Plant and Soil 202: 133–147, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 133 Leaf area expansion and assimilate production in sunflower ( Helianthus annuus L.) growing under low phosphorus conditions Daniel Rodr´ ıguez 1,4 , M.M. Zubillaga 1 , E.L. Ploschuk 2 , W.G. Keltjens 3 , J. Goudriaan 4 and R.S. Lavado 1 1 Fertilidad y Fertilizantes, Facultad de Agronom´ ıa U.B.A. Av. San Martin 4453, Buenos Aires (1417), Argentina, 2 I.F.E.V.A., Facultad de Agronom´ ıa U.B.A., 3 Sub-department of Soil Science and Plant Nutrition, Wageningen Agricultural University, Dreijenplein 10, 6703 HB, Wageningen, and 4 Sub-department of Theoretical Production Ecology, Wageningen Agricultural University, P.O. Box 430, 6700 AK, Wageningen, The Netherlands ∗ Received 30 June 1997. Accepted in revised form 24 April 1998 Key words: leaf expansion rate, modelling, phosphorus, photosynthesis, sunflower Abstract Reductions in leaf area and plant growth as a consequence of phosphorus (P) limitations have been attributed both to direct effects of P shortage on leaf expansion rate and to a reduced production of assimilates required for growth. Canopy assimilation and leaf area expansion are closely interrelated processes. In this work we used experimental and simulation techniques to identify and study their importance in determining leaf area on sunflower ( Helianthus annuus L.) growing under P-deficient conditions. Experiment 1 was done outdoors, in Buenos Aires, Argentina, and Experiment 2 in a glasshouse in Wageningen, The Netherlands. In both experiments we studied the effects of soil P addition on leaf appearance, leaf expansion, dry matter accumulation, and leaf photosynthesis of non-water stressed plants grown in pots containing a P-deficient soil. Before sowing the equivalent amounts of 0–600 kg of super phosphate ha −1 were added to the pots. Phosphorus deficiency delayed leaf appearance increasing the value of the phyllochron (PHY) up to 76%, the rate of leaf area expansion during the quasi-linear phase of leaf expansion (LER) was reduced by up to 74%, with respect to high P plants. Phosphorus deficiency reduced by up to 50% the rate of light saturated photosynthesis per unit of leaf area (AMAX) in recently expanded leaves, while at low levels of leaf insertion in the canopy, AMAX was reduced by up to 85%, when compared to that in high P plants. Phosphorus deficiency also reduced the duration of the quasi-linear phase of leaf expansion by up to eight days. The values of LER were related (r = 0.56, P < 0.05) to the mean concentration of P in all the leaves (Leaves P%) and not to the concentration of P in the individual leaf where LER was determined (r = 0.22, P < 0.4) suggesting that under P deficiency individual leaf expansion was not likely to be regulated by the total P concentration at leaf level. The values of AMAX of individual leaves were related (r = 0.79, P < 0.01) to the concentration of total P in the corresponding leaf (Leaf P%). LER showed a hyperbolic relationship with Leaves P% (R 2 = 0.94, P < 0.01, n = 13) that saturate at 0.14%. AMAX showed a hyperbolic relationship with Leaf P% (R 2 = 0.73, P < 0.01, n = 53) that saturated with values of Leaf P% higher than 0.22. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the effects of model parameters on the growth of sunflower plants under P-deficient conditions. With this model we identified the existence of direct effects of P deficiency on individual leaf area expansion. However, we calculated that under mild P stress conditions up to 83% of the reduction in the observed leaf area was explained by the particular effects of P% on the rate of leaf appearance, on the duration of the linear period of leaf expansion, ∗ Present address of corresponding author. FAX No: 31 317 484892. E-mail: Daniel.Rodriguez@staff.tpe.wau.nl