Seedlings of two sorghum (Sorghum bicolor (L.) Moench) genotypes with differential tolerance to salinity were exposed to 0 and 100 mM NaCl, gradually added in increments of 25 mM every 12 hours, in nutrient solution. Seven days after starting the salt treatment the growth of the shoot and root system and the inorganic and organic solutes contents were determined. Salinity reduced the dry matter yield and length of the shoot and root system in both sorghum genotypes, specially in the sensitive one. In general, it was observed an increase in Na+ and Cl- transfer to the shoot, in Na+ and Cl- accumulation and in the Na+/Cl- ratio but a decrease in the K+ and Ca2+ transfer to shoot and in the K+ and Ca2+ contents in the shoot, always with higher intensity in sensitive genotype. Apparently, the tolerance to high saline concentrations in sorghum seems to be related to the genotype ability to avoid accumulation of harmful levels of Na+ and Cl- and, or to maintain adequate levels of K+ and Ca2+, specially in the shoot. The soluble carbohydrates and amino acids constituted together over 98% of the total organic solutes and showed the greatest absolute increase in concentration during saline stress. Probably, the soluble carbohydrates were the most important organic solutes to contribute to the osmotic adjustment in the leaves and the amino acids in the roots. Under saline stress there was an expressive increase in proline contents, specially in the oldest leaves of sensitive genotype. The proline contents, however, even under salt stress, did not reach the levels of other organic solutes. Contrary to the general acceptance, proline does not seem to have an important role in the mechanism of salt tolerance, at least for these genotypes and under the experimental conditions applied here.
Salinity; salt stress; salt accumulation; organic solutes accumulation; Sorghum bicolor
