Sink size in maize is a function of kernel number and kernel weight which are both highly affected during the critical period around silking (Andrade et al., 1999; Borrás and Otegui, 2001). The genetic improvement in grain yield is highly associated with increased kernel number, rather than kernel size or kernel weight (Tollenaar et al., 1992; D. Duvick, personal communication, 2005). Kernel number in maize is a function of rate of DMA during a period bracketing silking, in other words plant growth rate (g DM d–1 plant–1) (Edmeades and Daynard, 1979; Tollenaar et al., 1992; Andrade et al., 1999). Rate of DMA during the period bracketing silking varies little among older and newer maize hybrids (Crosbie, 1982; Tollenaar et al., 1994a). Consequently it can be deduced that the increase in kernel number in newer hybrids is a result of greater partitioning of dry matter to the kernels during the sensitive period of kernel establishment.
According to Egli, 1998, and Andrade et al., 2001, sink size of an individual maize plant is also determined by ear size (kernel rows/ear and spikelets/row), ears per plant, and potential kernel size. Kernels per ear is determined by the proportion of the spikelets that develop into mature kernels and this is, in turn, related to source activity (plant growth rate) during the critical period for kernel number determination (Tollenaar et al., 1992; Andrade et al., 1993, 2000; Vega et al., 2001; Echarte et al., 2004). For maize, sink capacity is largely driven by final kernel number as a result of differences in kernel set or abortion (Schussler and Westgate, 1991; Cantarero et al., 1999). However, other sink differences can be attributed to changes in individual kernel size as a result of endosperm cell division rates, starch granule production, and enzymatic activities that peak during this critical period (Jacobs and Pearson, 1992; Lur and Setter, 1993; Cazetta et al., 1999). Many aspects of reproductive sink development are positively influenced by the N supply; thus, farmers routinely apply high levels of N fertilizer during maize cultivation to optimize grain yields.