The Physiological Mechanisms Of Silicon-enhanced Resistance Of Cotton To Tetranychus Turkestani

【Objective】Silicon (Si has been proven to be beneficial for healthy growth and development ofmany plant species and plays an important role in enhancing plant resistance against pests anddiseases, however, the mechanisms involved are still unclear. Traditionally, it is thought thatsilicon acts as mechanical or physical barrier in response to diseases or pests attack. However,recent studies show that silicon is associated with host defense responses and plays an active andphysiological role in enhancing resistance of the host plant. In the present study, cotton（Gossypium herbaceum cv. Xinluzao33） inoculated with T. turkestani was used as a case study todemonstrate the physiological and biochemical roles of silicon in improving pest resistance andprovide the theoretical basis for silicon-enhanced pest resistance to minimize pesticide residues insoil environments, and improve the ecol-environmental safety.【Methods】In the field experiment, there were3silicon treatments with or without pestinoculation at a density of,30heads per leaf: i.e. no Si added, foliar application of Si at1.7mmolSi·L^-1, and basal application of Si at a rate of0.725g·m^-2as Na2SiO3. The soil tested is grey desertsoil with pH8.02. In the solution-cultured experiment, cotton was grown hydroponically with3silicon treatments with or without pest inoculation at a density of,30heads per leaf: i.e. no Siadded, foliar application of Si at1.7mmol Si·L^-1, and basal application of Si1.7mmol Si·L^-1. Thedamage index, growth parameter, antioxidant defense enzyme activities, phenolic compounds andpathogenesis-related protein activities were tested in T. turkestan-inoculated cotton grown with orwithout application of Si.【Results】1) The damage index was much lower in plants grown with both foliar and basalapplication of silicon than in plants without silicon （control）. Folia and basal application of siliconrelieved the symptoms of damage by T. turkestani in cotton leaves.2) Damage index in foliar andbasal application of silicon treatments was43.05and55.07, respectively, but was68.72in controltreatment at the6thday after inoculation of T. turkestani. Under no inoculation conditions, siliconwas found to significantly promote the growth of cotton.3) Under pest-stressed conditions, the Siconcentration in plants receiving foliar and basal application of silicon was significantly higherthan that in plants receiving no silicon at the6thday after inoculation of T. turkestani. Under nopest-stressed conditions, the Si concentration was higher in plants receiving basal application ofsilicon than both in plants receiving folia application of silicon and in the control plants.4)Compared with control, under pest-stressed conditions, folia application and basal application ofsilicon significantly enhanced the activities of catalase （CAT）, superoxide dismutase （SOD） andascorbate peroxidase （APX） in cotton leaves, but decreased the contents of hydrogen peroxise（H₂O₂） and Malondialdehyde（MDA） at the6thday after inoculation of T. turkestani.5) Theperoxidase （POD） activity was lower in silicon treatments （foliar or basal application） than in thecontrol, but the polyphenoloxidase （PPO） and phenylalanine ammonia-lyase （APL） activities were higher under pest stress, suggesting that silicon regulate phenolic metabolism and thereby enhancethe resistance of cotton;6) Under pest-stressed conditions, both foliar and basal application ofsilicon significantly enhanced chitinase and β-1，3-glucanase activities in cotton leaves, showingthat silicon can increase the activity of pathogenesis-related proteins to improve the resistance ofcotton to T. turkestani.【Conclusion】Silicon is beneficial for plant growth. Silicon is involved in plant physiological andmetabolic activities by increasing antioxidant enzyme activity, promoting the phenolic metabolism,enhancing the pathogenesis-related protein activity to produce defense resistance to T. turkestaniin cotton. The role of silicon in enhancing resistance to T. turkestani in cotton is attributed to notonly the "physical barrier" effect, but also the silicon-mediated induced resistance which is anactive process.