Thesis (Ph.D.)--Chulalongkorn University, 2013

The CH2Cl2 and CH3OH extracts from seventeen plants, and the essential oils of ten plants were conducted for preliminary anti-rice pathogenic microorganism screening. Among them, Piper sarmentosum Roxb. and Polygonum odoratum Lour. exhibited promising antimicrobial activity. The CH2Cl2 extract and the essential oil of P. sarmentosum leaves and fruits, and the essential oil from P. odoratum displayed high potential antimicrobial activity. Using bioassay-guided, the separation of the CH2Cl2 fraction of P. sarmentosum leaves furnished an active compound: myristicin (21), whereas that of the fruits led to the isolation of brachystamide B (7), sarmentine (8), brachyamide B (10) and piperonal (29). The IC50 against two rice pathogenic fungi (Rhizoctonia solani and Bipolaris oryzae) of myristicin (21) and brachyamide B (10) exhibited the most potent antifungal activity while brachyamide B (10) and piperonal (29) displayed the most activity against two rice pathogenic bacteria (Xanthomonas oryzae pv. oryzae and X. oryzae pv. oryzicola). The major constituent of P. sarmentosum oil was myristicin (21). The essential oil of Polygonum odoratum contained aldehydes (dodecanal (2) and decanal (1)) as major constituents. The essential oil and dodecanal showed potential activity against all tested rice pathogenic microorganisms. According to structure anti-microbial activity relationship study, 2-dodecanone and 2-dodecanol displayed high significantly anti-R. solani, while 1-dodecanol and 2-dodecanol revealed significant antibacterial activity.