Rooting hormone should be totally unnecessary in air layering. I've done many air-layers with and without and find no meaningful difference in the rooting achieved. I've also experimented with the level of IBA applied, to as much as 4.5% IBA in a talc. In short order, it causes leaves to show fall colors, which are then even more intense when fall actually arrives. Logically, I think this should mean reduced photosynthesis and auxin production, yet I found no readily obvious effect on the adventitious rooting. Though somewhat aside, I'll also note that some varieties seem nearly impossible to root, my "Higasayama" is a case in point.