Radiation Use Efficiency of an Old and a New Maize Hybrid
Differences in dry matter accumulation among crop cultivars can be attributed to differences in either the absorptance of incident photosynthetically active radiation (PAR) and/or the conversion of absorbed PAR into dry matter. This study was conducted to quantify the contribution of radiation use efficiency (RUE, g dry matter MJ−1) to the difference in dry matter accumulation between an old and a new maize (Zea mays L.) hybrid. An old (Pride 5) and a new (Pioneer 3902) maize hybrid were grown during 1989 and 1990 at the Elora Research Station, Ontario, on a London loam (Aquic Hapludalf) soil that had been tile drained. Both hybrids were grown at their respective optimum plant density for grain yield (i.e., 7 plants/m2 for Pride 5 and 8 plants/m2 for Pioneer 3902) and, in addition, the new hybrid was grown at 5 plants/m2 which resulted in a leaf area index (LAI) similar to that of the old hybrid grown at its optimum plant density. Starting at silking, 2-wk crop growth rates (CGR, kg ha−1 d−1) were measured until physiological maturity, and incident PAR and absorptance of PAR by the three treatments were monitored during three 2-wk periods after silking. From silking to 6 wk postsilking, PAR absorption did not differ among the two hybrids grown at plant densities that resulted in similar LAIs, but crop growth rate was 19% larger for the new hybrid. The difference in crop growth rate between the old and the new hybrid grown at their respective optimum plant density was 33%; approximately 80% of the difference could be attributed to a higher RUE of the new hybrid. The difference in RUE was not associated with a change in the distribution of PAR across the canopy, i.e., extinction coefficients did not differ among treatments. Dry matter accumulation declined for all treatments after 8 wk postsilking, but the decline was steeper for the old hybrid and the decline was associated with rate of leaf senescence. In conclusion, increased dry matter accumulation of new maize hybrids after silking can be attributed, in a large part, to increased radiation use efficiency.
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