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Studies on the Possible Physiological Effects of Continued Application of MR Formulation on Muskmelon(Cucumis Meloncv. Non-Fa No.38)
棲息生態系統之影響等。基於此，本研究之目的，係以農發38號(Cucumis melon cv.
Non-Fa No. 38)甜瓜為測試材料，探討MR在連續施用下對其葉部之可能生理影響。雖
其結果發現Ascorbate peroxidase、glutathione reductase、與Superoxide dismutase等酵
The development of MR (methionine and riboflavin) formulation is a first successful attempt in the world of using photodynamic biocidal food constituents as main ingredients for the fungicide applications. One of the unique features of MR as a fungicidal formulation is that it is not likely to cause residual toxicity problems and thus is environmentally safe in the practical field application for disease control. The initial discovery of the microbial toxicity of MR was made by Tzeng and DeVay in 1984. During the past 10 years, a lot of efforts have been devoted to elucidate the involved molecular basis of the microbicidal activity; and a series of field trials have demonstrated the practical value of the developed concoction as a disease control agent. The formulation is currently on the process of commercialization in Taiwan, for the powdery mildew control on cucurbits, through the efforts of American Cyanamid Co. Taiwan Subsidiary However, so far the possible physiological or ecological impact of continued application of this novel fungicidal formulation remains largely to be explored. With this in mind, the purpose of this investigation was thus aimed at evaluating the possible physiological effects of a plant due to continued application of MR formulation. Using greenhouse grown muskmelons (Cucumis melon cv. Non-Fa No.38) as test meterial, possible important foliar function changes were monitored after the application of MR. Through the course of this study, none of the test plants showed observable morphological damages due to MR application. But, it was first noted that after each MR application, there was always a transient increase of ethylene production and proline contenet in treated foliar tissues indicating the presence of certain stressful effect due to the treatment. On the contrary, the stomatal conductivity as well as the contents of free amino acid, soluble protein, and chlorophyll were all not affected; and membrane permeability of the foliar tissues, as reflected by the electrolyte leakage detection, also seemed to remain normal. A followed SDS-PAGE analysis of the soluble proteins, however, revealed a transient increase of a 90kD protein in the MR treated foliar tissue. And a continued HPLC analysis of the free amino contents further indicated a transient surge of glycine content in accompany to the mentioned ethlene and proline increament. Although the plausible reason and the significance of changes of glycine metabolism all remained to be elucidated, the above results indicated clearly the lack of substantial, unrecoverable deteriorative effect on the vital function of test plants due to the MR application.
As a photodynamic biocidal agent, the effectivity of MR formulation for disease control resides mainly upon the production of various activated oxygen species. The lack of detectable damaging effect from MR treated foliar tissues implicated the existence of efficient protective mechanisms for the toxigenic oxygen species in the plant tissue. In order to learn how the test plants survive the oxygen toxicity associated with MR application, the changes of enzymatic as well as nonenzymatic biomolecules known to be of great potential in contributing to the defense mechanism of the MR treated foliar tissues were examined. In Mr treated foliar tissue, we noted that the activities of ascorbate peroxidase, glutathione reductase, and superoxide dismutase were not affected throughout two consecutive MR applications. The activity of peroxidase activity, however, significantly increased. Isozyme analysis by isolelectric frcusing electrophoresis further indicated that the change was mainly due to the increased activity of an isozyme at pI 10.0. In contrast to this, a transient decrease of catalase activity was detected from the foliar tissues after each MR treatment. Activity of this enzyme generally resume the level as the control plants 3 days after MR application. Analysis of the contained isozymes by Native PAGE, however, indicated a change of electrophoretic mobility of these enzyme molecules. In the same experiment, results obtained from HPLC analysis further revealed the lack of effect of MR treatment on contents of reduced form glutathione and carotenoid in foliar tissues, although a slight but significant decrease of oxidized form glutathione and ascorbate were both detected from test leaves after MR application. From the above discussed evidences, it seems clear that certain changes of active oxygen defensing biomolecules do exist in MR treated foliar tissue. However, except peroxidase which appeared to have somewhat prolonged effect during a two consecutive weeks experimental periol, the effect on other tested enzymatic or non-enzymatic molecules all appeared to be instant and rather transient. The lack of prolonged effect among these test biomolcules suggested that the protective effect of test plants resided mainly on the preexisting oxygen toxicity defensing mechanisms. What worth to mention more is that the increased activity of peroxidase and an accompanied decrease of catalase activity appeared to be common feature recently known to be associated with certain plants with systemically acquired resistance (SAR). Although whether or not this might also apply to the case of MR treated plants awaited to be explored, the evidence provided by this investigation does support fully of the use MR as an environmentally safe fungicide for field application.
|Appears in Collections:||植物病理學系|
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