| With the continuous development of economy and society, the earth’s environment is underincreasing pressure. A variety of ecological problems keep emerging and land salinization anddrought become the outstanding problems which affect human survival and agriculturaldevelopment. Sweet sorghum is a kind of energy crops which has excellent properties such assaline-alkaline tolerance and drought tolerance, however the salinity and drought environmentwill make a production decline. Therefore, the studies on salt resistance and drought resistanceare particularly important.In this paper, salt-tolerant and drought-tolerant sweet sorghums varieties have beenscreened based on the test about the salt resistance and drought resistance at the germinationstage and seedling stage and the relationship between salt resistance and drought resistance indifferent variety of sweet sorghums was evaluated. Otherwise, the influence of GA on thegermination of seeds under salt stress and drought stress was exmamined to look for the possiblereasons for the physiological mechanism of sweet sorghums in salt resistance and droughtresistance.The main results were as followings:1. The germination of different varieties of sweet sorghum seeds in100mmol·L-1NaCl andisoosmotic mannitol solution are restrained in different extents. There is no significant changesin germination rate under such salt stress and drought stress for most sweet sorghum seedsexcluding the genotypes5-3and2-10, which have a significant decline in germination rate undersalt treatment. Both germination index and vigour index have a significantly decline under saltstress and drought stress for all varitiess of sweet sorghums. This indicates that salt stress anddrought stress decrease the activity of seeds, and slow down the speed of seed germination.Otherwise, we also found that germination rate, germination index and vigour index of mostvarieties of sweet sorghum seeds have no significant differences in comparison of salt stress with drought treatment, which shows that the main factors inhibiting seed germination isphysiological drought which caused by osmotic stress rather than ionic effect.2.Comparing the growth parameters of NaCl-treated and isosmotic manitol-treated sweetsorghum seedlings, root length and dry&fresh weight of drought treatment seedlings weresignificantly higher than those of salt treatment seedlings, and contrastly the fresh weight of salttreatment seedlings had a significantly higher than that of drought treatment ones. Thusroot/shoot ratio in drought treatment groups is significantly higher than that in salt treatment.This shows that salt stress will cause certain damage to the seedlings roots of sweet sorghum andinhibit its growth. However, the seedlings roots under drought stress will promote the growth ofroots in order to absorb more water. Moreover, the decrease of the overground part (shoot) isalso in order to reduce the losses of water by transpiration. This is a kind of mechanism of plantresponse to drought environment. By measuring Na+, K+content in sweet sorghum seedlings,we found that Na+content in sweet sorghum roots is much higher than Na+content in leaf underthe salt treatment. This shows that sweet sorghum will store excess of Na+in the roots, thuslimiting this ion into the leaves via transpiration. Thus, sweet sorghum is a typically kindsalt-exclusion physiological mechanism.3. Salt injury indexes and drought injury indexes such as the germination rate, germinationindex, vigor index, root length, plant height, fresh weight of root and shoot under100mmol·L-1NaCl and isotonic mannitol solution were determined, and their salt resistance and droughtresitance were evaluated using Subordinate Function Value method for further study ofsalt-tolerant mechanism of Sorghum bicolor. The results showed that salt resistance of10varieties sweet sorghum sorting from strong to weak is5-13>5-9>7-1>5-15>5-7>2-8>3-10>1-22>2-10>5-3at the stage of germination, and drought resistance is in the order of5-13>5-15>5-7>7-1>5-9>2-8>2-10>3-10>5-3>1-22. Therefore, there is a positive correlation between salt resistance and drought resistanceof sweet sorghum genotypes at the stage of germination period, i.e.strong salt resistance varieties generally arealso strong drought resistance.4. Salt injury indexes and drought injury indexes such as root length, plant height, freshweight of root and shoot of the seedlings under100mmol·L-1NaCl and isotonic mannitolsolution were determined, and their salt resistance and drought resitance were evaluated usingSubordinate Function Value method for further study of salt-tolerant mechanism of Sorghum bicolor. The results showed that salt resistance of10varieties sweet sorghum at theseedlings stage is2-8>1-22>3-10>5-15>5-9>5-3>5-13>2-10>7-1>5-7and drought resistance is5-15>2-10>5-13>5-3>5-9>7-1>2-8>5-7>1-22>3-10. Therefore, the correlation between saltresistance and drought resistance in seedlings stage is very low. By comparing the salt resistanceand drought resistance at the stage of germination and seedlings of sweet sorghums varities,there is high positive correlation between salt resistance and drought resistance at thegermination stage but not at the seedlings stage. These results suggest that sweet sorghum aredifferently tolerant to salinity and drought at different developments stages.5. Applying a certain concentration of GA3significantly alleviated the inhibition effect onseed germination under high salt and high drought conditions. When exogenous GA3concentration less than20mg/L, seed germination under salt stress and drought stress wasmarkedly improved, while high concentrations of GA3inhibited the germination of seeds. WhenGA3concentration equals to20mg/L, the alleviation effect was the most significant. Whenadding the paclobutrazol (a inhibitor of GA biosynthesis) to sweet sorghum seeds in salt anddrought environment, seed germination rate dropped sharply. After GA3was added, thephenomenon can be reversed. These results suggest that decline in GA content in the seeds underboth salinity and drought is the key factor of the germination inhibition.By comparing the effectsof GA and paclobutrazol on sweet sorghum seeds under NaCl and isoosmotic mannitol solution,the results are similar.These results suggest that the main factors inhibiting seed germination iscaused by osmotic stress. |
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