Changes in maize hybrid photosynthetic response to leaf nitrogen, from pre-anthesis to grain fill

Improvement of nitrogen use efficiency in maize production requires quantifying genotypic variability in response to soil N. The objective of this held study was to quantify the relationship between leaf N concentration and leaf photosynthetic rate from pre-anthesis to grain fill for six maize (Zea mays L.) hybrids. Maize hybrids were grown at the Central Experimental Farm at Ottawa in Ontario (45 degrees 23'N, 75 degrees 43'W) in 1991, 1992, and 1993 at three N fertilizer rates (0, 100 and 200 kg N ha(-1)). A method of analysis was developed to relate leaf photosynthesis measurements at comparable irradiance levels to leaf N concentration measurements. Photosynthetic response to Leaf N nas characterized by two fitted coefficients, N-0, threshold leaf N concentration for photosynthesis, and P-3, photosynthetic rate at high (30 g N kg(-1) dry weight) leaf N concentration. There were no hybrid differences in No (P > 0.05). There were differences in P-3, notably a significantly lower P-3 value at anthesis for an old hybrid compared with recent commercial hybrids. In 1992, a recent commercial hybrid, Pioneer 3902, had the highest P-3 value, the second highest crop growth rate (CGR) and the highest grain yield, but hybrid differences in P-3 were not generally correlated with CGR or grain yield (P > 0.05). Growth stage had a significant effect on both N-0 and P-3; both declined at later growth stages (P less than or equal to 0.05). The decline in P-3 was greatest in Pride 5 in 1993. Photosynthate production during grain fill was potentially limited in all hybrids by a decline in P-3 as the season progressed, although the degree of decline varied with year and hybrid. This analysis quantified the relationships between Leaf N concentration and Leaf photosynthetic rate for different growth stages and hybrids for evaluation of hybrid N use efficiency.