Characterizing The Major U.S. Maize Heterotic Groups By Linking SPAD Values, N Use Traits, and Yield.

U.S. maize breeders have been very successful in improving maize yields year to year but rarely focus their decisions solely on yield alone. They consider how hybrids perform over a range of traits and advance hybrids based upon overall phenotype. In very large breeding programs this can be quite a demand on time and resources, especially for traits involved in N use which involve whole plant sampling and processing (N utilization and N uptake). Identifying measurements that circumvent the need to harvest whole plants and are indicative of differences in N utilization and N uptake might reduce the resources required to breed for improved maize N use efficiency.

The various U.S. maize heterotic groups have different genetic predispositions in how yield is achieved through either kernel number or kernel weight. Previous studies show that check plot yield (kernel number) and N response (kernel weight) are inversely related and that they are primarily influenced by N utilization and N uptake, respectively. The utility of the SPAD chlorophyll meter has been identified as a reliable tool to quantify leaf greenness/health and is correlated with leaf N concentration, applied N rate, and yield. Therefore, the use of this device as a non-destructive, in-season measurement of leaf health in response to N availability may aid in the physiological characterization of heterotic groups for N utilization and N uptake differences.

SPAD data was collected on 32 F₁ hybrids at V10, VT/R1, and R3 to quantify seasonal changes in leaf greenness. Hybrids were evaluated across two N rates (0 and 252 kg N ha^-1) using three replications and two site-years. The 32 hybrids were created by crossing four stiff stalk inbreds to eight non-stiff stalk inbreds in a factorial pattern. Each inbred represented one of the major U.S. heterotic groups. No interaction between hybrid and measurement timing was identified. SPAD values were generally positively correlated with yield at both N rates. However, SPAD values at low N were inversely related to SPAD values at high N, indicating that SPAD values at the two N rates were possibly representative of separate physiological processes. This hypothesis was confirmed when the SPAD values at low N were found to be positively correlated with kernel number and N utilization whereas the SPAD values at high N were positively correlated to kernel weight and N uptake.