Empirical evidence of a convection-diffusion model for pH patterns in the rhizospheres of root tips

A recently formulated convection-diffusion model predicted that root growth plus diffusion of protons in the neighbouring soil would lead to particula r pH patterns around the moving root tip. To test the predictions of this t heory, pH was measured at differing radial distances from the root surface after 24 h of growth in a medium with low diffusivity (sandy soil) and afte r a shorter period (55 min of growth) in a medium with high diffusivity (ag ar). In agreement with the theory, the growth zone was found to influence t he pH of the soil for distances less than 1 mm from the root surface (even after many hours) and the pH of the agar for a distance of at least 5 nun ( after only 1 h). The axial pattern of pH along the surface of soil-grown Ze a mays L. root tips was found to be the same for roots growing at different rates under different temperatures (2.23 nun h(-1) at 26 degreesC or 1.27 nun h(-1) at 20 degreesC). Thus, the plant can synchronize proton flux with growth to maintain a particular surface pH pattern within the growth zone. This implies that root tips growing at different rates in response to diff erent temperatures can carry the same microenvironment of pH through a homo geneous soil.