A comprehensive study of plant density consequences on nitrogen uptake, partitioning, and use efficiencies relative to biomass accumulation over time in maize
Maize (Zea mays L.) grain yield (GY) is closely associated with the quantity of nitrogen (N) taken up by plants. Past genetic improvements in maize GYs were associated with greater tolerance to crowding stress; as a result, farmers have consistently increased plant density (PD). This study attempted to better understand plant N uptake/partitioning processes at the individual-plant and plant-community levels, and the associated N use efficiency (NUE) responses of maize to progressively higher PD at varying degrees of N stress. For the first objective, from the review analysis, the most striking results were that: i) at the individual-plant level, GY and whole-plant N uptake showed an equivalent plateau for Old (1940-1990) and New (1990-present) Eras, with slightly greater N internal efficiency (NIE) for new Era hybrids, despite an average increase of 27% in PD from the Old to New Era; ii) even though overall grain harvest index (HI) and NHI was closely linked, the improvement in NIE over time was a consequence of dilution in grain N as GY increased with the New Era (across PD and N rate levels); and iii) the New Era hybrids displayed greater N stress tolerance and more yield response per unit of N fertilizer applied. In order to investigate the remaining research goals, the effects of crowding intensity (54,000, 79,000 and 104,000 plants ha -1) and N availability (0, 112-165 and 224-330 kg N ha-1) on maize growth and development were evaluated with different genotypes grown at two field sites (near West Lafayette and Wanatah, IN) during three growing seasons. Intensive study of the vegetative and reproductive period responses addressed present knowledge gaps at plant community and individual plant levels. At the plant-community level, major conclusions were i) treatment factor effects (genotype, PD and N rate levels) were reflected in unique BM trends for leaf, stem, and ear components over time while their respective N concentrations (%N) were dominantly affected by N rates; ii) as crowding and N stresses intensified, maize plants drastically reduced stem %N during vegetative growth (in their apparent attempts to sustain leaf %N), and drastically reduced shoot (leaf plus stem) %N during reproductive growth (in their attempts to sustain ear %N); iii) a trade-off was documented between vegetative N stored and reproductive shoot N remobilization, especially with high maize GY scenarios (leading to longer post-silking N uptake and later shoot N remobilization); iv) both BM and N uptake thresholds were found for the linear-plateau relationship for the respective dry mass and N partitioning indices (grain HI and NHI); v) both grain and non-grain %N showed equivalent partitioning as the plant N uptake increased at maturity; and lastly; vi) improvements in NUE can result when higher GY levels are pursued with a concomitant improvement in the grain N utilization (reflected in lower grain %N). At the individual-plant level, major conclusions were: i) dry mass and N partitioning from the plant to the ear around silking showed a tight interplay; ii) the latter parameters were also correlated with grain components and the ear strength (quantified as the kernel number –Kn- by kernel growth rate –KGR-); iii) the genotypes and management practices employed exerted a minor influence over the potential kernel number (Kn); iv) low Kn failure rate (KFR) was documented when ear dry mass and N allocation were at maximum levels from ear growth onset till maturity; v) maximum dry mass and N partitioning levels between plant and ear were already expressed early during the ear development, and remained fairly stable towards maturity; vi) N allocation within the plant around silking was more closely related to the ear strength than to vegetative or reproductive whole-plant N uptake rates; and vii) high N uptake rates during reproductive stage were associated with greater ear strength and late reproductive shoot N remobilization rates. However, adequate vegetative N status was essential for achieving superior ear strength, and consequently, GY. In summary, future interdisciplinary investigations should be directed to the improvement of maize plant N uptake by promoting a concomitant advancement in plant sink capacity while continuing to pursue understanding of the N dynamics under the interaction with other complex traits in nature (e.g. water use efficiency). (Abstract shortened by UMI.)^
Tony J. Vyn, Purdue University.
Biology, Botany|Agriculture, Plant Culture
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