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SUMMARY Balanced and unbalanced high-salt solutions were used to investigate the effects of osmotic stress on microbial activity in the rhizoplane and rhizosphere of barley. Bacteria in the root regions proved sensitive to both high ( - 1500 kPa) and low ( - 500 kPa) levels of each type of stress, but bacterial growth was most severely inhibited in the unbalanced treatments both of which contained high levels of sodium. Similar effects of stress were noted with barley. Distinct qualitative changes in microbial populations in the root zones were observed in the balanced stress treatments, but an increase in species diversity did not occur in the rhizoplane or rhizosphere following exposure to the unbalanced, high-sodium solutions. After five weeks of stress, a marked but transitory rise in rhizoplane counts accompanied death of the seedlings in the - 1500 kPa, high-sodium treatment indicating that at least some of the soil bacteria retain the potential for rapid growth at high salt concentrations.

Influence des conditions de stress induites par le sodium sur l'activite microbienne et la composition organique des exsudats racinaires de l'orge (Hordeum vulgare CV Clayton)

The effects of salt stress on levels of soluble carbohydrate, amino acids and proteins in the shoots, roots and exudates of barley were studied under sterile conditions using balanced and unbalanced, high-salt solutions at −500, −1000 and −1500 kPa of osmotic stress. Moderate and severe stress with the unbalanced, high-sodium solutions proved toxic to barley, but plants exposed to the low-sodium, balanced solutions remained green and the stems remained turgid after 7 days of treatment. Both types of salinity increased the content of soluble carbohydrate in the shoots at all levels of stress and in the roots at −500 and −1000 kPa of stress. Carbohydrate in the exudates increased over 20-fold in response to balanced stress, but an apparent 3-fold increase in the exudates from the unbalanced treatments was not significant. Sucrose, glucose and fructose remained the principal sugars in the roots and shoots regardless of the type or intensity of stress, but their relative contents varied with treatment. Galactose, maltose, ribose and rhamnose were the major sugars in all exudates. Protein contents in the tissues fell at each level of balanced and unbalanced stress, but significant changes in protein were not detected in the exudates. Stress increased the size of the free amino acid pool in the shoots; however, it stimulated the reverse trend in the roots. With the exception of the −500 kPa low-sodium treatment, stress also induced a marked decline in the free amino acid content of the exudates, thus initiating a major limitation on the supply of a key group of metabolites in the rhizosphere.