XVI International Botanical Congess

Clark-type polarographic oxygen microelectrodes were used to explore aspects of internal root aeration. A computer-controlled servo-assisted micro-driver and automated polarograph allowed the determination of radial profiles of oxygen distribution in the primary roots of intact seedlings. Profiles were recorded at various distances from the apex in roots to compare internal oxygen levels. Seedlings with aerenchymatous or non-aerenchymatous cortices were compared. Steep radial diffusion gradients were evident between (a) the epidermal/ hypodermal layers (E+H) and the cortex, and (b) the cortex and stele. The E+H layers were a significant barrier to radial diffusion and two-way gradients (in the from of a v-shaped dip) were sometimes found within this zone. In general the steepness of gradients increased basipetally. Steep inclines, and concentration differences of 6-8%, were recorded between cortex and stele close to the base of maize roots. The magnitude of this radial decline was smaller (ca., 2%), but equally significant, nearer to the tip. In non-aerenchymatous root tips this drop was often sufficient to bring the stele close to anoxia. The transport of oxygen longitudinally in the maize root cortex from the shoot was very rapid and consistent with gas-phase diffusion. Changes in the oxygen regime of the root apex occurred within 5 seconds of changing the oxygen regime around the shoot. The longitudinal decline (base to apex) in cortical oxygen concentrations in maize roots was significantly affected by the cortical structure. Concentrations fell from ca. 13.5% to 0.1% in non-aerenchymatous roots, and 16% to 3% when the cortex was aerenchymatous. Evidence of a circumferential pathway for oxygen transport in the cortex was seen from the radial profiles. There was also some indication that oxygen is transported axially through the central pith (medulla) which may be permeated by small intercellular gas-filled spaces, and in radial oxygen profiles the medulla often showed higher oxygen concentrations than in the surrounding vascular cylinder. Manipulation of the oxygen concentration around the shoot was used to induce stelar anoxia within the root apex. Both positive and negative incremental changes to the shoot oxygen supply enabled the determination of the critical oxygen pressure for respiration (COPR), and a value of 0.5 to 2% oxygen was detected in vivo. Below the COPR changes in shoot oxygen caused iterative oscillations in oxygen concentration prior to equilibrium.