Exogenous application of salicylic acid via seed soaking improved growth and photosynthetic efficiency by maintaining stomatal organisation, redox homeostasis, and antioxidant defense system in tomato (Solanum lycopersicum L.)

Salicylic acid (SA) is a versatile phenolic plant growth regulator (PGR) which is involved in regulation of several processes of plant growth and development. It confers tolerance against both biotic and abiotic stresses in plants by modulating different morphological and physio-biochemical aspects of plants. Therefore, the present experiment was intended to reveal the impact of SA by seed soaking in Solanum lycopersicum L. (varieties, S-22 and PKM-1). Seeds of both varieties were soaked in 0, 10⁻⁴, 10⁻⁵ or 10⁻⁶ M of SA for 3, 6 or 9 h, before sowing. The respective treated seeds were sown in nursery beds to create nursery and then seedlings were transplanted at 20 days after sowing (DAS) and at 40 days after transplantation (DAT), various growth, photosynthetic, microscopic, histochemical and biochemical attributes were assessed. It was observed that irrespective of the concentration and duration, treatment with SA through seed soaking had enhanced growth, photosynthesis, improved stomatal width, activity of antioxidant enzymes (peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT)), nitrate reductase (NR), carbonic anhydrase (CA) and greater accumulation of proline than the non-treated plants. Remarkably, SA supplementation reduced the accrual of reactive oxygen species (ROS; H₂O₂ and O₂^•− content) and also decreased the electrolyte leakage (EL). Soaking of seeds with SA improved growth and photosynthesis by regulating stomatal organisation, ROS levels and antioxidant enzymes. Among two dissimilar varieties of tomato and three different concentrations of SA, seed soaking of S-22 variety with 10⁻⁵ M for 6 h showed significant increase in growth and photosynthesis than PKM-1 variety.