Synergistic effects of humic acid and foliar application of micronutrients (Fe, Zn, Mn, Cu) on saffron (Crocus sativus L.) growth and biochemical compounds

Saffron, renowned for its economic importance in the medicinal plant market, presents unique challenges due to its weak root system, rendering its nutritional management critical. A field experiment was conducted from 2020 to 2022 in a randomized complete block design with three replications to evaluate the synergistic effects of HA along with irrigation water and foliar application of some micronutrients on various growth, yield, quality and physicochemical attributes of saffron. Experimental treatments were included three levels of HA (0, 3, and 6 kg ha−1) as main plot and foliar applications of micronutrients (control: water, Fe, Zn, Cu, and Mn) (Biozar Company) as subplot. The results unequivocally demonstrated that both HA and the micronutrients significantly impacted flower number, flower weight, stigma weight, leaf parameters, macronutrients uptake and the content of saffron bioactive compounds. The optimal combination of 3 kg HA ha⁻1 with foliar application of Cu resulted in substantial enhancements in flower number, stigma weight, and leaf length. Furthermore, the study revealed that HA positively influenced the chlorophyll a and b contents, while the micronutrients foliar application, particularly Cu and Mn, played a pivotal role in increasing chlorophyll a and carotenoid levels. Additionally, HA application was found to positively impact safranal production, with foliar application of Cu exhibiting a particularly pronounced effect on safranal content. The influence of micronutrients on crocin and picrocrocin content was more nuanced, with their effects varying depending on the applied HA rates. However, A notable enhancement in crocin and picrocrocin production was observed when foliar Mn was applied in combination with a 3 kg HA ha−1 treatment. In terms of nutrient uptake, both HA and micronutrients significantly elevated the concentrations of N, P, and K in saffron leaves. Overall, the findings of this study underscore the potential of HA and foliar application of micronutrients (especially Cu and Mn) to significantly improve saffron yield and quality through their synergistic effects on various growth and metabolic processes.