Hybrid Era and Management Impacts on Morpho-Physiological Traits Related to Grain Yield Components in Maize.

Canopy development rates, architecture and persistence in maize (Zea mays L.) are known to determine radiation interception and radiation use efficiency which strongly influence grain yield, kernel number (Kn) and kernel weight (Kw). Our research focused on better understanding N rate, plant density and hybrid effects on these canopy variables, as well as their correlations with grain yield, Kn and Kw. We conducted a 2-year and 2-location study in Indiana  focusing on anthesis silking interval (ASI), specific leaf area (SLA), specific leaf nitrogen (SLN), leaf biomass (leafB), leaf N concentration (leafN), leaf N content (leafNu) at silking as well as the decline in leaf area index (LAI), SPAD, and green leaf number  during the grain filling period. Our treatments included 2 N rates (N1- 55 kg N ha^-1 and N2 – 220 kg N ha^‑1), 3 densities (D1 – 54,000 pls ha^-1, D2 – 79,000 pls ha^-1, D3 – 104,000 pls ha^-1) and multiple commercial hybrids from previous decades (1967-2005). Considerable genetic variation was observed in ASI, SLN, SLA and SPAD at silking. High N increased SLN and SPAD while high density increased SLA and LAI but decreased SPAD and SLN. Newer hybrids had a higher Kw that was 39 mg kernel^-1 and 57 mg kernel^-1 higher than older hybrids in 2012 and 2013, respectively. Higher Kw in newer hybrids was related with their higher green leaf number retention, which ranged 0.5 ~ 0.6 more green leaf in 2012 and 0.3 ~ 0.5 more green leaf in 2013 during grain filling period. In 2013, SLN explained 83% of total variance of Kw in newer hybrids and 58% in older hybrids, and SLN was positively correlated with green leaf retention during grain filling. The lower Kn (in contrast to the higher Kw) in newer hybrids resulted from a different relationship between LAI and Kn which was parabolic for newer hybrids and linear for older hybrids. Averaged across treatment effects of N rate, density and hybrids, both leafB and leafNu at silking were most strongly correlated with grain yield and Kw, especially in 2013. These most positive correlations of LeafB and leafNu with grain yield were followed by less positive correlations of green leaf retention and SLN with grain yield in both years. However, LAI and SPAD at silking showed stronger correlations with grain yield and Kn when no water stress occurred.