Multivariate screening approach indicated adaptive tolerance to salt stress in the seedlings of an agroforestry tree, Eucalyptus tereticornis Sm.

Key message In the present study, the potential of in vitro screening for the identification of ST seedlings in Eucalyptus tereticornis was investigated. Gradual increase in salt concentration during the exposure period of seedlings resulted in acquiring tolerance up to 400 mM NaCl, which could be due to osmolyte accumulation and overexpression of genes encoding antioxidant enzymes. Eucalyptus tereticornis is one of the fast-growing agroforestry trees providing raw material for paper and plywood industries. Its plantations are hampered by rising soil salinity and identification of salt-tolerant (ST) seedlings could be beneficial to undertake plantation in such soils so to meet the growing demand of the industries. Three different in vitro screening approaches (direct, indirect, and gradual exposure to salt stress) were employed in the present study for the identification of ST seedlings. The screening via gradual exposure to salt stress (0-400 mM NaCl) was highly efficient and promoted tolerance level up to 400 mM NaCl. Shoot growth of seedlings demonstrated a 91.93% increase due to physiological adaptation to salt stress. The indirect screening approach was also effective but only a 12.8% rise in shoot length was recorded when cultured on a medium fortified with 400 mM NaCl. On the other hand, the direct screening approach could only identify seedlings with a tolerance level of 200 mM NaCl. Further, when salt-sensitive (SS) and ST seedlings were exposed to salt stress (0, 200, and 400 mM NaCl) for 14 days, a significant increase in chlorophyll, osmolyte accumulation, and antioxidant enzyme activity was only observed in ST seedlings. The tolerance of ST seedlings was also associated with significantly higher transcript levels of genes encoding superoxide dismutase, peroxidase, and catalase. Overall, these findings will be useful for faster screening of tree species and help in understanding the physio-biochemical basis of salt tolerance in the seedlings of E. tereticornis.