Effects of prohexadione-ca on the flavonoid metabolism of apple with respect to plant resistance against fire blight

Treatment of apple trees with the plant growth regulator prohexadione-Ca reduces the incidence of fire blight although there is no direct effect of this compound on the growth of Erwina amylovora. It could be shown, that prohexadione-Ca causes major changes in the flavonoid metabolism of apple. Within four hours after treatment, a flavonoid occurred in leaf and flower tissues which has not been described in apple so far. This flavonoid has been tentatively identified as luteoliflavan by HPLC-MS and NMR-spectroscopy. In addition, the accumulation of various intermediate metabolites such as eriodictyol could be observed. Prohexadione-Ca is known to inhibit distinct dioxygenases involved in gibberellin biosynthesis by mimicking 2-oxoglutarate which is required as a co-substrate. The changes in flavonoid metabolism are probably also related to inhibition of related dioxygenases active in this pathway. Since phenolic compounds have often been found to be involved in defence mechanisms of plants, further studies on their role in fire blight resistance are justified from our results.