Ultraviolet B radiation improves salt-induced responses in the facultative halophyte <i>Chenopodium quinoa</i>

In natural environments, plants are continuously exposed to multiple abiotic stresses, such as high salinity and excess ultraviolet (UV)-B radiation. While responses to individual stresses are well understood, less is known about their combined impact. Here, we treated quinoa (Chenopodium quinoa) seedlings with salt (0 and 200 mm NaCl) under either photosynthetically active radiation (PAR) or PAR supplemented with UV-B radiation (313 nm, 1 h/d, 1.71 W/m2) to investigate their response to combined salt and UV-B stress. While salinity had minimal effects on plant growth, it decreased stomatal conductance and photochemical efficiency by 36% to 47%. UV-B supplementation mitigated the negative effects of salinity, enhancing photosynthetic efficiency and water relations in UV-B- and salt-treated plants. Enhanced leaf water relations in the combined treatment were associated with altered ion translocation and shoot compartmentalization, especially for K+. Indeed, UV-B decreased K+ accumulation in epidermal bladder cells, suggesting a redistribution from epidermal bladder cells to other leaf tissues. UV-B treatment shifted plant metabolism toward producing hydroxycinnamic acid, while quercetin levels remained unchanged, indicating minimal stress. This study describes a protective mechanism in quinoa where UV-B radiation enhances ion translocation, water relations, and metabolic adjustments, mitigating salinity stress. Our findings offer key insights into plant resilience and physiological adaptation in salt-affected environments under elevated UV-B exposure.