Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.