The effect of water stress induced by polyethylene glycol (PEG 10000) on callus growth, callus water content, callus necrosis and regeneration was investigated on four cultivars of durum wheat considered to have good in vitro culture ability. 3-week old calluses, initiated on MS medium supplemented with 2 mg L-1 of 2,4- dichlorophenoxyacetic acid (2,4-D), were subcultured on media containing different concentrations of PEG (0, 10, 15, 20 and 25 %). After 4 weeks of culture, calluses growth and water content were determined. The results showed that increasing PEG concentration in the medium causes a gradual decrease in growth and water content of calluses. The results obtained for regeneration, after transferring stressed calluses to regeneration media without PEG, also showed that increasing osmotic pressure in the medium significantly reduces the percentage of regeneration and the number of plantlets per regenerating callus (NPRC). High concentrations of PEG caused callus necrosis as well. However in some cases we observed the development of a whitish, drought tolerant, and embryogenic secondary callus upon the necrotic callus which regenerated after transfer to regeneration medium.
