Phytotoxic and antiphytopathogenic compounds from Thai Alpinia galanga (L.) Willd. rhizomes
The management of weeds and diseases that are caused by phytopathogenic fungi is important for preventing the loss of agricultural products. The aim of the present study was to identify phytotoxic and antiphytopathogenic agents from the Thai Alpinia galanga rhizome. Extracts of the dried rhizomes of A. galanga (Zingiberaceae) were separated and tested for phytotoxic activity against lettuce (Lactuca sativa L. cv. Great Lakes) and Italian ryegrass (Lolium multiflorum Lam. cv. Wasefudou) and for antiphytopathogenic activity against Alternaria porri, Colletotrichum gloeosporioides, Fusarium oxysporum and Phytophthora nicotianae. 1′-Acetoxychavicol acetate (1) was identified as one of the main components, together with trans-p-coumaryl acetate (3) and trans-p-acetoxycinnamyl acetate (2). 1′-Acetoxychavicol acetate (1) was solvolyzed with 2% EtOH to yield trans-p-coumaryl ethyl ether (6), trans-p-coumaryl acetate (3) and trans-p-coumaryl alcohol (5). 1′-Acetoxychavicol acetate (1) completely inhibited the root growth of the lettuce seedlings at 50 μg mL–1, but had a weaker inhibitory effect on the growth of Italian ryegrass. 1′-Acetoxychavicol acetate also inhibited the growth of P. nicotianae and A. porri, with minimum inhibition concentration values of 15.6 and 31.5 μg mL–1, respectively. The plant growth-inhibitory activity and fungal growth-inhibitory activity of trans-p-coumaryl acetate (3), trans-p-coumaryl ethyl ether, trans-p-coumaryl alcohol (5) and trans-p-acetoxycinnamyl acetate (2) were lower than those of 1′-acetoxychavicol acetate. A structure–activity relationship suggested that the strong phytotoxic and antiphytopathogenic activity of 1′-acetoxychavicol acetate relied on the 1′-acetoxyl group.