The importance of non‐diffusional factors in determining photosynthesis of two contrasting quinoa ecotypes (Chenopodium quinoa willd.) subjected to salinity conditions

The broad distribution of quinoa in saline and non‐saline environments is reflected in variations in the photosynthesis‐associated mechanisms of different ecotypes. The aim of this study was to characterize the photosynthetic response to high salinity (0.4 M NaCl) of two contrasting Chilean genotypes, Amarilla (salt‐tolerant, salares ecotype) and Hueque (salt‐sensitive, coastal ecotype). Our results show that saline stress induced a significant decrease in the K+/Na+ ratio in roots and an increase in glycine betaine in leaves, particularly in the sensitive genotype (Hueque). Measurement of the photosynthesis‐related parameters showed that maximum CO2 assimilation (Amax) in control plants was comparable between genotypes (ca. 9–10 μmol CO2 m−2 s−1). However, salt treatment produced different responses, with Amax values decreasing by 65.1% in the sensitive ecotype and 37.7% in the tolerant one. Although both genotypes maintained mesophyll conductance when stomatal restrictions were removed, the biochemical components of Amarilla were impaired to a lesser extent under salt stress conditions: for example, the maximum rate of ribulose‐1,5‐ bisphosphate carboxylase/oxygenase (RubisCO; Vcmax) was not as affected in Amarilla, revealing that this enzyme has a higher affinity for its substrate in this genotype and, thus, a better carboxylation efficiency. The present results show that the higher salinity tolerance of Amarilla was also due to its ability to control non‐diffusional components, indicating its superior photosynthetic capacity compared to Hueque, particularly under salt stress conditions.