Studies On Eco-physiological Adaptation Mechanism And Endangering Mechanism Of Tetraena Mongolica Maxim. In Different Habitats

Tetraena mongolica, monotypic genus of Zygophyllaceae, is the super xerophyte and the relic shrub of Tethys. There is only a small part of them distributing in China. It is a special shrub endemic to the west of Ordos Plateau end its distributed area is very small, belonging to second-class national protected species. It has very important significance both in plant taxonomy and regional system study, and plays an important role as a windbreak, as well as in stabilizing sand and in conserving soil and water. It has also played a very important role in maintaining ecosystem in terms of preventing desertification. But in recent years, the landscape pattern has been fragmented and the habitat has been islanding because of deterioration of the environment and destruction by people. So the plant species has fallen into severe endangered. So the research on the conservation biology of endangered species T. mongolica has been taken great attentions. And analysis on habitat fitness of different population patches in distribution region and ecological adaptability of endemic species T. mongolica are of great significance. It will help to seek after the danger mechanism and offer science base for establishing the safeguard. Then five T. mongolica populations in different habitats (upland, hill tableland, high plain and terrace) and its congener species Zygophyllum xanthoxylon in the same condition were selected as our research objects. Through analyze the soil physical and chemical characters, combining with research on aspects of eco-physiology characters with season including water parameters characters, endogenous phytohormone, enzymatic antioxidant system, photosynthesis and water use efficiency responses, researches on eco-physiological adaptation mechanism of T. mongolica Maxim in different habitats to drought condition and endangering mechanism was carried out. The results are as follows:1.There was a significant difference in density, coverage, width-class and age-class structures of T. mongolica populations in different habitats. The habitats of Upland, hill and tableland population were fit for seedlings rebirthing and plants growth, and the density and coverage in these three populations were significantly higher than that of high plain and terrace populations. The habitats of these three populations is fit to establish nature preservation zone and shoud be given more effective way to conservation. But the habitats of high plain and terrace populations were not fit for seedlings rebirthing and plants growth. T. mongolica in these two populations has been replaced by Z. xanthoxylon that has stronger adaptability and will die out. So the habitats of these two populations should be given more attention.2. The content of organic matter, nutritional elements (N,P,K),micro-element (Fe,Mn,Cu,Zn,Mo,B) was very low, but the pH was high. This embodied the bad condition of barren soil and alkalization in desert. The physical and chemical index of soil in T. mongolica were cluster analyzed and T. mongolica population in five habitats were classified into two groups: upland, hill and tableland populations (group A) and high plain and terrace populations (group B). The content of organic matter, nutritional elements (N,P,K) in the group A was significantly higher than that in group B. The total content of micro- elements Fe,Mn,Cu Zn in soil is higher in the group A than in group B. The available content of six kinds of micro-element such as Fe,Mn,Cu,Zn Mo,B indicates the same results as above.3. T.mongolica is of lowΨ_s^sat,lowΨ_s^tlp , high AWC and high V_a/V₀, which shows that T.mongolica is of strong ability of drought tolerance. The seasonal change ofΨ_s^sat,Ψ_s^tlpandε^max showed May > July > September. And the AWC,V_a/V₀ showed May < July < September. The drought tolerance of T.mongolica continuously enhanced with season. This seasonal change is response to the phonological rhythm of plant. The ordination analysis for drought tolerance of T. mongolica in upland, hill,tableland, high plain, terrace was finished. The level of drought-tolerance among the five habitats above decreased in the following order: high plain and terrace>upland> tableland>hill. Based on three measurements of water parameters of T. mongolica in five habitats in P-V curves, the cluster analysis of drought tolerance of T. mongolica was finished. T. mongolica in five habitats can be classified into two groups: upland, hill and tableland is one group, high plain and terrace is the other group. The variance analysis showed that three measurements ofΨ_s^sat,Ψ_s^tlp and AWC of T.mongolica in high plain and terrace is lower than that of upland, hill and tableland. There are difference in osmotic adjustment mechanism among five different habitats. T. mongolica in high plain and terrace is of stronger ability of osmosis adjustment by improving the concentration of cellular solute, but T. mongolica in upland, hill and tableland is of stronger ability of holding water (high AWC).4. The change trend of ABA and GA3 in the five populations were distinct and regulative in the whole growthing season , i.g. August>June>May. Similarly, the content of IAA and ZR embodys distinct regulative changes, i.g. June> August>May. On the one hand, this proved IAA and ZR content decreased but ABA and GA3 increased under drought stress. On the other hand, this embodied interrelated relationships between endogenous phytohormone and plants growth . ABA played an important role as a signal under drought stress. The content of IAA in May, ABA and GA3 in August in group A were significantly higher than that in group B. The condition of endogenous phytohormone is adapted with its habitats, which plays an important role in plants growth and development.5. The seasonal change of MDA content showed August>June>May, which embodied the memberane lipid peroxidation became stronger. Both the activity and amount of five detected enzymatic antioxidant system SOD,POD,CAT,AsA-POD,GR and antioxidant AsA,GSH were increased significantly with the season, which mutually regulated to promote the growth of T.mongolica. The Enzymatic antioxidant systems have difference in five habitats. The memberane lipid peroxidation of growp A were lower than group B. The activity and amount of antioxidant system were lower than group A. It may be suggested that enzymatic antioxidant play a key role as one of the adaptation mechanism of T.mongolica in different habitats.6. Compared T.mongolica with its its congener species Z. xanthoxylon in group B, The MDA content of T.mongolica were higher which embody the memberane lipid peroxidation were higher than Z.xanthoxylon to eliminate the oxidation damage. The content of endogenous phytohormone ABA,IAA,ZR,GA₃ were lower. T.mongolica has lower ability of maintaining large turgor and low water potential , which embody lower ability of drought tolerance than Z. xanthoxylon. This explained that the ability of Z.xanthoxylon competing limited entironment resource to promote plant growth and drought tolerance, were higher than T.mongolica. These are reasons why T. mongolica has been replaced by Z.xanthoxylon and narrow distributing and endangering.7. Under wild conditions, Analysis on a series of physiological and ecological characteristics of photosynthesis of T. mongolica and Z.xanthoxylon during the growth season shows that the daily change pattern of net photosynthetic rate (Pn) and transpiration rate (Tr) of two plants can be expressed as a two-peak curve. The Pn and Tr reache the maximum at 11:00 and 15:00, Also the Pn and Tr were higher in the morning than in the afternoon. The decline of Pn and Tr of two plants in the noon were observed. The reasons of Pn decline were mainly caused by stomatal limitation. And the decline of Tr were a ecological adaptation way by closing down the stomatal to adapt high temperature in the noon. Correlation analysis and regression analysis showed the photosynthetic active radiation (PAR) and the stomatal conductance (Gs) are the two most influential factors on the Pn and Tr. Compared with T.mongolica, Z. xanthoxylon had higher Tr and water use efficiency (WUE), which may be an important reason why why T. mongolica has been replaced by Z.xanthoxylon and narrow distributing and endangering.