Genetic and agronomic approaches for improving nitrogen use and maize productivity.

摘要

Several factors have large effects on maize grain yield including environment, nitrogen (N) supply, and hybrid genetics. Efficient and environmentally responsible use of N fertilizer is a cornerstone of high?yielding maize production, and improvements in maize N use will require a combination of agronomic, breeding, and biotechnology approaches. A maize hybrid's N use and productivity are influenced by its grain yield at low N (check plot yield; no fertilizer N applied), and its response (i.e., increase in grain yield) to fertilizer N application. Maize N use can be improved by focusing on one or both of these parameters; however, better N use will not be sufficient by itself to increase maize productivity. Plant density tolerance also should be considered due to the possible interactions of hybrid, N rate, and plant density, as well as the need to increase plant density as a strategy to increase yield per unit area. The broad questions addressed by this research were i) how has past genetic selection for grain yield affected the N response characteristics of modern hybrids, ii) what is the variation for N use traits in current commercial hybrids, iii) how do hybrid, N rate, and plant density interact, and iv) has biotechnology (e.g., transgenic corn rootworm protection traits) already made an impact on N use and productivity of maize? An array of field-based phenotyping experiments (different levels of N and plant density) was conducted between 2008 and 2011 in Illinois using hybrids from various sources to address these questions.;Evaluation of old and new hybrids under various levels of N supply showed that improvement for grain yield at low N (56 kg ha-1 yr -1) has contributed to about two-thirds of the improvement in grain yield at high N (86 kg ha-1 yr-1). Despite past improvement for low N tolerance, current commercial maize hybrids vary widely in their grain yields at low N with an average genetic range of 2.2 Mg ha -1. Tolerance to low N was also closely associated with the ability to withstand high plant density. Grain yield at low N was mostly related to differences in genetic N utilization, which quantifies grain yield per unit of plant accumulated N under unfertilized conditions. Improved N uptake, however, also affected low N grain yield in experiments which focused on the role of biotechnology derived corn rootworm protection traits on maize N use.;Past genetic improvement for grain yield resulted in a more modest increase in N response (increase in grain yield between low N and high N treatments; 30 kg ha-1 yr-1) compared to that for low N tolerance. Current commercial hybrids also vary widely for magnitude of N response (avg. range of 1.9 Mg ha-1) and the optimum N rate (57% to 164% of the mean) at which this response occurred. N uptake is one factor that heavily influences the response of grain yield to fertilizer N. As such, in addition to improved grain yield at low N, corn rootworm protected hybrids had larger responses to applied fertilizer N as a result of improved N uptake efficiency in some genetic backgrounds and environments.;Individual kernel weight is a largely untapped resource for improving maize yields, and several chapters of this dissertation highlight its importance. Responses of kernel number and kernel weight to increased N supply were negatively correlated, and kernel weight required a greater optimum N rate compared to kernel number. Thus, improving the rate of kernel weight gain per unit of applied N may be a promising strategy for improving N use efficiency. Grain yield increases resulting from the addition of the HERCULEXRTM XTRA trait to hybrids formed from the intermated B73 x Mo17 recombinant inbred line (IBM RIL) population were mostly associated with improved individual kernel weight, which may be attributable to greater post-flowering N uptake and enhanced stay-green.;In conclusion, genetic and agronomic approaches for improving maize N use and productivity should focus on i) grain yield at low N (stress tolerance and yield stability), and ii) enhancing fertilizer N use through strategies that simultaneously optimize kernel number and kernel weight responses to N application under increased plant density.