Effects of Morphactin and Other Auxin Transport Inhibitors on Soybean Senescence and Pod Development

Because triiodobenzoic acid increases pod number, albeit variably, in soybean (Glycine max), we tested other auxin-transport inhibitors. Morphactins, especially methylchlorflurenol (MCF), were found to be very active (optimal concentration 10 micromolar) when sprayed onto the foliage. Applications at 1 week after the start of flowering were most effective, producing a 40% increase in pod number with little inhibition (12%) of stem elongation. MCF increased the number of pods initiated (reaching 1 cm length) at least partially by prolonging the initiation period, while pod abortion (failure of pods > 1 cm long) remained low. Generally, MCF did not increase seed yield (dry weight/plant); more, but smaller seeds, were formed by the treated plants. The promotive effect of MCF on pod initiation seems to be independent of its inhibition of stem elongation, which is insignificant at 10 micromolar. MCF delayed pod maturation by 3 to 4 days, while foliar yellowing, blade abscission, and petiole abscission were retarded by 2, 4, and 2 days, respectively. MCF has only a small effect on senescence and that could be indirect, due to a delay in pod development. Other auxin-transport inhibitors tested, including N-1-naphthylphthalamic acid, produced little or no increase in pod number; however, 0.1 millimolar 5-[2'-carboxyphenyl]-3-phenylpyrazole caused a 27% increase. These results implicate auxin as a potential regulator of pod development, and they show that soybean seed yield is not simply sink limited.