A Generic Equation for Nitrogen-limited Leaf Area Index and its Application in Crop Growth Models for Predicting Leaf Senescence

Appropriate quantification of leaf area index (LAI) is importantfor accurate prediction of photosynthetic productivity by cropgrowth models. Estimation of LAI requires accurate modellingof leaf senescence. Many models use empirical turnover coefficients,the relative leaf-death rate determined from frequent fieldsamplings, to describe senescence during growth. In this paper,we first derive a generic equation for nitrogen-determined photosyntheticallyactive LAI (LAI_N), and then describe a method of using thisequation in crop growth models to predict leaf senescence. Basedon the theory that leaf-nitrogen at different horizons of acanopy declines exponentially, LAI_N, which is counted from thetop of the canopy to the depth at which leaf-nitrogen equalsthe minimum value for leaf photosynthesis, is calculated analyticallyas a function of canopy leaf-nitrogen content. At each time-stepof crop growth modelling, LAI_Nis compared to an independentcalculation of the non-nitrogen-limited LAI assuming no leafdeath during that time-step (LAI_NLD). In early stages, LAI_Nishigher than LAI_NLD; but with the advancement of crop growth,LAI_Nwill become smaller than LAI_NLD. The difference betweenLAI_NLDand LAI_N, whenever LAI_Nis smaller than LAI_NLD, gives theestimate of leaf area senesced at the time-step; the senescedleaf area divided by specific leaf area (SLA) gives the estimateof senesced leaf mass. The method was incorporated into twocrop models and the models adequately accounted for the LAIobserved in field experiments for rice and barley. The novelfeatures of the approach are that: (1) it suggests a coherent,biologically reasonable picture of leaf senescence based onthe link with photosynthesis and leaf nitrogen content; (2)it avoids the use of empirical leaf-turnover coefficients; (3)it avoids over-sensitivity of LAI prediction to SLA; and (4)it is presumably of sufficient generality as to be applicableto plant types other than crops. The method can be applied tomodels where leaf-nitrogen is used as an input variable or issimulated explicitly. Copyright 2000 Annals of Botany Company Leaf area index, leaf senescence, canopy nitrogen, modelling