Interactive effects of salicylic acid and sodium nitroprusside on mucilage, tannin and phenolic contents in Borago officinalis L. under varying irrigation levels

Salicylic acid (SA) and nitric oxide (NO) are signaling molecules that play key roles in stress tolerance in plants. However, their interactive effect on mucilage and tannin production in medicinal plants remains unknown. Therefore, the present study assessed the individual and combined impact of SA and sodium nitroprusside (SNP as a NO donor) on the secondary metabolism of Borago officinalis L. under varying irrigation levels. The moderate and severe water deficit significantly decreased tolerance index (TI), total chlorophyll content (TChl), and relative water content (RWC) while increasing electrolyte leakage (EL) levels. The total phenol content (TPC) and tannin production increased, and mucilage content slightly reduced under moderate water deficit. The content of all these bioactive metabolites significantly declined under severe water deficit. Foliar application of SA (0.5 mM) and SNP (50 and 100 µM), particularly in combination, improved fresh and dry weights of roots and shoots, and biochemical attributes under all irrigation levels. The highest enhancements for TI (25.2%), TChl content (30.5%), RWC (21.8%), tannin content (20.20%), mucilage yield (18.8%), TPC (21.22%), DPPH scavenging activity (23.3%), and reducing power (15.2%) were recorded by spraying 0.5 mM SA and 100 µM SNP under moderate water deficit. Higher concentrations of SA (1 mM), particularly in combination with 100 µM SNP, negatively affected growth parameters, TChl content, and RWC in plants under non-stress and moderate water deficit conditions. However, this treatment improved the above attributes under severe water deficit. This study provides novel insights into the individual and combined impacts of SA and NO at appropriate concentration in enhancing the growth and antioxidant properties of borage plants, offering a promising strategy for their sustainable cultivation under water-deficit conditions.