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 O-2 deficiency, a property of wet soils, root growth and functions will be influenced by the respiratory demand for O-2 in root tissues, the transport of O-2 from the shoot to root and the supply of O-2 from the medium. In laboratory experiments with nodal roots of banana, we examined how these features influenced the longitudinal and radial distributions of O-2 within roots, radial O-2 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 O-2 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 O-2 supplied via aerenchyma, and reduced O-2 flow towards the tip of the supporting root. Stelar anoxia could be induced either by lowering the O-2 partial pressure in the bathing medium from 21 to 4 kPa (excised roots) or, in the case of intact roots, by reducing the O-2 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 O-2 concentration from a free-flowing aerated solution to the root surface. There was a steep decline in O-2 concentration across the epidermal-hypodermal cylinder and some evidence of a decline in the O-2 permeability of the epidermal-hypodermal cylinder with increasing distance from the root apex. The differences in O-2 concentration between cortex and stele were smaller than reported for maize and possibly indicated a substantial transfer rate of dissolved O-2 from cortex to stele in banana, mediated by a convective water flow component. An O-2 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 O-2 deficient environments, the stele would be among the first tissues to suffer anoxia and O-2 consumption within the root hair zone might be a major contributor to root anoxia/hypoxia in banana growing in temporarily flooded soils.