Effect of Altered Source‐Sink Ratio on Growth of Maize Kernels1

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Abstract
Grain yield in maize (Zea mays L.) is a function of the relationship between assimilate supply to the grain and inherent potential of the grain to accommodate this assimilate. In this study we sought to ascertain the effects of altering assimilate supply per kernel during early and late kernel development on kernel growth rate and final weight. We also sought to elucidate the effect of such alterations on the plant's compensatory responses, specifically kernel abortion and carbohydrate/ N redistribution. The single‐cross hybrid M14 ✕ W64A, selected because of intermediate weight per kernel was grown in field plots in 1979, 1980, and 1981. Assimilate enhancement per kernel was achieved by ear tip removal, which resulted in a 20 to 40% reduction in kernels per ear. Total defoliation was the assimilate reduction treatment. Both treatments were applied 12 days (early) and 24 days (late) after mid‐silking. Early defoliation greatly reduced both kernel growth rate and filling period duration, resulting in lower weight per kernel. These defoliation effects occurred despite large compensatory reductions in number of kernels per ear on treated plants. Soluble carbohydrate content of the internode above the ear of defoliated plants declined following defoliation, probably in conjunction with remobilization of stem reserves. This decline was more rapid for the later defoliated plants. In contrast, N concentration of this stem internode of defoliated plants was higher than in control plants and did not decline after treatment. Soluble carbohydrate and N concentration in the internode above the ear of plants in which ear size was reduced was higher than in control plants. Nitrogen concentration of the grain was also increased. However kernel growth rate, final kernel weight, and soluble carbohydrate concentration at maturity were not affected by this treatment. These results show that the remobilization patterns for stems in response to changes in assimilate supply, differs for N as compared to soluble carbohydrates. In addition, the data suggest that kernel growth rates and final seed size may be already approaching the upper limits of their potential in this hybrid. Undetermined factors other than assimilate supply appear to be restraining higher kernel growth rates and final weights.