Growth and antioxidant defence in hypocotyl-derived calli of two cotton cultivars with contrasting salt tolerance

Soil salinity is one of the major abiotic stress factors that limits cotton (Gossypium hirsutum L.) production worldwide. Although cotton is categorised as a salt-tolerant plant, its tolerance level can vary depending on the cultivar. The present study aimed to evaluate the growth and biochemical responses of two cotton cultivars (Carmen and NM-503) to salt stress under tissue culture conditions by using an in vitro selection technique. Hypocotyl explants were cultured on callus formation medium (full-strength MS (Murashige and Skoog medium) macro- and micro-salts + B5 (Gamborg medium) vitamins + 0.1 mg/L 2,4-D (2,4-dichlorophenoxyacetic acid) + 0.5 mg/L kinetin) containing 0, 100, 200 or 400 mM NaCl for 4 weeks. Compared to Carmen cultivar, NM-503 cultivar showed an improved growth performance under non-stress control conditions; however, both cultivars showed similar growth and browning tendencies when exposed to salinity stress. Although both cultivars showed a significant decrease in biomass accumulation and relative growth rate following treatment with 200 and 400 mM NaCl, they were tolerant to NaCl stress up to 200 mM in terms of callus survival rate. Carmen cultivar showed the lowest photosynthetic pigment content after treatment with 400 mM NaCl; however, both cultivars showed no significant differences in the photosynthetic pigment content between NaCl concentrations. Furthermore, in contrast to Carmen cultivar, NM-503 cultivar did not accumulate proline in response to 200 and 400 mM NaCl treatment. Under salinity stress, NM-503 cultivar exhibited lower lipid peroxidation level than Carmen cultivar. Both cultivars showed no significant differences in superoxide dismutase activity in control and NaCl-treated groups, except for the 400 mM NaCl-treated group in Carmen cultivar. Apart from constitutive enzyme activity, both cultivars showed similar catalase activities at all concentrations of NaCl treatment. In both cultivars, peroxidase and ascorbate peroxidase activities increased in response to an increase in NaCl concentrations. While glutathione reductase activity gradually increased in NM-503 cultivar in response to NaCl treatment, it gradually decreased in Carmen cultivar. In conclusion, both cultivars showed similar growth response under salinity stress, and this growth response was significantly restricted by 200 and 400 mM NaCl treatment. Among all studied parameters, the most distinct response of the cultivars to salinity stress was reflected by the lipid peroxidation level, which indicates the differences in the mechanism of cellular antioxidant protection between the cultivars. Despite the absence of a salt exclusion mechanism used by intact plants, the high level of resilience shown by the calli of NM-503 cultivar against severe salt stress conditions may be attributed to its effective cellular antioxidant defence mechanism.