Salicylic acid enhances photosynthesis in Borago officinalis plants exposed to water deficit by nitric oxide-mediated boosting carbonic anhydrase and antioxidant system

While the role of the interplay between salicylic acid (SA) and nitric oxide (NO) in modulating the antioxidant system of plants is well-documented, their combined effect on carbonic anhydrase (CA) activity has not yet been studied in any plants. To fill this gap, the current study for the first time, explored the impact of interplay between SA and sodium nitroprusside (SNP as a NO donor), on CA activity, photosynthetic efficiency, and the antioxidant defense system in Borago officinalis under water-deficit conditions. Water deficit decreased fresh and dry weight of plants, relative water content, chlorophyll contents, the Fv/Fm ratio and CA activity while increasing H₂O₂ and malondialdehyde levels in leaves. Foliar spraying with SA (0.5 mM), and SNP (100 µM), alone and particularly in combination, increased endogenous NO levels in leaves. This led to improvements in growth parameters, relative water content, chlorophyll a and b contents, photosynthetic efficiency, and CA activity under water deficit conditions. Additionally, these treatments reduced H₂O₂ and malondialdehyde levels, by boosting the activities of catalase, superoxide dismutase, peroxidase, and ascorbate peroxidase, and promoting proline accumulation in leaves. The application of a NO scavenger (cPTIO) negated the beneficial effects of SA, underscoring the role of NO in SA-induced enhancements of CA activity, photosynthetic efficiency, and antioxidant defense system. This research provides novel insights into the interplay between SA, NO, and CA activity in modulating photosynthesis in a C₃ plant. Furthermore, it underscores the potential of combined SA and SNP application as an effective strategy to enhance the sustainable productivity of medicinal plants in arid regions.