Empirical and Theoretical Studies of pH in the Rhizosheres of Growth Zones

University Of California-Davis, Davis CA

Investigators

Abstract

A recently published theory suggests that the root growth zone produces fluxes of substance that create a micro-environment of acidity (pH) around the root tip as it penetrates the soil (Kim, T.K., Silk, W.K., and A.Y. Cheer 1999. A mathematical model for pH patterns in the rhizospheres of growth zones. Plant, Cell and Environment 22: 1527-1538). Here I propose a pilot study to explore empirically the predictions of the theory. A number of plant species will be screened to find variation in ability to change the pH of soil substrates. Screening will be conducted with seedlings in miniature growth chambers. Root surface pH will be measured with microelectrodes at points along the root axis and in the soil substrate directly behind the root. To assess developmental trends, techniques will be developed to excavate growing parts of more mature root systems and insert the older root tips into the miniature growth chambers. Then root surface pH will be measured as for the screening experiments. These measurements will give temporal trends over longer periods in the development of the root system. Preliminary studies will be needed to determine whether the larger main roots or abundant fine roots will be accessible to the pH microprobe. If time permits, some older root systems will be used for nutritional studies. Spectrophotometric assays will be used to find nutrient concentration within the root as a function of axial distance from the root tip. The rate at which the plant deposits nutrient elements to support the growth of the tip will be calculated with an equation using growth rates and the nutrient concentration data. Genotypes found to induce slight and strong acidification of the soil surrounding the growth zone will be compared for their nutrient deposition rates. Since the ability of the root to change the pH of the soil in its immediate vicinity affects the uptake of both beneficial nutrients and phytotoxic metals, an understanding of root-induced pH changes in the surrounding soil is central to the study of nutrient cycling.