Variation in morpho-physiological and metabolic responses to low nitrogen stress across the sorghum association panel

Abstract

Background: Access to biologically available nitrogen is a key constraint on plant growth in both natural and agricultural settings. Variation in tolerance to nitrogen deficit stress and productivity in nitrogen limited conditions exists both within and between plant species. However, our understanding of changes in different phenotypes under long term low nitrogen stress and their impact on important agronomic traits, such as yield, is still limited.

Results: Here we quantified variation in the metabolic, physiological, and morphological responses of a sorghum association panel assembled to represent global genetic diversity to long term, nitrogen deficit stress and the relationship of these responses to grain yield under both conditions. Grain yield exhibits substantial genotype by environment interaction while many other morphological and physiological traits exhibited consistent responses to nitrogen stress across the population. Large scale nontargeted metabolic profiling for a subset of lines in both conditions identified a range of metabolic responses to long term nitrogen deficit stress. Several metabolites were associated with yield under high and low nitrogen conditions.

Conclusion: Our results highlight that grain yield in sorghum, unlike many morpho-physiological traits, exhibits substantial variability of genotype specific responses to long term low severity nitrogen deficit stress. Metabolic response to long term nitrogen stress shown higher proportion of variability explained by genotype specific responses than did morpho-pysiological traits and several metabolites were correlated with yield. This suggest, that it might be possible to build predictive models using metabolite abundance to estimate which sorghum genotypes will exhibit greater or lesser decreases in yield in response to nitrogen deficit, however further research needs to be done to evaluate such model.

Keywords: Hyperspectral; Metabolomics; Nitrogen stress; Sorghum.

Fig. 1

Phenotypic difference of morpho-physiological traits across two treatment conditions. Statistical significance of N treatment were determined by likelihood ratio test (LRT) on mix model with treatment denote as fix effect and genotype as random. Asterisks indicate p-value < 0.05. Red dots indicated values for genotypes selected for metabolomics analysis. HN - high nitrogen, LN - low nitrogen. a - i Comparison of distribution of nine traits under HN and LN conditions

Fig. 2

Components of traits variation. a shows the proportion of variance attributed to each component for each trait. b shows the magnitude of this variance relative to each trait’s mean, using the coefficient of variation (CV; the estimated variance divided by the squared mean of the respective trait). HN - high nitrogen, LN - low nitrogen

Fig. 3

Metabolomics profiling in 24 sorghum genotypes across two nitrogen conditions based on 145 confidently annotated metabolites. a Distribution of the 145 confidently annotated metabolites across two treatment conditions. b First two principle component (PC) from PCA. Values in bracket indicate amount of variance explained by each component. c Volcano plot showing the down regulated (yellow), up regulated (green), and unhanded (grey) metabolites under low nitrogen (LN) conditions compare to high nitrogen (HN). d Proportions of the metabolites with know structures more abundant in samples collected from plants grown under HN (green), more abundant in samples collected from plants grown under LN (yellow), and unchanged (gray)

Fig. 4

Proportion of variance attributed to each component for each metabolite. HN - high nitrogen, LN - low nitrogen

Fig. 5

Examples of unchanged a-b, changed but non-plastic c and plastic d-f metabolites. Each dot indicate genotypic mean and lines connect the same genotype across two treatment conditions. HN - high nitrogen, LN - low nitrogen

Fig. 6

Scatter plot of correlation values of each metabolite and yield in given conditions. Marked metabolites indicate significantly correlated metabolites from Pearson analysis ($p<0.05$). HN - high nitrogen, LN - low nitrogen