Comparative Studies On the Physio-Biochemical and Ionic Responses of Three Avocado (Persea) Species Under Salt Stress

The physio-biochemical responses of Persea gratissima, Persea americana, and Persea nubigena were investigated in response to NaCl stress (0, 3, 6, 9, and 12 dS·m^-1) in hydroponics. Growth (shoot/root fresh and dry biomass, leaf area, number of leaves), cell membrane stability index (CMSI), photosynthetic activity (A), stomatal conductance (gs), number of stomata (NS), chlorophyll contents (chl), ion accumulation (Na⁺, Cl^-, K⁺, Ca²⁺),  electrolyte leakage (EL), lipid peroxidation (LPO) and antioxidant enzyme activities (SOD, POD, CAT) were measured 15 days after salinity treatment. Salt stress significantly reduced the all above mentioned attributes in tested Persea species except Na⁺, Cl^-, LPO and antioxidant activities, which were elevated under saline regimes. Of the three species P. americana exhibited the least reduction in plant biomass, leaf area, CMSI, A, gs, NS, chl, K⁺, Ca²⁺. Similarly, P. americana had the least LPO and EL values and maximum antioxidant activities as compared to the other two species. The high CMSI and low LPO and EL in P. americana indicates that this species had the lowest salinity-induced membrane injury under NaCl stress, thus representing its high salt tolerance ability.  The reduction in the gas exchange attributes (A, gs, NS) was highly correlated with LA, and leaf Na⁺ and Cl⁻ content in tested Persea species. Although Na⁺ and Cl⁻ content increased in both roots and leaves with increasing salt stress, the accumulation of these two ions in P. americana shoots was lower than in the other two species. Likewise P. americana exhited the greatest leaf K⁺, Ca²⁺ contents under increasing salinity. P. americana had the greatest Na⁺ and Cl⁻ content in roots as compared to the other two species, which may be an adaptation to stop the movement of toxic ions to upper plant parts. P. americana showed the highest SOD, POD, CAT activities at all salinity levels as compared to the gratissima and nubigena species. The findings of the current investigation represent that among the investigated Persea species, P. americana was more salt tolerant than P. gratissima and P. nubigena to NaCl salinity. The relative salt tolerance of P. americana was associated with the higher Na⁺ and Cl^- exclusion capacity via roots and the increased antioxidant activities, which ultimately enhanced the osmotic adjustment potential of plants under stressed conditions.