Root nutation modelled by two ion flux-linked growth waves around the root

A new model of root circumnutation is proposed, based on the correlation between nutation and ion flux oscillations in the elongation zone. The model considers that, in a small region on one side of the root and within the elongation zone, the growth rate is different from the average. This local disturbance to growth may have been caused by mechanical, chemical or other stimuli, or it may be merely an effect of existing nutation, tropisms or other asymmetric growth. If this disturbance is a reduction in the local growth rate, the root will be bending towards that side. The model provides that the region of growth reduction enlarges and spreads as a wave along and around the root. As the wave of growth reduction moves away, the initially disturbed region recovers. Viewing the root in the cross section containing the original region, the growth reduction moves from that region with different speeds in the two opposite directions around the circumference of the root. The mathematical formulation for the movement of the growth disturbance is given. The resulting bending of the root can be quite complex. The proposed model allows us to describe typical features of nutating roots reported in the literature and observed in our experiments. Results of simulation show a high correspondence with experimental observations. Some components of a possible physiological mechanism for the model are discussed.