Evaluation of nitrogen use efficiency (NUE) in maize genotypes using in-season soil applications of isotopic nitrogen in field plots
The ability of modern maize (Zea mays L.) hybrids to accumulate more biomass and nitrogen (N) after the flowering period with extended photosynthesis capacity has been associated with improvements in grain yield (GY) and nitrogen use efficiency (NUE). The overall objectives of this research are to improve knowledge on N dynamics and physiological traits that are correlated with NUE using the “pulse-labeled” 15N application as a novel phenotyping strategy to characterize maize hybrids varying in yield stability and responsiveness to N under limited N environments. Three field experiments were established at Purdue University’s research farms during the 2013 and 2014 growing seasons; in each experiment five hybrids, including pairs of two modern “high yielding” commercial hybrids and two “high N responsive” hybrids (or potentially superior in NUE) and one old era hybrid from the 70’s were compared under zero N fertilizer application (0N) and a moderate rate of 112 kg of N per ha (112N). The “pulse-labeled” application of 15N at low N rates (2.1 to 3.2 kg 15 N ha-1) in the root zones on both sides of consecutive plants enabled precise estimation of N uptake efficiency and allocation of the recent 15N uptake (15Nu) to plant components at six development stages per location/year between V14 and R6. Plant recovery of 15N was determined in above-ground maize plant components sampled 4 to 6 days after pulse labeling. Before flowering, plants recovered about 45% of the 15N fertilizer applied and 15Nu was more evenly distributed in plant components with about 50% of the total 15Nu found in the leaves and ~ 40% in the stems; a small amount of 15Nu was found in the husks at late vegetative stage. After the flowering period, 15Nu decreased in the vegetative organs and increased in the ears and/or kernels as the season progressed. Even at the late R5 stage plants recovered about 20% (at 0N) or 30% (at 112N) of the new 15N applied, and approximately 70% of the total 15Nu was allocated to kernels. NUE hybrids allocated up to ~20% of 15Nu to the leaves even at this late stage. By analyzing total N uptake (Nu) and dry matter (DM) partitioning of plant components from V6 to the R6 growth stage (whether labeled with 15N or not) we observed that hybrids did not differ in total Nu or DM accumulation at all stages evaluated; however, NUE hybrids demonstrated stronger tendencies in accumulating DM and N to leaf and stem components both early and late in the growing season. NUE hybrids had higher leaf DM by physiological maturity in comparison to the other hybrids. Also, at maturity the NUE hybrid 2 showed potential for a more balanced contribution of grain Nu originating from post flowering Nu and remobilized N while the other hybrids showed larger fractions of grain Nu originating from the N remobilization process. The overall NUE values increased with NUE hybrids as they were more responsive to N rate (i.e. a larger GY gain from 0 to 112N). The latter NUE response was more positively correlated to a GHI increase with the 112N and larger kernel weights than it was associated with NHI or kernel number. With respect to morpho-physiological leaf measurements, the NUE hybrids showed higher leaf area index (LAI) around silking period, but lower specific leaf N (SLN) and SPAD values. Through the “pulse-labeled” 15N approach, NUE hybrids generally demonstrated a larger N storage capacity in vegetative tissue until later in the season. Therefore, these specific NUE hybrids displayed higher source: sink ratio during the grain filling period. Although plant component biomass and total N content determinations at multiple growth stages are helpful in tracking when and where plant N dynamics change in maize genotypes with potentially superior NUE, the pulse labeling approach with isotopic N helped to confirm just how and when hybrids vary in their uptake and allocation of the most recent fertilizer N available in the root zone.
Vyn, Purdue University.
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