This study examines the comparative effects of NaHCO 3 and Na 2CO 3 on young oat ( Avena sativa L.) plants to elucidate the species' physiological adaptive mechanisms to alkali stress. Factors considered are the intracellular influx and efflux of ions, ionic balance, osmotic adjustment, pH homeostasis, photosynthetic pigments and growth. Results show that, Na 2CO 3 had stronger effects than NaHCO 3, and that with increasing concentrations of both stresses the plant showed rising Na + influxes into the shoot resulting in Na + ion toxicity. This is tolerated by Na + sequestration in the vacuole; the accumulation mainly of Cl -, SO 42- and the synthesis of high concentrations of organic anions to maintain vacuolar ionic balance and, lastly by the synthesis of proline in the cytoplasm to avoid dehydration. Moreover, Na 2CO 3 stress inhibits growth more strongly, compared to NaHCO 3,because of the higher energy costs associated with Na + exclusion and compartmentalisation, the syntheses of organic anions, the syntheses of proline in the cytoplasm, reduced photosynthetic capacity and increased membrane permeability. Compared to the shoot, although the root had a similar response to both stresses, it showed a higher tolerance because high Na 2CO 3 stresses (>48 mmol L -1) resulted in significant increases in root tissue pH, but did not affect the pH homeostasis of the shoot. Additionally, while both stresses decreased root dry weight, they did not significantly affect root extension growth. This indicates that oat adopts an opportunistic guerrilla strategy by which it avoids resource-poor patches of soil (e.g. high alkali) while preferentially exploiting more favorable habitats by maintaining root extension.
