Inhibitory effects on Orobanche development by allelopathic
crops have been suggested before, but little was known about
the responsible metabolites. Inhibition of O. ramosa germination
was described in the wild tomato accession Lycopersicum
pennellii, suggesting that it was due to an excess of unidentified
stimulatory substances.31 Allelopathy by inhibition of O. crenata
seed germination was suggested as the mechanism responsible
for the reduction of parasitism in legumes intercropped with
fenugreek.32 Trigoxazonane was identified in fenugreek as an
active metabolite involved in the allelopathy against O.
crenata.33 Intercrops with berseem clover34 or with cereals12
reduced O. crenata parasitism in legumes, but the responsible
metabolites have not been studied so far. Inhibition of
development of Striga haustoria, although not of seed
germination, has been reported in intercropping with
Desmodium uncinatum.35 Both germination stimulants (uncinanone
B) and some inhibitory metabolites (uncinanone C
and isoschaftoside) have been identified in D. uncinatum.36,37
2-Benzoxazolinone (1) is known to interfere with germination
and early growth of several vegetable crops,16 which is in
agreement with our findings that (1) inhibits (over 30%) O.
crenata seed germination. In addition, we tested two
metabolites showing two different substituent groups at C-6
of (1). When a benzyloxy group was located at the C-6 position
as in (2), the inhibitory effect of (1) in O. crenata germination
was lost; however, when at the same position of the benzene
ring was substituted by a chloroacetyl group as in (3), the
inhibitory effect of (1) was greatly enhanced. The comparison
between the resonance structures of these two 6-substituted
derivatives of (1) (Supporting Information Figure S1) showed
a negative charge on the aromatic ring and a positive charge on
the oxazolinone nitrogen. In particular, the structure is greatly
stabilized by the presence of the electron-withdrawing
chloroacetyl group attached to the negatively charged carbon
of the aromatic ring. The same structure is destabilized by the
presence of the electron-donating benzyloxy group attached to
the same carbon.