Young Tomato Plants Respond Differently under Single or Combined Mild Nitrogen and Water Deficit: An Insight into Morphophysiological Responses and Primary Metabolism

This study aimed to understand the morphophysiological responses and primary metabolism of tomato seedlings subjected to mild levels of nitrogen and/or water deficit (50% N and/or 50% W). After 16 days of exposure, plants grown under the combined deficit showed similar behavior to the one found upon exposure to single N deficit. Both N deficit treatments resulted in a significantly lower dry weight, leaf area, chlorophyll content, and N accumulation but in a higher N use efficiency when compared to control (CTR) plants. Moreover, concerning plant metabolism, at the shoot level, these two treatments also responded in a similar way, inducing higher C/N ratio, nitrate reductase (NR) and glutamine synthetase (GS) activity, expression of RuBisCO encoding genes as well as a downregulation of GS2.1 and GS2.2 transcripts. Interestingly, plant metabolic responses at the root level did not follow the same pattern, with plants under combined deficit behaving similarly to W deficit plants, resulting in enhanced nitrate and proline concentrations, NR activity, and an upregulation of GS1 and NR genes than in CTR plants. Overall, our data suggest that the N remobilization and osmoregulation strategies play a relevant role in plant acclimation to these abiotic stresses and highlight the complexity of plant responses under a combined N+W deficit.

Automated summary

The study aimed to investigate the physiological responses and metabolism of tomato seedlings under mild nitrogen and/or water deficit. The combined deficit treatment showed similar effects as single nitrogen deficit, causing lower biomass, leaf area, chlorophyll content, and nitrogen accumulation, but higher nitrogen use efficiency. At the shoot level, both treatments induced higher C/N ratio, activity of nitrate reductase and glutamine synthetase, gene expression related to RuBisCO, and downregulation of specific gene transcripts. Interestingly, the root level responses differed, with combined deficit plants showing similar behavior to water deficit plants, including increased nitrate and proline concentrations, nitrate reductase activity, and upregulation of specific genes. Overall, nitrogen remobilization and osmoregulation strategies play a crucial role in plant adaptation to combined nitrogen and water deficit.