westerhoff and van Dam, 1987).
Received for publication March 10, 2001.
REFERENCES
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