Nitrogen supply and sink demand modulate the patterns of leaf senescence in maize

Abstract

Senescence is a key physiological process that can regulate crop grain yield. Patterns of leaf senescence and its association with grain yield for a short maturity maize hybrid were investigated in a factorial combination of two tillage treatments (conventional and no-till), three amounts of stubble (0, 3 and 5 t ha−1) and three N rates (0, 80 and 120 kg N ha−1) over three seasons in 2015 long rains, 2015/2016 short rains and 2016 long rains. Leaf senescence from flowering to harvest was assessed at (a) the whole-plant scale by the visual scoring of dry leaves and (b) the canopy-layer scale by measuring leaf greenness with a SPAD 502 chlorophyll meter. A bilinear model was used to quantify the patterns of senescence at the whole-plant scale. A logistic function was fitted to estimate the traits of senescence at three canopy layers (top, mid, bottom), including minimum and maximum SPAD, onset of senescence (EC90), time to loss of 50% maximum SPAD (EC50) and the rate of senescence in each layer. Nitrogen rate effect on patterns and traits of senescence were large and its interactions with stubble were more frequent than interactions between other treatments. Tillage and stubble amount had marginal effects. EC50 was delayed in the unfertilized controls compared with fertilized crops and was negatively correlated with grain yield. Rate of senescence was faster in fertilised crops compared with unfertilized controls at both whole-plant and canopy-layer scales. Grain yield, grain number and nitrogen remobilization efficiency were associated with faster rates of senescence in the top and mid leaves but with slower rates of senescence in the bottom layer leaves. We advance a sink-driven leaf senescence ideotype for high yield and efficient use of nitrogen for short maturity maize.