Parthenocarpy was induced in unfertilized pistillate flowers of cucumbers (Cucumis sativus L.) by the foliar application of N-1-naphthylphthalamic acid (Naptalam). These results substantiate the previous proposed hypothesis that potent auxin transport inhibitors in general are parthenocarpic agents for cucumber.
A hypothesis is proposed and tested for the mechanism of action of parthenocarpy induction in cucumber (Cucumis sativus L. cv. ‘GY3’) by the potent inhibitors of auxin transport 3,3a-dihy dro-2-(p-methoxy phenyl)-8H-pyrazolo [5,1-a] isoindol-8-one (DPX1840), methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin), N-l-naphthylphthalamic acid (naptalam), and 2,3,5-triiodobenzoic acid (TIBA). The transport of NAA-l-14C in 5-mm peduncle sections was strongly polar in the basipetal direction (i.e., out of the ovary toward the stem) and was inhibited within 0.5 hr by each transport inhibitor when applied at 500 to 1000 mg/1 to intact plants. Ether extractions of ovary tissue from plants treated in a similar manner contained significantly greater auxin activity as determined by the Avena coleoptile straight growth bioassay.
These results are compatible with the hypothesis that auxin transport inhibitors induce parthenocarpy in cucumber by rapidly blocking the natural outward flow of auxin from the ovary thereby resulting in an accumulation of auxin within the ovary sufficient to trigger parthenocarpy.
Parthenocarpy was induced in unfertilized pistillate flowers of cucumber, Cucumis sativus L., by the foliar application at early flowering of 10 to 1000 ppm of 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5,1-a]isoindol-8-one (DPX 1840) a new inhibitor of auxin transport. Treatment of a monoecious cultivar with ethylene or (2-chloroethyl)phosphonic acid (ethephon) increased the pistillate to staminate flower ratio and subsequent application of DPX 1840 increased the no. of fruit which developed parthenocarpically. Ethylene gas was just as effective as ethephon in altering sex expression. The threshold level of ethylene for this response was between 0.06 and 0.16 μl of ethylene per liter of air.