Breeding For Nitrogen-Efficient Maize Hybrids And Genetic Analysis On Roots Morphology In Response To Low Nitrogen Stress

Excessive nitrogen (N) fertilizer input in arable land may result in environmental pollutions, low income for farmers and low N use efficiency. Development of N efficient cultivars may be one of the ways to resolve such problems. Genotypic difference in N efficiency has been reported in many references, but breeding for N-efficient cultivar is still rare. Roots are important for N uptake, but few studies on the role of root in N uptake have been conducted in field conditions. And, less is known about how root traits are inherited at N stress. On the basis of selection for N-efficient inbred lines, the current study concentrated on breeding N-efficient maize hybrids. The role of roots in N uptake and yield formation of selected hybrids in the field was investigated in two N levels. Furthermore, response to low N stress of roots of both hybrids and their parent lines was compared to understand the physiological and genetic basis for root response to low N stress and possibility for root as a trait to breed for N-cfficient hybrids.Taking ND108 as a check, some N-efficient hybrids were developed among which NE1 is the most efficient one. Some other hybrids were proven to be N-inefficient. Three inbred lines with high combining ability in N efficiency were identified.Taking NE1 (N-efficient), ND108 (N-efficient), NE1 (N-inefficient) and SD19 (N-inefficient) as materials, biomass accumulation, N accumulation, root dry weight and root length were investigated under two N levels to understand the basis of N efficiency. It was found that N efficient hybrids did not show any advantages in N uptake and biomass accumulation at the early stage before silking. Efficient N uptake after silking was the charateristic of N-efficient hybrids. Between the two N-efficient hybrids, ND108 accumulated large portion of N after silking, with less translocation of pre-silking N into grains. While in NE1, both N uptake after silking and translocation of pre-silking N uptake contributed to its high N accumulation and yield. N-efficient hybrids had higher root biomass and root length, which might be the reason for its efficient N uptake.Although the large genotypic difference, low N generally suppresses shoot growth, increases root to shoot ratio with or without increasing root biomass of maize. Maize plants respond to N deficiency by increasing total root length and altering root architecture by increasing the elongation of individual axial roots, enhancing lateral root growth, but reducing the number of axial root. Inbreds showed weaker responses in root biomass and other root parameters than their crosses. Interaction between two parent inbreds had strong influence on the response of their cross to N stress.The genotypic variations of general combining ability (GCA) and specific combining ability (SCA) on root traits were observed under high and low nitrogen levels (HN and LN). N stress exerted an influence on the genetic ability of root traits, with decreasing of broad genetic ability and increasing of narrow genetic ability. The genetic of most root traits such as root weight, total root length and lateral root length were controlled by non-additive effect at HN, but by additive effect at LN.It is suggested that the root traits at later growth stage might be one of the selection criteria in selection for N-efficient cultivars. A big root system in a cross can be obtained by selecting parents lineswith large root weight, total root length, total length of lateral roots.