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 �mol 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.

Growth and root oxygen release by Typha latifolia and its effects on sediment methanogenesis

“…Environmental control of root morphology can also cause considerable intraspecific variation in root oxygen release, as in this study, with the high C plants having shorter roots that released less oxygen per unit dry weight than the low C plants (Table 3), although this difference was not statistically significant. Laterals, with their high surface area:volume ratios, are important sites for root oxygen release (Armstrong et al, 1991;Sorrell, 1994). The greater similarity of root oxygen release rates between the low C and high C plants when expressed on a surface area basis therefore provides further evidence that the amount of permeable surface area of the root system is the major morphological factor limiting root oxygen release, although the surface area of laterals were not included in our calculations.…”

“…They were accompanied by differences in root morphology known to be caused by oxygen stress in highly reducing sediments, such as shorter, thicker roots and reduced lateral root formation (Kludze et al, 1993;Armstrong et al, 1996). Short, thick roots are favoured when plants are under oxygen stress because they have low axial resistances to oxygen diffusion, whereas lateral roots are more difficult to support, having limited oxygen transport capacities due to their narrow diameter and low porosity (Armstrong et al, 1990;Sorrell, 1994). Hence, although T. latifolia produced similar total biomass in the low C and high C sediments, the smaller individual shoots and roots are a similar negative response to sediment oxygen demand to that seen in other emergent macrophytes (e.g.…”

Effects of silicon (Si) on arsenic (As) accumulation and speciation in rice (Oryza sativa L.) genotypes with different radial oxygen loss (ROL)

“…layers are thought to serve as barrier formation (Sorrell, 1994;Armstrong et al, 2000). In rice roots, the barrier against ROL is attributed to a suberized exodermis with casparian bands and lignified sclerenchyma cells (Kotula and Steudle, 2008).…”

Biological flora of New Zealand 11.Eleocharis sphacelata, kuta, paopao, bamboo spike sedge

“…Sorrell & orr (1993) established that very high rates of localised H + exchange at the lateral roots and tips of the main roots of emergent aquatic monocots can produce steep pH gradients near root surfaces. equally, oxygen leaked from rhizomes, as a result of gas transportation, dissipates into the surrounding rhizosphere (Sorrell 1994), enhancing microbiological activity in the soil. This provides an environment for micro-organisms, while the asphyxiating gases produced in the sediment can be released via the efflux culms (Brix et al 1992), an essential process for growth in deep water.…”