Because trijodobenzoic 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 prologing 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. number in soybean. In addition, these inhibitors may provide a tool for probing the role of auxin transport in regulating pod development and leaf senescence processes.In this study, we have used auxin-transport inhibitors to examine (a) potential involvement of auxin in regulating pod development and (b) the effect of increased pod number on total seed yield. 2Abbreviations: TIBA, 2,3,5-triiodobenzoic acid; Tween 80, polyoxethylene (20) sorbitan monooleate; CF, chlorflurenol or 2-chloro-9-hydroxyflurene-9-carboxylic acid; CPD, 1-(2' carboxyphenyl)-3-phenylpropane-1,3-dione; CPP, 542'-carboxyphenyl)-3-phenylpyrazole; DPX 1840, 3,3a-dihydro-24p-methoxyphenyl)-8H-pyrazolo (5,la) isoindol-8-one; MCF, methylchlorflurenol or methyl-2-chloro-9-hydroxyflurene-9-carboxylate; NPA, N-l-naphthylphthalamic acid; PBA, 24 1-pyrenoyl)benzoic acid. RESULTSWe attempted to increase pod set by spraying with 4 x 10-1 M TIBA in 0.05% Tween 80 at 1 week before flowering, at flowering, at 1 week after flowering started and when the most advanced pods were 1.0 cm long (data not shown). The earliest treatment produced stunted plants (fewer nodes) with more pods per node but not more pods per plant. These applications also delayed pod development and thereby, indirectly, senescence. The treatments given at flowering or 1 week later produced a small (0-15%) but
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