Four compounds, the flavone linarin
(1), the triterpene
lupenone (2), the tocopherol (vitamin E, 3), and the new natural alkaloid 1,2,3,4-tetrahydro-1,1-dimethyl-6,7-isoquinolindiol
(affineine, 4), were the major natural products isolated
from Zanthoxylum affine (syn. Zanthoxylum fagara, Rutaceae). Compound 1 is highly abundant in this plant and was isolated as a white precipitate
obtained from the acetone and methanol extracts. The structure of
these four compounds was established by 1D and 2D NMR spectroscopy
including 1H, 13C, DEPT, COSY, HSQC, and HMBC
experiments. The hexane, acetone, and methanol extracts, as well as 1, were evaluated for their potential phytotoxic effects in
pre- and post-emergent assays, as well as to identify their mechanisms
of action. As pre-emergent phytotoxic agents, the hexane, acetone,
and methanol extracts inhibited germination and residual growth (root
and stem elongation) of Lactuca sativa (lettuce) and Lolium perenne (perennial
ryegrass). As post-emergent agents, they inhibited dry biomass. Compound 1 acts as a pre-emergent herbicide, by inhibiting germination,
seed respiration, residual seedling growth and, notably, root hair
development. Furthermore, 1 inhibited the synthesis of
ATP and the electron transport chain of isolated spinach chloroplasts;
in this way, it behaves as a Hill reaction inhibitor. The site of
inhibition was located at the donor site of PSII from the oxygen evolving
complex to QA, thus acting as a multisite compound. These
results suggest that compound 1 can be used as a lead
for a potential green herbicide with different targets.
