Effect of p-CPA-parthenocarpic setting on the delayed ripening of netted-melon fruits

Application of plant growth regulators may induce parthenocarpic fruit setting, avoiding problems due to poor pollination. Their influence on the ripening of netted-melon fruit (Cucumis melo L.), however, is not yet well understood. The ripening behavior of parthenocarpic melon fruit (treated with 50 ppm 4-chlorophenoxy acetic acid (p-CPA) and 350 ppm gibberellic acid (GA3) at anthesis) was compared with that of hand-pollinated fruit. The ethylene production in parthenocarpic fruit peaked up about 10 days behind that of pollinated fruit but was greater. The delay was accompanied by a retardation in fruit softening, thereby leading to an extended edible period. The expression of ethylene synthesis-related enzymes: ACC synthase (ACS) and ACC oxidase (ACO) genes at the commencement of the ripening process was also evaluated. Northern blot and RT-PCR analyses revealed a higher level in the mRNA transcript abundance of ACS genes (auxin- and wound-inducible) but lower abundance of the ripening-related gene ACO in parthenocarpic fruit compared to that of pollinated fruit. Western blot analysis further confirmed the lower level of ACO polypeptide in parthenocarpic fruit. The retarded ripening of parthenocarpic melon fruit resulted from the delayed production of ethylene, which is likely a consequence of the low abundance of ACO. A possible inhibitory effect of auxin on ACO transcript as a result of p-CPA application is proposed.