Growth inhibitory effect of peel extract from Citrus junos

Extract from yuzu fruit peel (Citrus junos Sieb. ex Tanaka) strongly suppressed the germination of lettuce seeds while that from the peel of other citrus fruits such as navel orange (C. sinensis) and lemon (C. limon Burm. f.) had very little or no effect. The highest inhibitory activity was located in the peel followed by the segment but no significant activity was found in the seed extract of yuzu fruit. The effect of yuzu peel extract was tested on a variety of major crops including 38 crop species. Germination of seeds of tomato (Lycopersicon esculentum Mill.), celery (Apium graveolens L. var. dulce) and watercress (Nasturtium officinale R. Br.) was completely blocked by 27.8 mg dry peel equivalent ml−1 of yuzu peel extract, while that of cucumber (Cucumis sativus L.) and pumpkin (Cucurbita moschata [Duch. ex Lam.] Duch. ex Poir.) seeds was not affected at this concentration, showing a broad variation in the sensitivity of crop seeds to the extract. The effectiveness of yuzu peel was also tested on some harmful weeds. The extract inhibited the elongation of both radicle and hypocotyl in summer weed quinoa (Chenopodium quinoa) grown on agar plates. Yuzu peel powder added to the soil was also effective in suppressing the growth of C. quinoa, Sonchus oleraceus L. and Digitaria ciliaris, while it had little effect on the spring weed Alopecurus aequalis Sobol. A strong growth inhibitory activity of yuzu peel was found in the aqueous phase after solvent extraction and most of the activity was recovered in the neutral fraction that passed through cation and anion exchange resins.