Melatonin priming enhances quinoa drought tolerance via coordinated water status, photosynthesis, and hormones

Global food security is faced with one of its most serious threats, water scarcity, exacerbated by climate change. Quinoa is naturally tolerant to drought; however, extreme water deficit during its development negatively affects the growth and yield. The goal of this investigation was to evaluate the effect of exogenous seed priming with melatonin on water relations, photosynthesis, and hormonal balance of quinoa under drought stress over two growing seasons. A split-plot design experiment was conducted with four levels of water stress (100, 80, 60 and 40% of field capacity) as the main plots and three seed priming treatments (melatonin priming (20 µM), hydropriming, and no priming) as subplots across two growing seasons. The results showed that under severe water stress (40% FC), melatonin priming increased endogenous melatonin content by 2.66 and 2.54 times, and net photosynthetic rate by 1.47 and 1.40 times, while ABA levels were 1.35 and 1.26 times lower than in non-primed plants in 2019 and 2020, respectively. Melatonin priming also enhanced relative water content, leaf area index, transpiration rate, and root length density, thereby maintaining lower canopy temperatures and mitigating the physiological constraints induced by water stress. These coordinated changes were associated with higher grain yield under 40% FC, with melatonin-primed plants achieving 1.22 and 1.19 times higher grain yield than non-primed plants in 2019 and 2020, respectively. Beyond modulating endogenous abscisic acid concentrations, exogenous melatonin also functions as an integrative regulator that harmonize hormonal equilibrium, water relations, and photosynthetic performance, thereby promoting quinoa growth and yield under drought conditions. Overall, this research underscores the role of melatonin as an alternative and sustainable biostimulant for climate-smart quinoa production in arid regions.