Maize senescence under contrasting source-sink ratios during the grain filling period

Abstract

Grain growth and leaf senescence are the main processes during post-silking in maize. An unanswered question is how changes in sink strength (the number of grains to be filled) affect leaf senescence. We studied the effect of changes in the source-sink ratio during grain filling on the dynamics of green leaf area and photosynthetic rate. Maize plants were exposed to ear removal, pollination prevention, degraining or defoliation treatments during post-silking in two sowing dates. Green leaf area, leaf photosynthetic rate and leaf greenness were periodically measured. At maturity, yield, grain weight, and stem dry weight were determined. Green leaf area loss showed a first phase of slow senescence followed by a second phase of rapid senescence in all conditions. The response of green leaf area duration (GLAD) to the source-sink treatments depended on environmental conditions: ear removal increased GLAD while defoliations reduced it in the early sowing date, though GLAD was unaffected by these treatments in the late sowing date. But any change in senescence was less than proportional to the changes in the source-sink ratio. Leaf photosynthetic rate or leaf greenness (SPAD) did not change with the source-sink treatments. Defoliation caused a remarkable remobilisation of reserves from the stem, which did not fully mitigate the reduction in grain weight. It is concluded that (i) manipulations of source-sink balances modified slightly leaf longevity but not the functionality of the remaining leaf area (leaf photosynthetic rate), the response of GLAD being dependant on environmental conditions, and (ii) grain weight was more susceptible than GLAD to reductions in source-sink ratio.