Metabolite profiling reveals distinct changes in C-and N-metabolism of lentil (Lens culinaris Medik.) under CO2 enrichment in two contrasting growing seasons in the field

Elevated atmospheric [CO₂] (e[CO₂]) may alleviate the effects of water stress on plants. It is unclear however whether this results exclusively from changes in stomatal conductance and water savings or also reflects changes in metabolic pathways triggered by the extra carbohydrate supplies under e[CO₂]. To help address this knowledge gap, metabolite patterns were analysed in leaves and nodules of lentils grown in a Free-Air CO₂ Enrichment facility in a water limited agro-ecosystem over the course of two contrasting growing seasons, one with high (well above average), and one with low (well below average) rainfall. Metabolomic analyses of tissues sampled at flowering showed contrasting responses to e[CO₂] in the contrasting seasons. In the high rainfall season, e[CO₂] was associated with more pronounced signatures of active energy and amino acid metabolism in leaves as well as in nodules, and particularly increased abundance of proteinogenic amino acids in leaves and nodules, which suggested strong stimulation of nodule N₂-fixation and N supply to leaves. In the low rainfall season, e[CO₂] was associated with high abundance of stress responsive metabolites, including putative osmo-protectants such as sugars and polyols as well as some N-containing compounds (proline, γ-aminobutyric acid, putrescine), while the concentration of proteinogenic amino acids in leaves was reduced. In nodules, e[CO₂] was linked to lower concentrations of sugars, polyols and most proteinogenic amino acids, along with higher concentrations of N-containing stress metabolites. However, there was little evidence that e[CO₂] enhanced energy and amino acid metabolism in the low rainfall season. This study suggests that e[CO₂] amplifies rather than mitigates the effect of different seasons on lentil metabolism. Whilst in a high rainfall season e[CO₂] intensified metabolic patterns related to active growth and N-fixation, in a low rainfall season e[CO₂] strengthened stress response signatures.