Water stress is a serious environmental problem throughout the world. It is a consequence of both drought and salinity which may be relieved by breeding cultivars that can tolerate low soil water potentials. Alfalfa (Medicago sativa L.) is normally grown in arid and semi-arid zones. One approach to improving the ability of the crop tolerate drought is to make use of the variation provided by tissue culture derived from somaclonal variation. Seedlings of a moderately salt tolerant line CUF

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... derived from the commercial variety CUF 101 were used as a source of explants to initiate and regenerate tissue cultures. Regenerant plantlets were allowed to grow to maturity, selfed, and set seed. These seeds (R1) were germinated and grown in a nutrient medium containing 0, 200 and 250 g l-1 polyethylene glycol (PEG) 6000 and additional calcium as CaCl 2 (4.0 mol m-3) for 14 days to assess their ability to withstand stress in the progeny generation. Tolerance to PEG was assessed using measurements of root and shoot lengths. Proline levels and activity of the antioxidant enzymes, catalase (EC 1.11.1.6) and glutathione reductase (GR) (EC 1.6.4.2) were also determined. Selected somaclones were more tolerant than the parent. A large increase in the level of proline was observed in the somaclones compared with the parent variety in response to PEG stress. The activity of catalase and glutathione reduc-tase increased in the tolerant genotypes but remained unchanged in the parent when they were subjected to PEG stress.