The Synergistic Effect of Humic Acid Soil Application and Salicylic Acid Pretreatment on Morphological and Phenological Traits of Saffron Species (Crocus sativus L.).

Iran's high export value of saffron, the world's most expensive spice, is attributed to its cultivation in dry and semi-dry areas, which face moisture and soil organic matter deficits. To improve saffron yield, factors such as bulb size, planting time, fertilization, irrigation, and hormonal and organic treatments must be considered. Humic acid, an organic fertilizer, has positive effects on soil structure and plant growth, improving plant metabolism and resilience against drought and salinity. Salicylic acid, an organic fertilizer, induces plant responses to adverse environmental conditions, increasing plant growth and flexibility. This study aimed to determine the combined effects of soil-applied humic acid and salicylic acid pre-treatment on saffron's morphological and phenological traits.
 
A study was conducted at Payame Noor University's research farm in Marvast, using high-quality saffron corms from Torbat-Heydarieh. The experiment involved three replications over the 2019 and 2020 agricultural years. Treatments included four levels of humic acid (0, 5, 10, and 15 kg per hectare) and three levels of salicylic acid (0, 1, and 2 millimolar). The corms were planted manually in August 2019, and humic acid was applied with the first irrigation. Salicylic acid was pretreated for 8–10 hours in the dark. The study measured traits such as cormel number, leaf length, fresh and dry weights, flower and stigma yield, and phenological traits like sprouting and flowering times. Results showed that humic acid and salicylic acid treatments positively affected saffron growth and yield, indicating potential for improved cultivation practices.
 
The application of humic acid and salicylic acid significantly enhanced the total weight of corms, as well as the number and weight of cormels, particularly when used together. The highest total corm weight observed with the combination of 2 millimolar salicylic acid and 5 kg/ha humic acid was approximately 133% higher than the control. Similarly, the highest number of cormels was achieved with the 2 millimolar salicylic acid and 10 kg/ha humic acid treatment in the first year, around 108% more than the control. Increasing levels of humic acid at all salicylic acid levels resulted in higher average corm weights. Adding 15 kg/ha humic acid to 0, 1, and 2 millimolars of salicylic acid boosted average corm weight by 30.7%, 57.3%, and 83.4%, respectively, compared to control without humic acid. Synergistic effects were also noted on leaf traits. The greatest leaf length occurred with 2 millimolar salicylic acid and 5 kg/ha humic acid in the first year (38% increase) and with 2 millimolar salicylic acid and 15 kg/ha humic acid in the second year (46% increase). The highest dry leaf weight was observed in 2 millimolar salicylic acid and 15 kg/ha humic acid in the first year (79% increase) and in 2 millimolar salicylic acid and 10 kg/ha humic acid in the second year (103% increase). The maximum stigma length was recorded with 1 millimolar salicylic acid and 15 kg/ha humic acid, showing a 52% increase compared to the control. The combined treatment of 2 millimolar salicylic acid and 15 kg/ha humic acid led to the highest fresh stigma weight in the first year (132% increase) and the highest dry stigma weight in the second year (300% increase). Pearson correlation analysis revealed significant relationships between qualitative and quantitative traits of saffron. A positive correlation was observed between dry stigma weight and the weight of both mothers (r = 0.76) and cormels (r = 0.74). Conversely, fresh style weight had a negative correlation with time to sprouting (r = 0.56) and flowering (r = 0.43). Stepwise regression analysis indicated that the most influential traits on dry stigma weight were the weight of mother corms, days to flowering, and daughter corm weight. These factors explained 76%, 82%, and 86% of the variance in saffron performance, respectively.
 
This study showed that humic acid and salicylic acid had significant effects on the growth and flowering of saffron. Specifically, these compounds caused a notable reduction in the vegetative growth phase and flowering time of saffron. This result meant that the use of these compounds could lead to significant improvements in the morphological and phenological traits of saffron, ultimately increasing the number and quality of daughter corms and overall plant yield.