The mode of action of a morphactin,fluoren-9-carboxylic acid

Fluoren-9-carboxylic acid acts not only as an auxin but also as an gibberellin-antagonist. In the standard pea straight test (S₅ section) for auxin it stimulated elongation, the optimum concentration being 10 mg/l. On the other hand, it inhibited elongation at 0.1 mg/l. This inhibitory effect was more marked when younger tissue (S₁ section) which also responds to gibberellin was used. Interaction of FCA and IAA in the S₅ section has shown that at higher concentration of IAA there seemed to be a suppraoptimal effect, indicating that FCA acted as an auxin. However, in the S₁ section, the stimulating effect of GA₃ was markedly inhibited by 0.1 mg/l FCA; 10 mg/l FCA was either additive or less than additive to GA₃. In the cucumber hypocotyl test FCA itself was inactive up to 100 μg/plant, but it inhibited the GA₃-induced elongation. This inhibition was overcome by increasing the dosage of GA₃. In the same material, the IAA-induced elongation was not affected by FCA. These results indicate that whether FCA acts as an auxin or a gibberellin-antagonist depends on whether the tissue is sensitive to gibberellin and/or auxin.