Factors which influence the response of cuttings to treatment with root-promoting auxins are reviewed and new work is presented, with particular reference to the efficient entry of auxin into hardwood cuttings.
Auxin concentration is a principal factor in eliciting a rooting response, but other factors influence the response to concentration.
These are, in approximate order of importance, duration of auxin application, moisture tension within the cutting, position of application to the cut end and proximal epidermis, and depth of dipping in the auxin solution.
Effects due to depth of dipping appear to be a compromise between the need to avoid deleterious effects upon developing roots from auxin deposited on the epidermis and inadequate stimulation from very shallow dipping, which may be compensated for by run-off absorption when the epidermis is treated.
Wounding, and the tissue penetrability of the carrier also affect the response to auxin.
Re-treatment is valuable for improving rooting following an initial sub-optimal dose, but cannot be used to enhance rooting beyond that obtainable by a single treatment at optimal concentration.