Results obtained with graded concentrations of NaCl (20-200 m M) show decrease in the chlorophyll 'a' contents of Anabaena with increasing concentration of NaCl except at extremely low concentration of NaCl (5-20 m M). The rate of Hill activity and oxygen evolution are found to be stimulated by lower concentrations of NaCl, but not at higher concentrations of NaCl. Results have demonstrated that the O(2) evolution process is relatively more sensitive to NaCl stress than the Hill activity. Further, the results show NaCl induced an increase in the rate of RNO bleaching and loss of total thiol (-SH) contents. Taken together, these results suggest a NaCl-induced general oxidative stress. Results on the effect of oxygen radical quenchers reveal a predominant role of singlet oxygen in the NaCl-induced general oxidative stress as evident from a higher quenching effect of sodium azide than formate and histidine on the rate of RNO bleaching in Anabaena cells. However, the rate of lipid peroxidation and SOD activity show a declining pattern in response to increasing concentrations of NaCl. There is the possibility of a NaCl-induced decrease in the rate of lipid peroxidation when the SOD activity is also lower. But the NaCl-induced decline in the SOD activity does suggest that symptoms of general oxidative stress at elevated levels of NaCl are apparently owing to collapse of intracellular defense of the cells against the toxic oxygen radicals, not because of the higher rate of photosynthetic activity.

Results show that an isolated mutant of the cyanobacterium Anabaena doliolum is a fast-growing strain. It exhibits approximately twofold higher NaCl tolerance than the wild type. It also reveals cross-resistance against the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), drug bacitracin, and LiCl. Further, an improved LiCl tolerance property of both the mutant and wild-type strains at high concentration of NaCl (40 m M) may be interpreted in terms of competitive inhibition of the Li(+) uptake by Na(+) ions, whereas bacitracin resistance in these organisms is described to be the result of an alteration in the drug transporting channels of membrane. The multiple stress tolerance property of the A. doliolum may be attributed to altered membrane characteristics in the mutant strain, leading to reduced intake of such toxicants.

Effect of salinity (NaCl, 100 mM) on growth, nitrate reductase (NR) and glutamine synthetase (GS) activities, and uptake of NH 4 + was studied in the wild type (WT) and the NaCl-tolerant mutant type (MT) of cyanobacterium Anabaena doliolum. Results obtained in the presence of salt showed significant reduction in the growth rate of both WT and MT cells of A. doliolum by about 77.8 and 40 %, respectively over without NaCl. Similarly rate of NR activity in both WT and MT strains was reduced by 45.5 and 44.5 %, respectively. On the contrary, rate of GS activity of both the WT and MT strains in the presence 100 mM of NaCl increased by 34 and 159 %, respectively. The results of this study indicate that tolerance to NaCl in A. doliolum is more dependent on NH 4 + assimilation rather than on nitrate assimilation in relation to N-metabolism. The increased GS activity in MT cells of the cyanobacterium is possibly because of high rate of energy dependent NH 4 + uptake.