Studies On Salt-tolerant Mechanism Of Seeds And Seedlings In Melia Azedarach L.

The seeds and one-year-old seedlings in Melia azedarach L. from two provenances wereused as the tested materials. The seed germination and seedling growth were studied under saltstress with six concentrations. The physiological responsing mechanism of Melia azedarach L.was explored by investigating the effects of salt stress on physiological and biochemicalcharacteristics in seed germination and seedling growth.With the increasing salinity stress, germination rate and germination index fistly increased,and then decreased. When salt concentration was100mmolL^-1, the germination rate andgermination index were higher than that of the control, indicating that salt could promote seedgermination; but when salt concentration was higher than100mmolL^-1, it had an inhibiting rolein germination rate and germination index. The vitality index, seedling high, root length and dryweight gradually reduced with the increasing salinity stress. So the seedling quality wasrestrained by salt stress.The content of soluble sugar, soluble protein, free amino acid, proline, betaine in seed didnot change much under salt stress. In the late stage, the content of free amino acid and prolinealso declined slightly. The content of Ca²⁺, Mg²⁺and K⁺changed little, but the content of Na⁺increased significantly. The content of O₂^-· and MDA in seed did not increase significantly,antioxidant enzyme activities including SOD, POD and CAT increased, non-enzyme antioxidantcontent including GSH and ASA also increased. So the seeds of Melia azedarach L. were notonly subject to osmotic stress, but also ion toxicity under salt stress. However, osmotic stress wasmajor.Melia azedarach L. was a salt tolerant species, the salt tolerance of Binhai provenance wasstronger than that of Chuzhou provenance. At150mmol/L salt concentration, seedling survivalrate in Binhai provenance was79%. At100mmol/L salt concentration, seedling survival rate inChuzhou provenance maintained above77%. With the increasing of salt concentration, the height,ground diameter and dry weight of Melia azedarach L. seedlings gradually reduced, the higherthe concentration, the stronger the inhibition effect. With the inereasing of salt concentration, theincrements of the seedling height, ground diameter and dry weight expressed a decreasing trend.Salinity significantly inhibited the growth and development of Melia azedarach L. seedlings, andthe sensitivity to salt of different organs performed leaf>stem>root.Photosynthesis of Melia azedarach L. seedlings were significantly affected by salt stress.Under salt stress, the chlorophyll content of leaves decreased gradually. In the first period, withthe increasing of NaCl concentration, photosynthetic parameters of Melia azedarach L. seedlingsfrom two provenances changed significantly. The net photosynthetic rate（Pn）, stomatalconductance（Gs）, intercellular CO2concentrations（Ci） and transpiration rate（Tr） dropd, butstomatal limiting value （Ls） rised, which denoted that the the main factor inhibiting Pn was stomatal control. In the later stress period, Pn, Gs and Tr in the Melia azedarach L. seedlingsfrom Binhai provenance decreased, Ci firstly decreased and then increased, while Ls increasedfirstly and then decreased. So drop of Pn was mainly controlled by stomatal factor at low saltconcentration and drop of Pn was mainly controlled by non-stomatal factor at high saltconcentration. In the later stress period, Pn, Gs and Tr in the Melia azedarach L. seedlings fromChuzhou provenance decreased, Ci increased gradually, and Ls decreased, so drop of Pn wasmainly controlled by non-stomatal factors.The organic osmotic adjustment substances such as soluble sugar, soluble protein, freeamino acid, proline, betaine in Melia azedarach L. seedlings, had an identical changing trendunder salt stress, that is, all above substances increased. The content of soluble sugar was themost highest among all the organic osmotic adjustment substances, and the increment of theproline was the highest among all the organic osmotic adjustment substances under salt stress.Therefore, soluble sugar and proline were two effective organic osmotic adjustment substances.The content of Na⁺in leaves was the highest positive ion, Na⁺is a very important inorganicosmotica. The content of Ca²⁺, Mg²⁺and K⁺decreased, but this decline was not significant, andthe content of Ca²⁺, Mg²⁺and K⁺still maintained a high level. Therefore, seedlings of Meliaazedarach L. had strong nutrition balance control ability; salt injury might be caused mainly bythe single salt ion toxicity rather than malnutrition.Under salt stress, the content of O₂^-., H₂O₂and MDA in leaves increased, so the membranesystem was damaged. ROS in Melia azedarach L. seedlings were cleaned mainly throughAsA-GSH circulatory system and the high POD activity, which reduced the oxidative damagedegree of ROS to the membrane, CAT also had a certain contribution to clean ROS, but SODactivity was inhibited.