Under conditions of phosphorus deficiency, reductions in plant leaf area have been attributed to both direct effects of phosphorus (P) on the individual leaf expansion rate and a reduced availability of assimilates for leaf growth. Simulation techniques have been used to test the hypothesis of a non-trophic limitation to leaf expansion in wheat grown on P-deficient soils; this study provides further experimental evidence supporting this hypothesis. In a field experiment, we studied the effects of soil P additions (0–200 kg P2O5 ha-1) and assimilate availability (non-shaded and shaded treatments) on the expansion of individual leaves of wheat (Triticum aestivum L. cv. Oasis), light interception and radiation use efficiency. The soil was low in P (5.5 g P g-1), the crop was drip irrigated and nitrogen was applied at non-limiting rates during the experiment. At 61 days after emergence (DAE), phosphorus deficiency reduced above ground biomass 70 and 61°leaf area index 73 and 69°the percent intercepted radiation 63 and 46°and radiation use efficiency 21 and 31°in non-shaded and shaded plots, respectively. Leaves of shaded plants had lower C and lower accumulation of structural dry weight than non-shaded ones. In shaded leaves, there was a more important restriction to the accumulation of structural material in high-P than in low-P treatments. This and the fact that leaf expansion rate of individual leaves was negatively related to C led us to conclude that leaf expansion in low P plants is unlikely to be limited by the availability of assimilates.