THE CONCEPT OF A LIMITING ROOTING MORPHOGEN IN WOODY STEM CUTTINGS

A rooting morphogen, arbitrarily defined. can be assumed to induce adventitious roots in woody stem cuttings. Auxins generally promote rooting in herbaceous cuttings and are often considered to limit rooting, but they often have little or no effect when applied to difficult-to-root woody cuttings. They can move in a polar (basipetal) pathway but the polarity of root emergence equally reflects the activity of the vascular system in general, including the phloem, whose contents have a complex composition. The interactions between a morphogen of vascular origin and potential sites for root initiation are likely to be dynamic and variables such as flux rate or flux direction could be more relevant than static variables such as composition or concentration. The potential sites are not equally sensitive to the rooting morphogen since each cell has a unique lineage, ontogeny and position, and a prevailing insensitivity could restrict rooting. The contents of the vascular system are inaccessible to potential sites in the stem, so factors affecting efflux and the subsequent attenuation of the efflux, which depend on activity in the whole cutting, could be critical to rooting ability. No simply defined morphogen can be said to limit rooting, and the chemical identity of the morphogen (if definable) is probably unimportant.