A MECHANISTIC ANALYSIS OF LIGHT AND CARBON USE EFFICIENCIES

We explore the extent to which a simple mechanistic model of short-ter m plant carbon (C) dynamics can account for a number of generally obse rved plant phenomena. For an individual, fully expanded leaf, the mode l predicts that the fast-turnover labile C, starch and protein pools a re driven into an approximate or moving steady state that is proportio nal to the average leaf absorbed irradiance on a time-scale of days to weeks, even under realistic variable light conditions, in qualitative agreement with general patterns of leaf acclimation to light observed both temporally within the growing season and spatially within plant canopies, When the fast-turnover pools throughout the whole plant (inc luding stems and roots) also follow this moving steady state, the mode l predicts that the time-averaged whole-plant net primary productivity is proportional to the time-averaged canopy absorbed irradiance and t o gross canopy photosynthesis, and thus suggests a mechanistic explana tion of the observed approximate constancy of plant light-use efficien cy (LUE) and carbon-use efficiency. Under variable light conditions, t he fast-turnover pool sizes and the LUE are predicted to depend negati vely on the coefficient of variation of irradiance, We also show that the LUE has a maximum,vith respect to the fraction of leaf labile C al located to leaf protein synthesis (a(lp)), reflecting a trade-off betw een leaf photosynthesis and leaf respiration. The optimal value of a(l p) is predicted to decrease at elevated [CO2](a), suggesting an adapti ve interpretation of leaf acclimation to CO2. The model therefore brin gs together a number of empirical observations within a common mechani stic framework.