PROHEXADIONE-CA AND TRINEXAPAC-ETHYL: SIMILARITIES IN STRUCTURE BUT DIFFERENCES IN BIOLOGICAL ACTION

Prohexadione-Ca and trinexapac-ethyl are active only in their respective free acid form. The acid prohexadione is present as soon as its calcium salt is dissolved in water. Ammonium sulfate in the spray solution prevents re-formation of the relatively insoluble calcium salt. Acidification to approximately pH 4 to 5 accelerates absorption. Inside plants, prohexadione is primarily translocated acropetally. The ester trinexapac-ethyl is comparatively easily taken up. After absorption, it is translocated systemically with significant quantities reaching the root. Metabolic saponification is then required to form the active acid trinexapac. This process consumes time and depends on adequate temperature and light conditions. In their acid forms, pro¬hexadione and trinexapac are virtually identical in the type of translocation, in inhibiting the biosynthesis of gibberellins and ethylene, and in interfering with flavonoid metabolism. Due to the described differences, products containing pro¬hexadione-Ca and trinexapac-ethyl may perform differently in different plant species. When used as stem stabilizers in graminaceous species or to control shoot growth in turf grasses, the onset of action of prohexadione-Ca is, typically, faster, whereas trinexapac-ethyl is often giving a longer-lasting effect. Because of intense immediate activity, seedhead suppression in annual bluegrass (Poa annua) is achieved with prohexadione-Ca but not with trinexapac-ethyl. As compared with prohexadione-Ca, significantly higher dosages of trinexapac-ethyl are required in apple to obtain equivalent effects on shoot growth. Additionally, return bloom is often severely reduced in trinexapac-ethyl-treated trees. Most likely, this is due to blocking ethylene formation too intensely, thereby hampering the induction of flower buds.