357 Response of Growth of Tomato to Phosphorus and Nitrogen Nutrition C.C. de Groot 1 and L.F.M. Marcelis R. van den Boogaard Plant Research International ATO P.O. Box 16 P.O. Box 17 6700 AA Wageningen 6700 AA Wageningen The Netherlands The Netherlands H. Lambers School of Plant Biology University of Western Australia Crawley WA 6009 Australia Keywords: nutrient limitation, relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), cytokinin, dry-mass partitioning Abstract A detailed growth analysis has been conducted to unravel the separate effects of nitrogen and phosphorus nutrition on growth of young tomato plants (Lyco- persicon esculentum Mill. cv. Capita) and to study their interaction. We show that relative growth rate increased sharply with increasing plant P concentration before it levelled off, resulting in a broad plateau, while the response of relative growth rate (RGR, mg g -1 day -1 ) to increasing plant N concentration was gradual and levelled off at high N concentrations, resulting in a small plateau. Possible causes of this different shaped response are discussed. Furthermore, we show that the importance of net assimilation rate (NAR, g m -2 day -1 ) and leaf area ratio (LAR, m 2 kg -1 ) in explaining the effects of N and P on growth may depend on the severity of the nutrient limitation. Finally we discuss the hypothesis that the regulation of the measured increase in dry-mass partitioning to the roots with decreasing N and P supply and the measured decrease in plant N concentration with decreasing P supply may be mediated by cytokinins. This paper summarises and discusses some of the data described in earlier papers (De Groot et al., 2001; 2002; 2003) on the regulation of growth by P and N nutrition. INTRODUCTION Besides sunlight and water, nitrogen and phosphorus can be major limiting factors for plant growth. The use of N and P fertilisers has increased dramatically over the last four decades. In greenhouses, crops like tomato are often grown on artificial substrates (e.g. rockwool). Plants are irrigated with water that contains a surplus of nutrients and this water may be recirculated. The uptake of nutrients by the crop may deplete some nutrients while other accumulate. Depletion as well as accumulation of nutrients may negatively affect crop growth, production and quality of products. The regular renewal of process water to counteract these negative effects is a burden for the environment. An accurate tuning of the supply of nutrients to the demand by the plant is necessary to minimise accumulation and prevent depletion of nutrients. Furthermore, this may be used to control the product quality. To achieve this accurate tuning, detailed knowledge of the regulation of growth by nutrients and their interaction with environmental factors is necessary. Furthermore, information on interactions between these nutrients is essential. A detailed growth analysis has been carried out to unravel the separate effects of N and P on growth of young tomato plants, and to study their mutual interaction. This paper summarises and discusses some of the data described in earlier papers (De Groot et al., 2001; 2002; 2003) on the regulation of growth by P and N nutrition. 1 current address: Bejo Zaden B.V., P.O. Box 50, 1749 ZH, Warmenhuizen, The Netherlands Proc. XXVI IHC  Protected Cultivation 2002 Ed. A.P. Papadopoulos Acta Hort. 633, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA)