When aged kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) plants are replaced with young ones, the growth and productivity of the young plants were occasionally lower than expected. This symptom is typical of the replant problems reported in several fruit trees. Phytotoxic substances released from the plant are assumed to be involved in the replant problems. Although some substances contained in kiwifruit leaves have been reported to have phytotoxic activity, little is known about phytotoxicity and phytotoxic substances in kiwifruit leaves on the growth of the kiwifruit itself. Therefore, this study determined the phytotoxic activity of kiwifruit leaf extracts and soil extracts and identified a phytotoxic substance in kiwifruit leaves. Extracts of kiwifruit leaves inhibited the growth of Lepidium sativum, Lactuca sativa, Medicago sativa, Phleum pratense, Lolium multiflorum, Echinochloa crus-galli, and kiwifruit seedlings. The soil extracts also inhibited the growth of L. sativum. These results suggest that kiwifruit leaf extracts and soil extracts may possess phytotoxic substances. (-)-Epicatechin was isolated from the extracts with bioassay-guided separations using chromatography. The concentrations of (-)-epicatechin required for 50% growth inhibition of the hypocotyls and radicles of L. sativum were 27.9 and 10.7 mM, respectively, and those for the hypocotyls and radicles of kiwifruit were 13.9 and 7.6 mM, respectively. This substance may contribute to the phytotoxic activity of kiwifruit leaf extracts and may be involved in the kiwifruit replant problem.
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