Plant growth inhibitors from Baccharis sarothroides Gray and Haplopappus acradenius (Green) Blake.

Abstract

Plant growth inhibitors were isolated from Haplopappus acradenius (Green) Blake and Baccharis sarothroides Gray, two desert species, found at the Boyce Thompson Southwestern Arboretum. Leaf and stem tissues of B. sarothroides were extracted with 80% methanol (v/v). This extract was reduced to an aqueous phase in vacuo and partitioned with ethyl acetate at pH 7.3 (NF, neutral fraction), pH 2.8 (AF, acidic fraction), and again at pH 2.8 following hydrolysis at pH 11 (HF, hydrolyzed fraction). Thin layer chromatography (TLC) on silica gel H in chloroform:ethyl acetate:formic acid (CHCl₃:EtOAc:HCOOH) produced a region between R(f)'s 0.5 to 0.6 from AF of B. sarothroides which inhibited wheat seed coleoptile and radicle growth 52.7% and 66.5%, respectively, using 500 ul of a 1.9 mg/ul extract. This section inhibited wheat coleoptile straight growth 38.6% at the same concentration. Additional TLC, UV spectrophotometry, spray reagents, NMR, and GC/MS indicated that the compound was 3,8-dihydroxy-5,6,7-trimethoxy-2-(4-methoxyphenyl)-4H-1-benzopyran-4-one at a concentration of 265 ug/g fresh weight. This compound significantly inhibited the wheat coleoptile straight growth bioassay 18.4% using 2 to 3 ug/ul. An 80% methanol extract of H. acradenius leaves evaporated in vacuo produced an aqueous insoluble brown resin. This resin dissolved in absolute methanol and separated by TLC in CHCl₃:EtOAc:HCOOH contained a region between R(f)'s 0.6 to 0.7 that inhibited wheat seed coleoptile growth 71.8% and radicle growth 90.7% using 200 ul of 1.5 mg/ul solution. Wheat coleoptile straight growth was inhibited 53.7% in this region at the same concentration. Further examination of this region by the same methods as those used for B. sarothroides indicated the presence of a C-12 alkenyl alcohol (2 mg/ml), an aromatic heterocyclic hydrocarbon (4 mg/ml), and an alkyl substituted version of 7-hydroxycoumarin (5 mg/ml) at a concentration of 0.7, 1.4, and 1.8 ug/g fresh weight, respectively. A combination of these compounds inhibited the wheat coleoptile straight growth bioassay 41.1% using 11 ug/ul. A 2 M HCl extract of H. acradenius was partitioned with diethyl ether, which was evaporated and the residue resuspended in 95% methanol. TLC in CHCl₃:EtOAc:HCOOH separated an area between R(f)'s 0.5 to 0.6 where wheat seed coleoptile growth was inhibited 49.7% and radicle growth was inhibited 54.6% using 1000 ul of a 3.3 mg/ul solution. Identified in this region was 7-hydroxycoumarin at a concentration of 150 ug/g fresh weight. The wheat coleoptile straight growth bioassy was inhibited 13.2% using 2 to 3 ug/ul.