OPTIMIZATION OF TISSUE NITROGEN AND ROOT SHOOT ALLOCATION

A model is explored which describes the joint optimization of tissue nitrogen and root-shoot allocation in response to variation in nitrogen availability in the environment. The model plant is composed of root and shoot biomass and tissue nitrogen, and relative growth rate is the parameter maximized. A ‘source1 (uptake limited) model of joint tissue nitrogen and root-shoot allocation is reviewed (Hilbert, 1990) that optimally balances the uptake of carbon and nitrogen. Modifications including root respiratory costs and fixed root tissue nitrogen are explored. Then the role of tissue nitrogen in regulating ‘sink’ strength is combined with the balanced source model, and modified by assuming separate tissue nitrogen involved in source vs. sink. Generally, the results indicate that as available nitrogen increases, optimal root allocation declines and tissue nitrogen increases. These results appear to be robust for more complicated versions of the model provided that various internal nitrogen compartments ‘compete1 for the same nitrogen. © 1993 Annals of Botany Company.