| The Karst landscape, with its pronounced limestone geomorphology, is widerspread in the world, and it covers especially large parts of the southwest of China. It often features water limitaition to plants due to thin soil layer, high exposure of rocks, and poor water retention capacity. On the other hand, it often has abundant precipitation and climate modes predict an increase in the frequency and intensity of precipitation during the21th century and beyond. Thus, plants in karst suffer and will suffer from more repeated drying-wetting alternation. It has been proved that AM fungi can improve the tolerance of plants to drought stress. But it’s not clear that whether AM fungi stimulate plants growth under drying-wetting alternation (repeated drying and re-watering), and whether the mycorrhiza effect will change or not as drought time extend. In order to answer these scientific questions, we selected Lonicera japonica Thunb., an appropriate native species in karst region, to study the effects of AM fungi (Glomus mosseea) on plants growth, photosynthetic physiology and element absorption, by three-factors (injected AM fungi, water treatment, different drought time) randomized block design. Two drying-wetting alternations were designed:drying two weeks then re-watering (treatment1), drying four weeks then re-watering (treatment2). There were three levels in each group:CK, drought (D), drying-wetting alternation (DW). The main results are as follows:1. The group inoculated AM fungi increased leaf numbers, leaf surface area, node numbers of new branches, root length, root surface area and volume of root. The extension of plant root could help plant absorb water and nutrition from larger area. At drought stress, the growth of non-inoculated plant root is limited as plant chosed to reduce the number of leaves, leaf surface area to reduce consumption. At the same time, it could reduce the root diameter, extend root length to absorb more nutrients. However, the inoculated plant root diameter in the drought stress had no significant difference with CK group. In DW group, non-inoculated plant had compensation effect after re-watering in treatment1, but the compensation was weaken or even disappeared with the the prolonged drought in treatment2. The inoculations had no significant compensation on plant growth. Water stress had greater influence on the aboveground than underground. Aboveground biomass reduced significantly under drought condition, while AM fungi stimulated the underground biomass accumination more. The root and leaf ratio of plants were increased under both water stress condition and inoculated condition. Futhur, the ratio increased more with the prolonged drought time. Inoculation plant showed obvious higher root and leaf ratio than non-inoculation plants.2. At drought condition, the photosynthetic rate and transpiration rate decreased significantly. The effect of AM fungi on the photosynthetic rate was not significant in treatment1(1-D) compared with non-inoculation plants. However, significant differences appeared as time extend (2-D). Non-inoculation photosynthetic rate and transpiration rate increased, showing compensation effect to plants in treatment1, however, not to plants in treatment2. The inoculation showed no compensation in DW condition (treatment1and treatment2). Injected AM fungi greatly improved the stomatal density of plants, reduced plant transpiration rate in D and DW conditions. Stomatal limitation (Ls) of plant was higher than that of non-inoculation. The intrinsic water use efficiency of plants injected AM fungi significantly increased under drought conditions. In DW condition, AM fungi didn’t enhance the intrinsic water use efficiency in treatment1, but in treatment2, it increased with prolonged drought time. In short, inoculation plant improved stress resistance by increasing the stomatal density, improving the stomatal regulates ability under stress conditions, as well as reducing transpiration rate under DW conditions.3. When inoculated AM fungi, the P and N content of plant increased. In treatment1and treatment2, they all followed the rule of CK>D>DW. As for root P content, the trend turn to D>CK>DW with the prolonged drought time. The stiminated effects of inoculated AM fungi on mineral elements content decreased as the order of CK, D, DW. AM fungi promoted the absorption of three elements:K, Ca, Mg of plant leaf. Mg content of leaves had the stongest response as1-CK,1-D,2-CK,2-D increased3.16、1.93、2.07、2.25times respectively compared with whose of non-inoculated plants. K content was more sensitive to fungal inoculation than others. At1-DW and2-DW, K content increased1.43、1.23times. Leaf Ca and leaf Mg content didn’t changed by AM fungi. In root, inoculations significantly increased three kinds of mineral elements content. However, it’s not significant in DW condition. The root K and root Ca content in DW condition were not affected by inoculated AM fungi. However, root Mg was higher in DW than in drought, showing compensation effect. There had no compensation in Kn、Ca、Mg element absorption.Experiment showed that, in non-inoculated plants, the compensation was significant in treatment1, however the compensation weakened or even disappeared with the prolonged drought time in treatment2. Injected AM fungi could promote plant growth, photosynthetic and element absorption significantly under drought. But there was no significant compensation in most of the indexes under DW condition.While, the index increased more under drought condition and showed a promoting trendcy to plant growth under drying-and rewatering condition with the time prolonged. It’s benifitul for plants to growth under drought condition in karst when injected with AM fungi for a long time. Meanwile, promote the plants’adaptation to repeated drying-wetting alternation in karst. In short, AM fungi have a positive effect on vegetation restoration. |
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