Poinsettia (Euphorbia pulcherrima cv. 'Premium Red') as an ornamental pot plant is widely grown in peat-based substrates with high phosphorus (P) fertilization. The aim of the current study was to evaluate the P demand of poinsettia according to its P depletion ability during the growth stages by using a mechanistic simulation model. For this purpose, rooted poinsettia cuttings were grown in the medium with 80: 20 (V:V %) peat+mineral component (generally called as clay) and treated with

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... nt P levels of zero, 10, 35, 100, and 170 mg P [L substrate]-1. The yield and quality performance of the plants were evaluated thoroughly. Also, depletion of P around the root surface and the effect of buffering power on the depletion profile were assessed by means of mechanistic simulation model. The results showed that, in peat-based substrates, P was transported to the root surface mainly by mass-flow. The simulation approach also revealed that the well supplied plants cultivated in the peat-substrates needed a higher concentration gradient (30-50 µM) to drive the necessary flux and that the amount of plant available P (C s) was limiting at later growth stages. The optimum yield and quality of poinsettia was obtained at the P application rate of 35 mg L-1 substrate, with C s of 11-12 and 15-16 mg [L substrate]-1 at planting and 53 days after planting. It was concluded that, to ensure a sufficiently high concentration gradient, P had to be supplemented by frequent fertigation at later growth stages, but not at the early growing stage.