Waterlogging tolerance, root porosity and root anatomy were evaluated for 20 Trifolium accessions (species and subspecies, all annuals) selected from the eight Sections of the genus. Nine accessions were-sensitive [relative growth rate (RGR) reduced by up to 80%] to waterlogging, nine accessions were tolerant (RGR not reduced), and in two accessions RGR increased (up to 1.9-fold), when compared to drained controls. Growth of the main (i.e. tap) root axis was severely reduced in all accessions when waterlogged. Lateral roots formed the bulk of the root system of tolerant accessions when grown in waterlogged soil. Lengths of the longest lateral roots were up to three-times longer than the main root axis. Root porosity varied from 0.7-12% among accessions when grown in aerated solution and from 1.1-15.5% in plants grown in hypoxic (0.031-0.045 mol O-2 m(-3)) solution. In some accessions aerenchyma formed by cell lysigeny; in others it formed by schizogenous cell separation, or a combination of both processes. O-2 consumption rates of expanded lateral root tissues varied by up to 1.7-fold (on a mass basis) among the six accessions tested and was reduced by an average of 24% for roots of plants grown in hypoxic solution prior to measurements. Accessions with the highest root porosity tended to have longer roots when grown in waterlogged soil. Three accessions formed 'aerotropic roots' and the lateral root lengths of these plants exceeded those of all other accessions, suggesting enhanced O-2 movement to the submerged lateral root axis via the aerotropic roots. Waterlogging-tolerant accessions were identified in seven of the eight Sections in Trifolium, and the tolerant accessions tended to be those with extensive lateral root systems of relatively high porosity. (C) 2001 Annals of Botany Company.