Effects Of Decomposing Leaf Litter Of Juglans Regia L. On Growth Of Triticum Aestivum L.

The negative ecological effects of walnut (Juglans regia L.) allelopathy has been a concern for many years. The dominant narrative has remained roughly the same: Juglone is believed to be major allelochemical released by living walnut roots. Decomposition of plant litter is one of the most significant ways in which allelochemicals release. However, little information is known about the allelochemicals of walnut leaf litter. Therefore, this study identified the types of allelochemicals responsible for the allelopathic phenomenon and preliminary experiments were conducted regarding the ways that allelochemicals could influence plant growth. The objectives of this study were to gain in-depth knowledge of the walnut allelopathy. Furthermore, this study provided the reference thoroughly for choosing matching crop correctly, for adopting reasonable configuration way and management measures in a walnut-based agroforestry intercropping system.Pot experiments were conducted using wheat (Triticum aestivum L.) as receptor, to simulate the effects of decomposing leaf litter of walnut in natural environment on wheat. Experiments were divided into three groups.In experiment A, four different levels of leaf litter treatments were designed:CK (0g·pot-1), A1(30g·pot-1), A2(60g·pot-1), and A3(90g·pot-1), a total of48pots, each containing8kg soil mixed with prepared leaf litter. In experiment B, leaf litter was steamed for12h (Ls) to eliminate low-boiling-point compounds. In experiment C, leaf litter was extracted in80%acetone and alcohol for24h respectively, and then steamed again for12h (Le), to eliminate most of compounds. The other methods were the same as above. Leaf litter were identified by GC-MS analysis to determine the dynamic changes of secondary metabolites in leaf litter. The results of the study are listed as follows:1. In experiment A, during0～160d, the ground diameter, stem height, plant height, and dry weight of aboveground wheat were significantly inhibited by the walnut leaf litter, and the inhibition effect increased with increasing leaf litter addition; The allelopathic compound effects(CE) under all litter treatments were negative, and the inhibition effect increased during0-115d, after115d it decreased, the reason might be that the allelochemicals of walnut leaf litter were obviously reduced or almost no longer released; On180d, the total length, total surface area and biomass of the wheat root were significantly inhibited with increasing leaf litter addition.2. In experiment B, inhibitory effects on wheat were also observed for steamed leaf litter, and the RI averages for A1, A2, and A3were nearly identical to those for B1, B2, and B3, respectively (Table1). This finding revealed that the low-boiling-point and water-soluble components may not play a major role in the inhibitory effects.3. In experiment C, no significant difference was found in the ground diameter of stem and plant height between the extracted litter treatments and C0, and the RI absolute values of the dry weight aboveground for C2and C3were significantly lower than those for A2and A3, respectively (Table2). This result indicated that leaf litter may not significantly inhibit the wheat through its effects on the soil physical properties or as a physical barrier to root growth. Based on a comprehensive comparison of the results, the secondary metabolites of steamed leaf litter may be responsible for the allelopathic phenomenon.4. Resistance physiology of wheat were significantly influenced by decomposing leaf litter of walnut. During75～115d, the activity of superoxide dismutase (SOD) increased with increasing leaf litter addition, while the activities of catalase(CAT) and peroxidase(POD) were significantly lower than those for CK and no significant difference was found between leaf litter treatments; During75-95d, no significant difference was found in leaf Malondialdehyde(MDA) content; On115d, the soluble sugar contents of wheat leaf significantly increased with increasing leaf litter addition, while the protein contents showed a significant downward trend.5. Photosynthetic characteristics of wheat were significantly influenced by decomposing leaf litter of walnut. Photosynthetic pigments of wheat were significantly inhibited with increasing leaf litter addition (p<0.05); Pn and gs also decreased with the increasing amount of leaf litter, but Ci of A1、A2were higher compared with CK, and the difference between CK and A3were not significant (p>0.05); no significant difference was found in the Pn when PAR<200μmol·m-2s-1; With increasing PAR, the trend of Pn was CK>Ai>A2>A3; AQY, Pnmax, LSP, LCP and Rd of leaf litter treatments were significantly inhibited by the walnut leaf litter, and except for AQY and Rd of A3, the inhibition effect increased with increasing leaf litter addition6. Grain yield was significantly inhibited and yield components of wheat were significantly influenced by decomposing leaf litter of walnut. Kernels per spike were significantly inhibited with the increasing amount of leaf litter (p<0.05); but1000-Kernel weight increased by decomposing leaf litter of walnut, the RI was Ai> A3>A2>CK, and the differences of A1and A3were significant compared with CK (p<0.05).7. GC-MS analysis revealed that, the original leaf litter (Lo) contained14compounds (RA>1%), their relative amount represented94.84%of the total, and the major compounds were vitamin E (30.82%), squalene (26.74%) and y-sitosterol (13.52%), their relative amount accounted for nearly70%of the total. The low-boiling-point components of steamed leaf litter (Ls) were essentially eliminated, and there were six compounds (RA>1%) in steamed litter (Ls):vitamin E (37.30%), phytol (2.41%), eicosyl alcohol (1.77%), eicosane (3.73%), squalene (38.03%), γ-sitosterol (15.66%), and the total ion flow graph showed that the com Area of vitamin E, squalene and y-sitosterol decreased43.30%,33.32%and45.69%respectively compared with Lo. In extracted leaf litter (Le), the corr. Area of these six compounds was nearly90%lower than that in Ls.8. A comparison between the untreated aboveground wheat and the leaf-litter-treated wheat suggested that, eicosyl alcohol, eicosane, squalene, y-sitosterol and the new compounds produced or induced by the them may not be transported through xylem to aboveground wheat to exert toxic effects; therefore, their direct and indirect allelopathic effects are most likely due to their toxic effects on the root system; Stigmast-4-en-3-one, as the oxidation product of β-sitosterol, which is derived from squalene, was found in significant quantities in leaf litter after being mixed in soil for90d and180d after sowing and was observed in aboveground wheat. This finding suggested that stigmast-4-en-3-one may exert direct toxic effects on the root system. This remains to be studiedTaking into account of various parameters, phytol, eicosyl alcohol, eicosane, squalene, and sitosterol from the litter were the major potential allelochemicals responsible for the inhibitory effects on wheat. Except for phytol, the others and their degraded or transformed products such as stigmast-4-en-3-one, may only exert toxic effects on wheat root system. This not only inhibited wheat growth directly, but also inhibited wheat growth indirectly by disrupting the resistance physiology and inhibiting the photosynthetic characteristics, finally the grain yield was reduced by the inhibition of wheat growth.