Vertical profile of leaf senescence during the grain-filling period in older and newer maize hybrids

Grain yield improvement of maize (Zea mays L.) hybrids has been associated with delayed leaf senescence. The objective of this study was to quantity the vertical profile of leaf senescence during the grain-filling period in an older hybrid ('Pride 5') and two more recent maize hybrids ('Pioneer 3902' and 'Pioneer 3893'). Leaf senescence was rated visually from silking to maturity on each individual leaf across the vertical leaf-area profile along the stem of maize plants growing in the field at 1, 3.5, and 12 plants m(-2) near Elora, ON, Canada, during the 1999 to 2001 growing seasons. Maximum leaf area index (LAI) at silking was greater in newer hybrids than in the older hybrid. Rate of leaf senescence across hybrids and plant population densities progressed at a linear rate of 0.44% d(-1) during the first half of the grain-filling period, whereas the rate was 1.87% d(-1) during the second half of the grain-filling period. Rates of leaf senescence were 3.4 and 2.1 times greater in the older hybrid than in the newer hybrids during the first and second half of the grain-filling period, respectively. During the first half of the grain-filling period, leaf senescence increased from the medium to the highest plant population density, whereas rates of senescence during the second half of the grain-filling period declined with an increase in plant population density for the older hybrid and rates were lowest at the medium plant population density for the newer hybrids. A top-bottom profile of leaf senescence was observed during the second half of the grain-filling period, with leaves in the central section of the canopy being the last leaves to senesce, and this phenomenon was more marked in the newer hybrids.