硅被认为是植物生长的有益元素，研究表明，硅能缓解植物的非生物胁迫（铝、重金属和盐胁迫）和生物胁迫（病虫害）。硅能提高植物的抗病性已得到许多实验的证实，但是它的特性及其在植物中的作用方式仍不明确，在增强植物抗病性中的作用机理还有争议。本试验以黄瓜（Cucumis sativus L．）为材料，采用营养液栽培，研究了外源硅（K₂SiO₃·2H₂O 1.7 mM）对黄瓜细胞器质外体的抗氧化系统、离子分布、主要防御酶的影响及其叶片的电镜扫描分析，进一步阐明硅提高黄瓜抗病性的生理机制。所取得的主要结果如下：1．借助扫描电镜，分析了处理叶片的白粉菌菌丝体的分布生长情况。结果表明：在不加硅处理的叶片上，菌丝体集结成网状，还可以看见明显的分生孢子团，特别是易感白粉病品种。而加硅处理其叶片表皮的菌丝分布很少，只有零星的菌丝体细丝，而且抗病品种甚至没有菌丝体。因此加硅处理可以明显地抑制菌丝体的发展，起到物理屏障的作用。2．接种白粉菌（Sphaerotheca fuliginea）后，黄瓜抗／感病品种叶片质外体内过氧化氢酶（CAT）、抗坏血酸过氧化物酶（APX）、过氧化物酶（POD）、脱氢抗坏血酸还原酶（DHAR）等抗氧化酶活性明显升高，超氧化物歧化酶（SOD）活性显著下降，抗／感病品种的酶活性差异不显著；施硅能显著提高两个品种的APX、SOD、POD和DHAR的活性，而降低CAT的活性，特别是对感病品种酶活性的影响更显著。3．接种白粉菌（Sphaerotheca fuliginea）后，黄瓜抗／感病品种叶片质外体中K⁺、Ca²⁺、Mg²⁺的浓度均下降，同时硅处理可以改变黄瓜植株对各种元素的选择吸收能力，增加质外体内K⁺、Mg²⁺、Ca²⁺的含量，有利于质膜对离子的选择性，增强离子的区域化水平，提高植株的渗透调节能力。4．接种白粉菌（Sphaerotheca fuliginea）后，黄瓜抗／感病品种叶片中的防御酶过氧化物酶（POD）、几丁质酶和β-1，3-葡聚糖酶活性均增强，施硅后3种酶的活性继续增加，从而抵制以后白粉菌（Sphaerotheca fuliginea）的侵入。上述结果表明，这3种酶的活性与诱导系统抗病直接相关，能够显著降低白粉菌的危害程度。硅提高黄瓜对白粉病抗性的可能作用机理是：在黄瓜-白粉菌的相互作用中，硅参与了植物的代谢或生理活动，诱发植物产生对病原菌的一系列抗性反应，增强植株的系统抗病性，从而抑制白粉病菌对黄瓜的危害。Slicon（Si） is generally considered as a beneficial element for the growth of plants. Many previous studies have reported the roles-of Si in alleviating both abiotic and biotic stresses in plants. Its effects on the improvement of plants diseases resistance have been acknowledged by lots of experiments. Although Si has been a disease-resistant for a long time, its physiological and nutrient roles in plants are apparently minimal and poorly understood, its disease-resistant mechanism having been argued. In this paper, cucumber（Cucumis sativus L.） was grown hydroponically to study the effects of exogenous silicon （K₂SiO₃ 1.7 mM） on antioxidative system, ion distribution, the activities of the main defensive enzymes in apoplast and the analysis of leaves with scanning electron microscope. The objective was to clarify the possible physiological mechanisms of Si on increasing plant resistance to disease in cucumber. The main results are summarized as follows:1. The distribution and development of powdery mildew fungi mycelia of the sample leaves were analyzed by scanning electron microscope. The results indicated that dense mycelial mats and obvious conidial colonies were observed on the leaves of non-Si supplied plants, especially in the susceptible cultivar, while mycelial development at the leaf surface of Si supplied plants was reduced to a loosened network of fine hyphal filaments or no mycelium in the resistant cultivar. Thus the supplement with Si could strongly restrain the development of mycelia operating as physical safeguard.2. Hydroponic culture experiments were conducted with both susceptible and resistant cucumber cultivars to investigate the effects of Si and inoculation with Sphaerotheca Fuliginea on antioxidative enzymes in apoplast. The results showed that the inoculation enhanced the catalase（CAT）, ascorbate（APX）, peroxidase（POD） and dehydroascorbate（DHAR） activities in apoplast, but decreased the superoxide（SOD） activity, meanwhile the difference was unobvious between the resistant and the susceptible cultivar; while inoculation plus Si supply remarkably increased the activities of APX, SOD, POD and DHAR and decreased the CAT activity especially for the susceptible cultivar.3. The results showed that the inoculation increased K⁺, Ca²⁺ and Mg²⁺ concentrations, meanwhile the selective absorb capacity of lots elements was altered when silicon was