Allelochemicals Biologica... - Name

ALLELOCHEMICALS FROM AGERATUM CONYZOIDES AND ORYZA SATIVA 199
3.1. Rice antifungal allelochemicals
3. RICE ALLELOCHEMICALS AND THEIR
EFFECTS ON RELATED PATHOGENS
Rice (Oryza sativa L.) is one of the principal food crops in the world. Its production is
characterized by heavy use of herbicides and fungicides that may cause environmental
problems in the paddy ecosystem (Kim and Shin, 2000). Accordingly, rice
allelochemicals can potentially be used to improve weed and pathogen management
in rice production. Therefore, search for allelochemicals from rice has been extensively
studied (Chung et al., 2001; Kong et al., 2002b; Mattice et al., 1998; Rimando et al.,
2001). A range of phenolic acids was identified as potent allelochemicals from rice
tissues and root exudates (Chung et al., 2001; Mattice et al., 1998; Rimando et al.,
2001). However, these phenolic acids are unlikely to explain the allelopathy of rice
since their soil concentrations never reach phytotoxic levels (Olofsdotter et al., 2002).
More recently, an increasing number of studies have shown that a few flavones,
diterpenoids and other types of compounds are also the potent allelochemicals from
rice (Kato-Noguchi et al., 2002; 2003; Kong et al., 2002b; 2004d,e; Lee et al., 1999).
These allelochemicals can be biosynthesized in rice seedlings and then released into
their surroundings at ecologically relevant concentrations to inhibit the germination
and growth of associated weeds. Similarly, rice allelochemicals may participate in the
defense mechanisms of rice against pathogens.
In our laboratory, a flavone (5,7,4.-trihydroxy-3,5.-dimethoxyflavone), a
cyclohexenone (3-isopropyl-5-acetoxycyclohexene-2-one-1) and a liquid mixture of
low polarity, containing long-chain and cyclic hydrocarbons (Table 3), were isolated
and identified from leaves of allelopathic rice accession PI 312777 (Kong et al., 2004e).
Both the flavone and cyclohexenone significantly inhibited spore germination of
Pyricularia oryzae and Rhizoctonia solani, but the mixture containing low polarity
constituents did not show any inhibitory effect on them, even at high concentrations
(Figure 4a,b). The IC 50 values of the flavone on spore germination of P. oryzae and R.
solani were ca 50 and 70.g. g -1 , while the cyclohexenone were ca 75 (P. oryzae) and
95 (R. solani), respectively. At all concentrations tested, the inhibitory activity of the
flavone on pathogens was slightly higher than that of the cyclohexenone. The complete
inhibition of both compounds on spore germination of the pathogens was observed at
250.g. g -1 . (Figure 5a, b).
a. b.
5,7,4¡Ç-trihydroxy-3¡Ç,5¡Ç-dimethoxyflavone 3-isopropyl-5-acetoxycyclohexene-2-one-1
Figure 4. a. Flavone; b. Cyclohexenone.