Leaf senescence and resorption as mechanisms of maximizing photosynthetic production during canopy development at N limitation

1. A canopy N distribution model optimizing photosynthesis was combined with mechanisms for senescence and N resorption to predict canopy leaf area index (LAI) development as a function of canopy nutrient content, N-c . 2. Shedding of leaves at the bottom of the canopy was initiated when it increased canopy photosynthesis, through resorption and redistribution of N from lost leaves. The amount of N resorbed was modelled as a fraction (Rf ) of the N in the leaf prior to senescence. 3. For a fixed N-c , the LAI at which leaf shedding was initiated was calculated for different Rf s for canopies of Amaranthus cruentus , Glycine max , Oryza sativa and Sorghum bicolor . 4. Predicted LAIs exceeded optimal LAIs estimated without leaf shedding and N resorption. LAI increased with increasing Rf . For all four species, the model closely predicted the observed LAIs with Rf=0.7. 5. Area-based resorption efficiency increased and litter N concentration decreased with increasing LAI and average canopy N concentration (mol N m(-2) leaf). A reduction of Rf, e.g. in response to increased soil N availability, decreases resorption efficiency. 6. The model provides a mechanistic basis for interpreting plant nutrient-resorption-LAI relations.