Salt-tolerance Evaluation And Mechanism In Different Siberia Elm Clones

Siberian elm(Ulmus pumila), an indigenous tree species in China, has the great potential to be the main afforestation species in saline-alkaline lands due to its resistance to drought, extreme temperature, and alkali. In our study, 9 elm clones were chosen as the study materials based on the result of former study. The elm clones from plantlets in tissue culture, pot experiment and field experiment were used to study the salt-tolerance response mechanism under Na Cl and mixed salt. The growth characteristic, physiological changes, ion metabolism and gas exchange of elm clone were measured and used to seek the salt-tolerant mechanism of elm clones. The main conclusions are as follows:1. Low salt treatment could enhance the growth of Siberian elm clones. In clones # 7, 20, 50, 68 and 65225, the shoots increase rate was 20%(on average) higher in 3‰ Na Cl treatment compared with untreated controls. In clones # 7, 20, 51 and 105, branch increase rate was about 35% higher in 0.3% Na Cl treatment than the controls. In all elm clones, new shoot height was over 55% higher in 3‰Na Cl treatment compared with controls. Even in 5‰ Na Cl treatment,all growth parameters in clone #20 were greater than in the control plants. In the field experiments, tree height and DBH were 13% and 30% greater than the control respectively in high salt treatment after two years' experiment. In medium salt treatment, the tree height in clone# 7, 20, 30, 46, 51, 68, 105 and 65225 was 17% higher than control; the DBH was 12% larger than the control in clone 7, 20, 46, 51, 68, 105 and 65225.2. From the field study, the salt-tolerance ability of 9 elm clone are: #30>#51>#65225>#20>#50>#105>#46>#7>#68. The result indicates that the salt-tolerance ability of 9 elm clones from in vitro surrounding were the same with the field experiment under mixed salt treatment. Therefore, it proves that the new approach by using in vitro selection for the elm salt-tolerance screening is an effective way.3. The elm clones can increase their salt-tolerance ability through accumulated the soluble sugar and free proline in their cellular; when the salt stress reaches a certain threshold for these clones, the osmotic substance in the cellula will not keeping on accumulated or the content of these osmotic substance will decrease quickly which lead to some salt injury symptoms for the elm or even death. At the same time, with the increased of the salt concertation, the high content of MDA can damage the stabilized of cell membranes which increase the salt stress to the elm.4. The activities of antioxidant enzyme in elm clones increased quickly under low and medium salt treatment(both Na Cl and mixed salt) which could use to remove the excess active oxygen content in the cell. The highly activities of antioxidant enzyme can reduce the damage by the salt stress improve the salt resistant ability. While under high salt treatment, the increase of antioxidant enzyme slow down or even decreased by the effect of high salt stress. Consequently, it will lead to the salt injury symptoms or even death to the elm clones.5. Under the salt treatment(Na Cl and mixed salt) the inorganic ion distributes in different tissue by a certain order. The patterns of Na+ concertation in the elm was: root > shoots > leaf; The patterns of Cl- concertation in the elm was: leaf > root > shoot; The patterns of K+ concertation in the elm was: leaf > shoots > root; The patterns of Ca2+ and Fe2+ concertation in the elm was: root > shoots.6. Under the salt treatment, the chlorophyll content in the leaf reduced with the increase of the salt concertation. Under the Na Cl stress, the photosynthetic rate in pots experiment were inhabited when increase the salt concertation; the transpiration rate increased at low and/or medium Na Cl treatment, decreased at high Na Cl treatment. In mixed salt treatment, the photosynthetic rate and transpiration rate increased at medium(soil salt content 3.5‰-4.5‰) or high(soil salt content ?5‰) salt treatment. While, the changes of stomatal conductance, intercellular carbon dioxide concentration, water usage rate, apparent leaf mesophyll conductance and the percentage stomatal limitation were varying in each clone.7. By the comparative experiment between salt-tolerance elm and salt-sensitive elm clones, it is found: The soluble sugar content plays the key role to the salt stress by improve the elm's survival rate and salt resistance. The salt-tolerance clones show a high soluble sugar content(or keep increase) under higher salt concertation(Na Cl and mixed salt) and elm(in vitro plantlets without root and the plantlets with root in pots experiment) treatment. On the contrary, the content of soluble sugar in salt-sensitive clones decreased quickly at higher salt treatment, which cause a lower osmoticsubstance in the cellula. SOD enzyme play an important role to the salt stress in elm clones(with root). In the pots study, the activities of CAT and POD enzyme inhabit under higher salt treatment. Which means the excess active oxygen will not eliminate by the CAT and POD pathway. While, under high salt concertation, the activities of SOD enzyme in salt-tolerance clones still keep increase in the plants. The highly-activated SOD enzyme is benefit the eliminate of active oxygen and stabilized the cell. On the contrary, high salt treatment to the salt-sensitive clones will inhibit the activity of SOD enzyme, which lead to the large accumulation of active oxygen in the plants cause the damage or death to the elm. The transport and regulation of inorganic salt ions in elm were strongly controlled by the root. Under the high salt treatment, the Na+ content in the root of salt-tolerance clones was 8.9 times higher than that in salt-sensitives clones; for the shoots and leaf, the Na+ content in salt-tolerance clones were 2.4 and 1.6 times lower respectively than that in salt-sensitives clones. These shows that, the root play an important role in the regulation of the elm slat resistance ability. It can reduce the Na+ accumulation in the leaf and shoots by stock the salt ion into the root area, which could minimise the damage to the elm.