We assessed the impact of aeration and stagnation of the rooting medium on aerenchyma development and root porosity (total gas spaces) in four banana cultivars (cvs) using plantlets from tissue culture and from suckers. Aerenchyma formation in nodal roots of banana was expressed as percentage root porosity and cross sectional area of porosity because roots differed in thickness. Aerenchyma appeared to be constitutive and developed to within 100 mm of the root tip in roots up to 600 mm long and grown in medium with forced aeration; aerenchyma appeared to be fully developed at about 300 mm from the root tip. The cv Gros Michel (AAA) had most aerenchyma (15%) and cv Sugar (AAB) the least (5%). In roots up to 200 mm long that had grown into stagnant medium, aerenchyma developed to within 50 mm of the root tip and progressively increased to the root base. The calculated physical resistance to gaseous diffusion along the roots, R-p, increased by 2 to 5 fold from the base to the root tip in roots grown in nutrient solution with forced aeration. In this case the roots of Gros Michel had an R-p of about 36% of cvs Sugar, Williams or Goldfinger, which had similar values of R-p. Roots growing into stagnant medium had R-p about 20% to 30% of those growing in aerated medium but no differences were seen between the cvs. To evaluate tolerance to flooding the physical resistance to gaseous diffusion in the roots should be assessed, rather than the percentage porosity of the roots. (C) 1999 Elsevier Science B.V. All rights reserved.