Hydrotime analysis and gene expression reveal that cytokinin mitigates the detrimental effects of salinity on seed vigor

This study investigated the impact of salinity stress and cytokinin application on quinoa (Chenopodium quinoa Willd.) seed quality, germination behavior, and gene expression. Four quinoa cultivars with differing salinity tolerance were evaluated under varying salinity levels (0, 25, 40 dS m⁻¹) and cytokinin treatment (0 and 0.75 µM 6-BA). Seed germination was assessed under multiple water potentials, and the hydrotime model was applied to quantify responses. Salinity during maternal growth significantly influenced progeny seed quality, with some cultivars-particularly the salt-tolerant 'Titicaca'-exhibiting enhanced germination performance under stress. Cytokinin foliar application further improved germination rates, particularly in salt-sensitive genotypes, by modulating key hydrotime parameters; for instance, the base water potential (Ψ_b(50)) shifted from - 1.8 MPa in 'Q12' under high salinity to -1.3 MPa in 'Titicaca' under high salinity and cytokinin. Molecular analysis via RT-qPCR revealed genotype-specific upregulation of stress-responsive genes (SOS1, BADH, and NHX1). In the salt-tolerant 'Titicaca' under high salinity and cytokinin treatment, this was exemplified by a 23-fold increase in SOS1, an 18.7-fold increase in BADH, and a 12-fold increase in NHX1 expression, underscoring the mechanistic basis for the observed physiological resilience. These genes are involved in ion homeostasis, osmoprotection, and vacuolar Na⁺ sequestration, indicating their role in quinoa's adaptive mechanisms. Our results demonstrated that the combination of hormonal treatment and cultivar selection can enhance quinoa seed resilience under salinity stress. The study provided novel insights into integrating physiological modeling and molecular tools for quinoa cultivation in saline environments.