Research On The Allelopathic Effect Of Decomposing Walnut Leaf Litter

Agroforestry systems are receiving more and more concerns in recent years, because they have potential to promote the sustainable management and to use the land resource effectively. Walnut (Juglans regia), a traditional economic tree species in China, is often used as the purpose species in agroforestry systems. Recently, walnut is spread widely with the development of economic forests and the implement of Project of Conceding the Land to Forestry. The allelopathic effect of walnut has been paid attention to in spite of its good economic benefit. Researches related to its allelopathic effect has been a hotspot in the field of ecology throughout the world. Avoidance of the allelopathic effect of walnut on agricultural crops, and if possible, taking scientific measures to alleviate this effect are significant works in practice.On the basis of the study on the allelopathic effect of decomposing walnut leaf litter from Qinba, allelopathic effects of decomposing walnut leaf litter from the west and the Hilly Area of Sichuan Province were further investigated. These further studies aimed to broaden the sampling area of the donor material to provide more common conclusions on one hand, and on the other hand, aimed to investigate the allelopathic mechanism of the decomposing leaf litter more deeply and comprehensively and identify the potential action chemicals during the decomposition process. Also, based on the chemical analyzing technology (GC-MS) and bioassays, a comparative study on the allelopathic effects between leaf litter from Qinba and West Sichuan was also carried out to make sure whether environmental factors affect the expression of allelopathic effect of the donor plant. Main results were listed as follows,1. Walnut leaf litter from three areas all exhibited strong allelopathic effects during the decomposition process, with the growth of the receiver plants (Raphanus sativus, Spinacia oleracea and Brassica chinensis) obviously impacted. This not only embodied in the vegetative growth, but also in the delayed flowering dynamics of the receiver plant. However, the growth and allelopathic tolerance of the progeny of the receiver plant were hardly impacted by decomposing leaf litter.2. Generally, the allelopathic effect of walnut leaf litter collected from these three regions all increased with the increasing application dose, and decreased with the increasing decomposition duration. Of them, the effect of decomposing leaf litter from Qinba and West Sichuan on Brassica chinensis decreased when the decomposition duration exceeded 75 d, and the effect of decomposing leaf litter from West Sichuan and Hilly Area of Sichuan on Raphanus sativus and Spinacia oleracea decreased when the decomposition duration exceeded 120 and 135 d, respectively.3. To deal with the effect of decomposing walnut leaf litter on reactive oxygen species production, these three receiver plants were all able to change the activities of antioxidative enzymes when exposed to the decomposing leaf litter, and the lipid peroxidation product malonaldehyde (MDA) was kept from increasing to some extent. However, lipid peroxidation of membrane of the receive plants aggravated and the activities of antioxidative enzymes decreased with the increase of leaf litter and concomitant increase of allelopathic effect. Abnormal physiological metaboliam in vivo became one of the reasons leading to the inhibited growth of the receiver plants.4. Decomposition of walnut leaf litter might affect the water absorption of the receiver plant and cause osmotic stress on it, soluble sugar and soluble protein were thus stimulated.5. Decomposing walnut leaf litter was also observed to limit the accumulation of photo synthetic pigments, interfere the leaf gas exchange, and decrease the light assimilation and utilization of the receiver plant, resulting in compromised photosynthetic rate and subsequent limited growth.6. By gas chromatograph-mass spectrometry (GC-MS), a total of eight categories of secondary metabolites, including twenty-one chemicals that occupied a relative content of more than 1%, were identified from walnut leaf litter collected from the three areas, i.e., West Sichuan, Qinba Mountain, and Hilly Area of Sichuan. Of these 30 chemicals,12 belong to terpenes,4 belong to fatty acids,4 belongs to alcohols,3 belong to steroids,2 belong to phenols,2 belongs to esters,2 belongs to ketones, and the remaining one belongs to olefms. Most of these chemicals were reported to be allelopathic. Considering that steamed and organic dissolvents-extracted walnut leaf litter did not exert significant effects on the growth of the receiver plant, these secondary metabolites released from the leaf litter during the decomposition process rather than the alteration of soil gas and water permeability caused by litter addition should be responsible for the most of the results stated previously.7.The parallel experiment showed that the addition of steamed and organic dissolvent extracted leaf litter exerted slight effect on the growth of the receptor plant, indicating that the allelochemicals released during the decomposition process rather than the alteration of soil and water permeability caused by leaf litter addition should be responsible for the resul ts stated previously.8. The comparative study showed that decomposing walnut leaf litter from West Sichuan was stronger than that from Qinba Mountain in respect of the allelopathic effect, probably due to the worse environmental conditions in the former region, and concomitantly, higher relative content of allelochemicals in walnut leaf litter. By GC-MS, the potential allelochemicals like calarene, hinesol,β-eudesmol, hexahydrofarnesylacetone, linolic acid, stearic acid,1-octadecanol, y-sitosterol, and β-sitostenone were found higher in the relative content in walnut leaf litter from West Sichuan than that those from Qinba Mountain.On the basis of the study stated above, allelopathic effect of decomposing walnut leaf litter commonly exists in agricultural ecosystems, especially in regions that features bad environmental conditions.. It is recommended to remove too much walnut leaf litter in the inter-cropping systems, or take the leaf litter back to the field after a period of decaying to avoid detrimental effects on agricultural crops.