綠豆連作之相剋作用研究

Abstract

綠豆（Vigna radiata）屬於豆科一年生雜糧作物，臺灣的栽培以春作及夏作為主，秋作因溫度較低產量減少。由田間觀察得知，若於同一土地連續種植綠豆會使植株生長不良及產量降低。本研究的主要目的係瞭解綠豆連作減產的原因及分析其相剋物質，並藉綠豆栽培試驗、種子生物分析法及化學分析以瞭解綠豆植物體及其土壤之相剋物質活性，進而萃取、純化及鑑定其植物之相剋物質。在其植物化學物質之研究中除酚類化合物外，特針對綠豆中所含皂?（saponin）之相生相剋性質作研究。 種子生物分析中，將綠豆種子的浸泡時間延長為8至10小時，以降低幼根長短的偏差。取綠豆植株的水溶萃取液，對數種種子進行生物分析，以比較其相剋作用程度。結果顯示植株各部份對種子根生長的抑制程度為根＞葉＞莖，萃取液抑制萵苣種子根生長的作用大於對綠豆種子。根水溶萃取液在1％時抑制萵苣種子根生長達70％。故綠豆殘株具有相剋作用潛能。根圈土壤的水溶萃取液，則未有明顯的抑制能力。 經由色層分析法可從盆栽根圈土壤及所植綠豆植株中鑑定出p-hydroxycinnamic acid，此種酚類化合物占植體酚類化合物總量的15至30％，於土壤中則可達總量之50％，其中的差距與萃取過程及取樣材料有關。初步的盆栽試驗，在續種綠豆後估計相剋物質可能造成綠豆植株10至25％的生長抑制。另外，於盆栽中及田間均可觀察到土壤病原菌對綠豆植株莖及根部的侵害。經鑑定已知有Pythium spp.及其他數種真菌均會使植株生長受阻。土壤含氮量高易造成種植後的綠豆植株生長較差，甚至使綠豆苗倒伏。在病菌輕度感染後，仍能維持正常生長的綠豆植株中發現根部p-hydroxycinnamic acid之濃度較高，以此化合物及病菌處理土壤後，推測此種酚類化合物的濃度與綠豆植株受病菌感染之程度有關。 在皂?的製備方面，利用索氏萃取器及色層分析等方法分離後，可由綠豆苗中得到部分純化之皂?。此外，透析亦為可考慮使用的純化方法之一。以質譜儀鑑定得到soyasaponin I，其含量約占綠豆所含皂?總量之40％，為綠豆皂?的主要成份。將部分純化的綠豆皂?與土壤混合，並植入綠豆種子，作盆栽試驗，發現處理組的植株生長狀況比未加皂?者略佳，但產量並未明顯增加。然而，較高濃度的皂?溶液對綠豆種子幼根生長有抑制作用。綠豆皂?以0.2％的比例與土壤混合後，則有抑制綠豆植株生長的作用。 由田間試驗、盆栽試驗及植物化學物質等方面分析之結果，認為試驗中綠豆連作減產的原因為植物相剋物質與土壤病原菌作用的結果，因數間亦會互相影響。

Mungbean (Vigna radiata), a species of legume, is an annual food grain crop. In Taiwan, the main cultures of mungbean are in spring and summer. The reduced yield of the fall culture is due to the low temperature. Field observations has shown poor growth and reduced yield of mungbean plants occur after continuous cropping. The purpose of this study is to determine the reasons for reduced yield, the allelochemicals from mungbean plants, and to understand the phytotoxicity of mungbean plants and surrounding soils through cultured experiments in the field, seed bioassays and chemical analysis. Extraction, purification and identification of allelochemicals from mungbean plants are also included. In addition to phenolic compounds, saponin from mungbean is another emphasis. To reduce the variation of radicle length, the seed bioassay method was modified such that mungbean seeds were soaked in extracts for 8-10 hrs. The results of the bioassay experiment using several seeds treated by aqueous extracts of mungbean plant parts showed that the inhibition of parts to radicle growth is root> leaf> stem. Inhibition of lettuce seed growth is more severe than that of mungbean. 1% aqueous extract of mungbean root inhibits 70% of lettuce seed root growth. The results indicate the allelopathic potential of the mature mungbean plants. But aqueous extract of rhizospheric soil did not show significant results. Chromatography was used to identify phenolic compounds from soil and mungbean plants. p-Hydroxycinnamic acid was identified as a major compound, contaning 15-30% of total phenolics extracted from plant and up to 50% from rhizospheric soil. The variation may be caused by different extraction processes and samples. Preliminary pot experiments indicate that allelochemicals may contribute to 10-25% of the growth inhibition of successive crops of mungbean plants. Soil-borne pathogens were also found to be a cause for injured stems and roots of mungbean plants. Pythium spp., an identified fungus and other pathogens can also retard the growth of mungbeans. High content of nitrogen in the soil may cause poor growth of mungbean plants, and even damping of seedlings. The concentration of p-hydroxycinnamic acid in plant roots were higher in plants slightly infected by fungus but maintaining regular growth. The degree of fungus infection may be affected by this phenolic compound. Partly purified saponins were extracted from seedlings by utilizing Soxhlet apparatus, chromatography and other methods. Dialysis can also be used for purification. Soyasaponin I was identified by mass spectrometry, as the major mungbean saponin, its mass contributing about 40% of total mungbean saponins. The addition of saponin to the soils can enhance the growth of mungbean in pots, but the yield was not remarkable. However, high concentration of saponin solution showed inhibition to radicle growth of mungbean seed. Soil with 0.2% crude mungbean saponins showed growth inhibition of mungbean plants. The results from the field and pot experiments and analysis of phytochemicals indicate that the reasons for reduced yield of mungbean plant in continuous cropping include phytochemicals, pathogens and their interaction.