Inhibition of Malformed Incurved Flowers in the Cut Rose Cultivar ‘Yves Piaget’ by Methyl Jasmonate Spray Treatment of Flower Buds before Harvest

In the most popular fragrant rose cultivar in Japan, ‘Yves Piaget’, the petal edges are frequently malformed, curving toward the adaxial side. These malformed petals prevent normal flowering and weaken the flower fragrance, which significantly decreases the quality of this cultivar and increases financial losses of cut flowers. We refer to such malformed flowers as ‘incurved flowers’. It has been reported that jasmonic acid (JA) affects petal growth. Therefore, we attempted to control the number of incurved flowers by applying exogenous JA, methyl jasmonate (MeJA), during flower development before harvest. Two types of spray treatment were applied to the flower buds before flower opening; (1) 100 μM MeJA or (2) deionized water as a control. The 100 μM MeJA spray treatment before harvest reduced the incurved flower rate, with fewer incurved petals, and resulted in a significantly larger maximum flower diameter and longer stamen length. In addition, the 100 μM MeJA spray treatment before harvest tended to increase the number of days from the commercial harvest stage to full bloom and also significantly increased the maximum flower diameter of fully-bloomed flowers. We also analyzed the endogenous phytohormone content in the petals of normal and incurved flowers at each flower developmental stage. The results showed that at the beginning of the flower opening stage the petals of incurved flowers had higher indoleacetic acid (IAA) content and lower JA/jasmonoyl isoleucine (JA-Ile) content than those of normal flowers. In particular, the JA and JA-Ile contents in incurved petals were approximately one quarter of those in normal flowers. These results suggest that IAA, JA, and JA-Ile may be involved in the development of incurved flowers.