A detailed growth analysis has been conducted to unravel the separate effects of nitrogen and phosphorus nutrition on growth of young tomato plants (Lycopersicon 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, m2 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
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