The changes in plant growth, relative water content (RWC), stomatal conductance, lipid peroxidation, proline and antioxidant system in relation to the tolerance to polyethylene glycol mediated water stress were investigated in drought-sensitive common bean P. vulgaris L. accession FM 53 and drought-tolerant tepary bean P acutifolius Gray accession PI 321-638. For induction of water stress, the 35 days old bean seedlings were subjected to PEG 6000 of osmotic potential -0.40 MPa for 14 days. With regard to vegetative growth, PEG treatment caused more decrease in R vulgaris than in R acutifolius indicating a superior performance of wild species under water stress. Root and shoot DW increased in P. acutifolius while decreased in P. vulgaris on day 14. PEG treatment had no effects on relative water content (RWC) in P. acutifolius but. reduced RWC in R vulgaris. P. acutifolius maintained a greater stomatal conductance than P. vulgaris under water stress imposed by PEG treatment. In P. acutifolius constitutive level of lipid peroxidation was lower than in P. vulgaris and did not change at the end of the experiment. Constitutive activities of SOD, CAT, APOX and POX were higher in P. acutifolius than in the sensitive one and SOD, APOX and GR activities showed an enhancement in the former under water stress. Proline accumulation was also higher in P. acutifolius than in P vulgaris both under control and water stress conditions. These results possibly suggest that the drought-tolerant tepary bean P. acutifolius showed a better protection mechanism against oxidative damage by maintaining higher constitutive and induced activities of antioxidant enzymes, than the sensitive common bean P. vulgaris. (C) 2004 Elsevier Ireland Ltd. All rights reserved.