Study On The Alleviating Mechanism By Exogenous Salicylic Acid In Beta Vulgaris Under Salt Stress

Salicylic acid, a new type of plant hormones, has obvious alleviating effect to plants under various stress, such as low temperature, high temperature and salinity. In this thesis, we conducted a preliminary study on the mechanism of salicylic acid in alleviating growth inhibition of Beta vulgaris under salinity. The following are main results. 1. Screening the optimal concentration of salicylic acidB. vulgaris seedlings under 0 mM NaCl and 200 mM NaCl were treated with 0 mM, 0.25 mM, 0.5 mM, 0.75 mM, 1.0 mM, 2.0 mM and 3.0 mM SA for 15 days. The results of phenotype, dry weight and fresh weight as well as photosynthetic indexes showed that the most optimal concentration of salicylic acid was 0.5 mM. 2. Effects of exogenous salicylic acid on the growth of B. Vulgaris under salt stressWith the increase of NaCl concentration, the growth of B. Vulgarisis was inhibited gradually. The dry weight and fresh weight decreased gradually. The whole plant was small and wilting. However, with the spraying of exogenous salicylic acid, the growth of B. Vulgarisis was promoted. 3. Effects of exogenous salicylic acid on the photosynthesis and fluorescence of B. Vulgaris under salt stressWith the increase of NaCl concentration, the indexes of Pn, Gs decreased while Ls increased gradually. And with the increase of Na Cl concentration, the change of Ci tend to be gentle. So we can conclude that non-stomatal limitation contributes more to the decrease of Pn under high concentration of NaCl. Exogenous salicylic acid increased Gs and decreased Ci. In this way, we can conclude that it increased the capacity of CO2 assimilation.With the increase of NaCl concentration, ?PSII and qP decreased obviously, so we can know high concentration of NaCl decreases the light utilization efficiency. However, ETR and Fv/Fm showed no significant difference at 0 mM and 100 mM NaCl, while decreased obviously at 200 mM and 300 mM NaCl. Exogenous salicylic acid increased the light utilization efficiency to some extent and increases ?PSII, qP and ETR accordingly. 4. Effects of exogenous salicylic acid on the antioxidant system of B. Vulgaris under salt stressWith the increase of NaCl concentration, the content of H2O2 and O2.- increased gradually, this trend indicates that salinity promoted oxidative stress in B. Vulgaris. What's more, the activity of SOD increased firstly and decreased afterwards, but the activity of CAT did not change obviously. With exogenous salicylic acid, the content of H2O2 and O2.- decreased obviously, with the activity of SOD and CAT increased. 5. Effects of exogenous salicylic acid on the osmotic adjustment capacity and ion absorption of B. Vulgaris under salt stressWith the increase of NaCl concentration, the content of proline and soluble sugar increased gradually. With the spraying of exogenous salicylic acid, the content of them increased further. The absorption of B. Vulgaris to K+ and Na+ varied with NaCl concentration. The increase of Na+ and the decrease of K+ probably was due to the competitive absorption of K+ versus Na+. Exogenous salicylic acid decreases the absorption of Na+ and increases the absorption of K+, making the plant grow better. 6. Effects of exogenous salicylic acid on the rate of alternative oxidase respiration of B. Vulgaris under salt stressFrom the above results we found that B. Vulgaris under 300 mM NaCl was the most sensitive to exogenous salicylic acid, so we studied alternative oxidase respiration under 300 mM NaCl. Under salinity, cytochrome oxidase respiration was inhibited while the alternative oxidase pathway is activated obviously, leading to the increase of alternative respiration rate. Our experiment showed that the rate of total respiration decreased 47% under 300 mM NaCl and the alternative respiration rate increased 74%. With exogenous salicylic acid, the rate of total respiration increased about 35% and the rate of alternative respiration increased about 24%. 7. Effects of the inhibitor of alternative oxidase on the basic physiology of B. Vulgaris under salt stressWith the increase of NaCl concentration, the growth of B. Vulgarisis was inhibited gradually. With the spraying of SHAM, the growth of B. Vulgaris under 0 mM NaCl have no significant difference with that of control. But in the group of 100 mM, 200 mM and 300 mM NaCl, the plants sprayed with SHAM were much smaller than that of control. The fresh weight and dry weight of shoot and root decreased obviously.With the spraying of SHAM, the Pn, Gs, Ci, Ls and all the indexes of chlorophyll fluorescence of B. Vulgaris under 0 mM NaCl have no significant difference than that of no spraying. However, in the group of 100 mM, 200 mM and 300 mM NaCl, the Pn and Gs of plants spraied with SHAM decreased and the Ci increased than that of control.With the spraying of SHAM, the MDA content of B. Vulgaris under 0 mM NaCl had no significant difference with that of control. But in the group of 100 mM NaCl, 200 mM NaCl and 300 mM NaCl, the MDA content increased obviously, which suggests that alternative respiration plays important role in anti-oxidative stress.