Although breeding has increased genetic yield potential considerably, it remains unclear to what extent this has been done at the expense of Nitrogen Use Efficiency (NUE), which is effectively grain yield expressed on the basis of nitrogen supply. Fertilizer is now an expensive input for crop production and is associated with high greenhouse gas emissions and eutrophication in water courses run off. Variety trials assume that the amount of available nitrogen in the soil is the same and all varieties tend to have the same amount of nitrogen applied so NUE is just a re-scaling of yield, unless the available N in the soil can be assess for each plot several times during the growing season. In the absence of cheap and rapid methodologies to do that, we can gain an insight into NUE by breaking it into its components;
Nitrogen Uptake Efficiency (NUpE) – the amount of nitrogen in the above ground biomass at harvest / total nitrogen supply
Nitrogen Utilization Efficiency in the grain (NUtEg) – Yield at 100% dry matter / total nitrogen yield
Nitrogen Harvest Index (NHI) – grain nitrogen yield / total nitrogen yield
Nitrogen Remobilisation Efficiency (NRE) – total nitrogen yield at anthesis / total nitrogen in the canopy at anthesis
The winter barley gene pool reflects the effects of selection for two different end-uses; malting and hence low grain nitrogen, and high yield with no selection for grain nitrogen content. These contrasting selection strategies are likely to result in greater genetic variation in the components of NUE in the barley gene pool compared to wheat and the findings from barley study can therefore be used to inform studies of wheat.
We grew over 160 elite European winter barley and winter wheat cultivars were grown in two crop specific trial series. Each series was grown with three fertiliser regimes in a split plot design with partial replication; no nitrogen application, an intermediate nitrogen application and a full rate nitrogen application appropriate for maximum yield under a prophylatic fungicide regime. The barley series was grown for harvest years 2010-2011 and included recognised malting and feed two and six-row lines.
The wheat series, grown for harvest years 2012-13, included varieties bred for bread quality, biscuit quality and feed markets.
Standard agronomic traits were scored on the trials and grab samples were taken from a quadrat taken from the centre of each plot prior to harvest and the numbers of plants and fertile stems recorded. Samples were oven dried, separated into grain and straw and then weighed and analysed for nitrogen content and the grain sample was counted. These data allowed the calculation of measures of biomass production, components of yield and portioning characteristics such as harvest index, nitrogen uptake efficiency and utilisation efficiency (both total and grain). Analysis of the data revealed that there was considerable genetic variation for all the traits measured but that the interaction of genotype with fertiliser treatment was small in comparison to the genotype interactions with the site/season combination and the three way interaction with fertiliser. This suggests that breeding specifically for low nitrogen inputs over a wide range of environments is unlikely to be effective.
Note that the significantly greater genetic variation for grain number and thousand grain weight in barley compared to wheat is due to the inflation caused by having two and six-row lines in the barley germplasm. We can also compare the mean performance of each crop type for some nitrogen use parameters to see that wheat effectively has a higher nitrogen harvest index and hence nitrogen utilisation efficiency in the grain but that barley has a higher uptake efficiency.
All lines were genotyped using crop-specific Illumina Infinium arrays to provide 9,000 and 90,000 SNP markers for barley and wheat, respectively. We used this genetic information to examine the relationships between the genotypes within each crop and then combined it with the phenotypic data in genome wide association scans with correction for the population sub-structure that we detected in order to identify significant regions of the genome of each species that were associated with yield and parameters of nitrogen use efficiency. We then compared our results across both crops to highlight regions that are in common and represent conserved mechanisms of nitrogen uptake and/or distribution.
For further information on this project please contact Bill Thomas (firstname.lastname@example.org) from the James Hutton Institute.