Abstract:Some experiments had proved that 6-methoxy-2-benzoxazolinone (MBOA) was themain substance in phototropism of maize coleoptile. The physiological effects of MBOAwere studied by using the methods of bioassay and HPLC. The results showed that: 1. Thegrowth rate of hypocotyl of cress and cockscomb were inhibited obviously when treatedwith the substance excreted from maize coleoptile. The main component of the substancewas MBOA. 2. Antagonistic effects was found when the maize coleoptiles were treated with IAA and MBOA in combination．3．When an agar piece containing MBOA was placed on unilateral side of coleoptile with tip exised，the coleoptile could bend to growth．4 The content of MBOA increased when maize coleoptile was injured．It was shown that MBOA was the natural inhibitor which played important function in inducing plant phototropism as well as in protection against injury. It can be concluded that MBOA is a substance that antagonizes IAA and inhibits the plant growth．