@SmokingWings and I were discussing this in a thread about topping.Btw - on auxins - what if it were as simple as the physics of water in a vessel? Going vertically there’s more pressure on the water lift. Going horizontal eliminates that pressure and allows more efficient uptake. Would anything like that be a possible contributor? Sorry if that’s the dumbest question ever, lol.
From what I understood the growth tips produce auxins, and they're sent down the plant and are at greater concentration the lower you go. They promote apical dominance inhibiting the growth of branches and promoting root growth. Cytokinins are produced in the roots and sent up the plants. They promote growth in the branches.
The plant normally has a balance of them, causing a normal growth pattern, and topping temporarily tips that to the cytokinins favour.
I'm sure it's way more complex than that, but that's how I understood it.