Mineral nitrogen fertilization has improved crop yield over the last century but has also caused air and water pollution. Reduction of nitrogen inputs and maintaining high yields are therefore essential to ensure a more sustainable agriculture. Improving the nitrogen use efficiency (NUE) of crops is therefore needed. Rapeseed, Brassica napus, depends on nitrogen fertilization due to its low NUE, with the ratio of plant nitrogen content to nitrogen supplied often not exceeding 60 %. Here, we review the major phenotypic traits associated with NUE in B. napus, with special emphasis on winter oilseed rape. We discuss the genetic diversity available and potential breeding strategies. The major points are the following: (1) rapeseed seed yield elaboration is complex, with overlapping phases of nitrogen uptake and remobilization during the crop cycle; (2) traits related to nitrogen uptake, such as root length and the amount of nitrogen absorbed after flowering, and traits related to nitrogen remobilization, such as the "staygreen" phenotype, have been identified as possible levers to improve NUE in rapeseed; (3) a substantial body of studies investigating the genetic control of NUE traits have already published and potential candidate genes identified; and (4) rapeseed genetic diversity may be enriched by exploiting interpopulation genetic variation and the closely related gene pools of Brassica rapa and Brassica oleracea.
BackgroundNitrogen use efficiency is an important breeding trait that can be modified to improve the sustainability of many crop species used in agriculture. Rapeseed is a major oil crop with low nitrogen use efficiency, making its production highly dependent on nitrogen input. This complex trait is suspected to be sensitive to genotype × environment interactions, especially genotype × nitrogen interactions. Therefore, phenotyping diverse rapeseed populations under a dense network of trials is a powerful approach to study nitrogen use efficiency in this crop. The present study aimed to determine the quantitative trait loci (QTL) associated with yield in winter oilseed rape and to assess the stability of these regions under contrasting nitrogen conditions for the purpose of increasing nitrogen use efficiency.ResultsGenome-wide association studies and linkage analyses were performed on two diversity sets and two doubled-haploid populations. These populations were densely genotyped, and yield-related traits were scored in a multi-environment design including seven French locations, six growing seasons (2009 to 2014) and two nitrogen nutrition levels (optimal versus limited). Very few genotype × nitrogen interactions were detected, and a large proportion of the QTL were stable across nitrogen nutrition conditions. In contrast, strong genotype × trial interactions in which most of the QTL were specific to a single trial were found. To obtain further insight into the QTL × environment interactions, genetic analyses of ecovalence were performed to identify the genomic regions contributing to the genotype × nitrogen and genotype × trial interactions. Fifty-one critical genomic regions contributing to the additive genetic control of yield-associated traits were identified, and the structural organization of these regions in the genome was investigated.ConclusionsOur results demonstrated that the effect of the trial was greater than the effect of nitrogen nutrition levels on seed yield-related traits under our experimental conditions. Nevertheless, critical genomic regions associated with yield that were stable across environments were identified in rapeseed.Electronic supplementary materialThe online version of this article (doi:10.1186/s12863-016-0432-z) contains supplementary material, which is available to authorized users.
Field experiments have shown that water deficit can either increase or decrease white clover (Trifolium repens L.) seed yield depending on its intensity. In order to explain this behavior, we evaluated the effect of water deficit intensity on the components of plant reproductive potential: number of reproductive stolons produced per plant (NSr), inflorescences per reproductive stolon (NI), viable florets per inflorescence (NFv) and ovules per floret (NO). Four experiments were conducted in greenhouse or growth chamber on white clover plants grown in large or small soil columns, or in small pots with vermiculite. Water supply was managed to maintain relatively constant values for predawn soil water potential and midday leaf relative water content (RWC) during the deficit period (20 to 68 d). Water deficit treatments were classified as moderate (M) or severe (S) on the basis of the reduction of RWC compared with the well‐watered plants (C) and of previously established relationships between RWC, soil water potential and vegetative growth. The development of inflorescences, florets and ovules were largely unaffected in M plants, while vegetative growth was depressed by reductions in leaf number per stolon (up to 30%), leaf area (30 to 40%), and by inhibition of stolon branching. Moderate water deficits induced an increase in the percentage of reproductive stolons per plant and reproductive phytomers per stolon. Although this positive effect on the reproductive to vegetative balance was generally observed with S water deficit, the reproductive potential of S plants was strongly depressed relative to C and M plants because of reduced stolon branching, phytomer production, and increased inflorescence and floret abortion. These results show that inflorescence, floret and ovule development of white clover were not impaired by a moderate water deficit that reduced vegetative growth, and suggest good prospects to manage optimal soil‐plant status to maximize potential of seed production.
