Extended Abstract
Introduction: Quinoa (Chenopodium quinoa Willd) is a highly nutritious plant known for its tolerance to various abiotic stresses, including drought and salinity. This adaptability makes it an ideal candidate for cultivation in arid and semi-arid regions, where water scarcity poses a major challenge to agricultural productivity. Despite its resilience, drought stress negatively impacts physiological functions, morphological traits, and yield in quinoa, which requires effective strategies to mitigate these effects. Humic acid and silicon, both bio-stimulants, have been reported to enhance plant growth and stress tolerance by improving nutrient uptake and strengthening cell walls. However, the combined effect of these substances on quinoa under water-limited conditions remains insufficiently studied. This research aimed to evaluate the effects of foliar application of humic acid and soil application of silicon on the morphological characteristics, yield, and water use efficiency of quinoa under different irrigation regimes.
Materials and Methods:This study was conducted during the growing seasons of 2017–2018 and 2018–2019 at the research farm of Yasouj University, located in Yasouj, Iran (latitude 30°39′ N, longitude 51°33′ E). A split-plot factorial experiment based on a randomized complete block design (RCBD) with three replications was employed. The main factor consisted of three irrigation regimes: 100%, 70%, and 40% of the plant’s water requirement, determined based on the Class A evaporation pan. The sub-factor consisted of the foliar application of humic acid at three concentrations (0, 300, and 600 mg/L) and the soil application of silicon at 20 kg/ha, applied in a factorial arrangement. Quinoa seeds (variety Titicaca) were planted manually with a planting depth of 2 cm, and each plot was irrigated according to the respective irrigation treatment. Water application was calculated using the formula Id=InETcI_d = frac{I_n}{ET_c}, where IdI_d is the irrigation depth, InI_n is the amount of water applied in each irrigation cycle (in mm), and ETcET_c is the crop evapotranspiration. Various agronomic traits, including leaf area index, grain yield, and water use efficiency, were measured, and data were subjected to statistical analysis using SAS software.
Results and Discussion:The results demonstrated that irrigation regimes, humic acid, and silicon application significantly influenced quinoa’s morphological traits, yield, and water use efficiency. The 100% irrigation treatment resulted in the highest grain yield (2340 kg/ha), while the 40% irrigation treatment produced the lowest (616 kg/ha). Silicon application significantly improved leaf area index and grain number per plant under drought stress conditions, with the greatest improvements observed in the 70% and 40% irrigation treatments. Foliar application of humic acid at 600 mg/L increased leaf area index, grain yield, and water use efficiency, particularly under water-limited conditions. 
  
The interaction between irrigation and silicon application showed significant effects on the leaf area index and water use efficiency, with the highest efficiency observed in the 70% irrigation treatment combined with silicon application. Moreover, the combination of silicon and humic acid under 70% and 40% irrigation regimes resulted in improved stress tolerance by enhancing nutrient uptake and maintaining photosynthesis efficiency, ultimately leading to better performance in drought conditions. Water use efficiency was significantly enhanced by both silicon and humic acid, with the highest efficiency recorded in the silicon-treated plants under 70% irrigation (0.47 kg of grain per cubic meter of water). These findings are in line with previous studies, where silicon was shown to alleviate drought-induced oxidative stress by reducing hydrogen peroxide levels and improving cell wall integrity. Humic acid, through its chelating properties, improved nutrient availability, thereby enhancing plant growth and water retention during periods of water scarcity. This study highlights the importance of integrating these bio-stimulants into water-limited agricultural systems to optimize quinoa production in drought-prone regions.
Conclusions: The application of silicon and humic acid significantly mitigated the negative effects of drought stress on quinoa, enhancing morphological traits, yield, and water use efficiency. Combining these bio-stimulants with efficient irrigation practices can increase quinoa's resilience to water scarcity, offering a sustainable approach to improve productivity in arid regions.