The Response of Leaf Photosynthesis and Dry Matter Accumulation to Nitrogen Supply in an Older and a Newer Maize Hybrid

Nitrogen use efficiency is higher in newer than in older maize ( Zea mays L.) hybrids, but the physiological mechanisms underlying differences in N‐use efficiency are unknown. The objective of this study was to quantify differences between an older and a newer maize hybrid in their response to N availability throughout the life cycle at both the leaf and the whole‐plant level. An older and a newer maize hybrid were grown in a field hydroponic system located near Guelph, ON, in 2005 at a high and a low N level. Leaf carbon exchange rate (CER), chlorophyll index, and the thylakoid electron transport rate (ETR) were measured weekly from 2 wk presilking to 8 wk postsilking. Plant‐component dry matter and N content were determined from 1 wk presilking to maturity. At the leaf level, leaf CER declined during the grain‐filling period, and the decline was greater under low than high N availability. The decline in leaf CER during the grain‐filling period was less in the newer than in the older hybrid under both high and low N availability, and differences in leaf CER were associated most strongly with a reduction in leaf CER per unit absorbed photosynthetic photon flux density. At the whole‐plant level, reduction in grain yield in low vs. high N was greater in the older than in the newer hybrid. The hybrid × N interaction for grain yield was attributable predominantly to a greater decline in the proportion of dry matter allocated to the grain in the older hybrid.