'Vater availability is the main factor limiting agricultural productivity and may result in considerable yield reduction around the world. Therefore, identification of genetic variation for drought tolerance and exposure of this variation to appropriate drought conditions for selection are the main objectives of research for many crop plants. In this study twenty-five barley (Hordeum vulgare L.) populations consisting of accessions, landraces and modern varieties from different parts of the world, mainly arid, semi-arid and Mediterranean regions, were grown in PEG (6000) solutions in a controlled-temperature growth room. All measurements on physiological and genedc aspects of drought tolerance were made after one and two weeks' growth. Results from the screening trial indicated that increasing PEG concentrations caused significant reductions in root length, shoot length, fresh and dry weight of root and shoot of all populations. Root to shoot dry weight ratio increased with decreasing water potential. Importantly populations responded differently to PEG treatments. There was considerable variation amongst populations in response to drought stress. Estimation of broad-sense heritability (1t 2 B) for root and shoot traits showed that a great proportion of the variation in response to drought was genetically determined. Results from principle component analysis indicated that screening could be carried out on the basis of the root to shoot dry weight ratio, which had higher genotypic· coefficient variability and phenotypic coefficient variability under drought stress. Populations 'PK 30109', 'PK 30118' were relatively tolerant whilst cuItivar 'Rihane-03'and landrace 'Shair' were intermediate and 'Ir. line', with a lower relative root length and root to shoot dry weight ratio was relatively sensitive. Accumulation of most solutes in roots and shoots increased due to both drought and salinity. Proline and glycine-betaine in leaves increased in stress conditions, whereas the level of polyols in soil culture increased under drought stress, whilst it decreased in saline conditions. Total amino acids concentration increased in saline and PEG treatments, but there were no differences between controls and stressed-plants grown in soil culture. Populations varied in solute accumulation. Although both organic and inorganic solutes contributed to osmotic adjustment, the major contribution was made by inorganic solutes in drought conditions. Potassium content increased in drought conditions (PEG and soil culture), but was constant in saline conditions. Drought-stressed plants accumulated more sodium compared to control conditions. The ~/Na+ ratio reduced with the decrease in plant growth under drought stress, so that in the soil experiment landrace 'Shair' showed higher leaf ~lNa+ ratio in drought stress compared to 'Rihane-03' and 'Ir. line'. Potassium and total water soluble carbohydrates had higher concentrations than other solutes in drought conditions. Salinity decreased all aspects of growth, but was more severe in root characters including length, fresh and dry weight. In contrast drought stress caused greater reductions in the shoot than the root. Relative water content decreased due to drought and salt stresses, whilst osmotic adjustments increased in both stress conditions..Membrane stability index decreased with decrease in leaf water potential induced by PEG. Supplied by The British Library - 'The world's knowledge'