Morpho-Physiological Traits Associated with Drought Tolerance of Maize Hybrids Subjected to Different Water and Nitrogen Supply

University of Minnesota Ph.D. dissertation. November 2016. Major: Applied Plant Sciences. Advisor: Jeffrey Coulter. 1 computer file (PDF); xvi, 142 pages.Maize (Zea mays L.) hybrids reported as having tolerance to drought are expected to perform better than non-drought-tolerant (‘standard’) hybrids under drought stress, but few studies confirm this with a mechanistic justification. In addition, morpho-physiological traits that may confer drought tolerance have not been compared extensively between drought-tolerant and standard maize hybrids. This study was conducted to assess whether drought-tolerant and standard varieties differ in agronomic and morpho-physiological traits associated with drought tolerance. Three experiments were conducted on loamy sand field soil in central Minnesota, in which a drought-tolerant and a standard hybrid were compared under three levels of sustained moderate drought stress established using drip irrigation: no drought stress; drought stress from the 14 leaf collar maize phenological stage (V14) to maize physiological maturity (R6); and drought stress from the blister maize phenological stage (R2) to R6. The three fertilizer N rate treatments: sub-optimal (50%), 50% of the expected economically optimum N rate; optimal (100%), 100% of the economically optimum N rate; and supra-optimal (150%), 50% greater than the economically optimum N rate were applied to each combination of hybrid and drought stress. Grain yield was 10% greater for the drought-tolerant compared to the standard hybrid with drought stress from V14 to R6, but did not differ between hybrids when drought stress occurred from R2 to R6 or in the absence of drought stress. Grain and silage yields with the supra-optimal N rate were 7 and 12% greater for the drought-tolerant than standard hybrid, respectively. With drought stress from R2 to R6 and V14 to R6, kernels per square meter were 6 and 23% greater and kernel mass was 3 and 10% less with the drought-tolerant than standard hybrid, respectively. Aboveground N uptake was 9% greater with the drought-tolerant than standard hybrid at the supra-optimal N rate. Actual crop evapotranspiration (ETa) decreased with increasing duration of drought stress. Crop water use efficiency (CWUE) was 21% greater in the absence of drought stress compared to when drought stress occurred from V14 to R6 or R2 to R6. The drought-tolerant hybrid had 7 and 8% greater CWUE and irrigation water use efficiency (IWUE), respectively, than the standard hybrid with drought stress from V14 to R6. Irrigation water use efficiency for the standard and drought-tolerant hybrids was greater by 20 and 29%, respectively, with the supra-optimal than sub-optimal N rates. The basal crop coefficient (Kcb) during the mid-season was 1.08, 0.89, and 0.73, respectively, for no drought stress, drought stress from R2 to R6, and drought stress from V14 to R6 across hybrids in experiment where maize followed soybean with optimal N rate; and during the late season growth stage was 0.82, 0.61, and 0.61, respectively. Two phases of Kcb, an initial adaption phase with fluctuating Kcb followed by a response phase with stable Kcb, occurred when maize was exposed to drought stress. Compared to the standard hybrid, the drought-tolerant hybrid had a narrower anthesis-silking interval (ASI), greater leaf area index (LAI) for all N rates across experiments and drought stress at milk maize phenological stage, and lower leaf chlorophyll content in all levels of drought stress across experiments and N rates at silking and milk maize phenological stages. Drought stress and sub-optimal N rate increased ASI, and reduced LAI, leaf greenness, and leaf chlorophyll content. With drought stress from R2 to R6, photosynthesis (A) did not differ between hybrids but transpiration (E) and stomatal conductance (gs) were greater for the drought-tolerant hybrid suggests greater stomatal aperture. In the absence of drought stress, the standard hybrid had greater A, E, and gs in the experiment when maize followed winter rye. The drought-tolerant hybrid had more root length density (RLD) that was 0.4 to 1.5 mm in diameter in the 0- to 15-cm soil depth. Fine roots with a diameter 0 to 0.4 mm were more abundant in the entire sampled depth of 0 to 90 cm. Distribution of RLD was associated with water availability in the soil profile. Overall, the greater grain yield of the drought-tolerant maize hybrid when drought stress occurred from V14 to R6 was associated with greater aboveground biomass, N uptake, and kernel number, which were strongly correlated to IWUE. Results demonstrated expression of multiple morpho-physiological traits under drought stress; in particular, greater LAI, chlorophyll content, and stomatal conductance suggest that drought tolerant hybrids can conduct photosynthesis under drought stress conditions compared to standard hybrids. Further research is needed to enhance the knowledge of N management for drought-tolerant hybrids and the relationship of traits associated with drought tolerance, especially mechanisms related to photosynthetic carbon assimilation and crop water use