Effect of Nitrogen Fertilizer on Photosynthesis of Several Varieties of Winter Wheat

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Abstract
The rates of gross photosynthesis of the flag leaf and the next leaf below (second leaf) in crops of winter wheat were estimated from the 14C uptake of the leaves after exposure to short pulses of 14CO2. The photosynthetic rates of both leaves during the grain-filling period decreased with increase in nitrogen fertilizer because the intensity of photosynthetically active radiation was less at the surface of the leaves in the dense crops with additional nitrogen. In addition, the rate of photosynthesis at saturating light intensity was slightly decreased by nitrogen. The effects of nitrogen, in decreasing the rate of photosynthesis per unit area of leaf and in increasing the leaf-area index of the top two leaves, were such that the photosynthetic productivity per unit area of land of the flag leaf was increased by nitrogen but the productivity of the second leaf was unaffected. Applying 180 kg N ha−1 increased the productivity of the top two leaves by a factor of 2.3 but increased grain yield by only 1.8. The photosynthetic productivity of the second leaf during the grain-filling period was about half that of the flag leaf. There was no difference in photosynthetic rate per unit area of leaves of Cappelle-Desprez and Maris Huntsman which could account for the larger yield of the latter cultivar. There was a slight indication that the leaves of the semi-dwarf cultivars Maris Fundin and Hobbit photosynthesized faster than those of Maris Huntsman.