Root porosity and oxygen movement in waterlogging-tolerant Trifolium tomentosum and -intolerant Trifolium glomeratum

Trifolium tomentosum and T. glomeratum are small (< 0·5 mg) seeded pasture legumes which are considered to be waterlogging tolerant and intolerant, respectively. The root porosity of the two species was compared for plants raised for 10 d in aerated nutrient solution and then transferred to either aerated (0·25 mol O₂ m^–3) or ‘hypoxic’ (0·031–0·069 mol O₂ m^–3) solutions for a further 7 and 21 d. After 21 d, T. tomentosum developed a root porosity of 11·2% in ‘hypoxic’ solution, which was significantly higher than the 6·1% developed by T. glomeratum. When grown in aerated solution, T. tomentosum also had a larger constitutive porosity (6·7%) than T. glomeratum (3·9%). Cylindrical root-sleeving O₂ electrodes were used to measure the rates of radial O₂ loss (ROL) from roots of the two species when in an O₂-free medium. In general, roots previously grown in ‘hypoxic’ solution had higher rates of ROL than roots grown in aerated solution. Moreover, the rates of ROL along the main root of T. tomentosum were ≈ 5-fold faster than from equivalent locations along roots of T. glomeratum. Manipulations of the shoot O₂ concentration resulted in rapid changes in ROL near the root tip of T. tomentosum plants raised in aerated or ‘hypoxic’ solutions, whereas for T. glomeratum ROL only increased for roots of plants raised in ‘hypoxic’ solution. Thus, the cortical air spaces in roots of both species raised in ‘hypoxic’ solution formed a continuous, low resistance pathway for O₂ diffusion from the shoots to the roots. ROL from the lateral roots was also evaluated and it was 3-fold faster from T. tomentosum than from T. glomeratum. Moreover, ROL from lateral roots of T. tomentosum was 10–20-fold higher than from a position on the primary root axis the same distance from the root/shoot junction. Relatively, high rates of ROL were also recorded for young (40 mm in length) lateral roots of T. glomeratum which were previously grown in ‘hypoxic’ solution, but the ROL was low for the older lateral roots of this species. The substantial amounts of ROL from the lateral roots may limit O₂ supply to the lower parts of the primary root axis, so that the laterals probably become the main functional root system in waterlogged soils.