Microelectrode and Modelling Study of Oxygen Distribution in Roots

Abstract

The use of polarographic microelectrodes to explore the radial distribution of oxygen in maize roots aerated chiefly via the shoot is described. Also given is a brief description of a mathematical model of root aeration which introduces two simultaneous oxygen sources (the shoot and the rooting medium), and mimicks more realistically than hitherto the diffusive characteristics of the non-porous epidermal/hypodermal shell.In keeping with modelling predictions, steep radial diffusion gradients were a characteristic of the non-porous epidermal/hypodermal shell and stele, while shallow profiles generally characterised the cortex. Stelar anoxia, previously forecast by earlier mathematical modelling, was inducible by manipulating the oxygen concentrations around the shoot.The stelar diffusion gradients always indicated an inward diffusion from the cortex. However, two-way gradients were sometimes evident in the epidermal/hypodermal annulus, and were also found in the sheathing extensions of the root cap. These indicated diffusion into these annuli simultaneously from both the cortex and the rooting medium. Elsewhere the gradients in the epidermis/hypodermal cylinder were either solely positive or negative depending upon the oxygen concentrations within the root cortex and surrounding agar. Examples of two-way and one-way diffusion in the epidermal/hypodermal cylinder were created satisfactorily by the model.Finally, the results indicate that with roots in stagnant anaerobic media, the bulk of the root cap proper is likely to be anoxic, and that some anoxia may be the norm within the basipetal extensions of the cap which ensheath the root apex.