A model of the coupling between respiration, active processes and passive transport

Abstract

A biochemically-aggregated model is introduced which captures the essential features of the coupling between respiration and active (energy-requiring) plant processes. Each active process is characterized as the conversion of ATP and NADPH (represented by X*) and a substrate (S) to ADP and NADP (represented by X) and a product (P) (e.g. for protein synthesis, S = amino-acids, P=protein). For each process, respiration generates X* and CO2 from glucose (C) and X. Respiration and active processes are thus coupled through the turnover of ATP and NADPH, with C and S representing, respectively, the main energetic and material substrates of the overall reaction C + S → CO2 + P. The model assumes mass action kinetics for the reaction rates, and incorporates passive transport of C and S to the reaction sites from an external region (e.g. phloem) with substrate concentrations C(e) and S(e). The behaviour of this coupled respiration - active process - passive transport model is explored analytically. The main results are as follows: (1) In general, the respiration rate coupled to a given active process S → P has a non-rectangular hyperbolic dependence on C(e) and S(e). (2) Because glucose provides both the energetic and material substrates for structural growth (cellulose synthesis), the associated respiration rate is proportional to C(e). (3) When the passive transport of C and/or S for the process S → P becomes limiting, the associated respiration rate reduces to a 'Blackmann response' which is either entirely C-limited or entirely S-limited, depending on the relative availability of C(e) and S(e). (4) These predictions are used to interpret empirically-derived growth and maintenance respiration coefficients, as well as widely-reported observations concerning the respiration/photosynthesis ratio and the response of respiration to carbohydrate concentration. (5) It is concluded that the model provides a simple, realistic, physiologically-based representation of the components of respiration, which can be used in plant growth models that separate substrates from structure. (C) 2000 Annals of Botany Company.