Physiological effects of nano-iron and humic acid on the growth of trifoliate orange (Citrus trifoliata) rootstock seedlings

Poor iron availability and nutrient imbalance can affect the growth and overall quality of trifoliate orange rootstock seedlings during the nursery stage. Application of nano-iron along with humic acid may help improve nutrient uptake, promote healthy seedling growth and produce better-quality rootstocks for citrus propagation. Based on this, an experiment was conducted on a farm in Kufa, Najaf Governorate, Iraq, from 15 March to 15 November 2025 to evaluate the growth response and selected physiological traits of trifoliate orange (Citrus trifoliata) rootstock seedlings to foliar applications of nano-iron and humic acid. The factorial experiment was arranged in a randomised complete block design with three replicates and included three nano-iron concentrations (0, 15, and 30 mg/L) and three humic acid concentrations (0, 1, and 2 mL/L), with four seedlings per experimental unit, giving a total of 108 seedlings. The reported means and least significant difference values indicated improvements in the studied traits as the concentration of each factor increased. Nano-iron at 30 mg/L produced the highest mean increases in seedling height and main stem diameter, leaf area, and leaf total chlorophyll and carbohydrate contents, reaching 43.30 cm, 5.96 mm, 1162.82 cm², 98.77 mg/100 g fresh weight, and 27.70 mg/g dry weight, respectively. Humic acid at 2 mL/L produced the highest numerical means for the same traits, reaching 41.31 cm, 5.92 mm, 1139.86 cm², 97.29 mg/100 g fresh weight, and 26.38 mg/g dry weight, respectively, with no significant difference from 1 mL/L in height increase or carbohydrate content. Combining the highest concentrations of both factors produced the highest numerical values for all traits. These results indicate the potential to improve seedling growth and chlorophyll and carbohydrate contents using the studied treatments, with inference restricted to the concentrations and experimental conditions examined.