Ecological Adaptability Of Typical Woody Plant Seedlings Under Drought Stress In Mining Areas Of The Loess Plateau

Soil moisture is the main limiting factor affecting vegetation growth and ecological restoration in mining areas of the Loess Plateau. In order to reveal the ecological adaptability of typical woody plant seedlings to drought stress in the mining areas of the Loess Plateau, according to our preliminary studies on naturally colonized and planted community in the mining areas, this study took the one-year seedlings of seven typical woody plant species(Alianthus altissima, Robinia pseudoacacia, Platycladus orientalis, Pinus tabuliformis, Amorpha fruticosa, Hippophae rhamnoides, Vitex negundo var. heterophylla) with strong drought and barren soil resistance as research subjects and the raw mixed-loess soils collected from coal Shanxi mining areas seedlings as soil matrix, and by conducting the greenhouse planting experiment for water control, three soil moisture treatments (Control:normal water with 75±5% of field capacity levels, medium drought with 55±5% of field capacity levels and severe drought with 35±5% of field capacity levels) were set by simulating soil moisture conditions of the mining areas and each treatment repeated three times. By using biochemical techniques and photosynthesis system (Li-6400XT), the effects of different drought stress on leaf water physiological index, chlorophyll content, antioxidant enzyme system, photosynthetic characteristics and soil enzyme activity of the woody plant seedlings were researched. The research revealed their response characteristics under drought stress, and at the same time, we evaluated their ecological adaptability under drought stress. The main results were as follows:(1) The relative water content (RWC) and leaf water-holding capacity of seven woody plant seedlings and chlorophyll content of five woody plant seedlings(Alianthus altissima, Robinia pseudoacacia, Amorpha fruticosa, Hippophae rhamnoides, Vitex negundo var. heterophylla) decreased with the aggravation of drought stress, while their leaves relative water deficit (RWD) increased. The anti-dehydration ability of tree seedlings of Platycladus orientalis and Pinus tabuliformis was higher than that of Ailanthus altissima and Robinia pseudoacacia seedlings, and the anti-dehydration ability of shurb seedlings of Amorpha fruticosa was higher than that of Hippophae rhamnoides seedlings.(2) Except the malondialdehyde (MDA) content of Pinus tabuliformis decreased with the aggravation of drought stress, the MDA and membrane lipid peroxidation of the other six woody plant seedlings increased under drought stress. Among them, the MDA content of Pinus tabuliformis and Platycladus orientalis seedlings changed less than that of Robinia pseudoacacia and Alianthus altissima seedlings, and the MDA content of Vitex negundo var. heterophylla seedlings changed less than that of Amorpha fruticosa and Hippophae rhamnoides seedlings. Compared with the Control (normal water), under severe drought stress, the seedlings of Alianthus altissima, Robinia pseudoacacia and Amorpha fruticosa could resist drought stress by increasing superoxide dismutase (SOD) and peroxidase (POD) activity to remove mass accumulated reactive oxygen in their bodies; And the seedlings of Pinus tabulaeformis and Vitex negundo var. heterophylla seedlings could resist drought stress by increasing SOD and POD activity, respectively. The seedlings of Alianthus altissima, Robinia pseudoacacia, Platycladus orientalis, Pinus tabuliformis and Amorpha fruticosa could resist drought stress by increasing their osmotic regulation substances-soluble protein (SP) contents. These results showed that different woody plant seedlings could resist or bear drought stress through different mechanisms, Among the tree seedlings, Platycladus orientalis seedlings had stronger drought resistance ability than that of Pinus tabuliformis, Alianthus altissima and Robinia pseudoacacia, and among the shrub seedlings, Amorpha fruticosa seedlings had stronger drought resistance ability than that of Hippophae rhamnoides and Vitex negundo var. heterophylla.(3) The net photo synthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) of five woody plant seedlings (Alianthus altissima, Robinia pseudoacacia, Amorpha fruticosa, Hippophae rhamnoides, Vitex negundo var. heterophylla) dropped with increasing of drought stress levels. Compared with the Control (normal water), under severe drought stress, the intercellular CO2 concentration(C;) of Ailanthus altissima and Amorpha fruticosa seedlings decreased and the Pn decline of them mainly caused by stomatal limitation, while the Ci of Robinia pseudoacacia, Hippophae rhamnoides and Vitex negundo var. heterophylla seedlings increased, the Pn decline of them mainly caused by non-stomatal limitation. Under severe drought stress the water use efficiency (WUE) of Alianthus altissima and Amorpha fruticosa seedlings was higher than that of others, and both of them could use water more economically.(4) The modified rectangular hyperbola model could fit light response curves of the five woody plant seedlings under different drought stress very well. Under the same photosynthetically active radiation (PAR), the net photosynthesis rate (Pn) under normal water>that of medium drought> that of severe drought. With the aggravation of drought stress, the apparent quantum yield (AQY), the maximum photosynthetic rate (Pmax), the dark respiration rate (Rd) and the light saturation point (LSP) of the five woody plant seedlings decreased, while the light compensation point (LCP) of them increased with the degree of drought stress aggravated, and among them, Ailanthus altissima, Amorpha fruticosa and Hippophae rhamnoides had a wide range to use light than others. Under drought stress, the adaptability of Ailanthus altissima seedlings to strong light was stronger than that of the Robinia pseudoacacia, and the adaptability of Amorpha fruticosa and Hippophae rhamnoides seedlings to strong light was stronger than that of the Vitex negundo var. heterophylla seedlings.(5) Compared with the Control (normal water), under severe drought stress, the soil enzymes activities of catalase, urease, alkaline phosphatase and glucose in the rhizosphere of seven woody plant seedlings were inhibited and their soil enzymes activities of alkaline phosphatase and catalase changed greatly. Under severe drought stress, among the four tree seedlings, soil enzymes activities in the rhizosphere of Platycladus orientalis seedlings changed relatively little, catalase activity in the rhizosphere of Pinus tabuliformis seedlings decreased greatly, urease and glucose activities in the rhizosphere of Alianthus altissima seedlings decreased greatly, while alkaline phosphatase and glucose activities in the rhizosphere of Robinia pseudoacacia seedlings changed obviously. Under severe drought stress, among the three shrub seedlings, soil enzymes activities of catalase, urease and alkaline phosphatase in the rhizosphere of Hippophae rhamnoides and Vitex negundo var. heterophylla seedlings greater decreased than that of Amorpha fruticosa. This result showed that soil enzyme activity influenced by the woody species, the enzyme categories and soil moisture.(6) Ecological adaptability of the four tree seedlings under medium and severe drought stress was evaluated by the method of subordinate function and their rank order from high to low were:Platycladus orientalis> Pinus tabuliformis> Alianthus altissima> Robinia pseudoacacia. Ecological adaptability of the three shrub seedlings under medium and severe drought stress was also evaluated by the method of subordinate function and their rank order from high to low under medium and severe drought stress were: Amorpha fruticosa> Vitex negundo var. Heterophylla> Hippophae rhamnoides.Overall, the seedings of Platycladus orientalis, Pinus tabuliformis and Amorpha fruticosa have strong ecological adaptability under drought stress, which are the best woody plant species for ecological restoration in mining areas of the Loess Plateau. And the seedlings of Robinia pseudoacacia are suitable for planting in the area with good soil moisture condition.