Physiological Responses of Thai neem (Azadirachta siamensis Val.) to Salt Stress for Salt-tolerance Screening Program

Abstract

The objective of this investigation is to study the physiological responses of Thai neem (Azadirachta siamensis Val.) to salt stress for salt tolerance screening and for establishing a salt tolerance index (STI). The physiological responses of photoautotrophic seedlings under salt stress condition, especially, net photosynthetic rate (NPR) were evaluated as STI. Fifty-six-day-old photoautotrophic seedlings were treated in-vitro with 0 (control), 0.34, 0.68 and 1.02 M NaCl solution contained in the culture media. Physiological characteristics, namely water use efficiency [water potential (Ψw), solution potential (Ψ s) and pressure potential (Ψp)], photosynthetic ability [chlorophyll a (Chla), chlorophyll b (Chlb) and NPR] and growth efficiency [fresh weight, dry weight, leaf area and leaf area ratio (LAR)], were measured after treatment for 28 days. The increase of NaCl concentrations in the culture media decreased Ψw and Ψ s, but, increased Ψp in the leaf tissues. Chlorophyll a and b concentrations of seedlings cultured under salt stress condition (0.34-1.02 M NaCl) were significantly lower than those of seedlings cultured under control conditions (0 M NaCl). The chlorophyll degradation is positively related to NPR reduction (r = 0.98), resulting in the low survival percentage (r = 0.95). Thus, the lower water use efficiency and lower photosynthetic ability of seedlings cultured under salt stress condition led to growth reduction as shown by fresh weight, dry weight, leaf area and LAR. The photoautotrophic seedlings of Thai neem were approximately 50% inhibited at 0.34 M NaCl concentration. So this concentration, which was the 50% lethal dose (LD50), was applied for the salt-tolerant screening program. Six hundred and sixty clones of Thai neem were collected, disinfected and were then germinated in-vitro on PhytagelR-solidified MS sugar-free media (photoautotrophic condition). The culture media of those clones were adjusted the NaCl concentration to LD50. Nine clones of Thai neem still survived under salt stress condition (0.34 M NaCl) for 28 days and were classified as the salt tolerant clones. Salt tolerant ability was evaluated by the NPR reduction under salt stress condition. Salt tolerant index (STI = NPR at 1.02 M NaCl / NPR at 0 M NaCl) was investigated by the ratio of NPR under salt stress (1.02 M NaCl) to control (0 M NaCl) conditions. Nine salt tolerant clones and one salt sensitive clone were assayed for salt tolerant ability using STI. The STI of the salt tolerant clones were higher than those of the salt sensitive clone by a factor of 1.3-1.5 times. The STI is a rapid technique to assay the salt tolerant ability of many clones of Thai neem.