Activity and Allelopathy of Soil of Flavone O-Glycosides from Rice

Two flavone O-glycosides were isolated from allelopathic rice seedlings and have been identified as 5,4'-dihydroxy-3',5'-dimethoxy-7-O-beta-glucopyranosylflavone and 7,4'-dihydroxy-3',5'-dimethoxy-5-O-beta-glucopyranosylflavone. Considerable levels of these glycosides could be found in allelopathic rice tissues. They could not be detected in the soils growing these allelopathic rice seedlings. Only their aglycone, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, could be found in the soil. Further experiments showed that two flavone O-glycosides were exuded from allelopathic rice roots to the rihzosphere and then transformed into their aglycone form, that is, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, with a great diversity of biological activities on associated weeds and microbes by soil interactions once released. The glycosides degraded rapidly (t1/2 < 2 h), whereas their aglycone was more resistant toward degradation in paddy soils, in which the half-life (t1/2) at low (25 mug/g) and high (200 mug/g) doses reached 19.86 +/- 3.64 h (r 2 = 0.97) and 28.78 +/- 3.72 h (r 2 = 0.98), respectively. Furthermore, the mobility of both glycosides and their aglycone in paddy soil was evaluated by soil TLC with bioassay. The mobility of the glycosides (Rf = 0.418 +/- 0.069, n = 18) is higher than that of the aglycone (Rf = 0.361 +/- 0.048, n = 18). The results suggested that two flavone O-glycosides are formed in rice biosynthesis and that storage of the allelochemicals and their aglycone 5,7,4'-trihydroxy-3',5'-dimethoxyflavone is the agent of alleloapthic rice which interferes with weeds or microbes in paddy soil.