Abstract: Salinization and alkalization of soil are a widespread environmental problem, including salt-stress and alkali-stress in terms of the salt characteristics. In fact, the destructive effects caused by alkali-stress are more serious than that by salt-stress. The resistance to salt- and alkali-stress of sunflower (Helianthus annuus L.) is stronger than that of the other crops, as some sunflowers are able to grow on alkalinized soil. The stress conditions of 6 salinities and 6 alkalinities were simulated by mixing two neutral salts (NaCl and Na₂SO₄) and two alkaline salts (NaHCO₃ and Na₂CO₃) both in the molar ratio of 9:1 respectively. The concentration range of treatments was from 40 to 240 mmol L^-1 for the salt-stress, and 10 to 60 mmol L^-1 for the alkali-stress. A sunflower cultivar Baikuiza 6 with higher resistance to salt-alkaline stresses, was used to determine emergence time, emergence rate, survival seedling rate, and physiological indexes including leaf area, water content, biomass of seedlings, and content of Na⁺ and K⁺ under salt- and alkali-stresses. The results showed that the maximum intensity of salt-stress for baikuiza 6 germination and seedling growth was pH 6.98 and electric conductivity of 1.311 dS m^-1 in the culture medium, and alkali-stress was pH 9.71 and electric conductivity of 0.360 dS m^-1. Under the salt or alkali conditions that seedlings could be able to grow, not only the emergence rate and survival seedling rate decreased, but also the time of emergence delayed significantly with the increasing intensity of salt-stress. Whereas the survival seedling rate decreased and the emergence time didn’t delay with the increasing of alkali-stress intensity. Moreover, various growth indexes of seedling decreased with increasing intensity of salt- or alkali-stress, and the extent of decrease was greater under alkali-stress than under salt-stress. In conclusion, the mechanisms of salt- and alkali-stresses effects on germination and seedling growth of sunflower are different, and the tolerance and response of sunflower to the both stresses are different during the two growing stages, proving that salt-stress and alkali-stress are two kinds of stresses.

Key words: Sunflower, Salt-stress, Alkali-stress