PROHEXADIONE-CA: MODES OF ACTION OF A MULTIFUNCTIONAL PLANT BIOREGULATOR FOR FRUIT TREES

Prohexadione-Ca is a plant bioregulator that is primarily used to inhibit excessive vegetative growth in fruit trees and other crop plants. Additionally, effects on the incidence of bacterial and fungal diseases and on insect pests are often observed. Furthermore, the compound may reduce abortion of fruitlets, thereby increasing fruit set. Prohexadione-Ca is relatively short-lived and possesses very favorable toxicological and ecotoxicological features. The compound acts as a structural mimic of 2-oxoglutaric acid. Therefore, distinct dioxygenases involved in gibberellin biosynthesis, which require this compound as a co-substrate, are blocked (e.g. GA₂₀ 3ß-hydroxylase). As a result, less growth-active gibberellins are formed and treated plants remain compact. 2-Oxoglutaric acid-dependent dioxygenases are also involved in flavonoid metabolism. In shoots of apples and pears, prohexadione-Ca causes considerable changes in the formation of flavonoids and their phenolic precursors by inhibiting flavanone 3-hydroxylase. Prohexadione-Ca triggers pathogen resistance by inducing the formation of 3-deoxyflavonoids, in particular luteoforol, with phytoalexin-like properties. A similar mechanism may explain the lowered incidence of insect pests. Finally, treatments with prohexadione-Ca lead to reduced ethylene formation. This is due to structural similarities with ascorbic acid, the co-substrate of aminocyclopropanecarboxylic acid oxidase, another dioxygenase, which is involved in ethylene biosynthesis. Lowered ethylene levels, together with elevated availability of assimilates that are no longer needed for shoot growth, explains increases in fruit set.