Effects Of NaCl Stress On Seeds Germination And Seedlings Physiological Characteristics Of Hibiscus Hamabo Sieb.et Zucc.

In order to study the effects of salt treatment with different NaCl concentration on the seed germination and the content of Na⁺ and K⁺ in the seed coat and embryo of Hibiscus hamabo Sieb.et Zucc., the seeds were cultivated by 1/4 Hoagland nutrition solution with 0, 50, 100, 150 and 200 mmol·L^-1 NaCl, the germination rate and imbibition rate were measured every day, the content of Na⁺ and K⁺ in seed coat and embryo was determined when the seeds were treated for 5 and 10 days respectively; and after 15 days , the ungerminated seeds in 1/4 Hoagland nutrition solution with NaCl were cultivated by 1/4 Hoagland nutrition solution without NaCl. The results showed that the seed germination rate decreased gradually as the NaCl concentration increased .The seed germination rate and imbibition rate was lowest when the seeds were treated with 200 mmol·L^-1NaCl; the recovery rate of ungerminated seeds in 150 and 200 mmol·L^-1NaCl had no significant difference with the control and it was more higher than the seeds in100 mmol·L^-1NaCl. The Na⁺ content in seed coat and embryo increased gradually as the NaCl concentration increased and days progressed, and Na⁺ content in the embryo was significantly lower than which in the seed coat under the same NaCl concentration. As the NaCl concentration increased and days progressed, the K⁺ content in the seed coat increased gradually while it decreased gradually in the embryo, but the latter was significantly higher than the former. In addition, the ratio of Na⁺/K⁺ in seed coat and embryo increased gradually as the NaCl concentration increased and days progressed, but the ratio in embryo was significantly lower than in seed coat. These results suggested that higher concentration of NaCl could cause the seed dormancy and accelerate the germination of ungerminated seeds. Perhaps there was a mechanism that the seed coat or embryo could prevent Na⁺ from entering into the embryo so as to keep higher K⁺ and lower Na⁺/K⁺ in the embryo. And it might be one of the physical mechanisms that the seed of Hibiscus hamabo Sieb.et Zucc. could resist high salinity stress. In addition, the annual seedlings of Hibiscus hamabo Sieb.et Zucc. were respectively treated with different concentration of NaCl solution. The effects of NaCl stress on the Na⁺ and K⁺ content of roots, mature leaves and young leaves, antioxidative system and osmotic regulation in the leaves were studied in this part. The result showed that, with the increasing of NaCl concentrion, Na⁺ content and ratio of Na⁺ /K⁺ in roots and leaves increased; K⁺ content decreased firstly and then increased, but it was always lower than the control. Na⁺ and higher Na⁺ /K⁺ was harmful to the plant, the harm could be alleviated because of organic osmolyte content and absorbing capacity increasing, but this must be at the expense of sacrificing growth. At the same time, the content of Na⁺ in leaves was distinctly lower than roots and the content of K⁺ was the reverse under the same NaCl concentration treation.So, the ratio of Na⁺ /K⁺ in leaves was evidently lower than in roots. The results indicated that the seedlings of Hibiscus hamabo Sieb.et Zucc. could prevent Na⁺ from transporting to upground by withholding Na⁺ in roots. The Na⁺ and K⁺ content of mature leaves was higher than young leaves when Hibiscus hamabo Sieb.et Zucc. was treated by the same concentration of NaCl, this might be due to varying intensity caused by transpiration. Under the treatment of 400 mmol·L^-1NaCl , the ratio of Na⁺ /K⁺ in young leaves was remarkably higher than mature leaves , and the plant growth was inhibited significantly.The activities of superoxide dismutase (SOD), peroxidase (POD), ascorbic acid peroxidase (APX), Glutathione reductase (GR) and Glutathione- S-transferase (GST) had similar trend, which were increasing firstly, then decreasing with NaCl concentrations from 0 up to 400 mmol·L^-1. However, the concentrations of NaCl in which the enzymes activity came to the highest level were different; The catalase (CAT) activity decreased with the increasing of NaCl concentrations; The MDA content and electrolyte leakage(%) decreased firstly and then increased with the increasing of NaCl concentrations, the fomer was always lower than the control level. The content of AsA and GSH was the highest when the NaCl concentration was 200 mmol·L^-1; The content of proline (Pro) and soluble sugar(SS) increased evidently with the increasing of NaCl concentration as well. It was thus suggested that the high-salinity resistance of Hibiscus hamabo Sieb.et Zucc. was related to its stronger antioxidance and osmotic regulation capacity.