The interplay of salinity stress and nitrogen management on gas exchange in quinoa

This study evaluates the quinoa physiological responses to varying irrigation water salinity levels and nitrogen application rates. Over two years, a field experiment was conducted at Shiraz University in Iran using a Factorial Randomized Block Design with three replications to investigate the combined effects of irrigation water salinity levels (5, 10, 20, and 25 dS m⁻¹) and nitrogen rates (0, 150, and 300 kg N ha⁻¹). The study focused on key parameters, including gas exchange, leaf area index, leaf water potential, and root characteristics. The results indicated that the maximum leaf area index (LAImax) decreased by 25% in 25 dS m⁻¹ compared to 5 dS m⁻¹, with no significant differences between 5 and 10 dS m⁻¹. Photosynthesis rate (An) reduced by 20% and 23% at 20 and 25 dS m⁻¹, respectively, compared to 5 dS m⁻¹, with minimal differences between 5 and 10 dS m⁻¹ or 20 and 25 dS m⁻¹. It also increased by 20% in 150 kg N ha⁻¹ and 43% in 300 kg N ha⁻¹ compared to non-fertilized control. Stomatal conductance (gs) in 5 dS m⁻¹ increased by 25% and 100% by N application rate of 150 and 300 kg N ha⁻¹, respectively. The 300 kg N ha⁻¹ treatment improved gs under salinity levels (5, 10, 20, and 25 dS m⁻¹) by 100%, 136%, 64%, and 70%, respectively, compared to non-fertilized crop. Leaf transpiration rate was enhanced by 10% in 150 kg N ha⁻¹ and 38% in 300 kg N ha⁻¹ compared to non-fertilized treatments. Therefore, although salinity harmed gas exchange parameters, nitrogen application mitigated the negative effect and improved these parameters. Despite these improvements, the 300 kg N ha⁻¹ treatment reduced water-use efficiency (An/gs) due to disproportionately higher increases in gs and leaf transpiration (Tr) compared to An. Leaf water potential was stable in salinity levels of 5 and 10 dS m⁻¹ but decreased significantly by 9% and 12% in 20 and 25 dS m⁻¹, respectively. Our findings showed a significant reduction in root length and weight density (RLD and RWD, respectively) in salinity above 10 dS m⁻¹, with greater reductions in higher salinity levels. Generally, higher salinity levels reduced RLD and RWD; however, applying higher nitrogen rates mitigated this negative effect. For instance, in salinity level of 25 dS m⁻¹, RLD and RWD decreased by 68% and 22% in non-fertilized treatments, respectively, whereas in fertilized treatments, the reduction was only 50%.