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 important for accura te prediction of photosynthetic productivity by crop growth models. Estimat ion of LAI requires accurate modelling of leaf senescence. Many models use empirical turnover coefficients, the relative leaf-death rate determined fr om frequent held samplings, to describe senescence during growth. In this p aper, we first derive a generic equation for nitrogen-determined pholosynth etically active LAI (LAI(N)), and then describe a method of using this equa tion in crop growth models to predict leaf senescence. Based on the theory that leaf-nitrogen at different horizons of a canopy declines exponentially , LAI(N), which is counted from the top of the canopy to the depth at which leaf-nitrogen equals the minimum value for leaf photosynthesis, is calcula ted analytically as a function of canopy leaf-nitrogen content. Ar each tim e-step of crop growth modelling, LAI(N) is compared to an independent calcu lation of the non-nitrogen-limited LAI assuming no leaf death during that t ime-step (LAI(NLD)). In early stages, LAI(N) is higher than LAI(NLD); but W ith the advancement of crop growth, LAI(N) will become smaller than LAI(NLD ). The difference between LAI(NLD) and LAI(N), whenever LAI(N) is smaller t han LAI(NLD), gives the estimate of leaf area senesced at the time-step; th e senesced leaf area divided by specific leaf area (SLA) gives the estimate of senesced leaf mass. The method was incorporated into two crop models an d the models adequately accounted for the LAI observed in held experiments for rice and barley. The novel features of the approach are that: (1) it su ggests a coherent, biologically reasonable picture of leaf senescence based on the link with photosynthesis and leaf nitrogen content; (2) it avoids t he use of empirical leaf-turnover coefficients; (3) it avoids over-sensitiv ity of LAI prediction to SLA; and (4) it is presumably of sufficient genera lity as to he applicable to plant types other than crops. The method can be applied to models where leaf-nitrogen is used as an input variable or is s imulated explicitly. (C) 2000 Annals of Botany Company.