Phytotoxic activity of bibenzyl derivatives from the orchid Epidendrum rigidum

A whole plant chloroform-methanol extract of the orchid Epidendrum rigidum inhibited radicle growth of Amaranthus hypochondriacus seedlings (IC ₅₀ = 300 μg/mL). Bioassay-guided fractionation furnished four phytotoxins, namely, gigantol (1), batatasin III (2), 2,3-dimethoxy-9,10- dihydrophenathrene-4,7-diol (9), and 3,4,9-trimethoxyphenanthrene-2,5-diol (11), along with the known flavonoids apigenin, vitexin, and isovetin and the triterterpenoids 24,24-dimethyl-9,19-cyclolanostane-25-en-3β-ol (14) and 24-methyl-9,19-cyclolanostane-25-en-3β-ol (15). Stilbenoids 1, 2, 9, and 11 inhibited radicle growth of A. hypochondriacus with IC₅₀ values of 0.65, 0.1, 0.12, and 5.9 βM, respectively. Foliar application of gigantol (1) at 1 βM to 4 week old seedlings of A. hypochondriacus reduced shoot elongation by 69% and fresh weight accumulation by 54%. Bibenzyls 1 and 2, as well as synthetic analogues 4′-hydroxy-3,3′,5-trimethoxybibenzyl (3), 3,3′,4′,5-tetramethoxybibenzyl (4), 3,4′-dihydroxy-5- methoxybibenzyl (5), 3′-O-methylbatatasin III (6), 3,3′,5- trihydroxybibenzyl (7), and 3,4′,5-trihydroxybibenzyl (8), were tested for phytotoxicity in axenic cultures of the small aquatic plant Lemna pausicostata. All bibenzyls derivatives except 7 and 8 inhibited growth and increased cellular leakage with IC50 values of 89.9-180 and 89.9-166 μM, respectively. The natural and synthetic bibenzyls showed marginal cytotoxicity on animal cells. The results suggest that orchid bibenzyls may be good lead compounds for the development of novel herbicidal agents.