Effects of source and sink manipulations on the nutrient partitioning of maize (Zea mays L.)

It has not been clearly demonstrated that whether the source supply or the sink demand is the main factor in determining grain quantity and quality of maize. The effect of an altered sink capacity on dry matter accumulation and partitioning was studied by modifying the endosperm genotype of maize hybrids differing in kernel dry matter accumulation kinetics and N-responsiveness. Another purpose of this research was to understand if the relative content of carbon (C) and nitrogen (N) in kernels is source or sink determined through genetic, physiological, and developmental approaches. The effect of an altered sink on kernel weight, kernel protein content and grain yield is hybrid-specific. The kernel weight, kernel protein content, and grain yield of P3732 were significantly increased by modifying the endosperm genotype with B73 x Mo17 pollen. On the other hand, no increase in kernel weight, kernel protein content, and grain yield was observed when the endosperm genotype of B73 x Mo17 was modified by cross-pollination with P3732 pollen. The increases in dry matter and N accumulation in the crossed kernels of P3732 were the result of a prolonged duration of grain-fill rather than a higher rate of dry matter and N accumulation during grain-fill. The additional assimilates which accumulated in the crossed kernels were attributable to increased assimilation rather than improved partitioning efficiency. In addition, the N use efficiency of P3732, a low N response hybrid, was improved by an increased sink capacity. Analysis of kernels obtained from reciprocal crosses between IHP (Illinois High Protein) and ILP (Illinois Low Protein), and between IHP and Mo17 show a clear maternal effect in determining the kernel C/N ratio. Furthermore, the C/N ratio of individual F2 or F3 kernels from the same ear did not segregate. In a separate experiment, the C/N ratio of nutrients entering 14, 21, and 28 day-old endosperms of B73 x LH51, incubated in vitro with radioactive C and N substrates, was largely dependent upon the C/N ratio of reaction mixtures. In the field study, the kernel C/N ratio of B14 x B37 grown without supplemental N was higher than with 240 Kg N/ha at later stage of grain-fill. Based on these observations, it is concluded that the kernel C/N ratio is most likely determined by source supply rather than by sink demand