Allelopathic Effects Of Walnut On Agroforestry System In Loess Area Of Northern Wei River In Shaanxi

The ecological environment of Loess area of China is fragile, and soil erosion is serious. Agroforestry is one of the main measures to control the Loess Plateau and is an effective way to resolve this vicious circle. Researches on the relationship between species of Agroforestry will help us understand the deeper structure and function of ecosystem stability, explore the rational and efficient resource use patterns, and provide theoretical guidance for agroforestry systems selection and configuration.Walnut (Juglans regia L.) is an important economic tree species in China, it has long history of cultivation, widely distribution and abundant resources. Walnut is the first plant we found that has allelopathy, its secondary metabolite-Juglone can inhibit the growth of other plants. Allelopathy is obvious selectivity and specificity, walnut tree is the typical allelopathic plant, but as long as we choose suitable plants intercropping with walnut, agroforestry will be able to give full play of the advantages grown. For a long time walnut and some crops have been applied for intercropping, however so far there are few reports on intercropping patterns between walnut and Chinese medicinal plants. Therefore, Bioassay methods were used to probe into the walnut allelopathic effect on Chinese medicinal plants Atractylodes macrocephalak, Isatis tinctoria, Polygala tenuifolia and Salvia miltiorrhiza. The objectives were to clarify the main allelopathic substance releasing routes of walnut and allelopathy function mechanism on different receptor plants, to understand the time and spatial change rule of the enzyme activities in rhizosphere soil of walnut, to discuss the feasibility of intercropping between the Chinese medicinal plants and walnut, and to provide a theoretical basis for maximizing the ecological and economic benefits of agroforestry. The main results are as follows:1. Allelochemicals of walnut had promote effects, inhibit effects and double concentration effects on A. macrocephalak, I. tinctoria, P. tenuifolia and S. miltiorrhiza, there exsited different effects with concentration of walnut leaf extracts and receptor plant species, so allelopathic effects exhibited different selectivity and specificity.2. Low concentration (5 mg/mL) of walnut leaf aqueous extract could stimulate the stem length, root collar diameter, chlorophyll content, net photosynthetic rate and stomatal conductance of I. tinctoria, high concentration (100 mg/mL) of walnut leaf aqueous extract inhibited the stem length, root collar diameter, chlorophyll content, net photosynthetic rate and stomatal conductance, therefore, the receptor showed the double effects of concentration.3. Concentration of 0.025, 0.05, 0.1, 0.2 and 0.4 g/mL walnut leaf aqueous extract had obvious damaging effect on the cell membrane permeability of receptor A. macrocephala and I. tinctoria. When the concentration increased, the cell membrane permeability increased, cell inclusions extravasated, resulting in increasing the conductance rate of extracellular osmose fluid.4. The main allelopathic substance releasing routes of walnut were volatile matter aboveground and eluvial matter of walnut, next were root exudates and plant debris decoposition underground. The synthetic effects (SE) of allelopathic substance releasing routes of walnut on different receptor plant were: the SE of A. macrocephalak was 16.77, the SE of I. tinctoria 19.00, the SE of P. tenuifoli 12.10 and the SE of S. miltiorrhiza 27.25 respectively, i.e., the synthetic inhibitory effect of S. miltiorrhiza>that of I. tinctoria>that of A. macrocephalak>that of P. tenuifolia. The synthetic effects of seed germination, shoots length and roots length on four different receptor plants were: the SE of seed germination was 7.82, the SE of shoot length 23.94 and the SE of root length 24.59 respectively, that is the synthetic inhibitory effect of root length>that of shoots length>that ofseed germination.5. Walnut allelochemicals inhibited the receptor plants'(I. tinctoria and A. macrocephalak) soluble protein synthesis, resulting in increasing soluble sugars, PRO and MDA content and decreasing chlorophyll content, and destroying the normal metabolism of plant growth; simultaneously, reducing the POD, SOD, CAT and other antioxidant enzymes activity, breaking the balance of normal defense system, weakening the resistance, and inhibiting the receptor plants growth.6. In the Loess area, the catalase, urease, phosphatase and saccharase activities in rhizosphere soil of 6 years old walnut decreased with the depth of spatial profile, indicated that the soil enzymes activities may be related with the root secretion. Along with sampling time's difference, variation rules of different soil enzyme activities were inconsistent, this was because of rhizosphere of the walnut secretion types and quantity, and microorganism that displayed the different functions to each kind of soil enzyme's synthesis.