MODELING CROP GROWTH AND BIOMASS PARTITIONING TO SHOOTS AND ROOTS IN RELATION TO NITROGEN AND WATER AVAILABILITY, USING A MAXIMIZATION PRINCIPLE .1. MODEL DESCRIPTION AND VALIDATION

Many crop models relate the allocation of dry matter between shoots an d roots exclusively to the crop development stage. Such models may not take into account the effects of changes in environment on allocation , unless the allocation parameters are altered. In this paper a crop m odel with a dynamic allocation parameter for dry matter between shoots and roots is described. The basis of the model is that a plant alloca tes dry matter such that its growth is maximized. Consequently, the de mand and supply of carbon, nitrogen, and water is maintained in balanc e. This model supports the hypothesis that a functional equilibrium ex ists between shoots and roots. This paper explains the mathematical co mputation procedure of the crop model. Moreover, an analysis was made of the ability of a crop model to simulate plant dry matter production and allocation of dry matter between plant organs. The model was test ed using data from a greenhouse experiment in which spring wheat (Trit icum aestivum L.) was grown under different soil moisture and nitrogen (N) levels. Generally, the model simulations agreed well with data re corded for total plant dry,matter. For validation data the coefficient of determination (r(2)) between simulated and measured shoot dry weig ht was 0.96. For the validation treatments r(2) was slightly lower, 0. 94. In addition to dry matter production the model succeeded satisfact orily in simulating the dry weight of different plant organs. The resp onse of simulated root to shoot ratio to the level of soil moisture wa s mainly in accordance with the measured data. In contrast, the simula ted ratio seemed to be insensitive to the changes in the levels soil N concentration used in the experiment. The data used in the present st udy were not extensive, and more data are needed to validate the model . However, the results showed that the model responses to the changes in soil N and water level were realistic and mostly agreed with the da ta. Thus, we suggest that the model and the method employed to allocat e dry matter between roots and shoots are useful when modelling the gr owth of crops under N and water limited conditions.