Keywords: chalcone, naringenin, plant growth.Hence, we propose that the lignin biosynthesis pathways involving 4CL might have the potential as novel action sites of plant growth suppression in selected species. The objective of the present study was to investigate the sensitivity of a wide range of annual plant species to naringenin and chalcone. Compounds and plantsNaringenin (4¢-, 5-, 7-trihydroxyflavanone, 95%) and chalcone (1,3-diphenyl-2-propen-1-one, 97%) were commercially obtained from Sigma-Aldrich Co. (Tokyo, Japan) and Acros Organics Co. (Tokyo, Japan), respectively. Twenty annual species (Table 1), which cover 10 families, were evaluated for their responses to naringenin and chalcone. Crop seeds were purchased from Sakata Seed Co. Ltd. (Yokohama, Japan). Weed seeds of common chickweed (Stellaria media Villars), great burdock (Arctium Lappa L.) and early watergrass (Echinochloa oryzicola Vasing) were obtained locally from our laboratory. Crop and weed species were evaluated and their respective scientific and family names are listed in Table 1. Plant growth and compound treatmentsSeeds were rinsed with distilled water four times and germinated in tissue culture dishes (F100 mm) containing wet muslin in the growth chamber in darkness at 30∞C for 48 h. The germinated seedlings were grown on wet muslin in plastic plates (200 ¥ 300 mm) containing nutrient solution (1/1000 of Hyponex, Type: 5-10-5 [N : P : K = 10 : 10 : 5%], Hyponex, Japan) in a greenhouse under natural light at about ±25∞C. At the second-leaf stage, the seedlings were transferred into bottles (200 mL) and sponge was used to support the seedlings so that the roots were exposed to Hyponex nutrient solution with (i.e. treatment) or without (i.e. control) 0.1 mmol L -1 naringenin or chalcone (final concentra-

To investigate the selectivity and safening action of the sulfonylurea herbicide pyrazosulfuron-ethyl (PSE), pyrazosulfuron-ethyl O-demethylase (PSEOD) activity involving oxidative metabolism by cytochrome P-450 was studied in rice (Oryza sativa L cv Nipponbare) and Cyperus serotinus Rottb. Cytochrome P-450-dependent activity was demonstrated by the use of the inducers 1,8-naphthalic anhydride and ethanol, the herbicides PSE, bensulfuron-methyl, dimepiperate and dymron, or the inhibitor piperonyl butoxide (PBO). Growth inhibition in C serotinus seedlings was more severe than that in rice seedlings. O-Dealkylation activities of PSE were induced differently in rice and in C serotinus, with distinctly higher activity in rice seedlings. The induced PSEOD activities were slightly inhibited by PBO in rice seedlings, whereas they were strongly inhibited in C serotinus seedlings. Dimepiperate and dymron were effective safeners of rice against PSE treatment. Treatments with herbicide alone resulted in less induction of PSEOD activity compared with combined treatments of the herbicide and safener. PSEOD activity in rice seedlings induced with herbicide alone was strongly inhibited by PBO, whereas it was weakly inhibited in rice seedlings induced with combinations of PSE and two safeners. These results suggest that O-demethylation by cytochrome P-450 enzymes may be involved in the metabolism of PSE and may contribute to its selectivity and safening action. Furthermore, these results suggest the existence of a multiple form of cytochrome P-450 in plants.

The 0-dealkylating activities in rice were investigated to obtain fundamental information relating to the selectivity and safening action of sulf onylurea herbicides by measuring 7-ethoxycoumarin 0-deethylase (ECOD) and 7-methoxycoumarin 0-demethylase (MCOD) activities in microsomes taken from rice shoots. NADPH cytochrome P-450 (c) reductase (NADPH cyt. c red.) activity was measured to confirm the effect of electron transfer system on cytochrome P -450 reaction. Cytochrome P-450 dependence of the reactions was demonstrated by use of safeners: 1, 8-naphthalic anhydride (NA) and ethanol; inhibitors: 1-aminobenzotriazole (1-ABT) and piperonyl butoxide (PBO); herbicides: dimepiperate, dymron, pyrazosulf uron ethyl (PSE) and bensulfuron methyl (BSM). The 0-dealkylation was induced by the treatment of not only safeners (NA, ethanol) but also herbicides (dimepiperate, dymron, PSE, BSM). Furthermore, it was inhibited to varying degrees by both inhibitors (1-ABT, PBO) of induced cytochrome P-450 enzymes and by sulfonylurea herbicides (PSE, BSM). In contrast, NADPH cyt. c red. activity was not significantly changed by either induction or inhibition. These results suggest that cytochrome P-450 is involved in the oxidative detoxifying metabolism of sulfonylurea her-bicides like PSE and BSM, so that these herbicides may be a substrate or competitive inhibitor of microsomal oxygenases.

Introduction: Various flavonoids may regulate the activity of itdolacetic acid oxidase (IAA-oxidase) in vitro. 4-coumarate: CoA ligase (4CL) catalyzes the activation of cinnamic acid, coumaric acid, caffeic acid, ferulic acid and sinapic acid, and it is considered to be a key enzyme for lignin and phenylpropanoid biosynthesis. In our previous investigations, it was shown that naringenin and chalcone could inhibit the growth of annual plants of 20 species out of 11 families. It was proved that chalcone suppressed maize growth and inhibited lignin biosynthesis, and it was also supposed that 4CL would be the action site of chalcone in maize. The objective of this study is to clarify the effects of chalcone and illumination on 4CL activity in pea seedlings.