Beyond the ionic and osmotic response to salinity in Chenopodium quinoa: functional elements of successful halophytism

Chenopodium quinoa Willd. is a halophyte for which some parameters linked to salt tolerance have been investigated separately, in different genotypes, and under different growth conditions. In this study, several morphological and metabolic responses were analysed in parallel after exposure to salinity. Upon 150 mM NaCl treatment in-vitro seed germination was initially delayed but then reached control (0 mM NaCl) levels, whereas seedling root growth was enhanced; both parameters were moderately inhibited (ca. 35-50%) by 300 mM NaCl. In pot-grown plants, plant size was reduced by increasing salinity (0 to 750 mM NaCl). Transpiration and stomatal conductance declined at the highest salinities, consistent with reduced stomatal density and size. Density of epidermal bladder cells (EBCs) on the leaf surface remained unaffected up to 600 mM NaCl. Tissue contents of Na+ and Cl- increased dramatically upon salt treatment, but only resulted in a 50% increase in Na+ from 150 to 750 mM NaCl. Internal K+ was unaffected up to 450 mM NaCl, and increased at highest salinity levels. Excretion through sequestration into EBCs was limited (generally ≤ 20%) for all ions. A dose-dependent, albeit modest, proline accumulation, together with reduction in total polyamines and putrescine efflux occurred in NaCl-treated plants. Results confirm the importance of inorganic ions for osmotic adjustment, the plant–s ability to maintain K+ levels, and the involvement of putrescine efflux in maintaining ionic balance under high salinity. Conversely, ion excretion and proline appear to play a minor role. Taken together they indicate which parameters could be used for future comparison among different genotypes.