Studying the biochemical characteristics, seed, and oil yield of safflower (Carthamus tinctorius L.) in response to foliar spaying of amino acid, iron, and urea in the condition of withholding irrigation in the reproductive stage

IntroductionDrought is one of the most significant abiotic stresses that negatively affects agricultural productivity worldwide. More than 60% of global agricultural production is directly or indirectly impacted by drought stress. Water deficiency during any stage of plant growth, especially during critical periods such as flowering or seed filling, can result in substantial and often irreversible yield losses. The severity of drought's impact varies depending on the plant species and cultivar, ranging from high sensitivity to moderate or high levels of tolerance. In this context, adopting strategies that can enhance drought tolerance in crops is of paramount importance. Among such strategies, foliar application of essential nutrients—including iron (Fe), urea, and amino acids—can play a critical role by improving nutrient use efficiency and enhancing plant stress tolerance. Foliar feeding, due to its rapid absorption compared to soil-based nutrient application, is particularly important under water-deficit conditions. Urea, owing to its small molecular size, non-ionic nature, and high solubility, is readily absorbed through the leaf surface and plays a significant role in plant metabolism and growth. Iron is a vital micronutrient involved in key physiological processes, especially chlorophyll synthesis and photosynthesis. Its deficiency, particularly in calcareous soils, can cause chlorosis and decreased productivity. Foliar application of iron in the form of iron chelate (Fe-EDTA) is a widely used method to correct iron deficiencies in field crops. Moreover, amino acids are increasingly recognized as biostimulants that enhance plant physiological functions, improve growth parameters, and increase tolerance to environmental stresses such as drought. They contribute to improved antioxidant activity, membrane stability, and nutrient uptake, which are essential during stress conditions. Safflower (Carthamus tinctorius L.) is an annual oilseed crop that has gained attention due to its wide adaptability and multi-purpose use in the food, pharmaceutical, dye, and oil industries. Owing to its deep root system and moderate drought tolerance, safflower is a suitable candidate for evaluating the effectiveness of foliar nutrient applications under water-deficit conditions.Materials and methodsThis experiment was carried out as a split-plot design based on randomized complete blocks in the research farm of the Faculty of Agriculture of Tarbiat Modares University, during the 2022-23 growing season. The treatments were include three levels of irrigation regimes [no interruption of irrigation (full irrigation), withholding irrigation from the flowering stage, withholding irrigation from the seed filling stage] in the main plots and nine levels of Foliar spraying (no foliar spraying, distilled water, iron chelate 6% (Fe), urea 1% (U), amino acids 1 g.lit-1 (AA), Fe+U, Fe+AA, U+AA, Fe+U+AA) were placed in sub-plots. Analysis of variance (ANOVA) and also mean comparisons were accomplished using the general linear model (GLM) procedure. LSD procedure at a probability level of 0.05 was used to determine statistically significant differences among treatment means.Results and discussionThe results showed that the highest oil content with 15.92% was obtained from Fe foliar application in the condition of withholding irrigation from the seed filling stage. Also, the interaction of foliar application and irrigation regimes had a significant effect on safflower seed yield, and foliar application of Fe+AA+U increased safflower yield. The highest amount of oil yield was produced from the Fe+AA+U in the condition of withholding irrigation from the seed-filling stage with 359.08 kg.ha-1. The highest amount of seed yield in the condition of full irrigation was obtained from the AA+U with 2333.33 kg.ha-1.ConclusionThe results of this study highlight the positive impact of foliar application of iron chelate, urea, and amino acids—particularly when applied in combination—on the growth, seed yield, and oil content of safflower under both normal and drought-stressed conditions. The combined foliar treatment of iron, urea, and amino acids (Fe+U+AA) proved to be the most effective in enhancing oil yield and overall crop performance, especially under irrigation withholding from the seed filling stage. Additionally, the amino acids and urea combination (AA+U) performed best under full irrigation, suggesting a role in promoting productivity even in the absence of stress. These findings support the use of targeted foliar nutrition as an efficient strategy to mitigate the adverse effects of water stress in safflower cultivation. The synergistic effect of micronutrients and biostimulants can not only improve plant tolerance to drought but also sustain economic yield levels under limited water availability. Therefore, it is recommended that safflower farmers, particularly in semi-arid and arid regions, consider integrating foliar application of iron, urea, and amino acids into their agronomic practices. Moreover, moderate irrigation management, such as withholding water from the flowering stage along with foliar feeding, may optimize resource use and yield outcomes in water-limited environments.