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

Abstract

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.