Abstract: By a solution culture experiment, cucumber seedlings were treated with different concentration (0, 25, 50 and 75 mmol·L^-1) NaCl and NaHCO₃, and their physiological responses were studied. The results showed that with increasing NaCl and NaHCO₃ stress, the shoot and root growth, the leaf chlorophyll content and relative water content of the seedlings decreased dramatically, and the decrements were greater under NaHCO₃ than under NaCl stress. The shoot Na⁺ content increased significantly with increasing NaCl and NaHCO₃ stress, while shoot K⁺ content was in adverse. At the same concentration of Na⁺ in culture solution, the addition of NaHCO₃ induced a greater decrement of shoot K⁺ content, and thus, a lower shoot K⁺/Na⁺ ratio. Under NaHCO₃ stress, the leaf electrolyte leakage rate and the leaf MDA, proline, and soluble sugar contents increased much greater, compared with those under NaCl stress. Under NaCl and NaHCO₃ stress, the leaf superoxide dismutase (SOD), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) activities were induced significantly, while the leaf peroxidase (POD) activity was inhibited obviously.

Key words: Zhongjieshan Islands, niche breadth, niche overlap