Prohexadione-Ca in Fruit Trees: Modes of Action of a Multifunctional Bioregulator

APOGEE and REGALIS have recently been introduced in a number of countries for use in pome and other fruit trees. These products contain 27.5% and 10% of prohexadione-Ca (ProCa), respectively. As a result of inhibiting excessive vegetative growth, less summer and dormant pruning is required, the ratio between vegetative growth and fruit formation is improved, and crop protection is facilitated due to the reduction of tree row volume and a more open canopy. Additionally, a lowered incidence of diseases such as fire blight and scab is observed, which is not due to a direct bactericidal or fungicidal effect of the compound. Further, the compound may reduce fruit drop early in the season. Prohexadione is a structural mimic of 2-oxoglutaric and ascorbic acid. Therefore, distinct dioxygenases are blocked, which require these compounds as a co-substrate. Such enzymes catalyze late steps in gibberellin biosynthesis. After treatment with ProCa, less growth-active gibberellins are formed and treated plants remain more compact. ProCa also affects ACC oxidase, another dioxygenase. The resulting reduction of ethylene formation, in addition to the availability of more assimilates for fruit growth, is most likely the cause of reduced fruit drop. 2-Oxoglutaric acid-dependent dioxygenases are also involved in the metabolism of flavonoids and their phenolic precursors: In shoots of apples and pears, ProCa causes considerable changes by inhibiting flavanone 3-hydroxylase. Convincing evidence is now available that ProCa triggers pathogen resistance by inducing the formation of 3-deoxyflavonoids, in particular luteoforol, with phytoalexin-like properties. Morphoregulatory effects caused by ProCa are only of secondary relevance for the reduction of disease incidence.