NUE in Maize

Research interests of NUE-CROPS partners working on maize were mainly focused on Nitrogen Use Efficiency (Nit-UE). The partner institutions included the crop breeding company KWS (Germany), University of Düsseldorf (HHUD, Germany), AgroBio Institute (ABI, Bulgaria) and the Chinese Academy of Agricultural Sciences (CAAS).

Previous studies using Central European maize populations clearly demonstrated that selection of maize genotypes under low N-input results in higher Nit-UE without reductions in yield. Similarly, recent maize breeding projects in China have shown that the average yields of hybrids developed under low N conditions were about 10% higher (higher NUE) both at high and low N supply than those selected under high N conditions. The maize on the top of Fig. 1 shows that selection for high N shows a reduced fertility and a remarkable grain loss due to kernel abortion in the first stages of grain filling when grown under low N supply. Another variety (on the bottom of Fig. 1) selected under lower levels of N supply does not experience the same kernel abortion. This provides strong evidence that it is possible to select for varieties that will yield well under reduced levels of N input.

Fig. 1. Response of maize hybrids to varying levels of N supply

Activities in WP2 therefore focussed on:

Many studies highlight the essential role of root traits in N-acquisition. Partners in SP2 searched for genetic loci that may influence root traits. They used a "paper roll" method to characterize maize roots at different levels of N supply (Fig. 2). Root architecture was studied using WinRhizo software. This resulted in a few QTL clusters being identified which showed potential in marker assisted breeding.

Fig.2 Dramatic differences in root architecture for two maize genotypes with differing NUE

Shovelomics
Fig.3 Shovelomics: phenotyping of maize root architecture in the field (HHUD and KWS)

Partner CAAS (WP2.1) applied a similar strategy. The population of 200 recombinant inbred lines (RIL) derived from the cross of Ye478, the large-root maize inbred line, and Wu312, the small-root maize inbred line, was tested over 2 years at two locations with two N levels. In parallel the root phenotypes of the seedlings were analysed in hydroponics and in paper roll tests to identify quantitative trait loci (QTL) for root traits. Several QTL clusters  for maize root growth and development at seedling stage were identified; three stable QTLs detected lateral root density, seminal root number, and dry root weight.

Maize Seedlings
Fig.4 Maize seedling growth in paper rolls at CAAS

The QTL approach taken by the Partner ABI explored the genetic variation for NUE traits generated by chemical mutagenesis in an elite temperate breeding line (WP2.1). The mutant inbred XM 87-136 was obtained by chemical mutagen treatment of the line B37 to induce variations, followed by recurrent reciprocal breeding to evaluate and fix the desirable mutations in homozygous state. A DH Population, derived from a cross between B37 and the mutant line, was developed in collaboration with KWS. In order to study tolerance to low soil N availability at the hybrid level each of the 192 DH lines of the population was crossed to an unrelated tester (Mo17). The tester was chosen for its high combining ability for grain yield with both parental lines of the DH population. The test-cross hybrids were evaluated in field trials across 12 environments with 2 N levels including 2 irrigation regimes (rain fed and irrigated) in one location and 2 N levels with rain-fed conditions in a second location during 2010 and 2011.

Variation of Yield
Fig.6 Variation for yield in testcross hybrids of the population derived from the chemically mutagenized line at high and low nitrogen availability (ABI)

Results of the mycorrhizal research were disappointing with overall only a small effect from inoculation. This led partners to conclude that this was not an economical strategy for European production systems, although benefits from inoculation have been observed in systems where inherent soil fertility or moisture supply is very low.

Activities in this workpackage resulted in the following publications:

Jia X, Peng L (2014) Effect of different nitrogen and irrigation treatments on yield and nitrate leaching of summer maize (Zea mays L.) under lysimeter conditions. Agric Water Manag 137:92-103

Pan X, Lin Z, Yuan L, Wen Y, Zhao BQ (2013) Nitrogen uptake and use of summer maize under different soil fertility levels. Soil and Fertilizer Sciences in China 1:8-13

Qi W, Liu P (2012) Morphological and physiological characteristics of corn (Zea mays L.) roots from cultivars with different yield potentials. Eur J Agron 38:54-63

Qi W, Liu P (2012) Temporal and spatial distribution characteristics of super-high-yield summer maize. Journal of Plant Nutrition and Fertilizer Science 18 (1):69-76

Tsonev S (2013) Development of multiplex primer sets for cost efficient SSR genotyping of maize (Zea mays) mapping populations on a capillary sequencer. Bulgarian Journal of Agricultural Science 19 (2):5-9