Controlled-Release Salicylic Acid-Loaded Microspheres Enhances Physiological Performance and Organic Acid Metabolism of Lettuce (Lactuca sativa L.) under Salt Stress

Excessive salinity is a great setback when growing vegetables in greenhouses, so salicylic acid (SA) from external sources is used to mitigate the damage caused. At the same time, traditional applications might result in phytotoxicity and varying physiological responses, depending on the dose. In this study, a comparative effect of the synthesized controlled-release system, SA-loaded sodium alginate (NaAlg) microspheres as controlled-release salicylic acid (CRSA), on lettuce (Lactuca sativa L. cv. Paris Island) grown under NaCl-induced stress was evaluated. NaAlg–SA microspheres were characterized by ATR-FTIR (Attenuated Total Reflectance–Fourier Transform Infrared), particle size, zeta potential, and entrapment parameters. FTIR spectra confirmed successful incorporation of SA into the alginate matrix while entrapment efficiency and loading capacity increased with initial SA concentration, reaching values of 48.38 and 4.40% in NaAlg-SA4, respectively. In vitro release at pH 6.5 exhibited a biphasic profile with an initial burst followed by sustained diffusion with approximately ~24% of SA released after 24 h, indicating further availability under soil-like conditions. Pot experimentation was carried out at four salinity levels (0, 50, 100, 150 mM NaCl) with the free SA (1, 2, 4 mM) in combination with CRSA at equivalent doses. The ANOVA confirmed that salinity significantly reduced chlorophylls and carotenoids, as well as increased total phenolics and antioxidant activity, in a dose-dependent but non-linear manner. Both SA and CRSA mitigated the loss of pigments and increased the accumulation of phenolic compounds, though at 2 mM, CRSA produced the most stable enhancements across salinity levels. Organic acid profiling and clustered heatmap analysis indicated that the CRSA was relatively more favorable than the free SA, particularly at 2 mM, in keeping a balanced tricarboxylic acid (TCA) cycle and osmolyte profile while causing very strong, dose-oriented metabolic oscillations. In conclusion, the results suggest that NaAlg microspheres can be an efficient, controlled-release source of SA for improved salt tolerance and salinity-induced metabolic stability in lettuce.