Nitrogen metabolism plays a major role in the adaptation of the halophytic forage species Sulla carnosa to water deficit and upon stress recovery

Our study aimed to evaluate the changes in the nitrogen compounds and enzyme activities in a forage halophytic species, Sulla carnosa, subjected to water deficit stress and recovery. Seedlings were individually cultivated for two months under two irrigation modes: 100% and 33% of field capacity. Our results showed that water deficit stress induced a strong decline in both leaf biomass production (-70%) and leaf water content. Besides, S. carnosa plants exposed to water deficit stress preferentially allocated photoassimilates to roots while showing a decrease in leaf osmotic potential. Moreover under water deficit conditions, leaf ammonium content remained unchanged, whereas a significant increase in leaf nitrate content was observed. Water deficit stress also triggered an accumulation of free amino acids, mainly proline and asparagine (respectively 18-and 70-fold increases as compared to controls), suggesting their potential role in osmotic adjustment. Despite its increase in leaves of stressed plants, glycine did not exceed 2% of the amino acid pool. The accumulation of glutamate, as a consequence of a high glutamine synthetase (GS) activity and the marked amino acid accumulation in leaves strongly suggest that glutamate turnover is of adaptive significance for S. carnosa, confirming the involvement of glutamate metabolism in the accumulation of adaptive metabolites in plant response to stress conditions. After rehydration, a partial growth re-establishment was observed in S. carnosa, and was concomitant with a better tissue hydration and a decrease in leaf nitrate content, involving a significant increase in GS activity, and a decrease in glutamate dehydrogenase (GDH) deamination activity. Overall, S. carnosa is able to conserve the functional integrity of nitrogen-related metabolic pathways after exposure to stressful conditions.