1. Introduction: Persian leek (Allium ampeloprasum ssp. persicum), a member of the Alliaceae family, is a traditional leafy vegetable widely used in Iranian cuisine. Its growth and development are strongly influenced by environmental factors, particularly temperature and light, which affect its morphology, physiology, and nutritional quality. Optimal germination and vegetative growth in Allium species generally occur at temperatures between 12°C and 24°C. While high temperatures above 30°C may inhibit early growth in some species like garlic, other studies report enhanced vegetative growth at around 30°C, indicating species-specific and cultivar-specific variability in thermal response. The determination of an appropriate planting date plays a critical role in aligning crop development with favorable environmental conditions, thus improving both the quantity and quality of yield. Given that climatic conditions vary regionally and that different cultivars respond differently to these conditions, it is essential to identify the best sowing time based on local climate, cultivar characteristics, and production goals. To date, most studies on Persian leek have focused on seed germination under controlled conditions and its physiological or nutritional properties. However, limited information is available on its phenological development and yield performance in open-field cultivation. Since the harvestable parts of Persian leek are its vegetative organs, understanding how different planting dates affect its early growth stages and final yield is essential. This study was conducted in Gorgan County, Golestan Province, over six months to monitor phenological stages and yield under varying planting dates, aiming to identify the optimal sowing time for Persian leek under local field conditions.
2. Materials and Methods: To investigate the phenological stages and yield of Persian leek under field conditions, the present study was conducted over 12 planting dates (one in each month of the year) with three replications at the research farm of Gorgan University of Agricultural Sciences and Natural Resources. Seed sowing was carried out on the 15th day of each month from Farvardin to Esfand (March to February). Daily minimum and maximum temperatures were recorded in the field at 8:00 AM, and the average temperature was calculated for each day and subsequently averaged monthly. Upon seedling emergence, phenological stages of the seedlings, including eight stages (emergence, hooked, green hooked, scape elongation, first true leaf appearance, first true leaf formation, second true leaf appearance, and second true leaf formation), were recorded according to the BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie) scale. The date of occurrence for each phenological stage was recorded when 50% of the plants reached that stage. Yield measurements for each planting date were conducted monthly for six months. Finally, data were analyzed using SAS software version 9.4; mean comparisons were performed using Duncan’s multiple range test. Graphs were plotted using Microsoft Excel, and correlation analyses were performed with R software.
3. Results and Discussion: The results showed that the highest number of seedlings emerged per day occurred in July and August, with an average of 10.4 seedlings per day, followed by June and September with an average of 7.8 seedlings per day. The lowest number of seedlings emerged per day, with averages of 1.3 and 1.28, respectively, belonged to February and March. The highest number of seedlings entering the hook and green hook stages was also recorded in July and August, with an average of 7.8 seedlings per day. For the whip stage, the highest average daily number of seedlings entering this phase occurred in June with an average of 6.1 seedlings per day, and in July, August, and September with an average of 6.34 seedlings per day. Examination of the timing of phenological stages (days after planting) across different months revealed that the longest duration to reach the phenological stages of emergence, start of the hook stage, start of the green hook stage, and start of the whip stage was observed in January with an average temperature of 12.94 °C, while the shortest duration occurred in July and August with average temperatures of 29.37 °C and 30 °C, respectively. The highest number of seedlings entering the first true leaf emergence stage was recorded in June, July, August, and September, with average numbers of 5.27, 5.7, 5.7, and 5.27 seedlings per day, respectively. For the first true leaf formation stage, the highest numbers of seedlings entering this phase in July, August, and September were 3.9, 3.9, and 3.53 per day, respectively. The highest average numbers of seedlings entering the second true leaf emergence and formation stages per day were recorded in July and August, with values of 2.61 and 2.17, respectively. Based on the results, the shortest number of days after planting to reach the first and second true leaf emergence and formation stages occurred in July and August, whereas December exhibited the greatest delay in phenological development, followed by November. The highest harvestable organ yield was recorded in March and April with 1.12 and 1.07 kg/m², respectively, and the lowest yield was observed in November with 0.54 kg/m². Correlation analysis indicated that the emergence and formation stages of the first and second true leaves had a significant positive relationship with yield.
4. Conclusion: One of the effective strategies for optimal use of environmental factors, inputs, and increasing yield is to pay attention to the determination of planting dates and the selection of suitable cultivars. Based on the results obtained from the present study, it can be stated that under the conditions of Gorgan County, Golestan Province, the increase in temperature during March and April leads to the acceleration of phenological stages, which in turn increases the growth rate and ultimately enhances the final yield of the vegetative organs. In planting dates of November, December, and January, the emergence and formation of the first and second true leaves are delayed, and the growth period of Iranian leek is not completed. Conversely, in the planting dates of June, July, and August, due to the onset of intensified heat and water evaporation, leaf development stages are reduced. Therefore, the suitable planting dates for Iranian leek are March and April.