Source-Sink Manipulations during Grain Filling in Maize Grown Under Contrasting N and Temperature Regimes.

Maize yield depends of the interaction between the number of grains set and their average individual weight. It is clearly established that grain number is determined by plant growth during the critical period around silking. However, physiological causes for grain weight determination, in particular in interaction with the number of grains set, are far less clear. Final grain weight depends on the potential size established at around silking, as well as on its realization during the effective grain filling period. Some controversies exist regarding the degree of source-limitation during grain filling in the literature. Most previous source-sink relationship research has been done through modifications in crop management affecting the strengths of both sources and sinks, as well as grain weight potential. In this study we reported results from field experiments for determining the responsiveness of grain weight to source-sink ratios by imposing defoliation and de-graining treatments. Treatments were imposed 15 days after silking to maize plants, and grown under a wide range of N and heat stress conditions. De-graining was performed through a novel approach (also including placebos), carefully opening the husks and provoking alternate rows of grains of the ear to die, and spraying a broad spectrum fungicide, and finally returning the husks to their original position. Final grain number was approximately 50% of control. Grain weight was largely unresponsive to increases in source availability but strongly diminished by reducing the leaf area. These results from direct manipulations strongly agree with conclusions from data available from other treatments. Heat stress directly affected grain growth capacity. Neither defoliation increased nor de-graining diminished the penalty imposed by heat, which in turn was similar for grains of different potential size. Therefore, this study, based on direct manipulations of source-sink relationships under contrasting growing conditions, supported the idea that yield in maize would be limited by the sink-strength during grain filling.