Modelling the components of plant respiration: Some guiding principles

Respiration is poorly represented in whole plant or ecosystem models relati ve to photosynthesis. This paper reviews the principles underlying the deve lopment of a more mechanistic approach to modelling plant respiration and t he criteria by which model behaviour might be judged. The main conclusions are as follows: (1) Models should separate C substrate From structure so th at direct or indirect C substrate dependence of the components of respirati on can be represented. (2) Account should be taken of the fact that some of the energy for leaf respiration is drawn from the light reactions of photo synthesis. (3) It is possible to estimate respiration associated with growt h, nitrate reduction, symbiotic N-2 fixation, N-uptake, other ion uptake an d phloem loading, because reasonable estimates are available of average spe cific unit respiratory costs and the rates of these processes can be quanti fied. (4) At present, it is less easy to estimate respiration associated wi th protein turnover, maintenance of cell ion concentrations and gradients a nd all forms of respiration involving the alternative pathway and futile cy cles. (5) The growth-maintenance paradigm is valuable bur 'maintenance' is an approximate concept and there is no rigorous division between growth and maintenance energy-requiring processes. (6) An alternative 'process-residu al' approach would be to estimate explicitly respiratory fluxes associated with the six processes listed in (3) above and treat the remainder as a res idual with a phenomenological 'residual maintenance' coefficient. (7) Maint enance or 'residual maintenance' respiration rates are often more closely r elated to tissue N content than biomass, volume or surface area. (8) Respir atory fluxes associated with different processes vary independently, season ally and during plant development, and so should be represented separately if possible. (9) An unforced outcome of mechanistic models should be a cons trained, but non-constant, ratio between whole plant gross photosynthesis a nd respiration. (C) 2000 Annals of Botany Company.