Salinity tolerance mechanisms during germination and early seedling growth in Chenopodium quinoa Wild. genotypes with different sensitivity to saline stress

Quinoa (Chenopodium quinoa Willd.) is a facultative halophyte which has taken great importance in recent years due to the nutritional characteristics of its seed. In environments with moderate to high salinity, both the decrease in water potential and the accumulation of Na+ and Cl- ions can constitute stress factors that limit germination and seedling establishment. In the present study, we investigated the influence of NaCl solutions (ranging from 0 to 400 mM) on different components of the antioxidant metabolism during seed germination and seedling emergence, in three quinoa genotypes (CICA, Villarrica and Chadmo) differing in their germination responses and tolerance to salinity. To further explore the mechanisms involved, seed coat characteristics were microscopically analyzed, and seed hydration rates together with changes in the distribution of different ions in selected seed tissues were monitored. Finally, because the effect of NaCl on the accumulation of betalains in young seedlings differed among genotypes, the role of these pigments in salt stress tolerance was also investigated. Among the three genotypes, CICA was the most tolerant as indicated by its higher maximum and normal germination percentages at the highest salt levels tested. Surprisingly, this response was not correlated to the activity profile of antioxidant enzymes, most of which were up-regulated to a larger extent in the less tolerant genotype (Chadmo). Rather, seed coat characteristics that favor a rapid hydration rate, together with mechanisms aimed at preventing excess osmotic and ionic imbalances seem to play a predominant role. Interestingly, the presence of salt decreased the concentration of betalains in the seedlings, though to a lesser extent in CICA than in the other two genotypes. The down-regulation of their synthesis through germination under complete darkness, impaired the percentage of normal germination and increased lipid peroxidation in CICA seedlings exposed to 300 mM NaCl, indicating that these pigments may also contribute to salt stress tolerance.