Airspace structure and mathematical modelling of oxygen diffusion, aeration and anoxia in Eleocharis sphacelata R. Br. Roots

Abstract

A quantitative description of the structure of the roots of Eleocharis sphacelata is presented, forming the basis of a mathematical analysis of their aeration via the intercellular airspace system. The mature aerenchymatous roots have cortical porosites as high as 70% and resistances to axial diffusion of 0.015-0.04 Ms mm^-3 per mm root length. The corresponding resistance in the younger, non-aerenchymatous tissue just behind the apex is 0.08-0.14 Ms mm^-3 per mm, root length. The observed maximum length of the roots (about 0.3 m) is not caused by the oxygen limitations at the apical meristem of the main axis because axial fluxes could theoretically support the meristem in much longer roots. However, the phloem and pericycle of the stele become hypoxic at 0.25 to 0.3 m, suggesting that length could be limited by the need to prevent excessive hypoxia in these tissues. Rates of root oxygen release into the sediment are predicted to be as high as 2.5 µmol h^-1 per 0.3-m-long root and higher still for shorter roots. The prevention of anoxia depends greatly on the basal oxygen concentration at the root base: oxygen partial pressures below 8 kPa in the rhizome would cause meristematic anoxia in 0.3 m-long roots. A better resolution of fine-scale variations in respiratory activity in the roots and in the sediment rhizosphere could improve the accuracy of the model, but it does nevertheless indicate that roots of E. sphacelata would normally remain aerobic and significantly oxidize anaerobic sediments in situ.