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The effect of phosphorus deficiency on yield formation in spring wheat (Triticum aestivum cv. Prointa Oasis) wits investigated, focusing on Crop growth and dry matter partitioning during the spike growth period (SGP), which is critical for grain number determination. Two experiments combining shading and P deficiency were performed at Balcarce, Argentina (37 degrees 45’s). The main treatments were two levels of son P-availability: low P, it naturally low P fertility son (7.0 and 5.5 mg Bray extractable P/kg son, in the first and second experiments respectively) and high P, it P dose that does not limit growth. The sub-treatments were two levels of radiation (shaded and control). Phosphorus deficiency affected yield mainly through the number of grains/unit surface (m(2)). Differences in grain number/m(2) were related to differences in dry weight of spikes/m(2), measured 7 days after anthesis, excluding grain weight. The duration of the SGP did not change much as result of P deficiency: 27 days with high P and only 3 days more with low P. Therefore, changes in spike dry weight were mainly due to differences in spike growth rate. In turn, the spike growth rate of all treatments was linearly related to crop growth rate, with little effect of dry matter partitioning to spikes. Finally, differences in crop growth rate between P treatments were mainly determined by the amount of intercepted radiation. It was concluded that P deficiency resulted in a reduction in intercepted radiation during the SGP, thus causing it reduction in grain number and crop yield.