Oxygen distribution and movement, respiration and nutrient loading in banana roots (Musa spp. L.) subjected to aerated and oxygen-depleted environments

Excessive soil wetness is a common feature where bananas (Musa spp.) evolved. Under O2 deficiency, a property of wet soils, root growth and functions will be influenced by the respiratory demand for O2 in root tissues, the transport of O2 from the shoot to root and the supply of O2 from the medium. In laboratory experiments with nodal roots of banana, we examined how these features influenced the longitudinal and radial distributions of O2 within roots, radial O2 loss, solute accumulation in the xylem, root hydraulic conductivity, root elongation and root tip survival. In aerated roots, the stele respired about 6 times faster than the cortex on a volume basis. Respiratory O2 consumption decreased substantially with distance from the root apex and at 300–500 mm it was 80% lower than at the apex. Respiration of lateral roots constituted a sink for O2 supplied via aerenchyma, and reduced O2 flow towards the tip of the supporting root. Stelar anoxia could be induced either by lowering the O2 partial pressure in the bathing medium from 21 to 4 kPa (excised roots) or, in the case of intact roots, by reducing the O2 concentration around the shoot. The root hair zone sometimes extended to 1.0 mm from the root surface and contributed up to a 60% drop in O2 concentration from a free-flowing aerated solution to the root surface. There was a steep decline in O2 concentration across the epidermal-hypodermal cylinder and some evidence of a decline in the O2 permeability of the epidermal-hypodermal cylinder with increasing distance from the root apex. The differences in O2 concentration between cortex and stele were smaller than reported for maize and possibly indicated a substantial transfer rate of dissolved O2 from cortex to stele in banana, mediated by a convective water flow component. An O2 partial pressure of 4 kPa in the medium reduced net nutrient transfer into the vascular tissue in the stele within 1 or 2 h. Hypoxia also caused a temporary decrease in radial root hydraulic conductivity by an order of magnitude. In O2 deficient environments, the stele would be among the first tissues to suffer anoxia and O2 consumption within the root hair zone might be a major contributor to root anoxia/hypoxia in banana growing in temporarily flooded soils.