Plant growth regulators ameliorate the ill effect of salt stress through improved growth, photosynthesis, antioxidant system, yield and quality attributes in Mentha piperita L.

The roles of plant growth regulators (PGRs) in plants are well documented. However, there is a little information regarding their roles in alleviating salt stress in plants, particularly peppermint. This necessitated the evaluation of the efficiency of three selected PGRs in counteracting the ill effect of salt stress by conducting a pot experiment on peppermint (Lamiaceae). Three uniform size suckers were transplanted in each pot containing proper nutrients. Thirty day old plants were subjected to 4 levels of salinity, viz. 0, 50, 100 or 150 mM NaCl. Salt stress was given at 30 days after their transplantation (DAT). Plants were sprayed twice, i.e., at 60 and 75 DAT with 10−6 M each of gibberellic acid (GA3), salicylic acid (SA) or triacontanol (Tria). The sampling was made at 100 DAT and harvesting at 120 DAT. The graded levels of salinity decreased growth, photosynthesis, carbonic anhydrase (CA) activity, NPK content, peltate glandular trichome (PGT) density, essential oil (EO) and menthol content and herb, EO and menthol yield, but increased catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD) activities and proline content linearly. Spray of PGRs particularly SA improved all parameters under both salt and salt free conditions. The maximum values for yields of herb, EO and menthol were noted with 0 mM NaCl × SA. However, antioxidants, proline content, PGT density and EO content were found to be maximum with 150 mM NaCl × SA.