A 3-year field lysimeter experiment was performed to determine transformations of 15N-labeled cauliflower (Brassica oleracea) residues incorporated into lysimeter topsoil in a potato (Solanum tuberosum)/cauliflower rotation. Only the potato crop received 150 kg mineral N ha −1 y −1 . Cauliflower yields were high (12-13 t fresh matter ha −1 ), and N returned to the soil represented 51% of the aboveground plant N uptake. The 15N recovery by the potato/cauliflower rotation began at 46%, then decreased sharply to 12 and 6% for the second and third year, respectively. The cumulative 15N leaching rate was only 3%; 63% remained in the soil 3 years after incorporation. Soil N mineralization rates described by a parallel first-order kinetic model predicted 27, 7 and 6% of residual N lost annually during the first, second and third year, respectively. Thus, a potato/cauliflower rotation with moderate N fertilization optimizes N recovery of crop residues and can control leaching loss efficiently.
One challenge in plant breeding is to ensure optimized production under fluctuating environments while reducing the environmental impacts of agriculture. Thus, new rapeseed varieties should be adapted to a wide range of pedoclimatic conditions and constraints. Addressing this issue requires identifying the critical factors limiting production and the genotype by environment (G × E) interaction. Our goal was to characterize the effects of environment and G × E interaction on the seed yield of rapeseed grown over a large field network. First, we defined a pedoclimatic indicator set with the ability to highlight the potential limiting factors along the crop cycle by analyzing the yield of two genotypes grown under 20 environments. Out of the 84 pedoclimatic indicators, 10 were identified as limiting after a partial least squares regression analysis. The environments were then clustered into five envirotypes, each characterized by few major limiting factors: low winter temperatures and heat stress during seed filling (1); low solar radiation during seed filling (3); vernalization conditions during winter (4) and high temperatures at flowering (5). A larger genetic diversity was evaluated in a subset of 11 environments to analyze the impact of envirotyping on genotype ranking. Their results were discussed in light of field network management and plant breeding purposes.
Maintaining seed yield under low N inputs is a major issue for breeding, which requires thoroughly exploiting the genetic diversity of processes related to Nitrogen Use Efficiency (NUE). However, dynamic analysis of processes underlying genotypic variations in NUE in response to N availability from sowing to harvest are scarce, particularly at the whole-plant scale. This study aimed to dynamically decipher the contributions of Nitrogen Uptake Efficiency (NUpE) and Nitrogen Utilization Efficiency (NUtE) to NUE and to identify traits underlying NUpE genetic variability throughout the growth cycle of rapeseed. Three experiments were conducted under field-like conditions to evaluate seven genotypes under two N conditions. We developed NUE_DM (ratio of total plant biomass to the amount of N available) as a new proxy of NUE at harvest, valid to discriminate genotypes from the end of inflorescence emergence, and N conditions as early as the beginning of stem elongation. During autumn growth, NUpE explained up to 100% of variations in NUE_DM, validating the major role of NUpE in NUE shaping. During this period, under low N conditions, up to 53% of the plant nitrogen was absorbed and NUpE genetic variability resulted not from differences in Specific N Uptake but in fine-root growth. NUtE mainly contributed to NUE_DM genotypic variation during the reproductive phase under high-N conditions, but NUpE contribution still accounted for 50–75% after flowering. Our study highlights for the first time NUpE and fine-root growth as important processes to optimize NUE, which opens new prospects for breeding.
The reproductive potential of white clover (Trifolium repens L.) plants is favored by moderate water deficits that reduce vegetative growth without impairing development of inflorescences, florets, and ovules. We examined the effects of this type of water deficit on reproductive characters involved in pollination, ovule fertilization, seed set, and seed filling. Two experiments were conducted under controlled conditions with four genotypes either in soil columns or vermiculite, and with various levels of water deficit that were maintained for 68 d. Treatments were classified as moderate (M) and severe (S) water deficit on the basis of the reduction of leaf relative water content (RWC) compared with the well‐watered plants (C) and of previously established relationships between RWC, soil water potential and vegetative growth. Florets were hand pollinated and seed set was determined in all possible cross‐pollinations between pollen and florets from plants subjected to C, M, or S treatments. Pollen viability declined as water deficit increased. Control and M plants had similar high levels of nectar production. In S plants, the proportion of florets containing nectar was reduced by 60 to 70%. Pollen from S plants induced a lower fertilization efficiency than pollen from C or M plants. Most of the reduction of seed set by severe water deficit, however, was linked to the decrease in female plant water status resulting in abortion of embryos. These results show that pollination, fertilization, seed setting, and seed filling can be limited by a severe water deficit applied during reproductive development. A moderate water deficit characterized by an avoidance of leaf dehydration and a reduction of vegetative growth did not impair fertilization efficiency and resulted in maximum seed set and thousand seed weight, compared to C and S plants.
deficits reduce the number of stolons produced per plant and the number of phytomers (defined as a structural Field experiments have shown that water deficit can either increase subunit that comprise a leaf, a node, an internode, and or decrease white clover (Trifolium repens L.) seed yield depending the associated axillary bud, Moore and Moser, 1995) on its intensity. In order to explain this behavior, we evaluated the produced per stolon (Belaygue et al., 1996). This reeffect of water deficit intensity on the components of plant reproduc-
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