Foliar serotonin and allantoin mitigate tellurium toxicity in Quinoa ( <i>Chenopodium quinoa</i> Willd.) through antioxidant activation and nutrient homeostasis to enhance yield and grain quality

Tellurium toxicity severely impairs soil health, plant physiology and crop productivity, posing a critical threat to global food security, ecosystem sustainability and human health. The exogenous application of phytohormones and osmoprotectants offers a sustainable strategy for mitigating heavy metal-induced phytotoxicity and restoring agricultural viability. A comprehensive experiment was conducted to evaluate the synergistic efficacy of serotonin (5-HT) and allantoin (ALL) in ameliorating tellurite stress in quinoa plants. Plants subjected to 50 µM sodium tellurite soil drenching were treated with 50 µM 5-HT and 100 µM ALL via foliar spray under a randomized complete block design with a factorial arrangement and four replications. Tellurite stress alone drastically reduced morphological traits and overall yield, while simultaneously elevating oxidative damage. However, combined 5-HT and ALL application under stress increased grain yield by 90.41%, photosynthetic rate by 78.68%, stomatal conductance by 80.30%, relative water content by 33.52%, and antioxidant enzyme activities by up to 84.86%. Furthermore, it decreased malondialdehyde content by 58.20%, hydrogen peroxide by 61.16%, tellurium root accumulation by 52.48%, shoot translocation by 60.41%, and grain residue by 66.74% compared to stressed controls, while concurrently improving soil microbial biomass carbon by 63.71%, dehydrogenase activities by 88.49%, and available nitrogen by 39.48%. The integrated treatment significantly restored cellular ion homeostasis, enhanced complex root architectural traits, and effectively restricted heavy metal translocation to the edible harvested grains. Consequently, the co-application of 5-HT and ALL is highly recommended as a robust agronomic intervention to safeguard crop productivity, improve soil health, and ensure long-term food safety in severely Te-contaminated agricultural environments.