Inhibitory Activities of 3-Thiophenecarboxylic Acid and Related Compounds on Plant Growth

3-Thiophenecarboxylic acid (1) showed strong growth-inhibitory activity toward the following plants but not Glycine max Merrill; Brassica campestris subsp. rapa Hook. fil. et Anders, Sesamum indicum L., Lactuca sativa L. var. longifolia Lam, Echinochloa utilis Ohwi et Yabuno and Allium tuberosum Rottler. Compound 1 strongly inhibited the growth of roots of S. indicum and L. sativa even at the low concentration of 5.0 × 10−5 m. The growth-inhibitory activity of 1-related compounds (2–6) on S. indicum was also studied. Among the compounds, 3-thiopheneacetic acid (6) showed the strongest inhibitory activity, but 3-thiophenecarboxaldehyde (2), 3-thiophenemethanol (3), and 3-thiophenecarboxamide (5) showed no activity. The radicles of plants treated with these active compounds showed negative geotropism.