Does assimilate supply limit expansion in wheat grown in the field under low phosphorus availability

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.