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

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 profess 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.