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

Chenopodium quinoa Willd. (quinoa) 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. In vitro seed germination was initially delayed by a 150 mM NaCl treatment but eventually reached the same level as the control (0 mM NaCl), whereas seedling root growth was enhanced; both parameters were moderately inhibited (similar to 35-50%) by 300 mM NaCl. In pot grown plants, plant size was reduced by increasing salinity (0-750 mM NaCl). Transpiration and stomatal conductance were decreased at the highest salinity levels tested, consistent with reduced stomatal density and size. The 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 with salt treatment, but resulted in only a 50% increase in Na+ from 150 to 750 mM NaCl. Internal K+ was unaffected up to 450 mM NaCl but increased at the highest salinity levels tested. Excretion through sequestration into EBCs was limited (generally <= 20%) for all ions. A modest dose-dependent proline accumulation, and concomitant 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 conditions. Conversely, ion excretion and proline appear to play a minor role. Taken together these results indicate which parameters could be used for future comparison among different genotypes.