Studies On Physiological Mechanisms Of Powdery Mildew Improved By Silicon In Cucumber (Cucumis Sativus L.)

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 added to cucumber nutrient solutions. As a result, Silicon enhanced K+, Ca²⁺and Mg²⁺ concentrations in apoplast that may contribute to the ion selectivity, ion distribution, ion homeostasis in cells and the increase of osmotic adjustment.4. The defensive enzymes eroxidase (POD),chitinase andβ-1, 3-glucanase activities of both susceptible and resistant cucumber cultivars were increased after inoculation with Sphaerotheca Fuliginea.. Their activities continued increasing with Si supply, thus these leaves could resist powdery mildew caused by Sphaerotheca fuliginea. Our research suggested that the activities of POD, chitinase andβ-1, 3-glucanase are related to induced systemic resistance of cucumber plant, remarkably decreased the damage brought by powery mildew.The physiological mechanism of Si enhanced resistance of cucumber against mildew may be that Si is involved in the metabolic or physiological changes after powdery cucumber leaves inoculated with Sphaerotheca fuliginea. Si induced plant defence to Sphaerotheca fuliginea and enhanced systemic resistance of cucumber plants.