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

^a Dep. of Plant Agriculture, Crop Science Bldg., Univ. of Guelph, Guelph, ON, Canada, N1G 2W1. Current address of L. Echarte, INTA Balcarce-Universidad Nacional de Mar del Plata, CC 276, 7620 Balcarce, Argentina
^b Dep. of Molecular and Cellular Biology, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1. This study was supported, in part, by the Natural Sciences and Research Council of Canada; Ontario Ministry of Agriculture, Food, and Rural Development; and Syngenta Biotechnology, Research Triangle Park, NC

^* Corresponding author (mtollena{at}uoguelph.ca).

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 x 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.

Abbreviations: CER, carbon exchange rate • CER Abs^–1, CER per unit absorbed photosynthetic photon flux density • CHU, crop heat units • ETR, thylakoid electron transport rate • HI, harvest index • HLN₁, high N level switched to low N level at 1 wk presilking • HLN₂, high N level switched to low N level at 2 wk postsilking • HN, high N availability • LN, low N availability • Na, N content per unit leaf area • PPFD, photosynthetic photon flux density • PSII, Photosystem II

Received for publication June 28, 2007.