Pask, Alistair
(2009)
Optimising nitrogen storage in wheat canopies for genetic reduction in fertiliser nitrogen inputs.
PhD thesis, University of Nottingham.
Abstract
Wheat (Triticum aestivum L.) is the major UK arable crop with a total annual production of about 15 Mt. Intensive cultivation of wheat requires large inputs of fertiliser nitrogen (N), and of the total annual UK use of 1.1 Mt of N fertiliser, 0.36 Mt are applied to wheat crops. However, current cultivars are only able to take up a limited proportion of this applied N (50-60% in NW Europe; Bloom et al., 1998), and large amounts of N are lost to the environment. These fertilisers represent a cost to the grower, and have negative environmental impacts through nitrate, ammonia and greenhouse-gas emissions. There is therefore an increasing need to reduce fertiliser N inputs whilst maintaining or increasing yields. Developing new N-efficient genotypes is an important approach, and could be achieved by increasing the crop N-uptake efficiency (UPE; above-ground N uptake / N available) and/or the N-utilisation efficiency (UTE; grain DM yield / above-ground N uptake). Since only around 50% of the total canopy N is in the leaf lamina at anthesis, there may be scope to reduce the remainder, in particular the significant quantities of N contained in the true stem (up to 25% of canopy N). The overall aim of the present study was to investigate the physiological basis of yield responses to N supply in winter wheat and how cultivars differ in their responses, and to identify breeding targets for new cultivars with lower fertiliser requirements.
Three field experiments were carried out: the first (sown October 2005) and third (sown October 2006) were ADAS Terrington, near King’s Lynn, UK, and the second (sown June 2006) was at the Institute for Crop and Food Research, Lincoln, New Zealand. At Terrington, six N fertiliser treatments were randomised on main plots and four cultivars of winter wheat (Istabraq, Atlanta, Claire, and Savannah) were randomized on sub-plots in a split-plot design with three replicates. The cultivars were chosen to contrast for N partitioning amongst plant organs at anthesis according to previous data sets. At Lincoln, six N fertiliser treatments were randomised on plots with six replicates for one cultivar (Istabraq). Plots were sampled at five developmental stages, with particular emphasis on anthesis and harvest. At each sampling, crop growth (above-ground N uptake, green canopy area, above-ground dry matter, and DM and N partitioning) was assessed, as well as fractional interception of photosynthetically active radiation (PAR).
Data for N uptake and crop DM growth were related to the canopy N requirement (Sylvester-Bradley et al., 1990a) and critical N concentration (Justes et al., 1994) models for winter wheat, and the crop N status at anthesis was quantified according the N nutrition index (Lemaire et al., 1989). The crop N content at anthesis was allocated to three conceptual N pools: structural (SN), photosynthetic (PN) and reserve (RN); the reserve N pool was sub-divided into ‘storage’ (remobilised post-anthesis) and ‘accumulation’ N (not remobilised post-anthesis) pools (Staswick, 1994). Two N sourcesink manipulation treatments were imposed in the experiments approximately two weeks after flowering: defoliation (removal of leaf 3 and below on each shoot) and degraining (removal of all the spikelets from one side of the ear), to test responses of remobilisation of canopy N to changes in grain N source-sink balance.
Results showed that NUE (grain dry matter yield / N available) decreased with N supply. Between the unfertilised and optimally fertilised N treatments the decrease was approximately equally associated with declining UPE and UTE. However, above the optimally fertilised N treatments only UPE continued to decline. The main driver of lower UTE was the biomass production efficiency (BPE; above-ground DM / aboveground N), and varietal differences in BPE at Terrington in 2006/7 indicated the potential to breed for superior UTE. The amount of fertiliser N required to maximise above-ground DM at anthesis was considerably less than that required to optimise yields at harvest (N opt-trt), and reserve N was observed to accumulate within the canopy at anthesis in all N treatments. This reserve N accounted for 41 and 44% of above-ground N (AGN) at the optimal and supra-optimal N rates, respectively.
Reserve N was particularly located in the leaf lamina and true stem. The leaf lamina showed the highest PN content. However, the relationship between radiation-use efficiency (RUE; above-ground DM / PAR) during stem elongation and specific leaf N content (all culm leaves) at anthesis showed that the concentrations of N at the optimal and supra-optimal N treatments exceeded that required for effective photosynthesis, which was ca. 2 g N m-2, and indicated that the crop may be using these tissues as RN capacity, most likely in the photosynthetic enzyme ‘Rubisco’. Results showed that a large quantity of N is loaded in the true stem at anthesis (ca. 25% of AGN at the N opt-trt). The true stem had the highest SN content, but also contained considerable quantities of RN at all N treatments, particularly at the optimal and supra-optimal N treatments (averaged across experiments at 45 and 45 kg N ha-1, respectively; representing 42 and 38% of crop RN, respectively). The large physical capacity, central position and vascular role of the true stem makes this organ particularly suited to a RN function. Overall there was little genetic variation in N partitioning to the SN, PN and RN pools at anthesis (at the N opt-trt in the ranges 0.21-0.22, 0.42-0.44 and 0.35-0.37, respectively). This may have reflected the relatively narrow genetic basis of the germplasm used in this study (i.e. four elite UK cultivars with similar dates of release and end-use).
Large quantities of N were remobilised post-anthesis (overall in the range 90-153 kg N ha-1 across the three experiments). Most N was from the leaf lamina - contributing 29- 35% to the grain N at harvest, with leaf sheath and true stem also providing 10-14% and 9-17%, respectively. This was relatively consistent across varieties. The N remobilised in the post-anthesis period (NR) appeared to be drawn mostly from RN pool in the first half of the grain-filling and then from PN pool in the second half of the phase. The timing and rate of canopy senescence was associated with canopy RN accumulation at anthesis, with senescence occurring predominantly after mid-grain filling in the well fertilised treatments when canopy RN capacity had declined. Senescence was also faster or slower where post-anthesis N remobilisation was increased or decreased in response to defoliation or degraining treatments, respectively. Present results showed that proportionally less true stem N at anthesis was remobilised during the grain filling period (i.e. lower N remobilisation efficiency; NRE) compared to the leaf lamina and leaf sheath, with little genetic variation observed in the Terrington experiments. Therefore the true stem contained considerable quantities of accumulation N at harvest at the optimal and supra-optimal N treatments (overall 12 and 17 kg N ha-1), and would appear to provide a realistic breeding target for reducing canopy N requirement. However, responses in the defoliation treatments demonstrated that true stem NRE could be significantly increased (overall by 20%) compared to the control, whilst the degraining treatments showed that grain N was mainly source limited, up to the upper limit of 1.1- 1.2 mg N grain-1 when it became sink limited.
Overall observed genetic variation in UTE and underlying traits related to canopy N loading in the pre-anthesis phase and canopy N unloading in the post-anthesis phase was small in the present study. Nevertheless, several candidate traits were identified with potential to reduce fertiliser requirements in feed varieties. Firstly, increasing true stem RN capacity as means to increase the maximum rate of N uptake (kg N per day) during stem elongation may be feasible through optimisation of traits such as stem length and wall thickness. Secondly, modifying true stem RN unloading by increasing storage N in relation to accumulation N may offer a realistic mechanism for improving crop BPE and thus UTE. Such an increase in true stem NRE might be achieved through manipulation of key N assimilation enzymes. Thirdly, it may be possible to select for ‘stay-green’ traits associated with lower leaf lamina NRE and lower grain N% to boost UTE. However, in each case further phenotyping studies are required to characterise genetic variability, identify the most appropriate germplasm resources for genetic studies, and to identify appropriate genetic sources of variation for breeding.
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