Continuous Drought Stress Study Of Toona Sinensis Seedling

The study has made a continuous drought treatment to the selected fine two-year-old Toona sinensis seedling and conducted observation and determination on the phenotypic responses, photosynthetic characteristics, biochemical indicators, endogenous hormones, moisture contents of plant parts, biomass and survivability of root system of seedlings in the continuous drought process. Then the rehydration is executed to explore the impacts of the soil moisture content on the growth, physiological and biochemical index changes of seedlings and the correlation between the soil moisture content and the drought resisting ability of seedlings. And the critical values of the soil moisture content and the moisture contents of plant parts when seedlings close to death are figured out via rehydration. In order to raise the survival rate of Toona sinensis and provide scientific references to the seed selection of drought resisting breeds, the research results are concluded as follows:1. In continuous drought process, the external morphological characteristics of Toona sinensis seedlings with different injured grades are obvious, and the top buds, leaves (leaflet, old leaf, tender leaf) and stems all have different symptom expressions.2. The chlorophyll contents of Toona sinensis seedlings decrease among the three moisture gradients of the continuous drought process. In the 1～11d of the continuous drought process, the net photosynthetic rate (Pn), transpiration rate (Tr) and stomatal conductance (Gs) of Toona sinensis seedlings show an overall trend of decline, and the intercellular CO2 concentration shows a wavelike decrease in the early period but shows a rising trend in the later period. When the relative rehydration of soil is below 49.01%, the plants will not be able to carry out photosynthesis.3. Among the three moisture gradients of the continuous drought process, the soluble protein contents of Toona sinensis sprouts are higher than the reference, which are characterized as "high-low-high", but the change is not obvious. The soluble sugar contents are on the rise and are higher than the level of the reference, and the differences are very significant. The free proline contents are on the rise and are higher than the reference. The malondialdehyde contents are increasing and are higher than the reference. The flavonoids contents are increasing and the differences are extremely significant. The total alkaloid contents are on the rise and are higher than the level of the reference. The gallic acid contents are declining. And the relative rehydration of the soil is 65.54% which is much higher that the reference.4. Among the processing of the three water gradients of continuous drought process, the IAA contents are significantly different with a feature of "low-high-low". The ZR contents show a trend of rising first and declining later, and are all below the average level. The GA contents are declining and are lower than the average level. The ABA contents are increasing constantly and are higher than the average level. The IAA/ABA ratio rise first and declines later.5. As the degree of drought deepens, the water contents of plant parts show a declining trend and are below the average level. Compared with the reference, the fresh weight ratio and dry weight ratio between the aboveground part and underground part of plants with different injured grades, and the fresh weight ration is lower than the dry weight ratio.6. As the degree of drought deepens, the survival rates of the fresh lateral roots at all levels of the injured plants are declining but with different decline range. The survival rates of primary lateral roots are generally higher than those of the secondary and third-level. The third-level roots have the lowest survival rate.7. After rehydration, the sprout time and stem number of different injured grades of Toona sinensis seedlings are different. The first four injured grades have the same sprout position, and show no dryness, but have different stem numbers. In ld-4d after all-brown, the seedlings have the same sprout time, while the sprout time of other rehydrated plants extend with the expansion of all-brown time.8. The plants with 1/3,1/2 or 2/3 of individual yellow leaf and withered apical bud can recover fully in 5d after rehydration, and begin to sprout in 6d after rehydration with 100% of survival rate. In 1d～4d after all-brown, the survival rate of all-brown plants after rehydration is 100%. In 10d after all-brown, the survival rate of all-brown plants after rehydration is 0.00%.9. The actual water content range in the death leaves of Toona sinensis seedlings is 9.26%-9.37%, and the relative water content range is 14.10%～14.26%. The actual water content range in the stem is 32.70%～33.82%, and the relative water content range is 58.97%～60.99%. The actual water content range in the root is 39.51%～40.96%, and the relative water content range is 61.96%～64.23%. The relative water content range of the death soil is 27.33%～23.95%.10. According to the drought resistance of Toona sinensis seedlings response to drought, the selected fine Toona sinensis seedling which drought resistance can be divided into sensitive type, sensitive type, general type, type of the drought, drought types and extremely drought types, a total of six grades, and can be used as Toona sinensis seedling drought resistance selection evaluation basis.