Physiological Evaluations of Recent Drought-Tolerant Maize Hybrids at Varying Stress Levels

Maize (Zea mays L.) improvement in drought-stress tolerance poses a great challenge as the global need for food, feed, fiber, and fuel increases. Seed companies are developing and promoting drought-tolerant hybrids, but their physiological drought-tolerance mechanisms are not well understood. The research objective was to investigate the plant traits related to yield improvement for similar maturity hybrids classified as either drought-tolerant (non-transgenic) or conventional at varying plant density (PD) (two levels) and N rates (four levels) over 2 site-years in northwestern Indiana. Physiological measurements included photosynthesis (A), transpiration (E), and leaf area index at multiple growth stages, as well as anthesis-silking interval, potential kernel number, grain yield (GY) and its components. Intensive heat and drought stress occurred in the 30-d period before and during flowering in 2012, but not in 2011. Overall, similar maturity drought-and non-drought-tolerant hybrids did not markedly differ in GY or most other traits, and hybrid responses to varying PD and N rates were similar. In both seasons, GY was impacted most by N rates. A complex N rate effect on A and E was tightly related to water supply (i.e., higher N had positive impact under non-drought conditions). Hybrid differences in A and E were not significant at the leaf-scale, but one drought-tolerant hybrid had lower estimated cumulative A and E at the season-long canopy scale. Under the non-drought and specific-drought conditions in these single-location trials there was no indication that designated drought-tolerant hybrids were more tolerant to high crowding intensity and/or low N stresses.