Introduction
Sesame (Sesamum indicum L.) is one of the oldest oilseed crops cultivated for purposes relating to industry and household consumption. It is widely grown in tropical and subtropical regions around the world. Sesame productivity can be highly variable depending upon the growing conditions, cultivars, and cultural practices. Sowing date is a critical field crop management practice that influences the intensity of other practices applied in crop growth and production. Seed yield significantly influences by sowing date and cultivar. The optimal sowing date is one of the most important and low cost field management practices affecting crop growth and yield. In general, early sowing dates correspond to higher growth and seed yields. The aim of this study was to explore an optimum sowing date for new indehiscent sesame cultivars and obtain new superior cultivars with the highest yield potential and adaptation to different sowing date in Dezful region.
 Materials and Methods
In order to study the effects of different sowing dates on phonological traits, photosynthetic parameters, and seed and oil yields of dehiscent and indehiscent sesame cultivars, a field experiment was carried out in a split plot arrangement in randomized complete block design with four replications in 2021-2022. The study site was located in Dezful, Iran. The main plots included three sowing dates, June 5, June 20, and July 20, and the sub-plots consisted three indehiscent (Chamran, Mohajer, Barkat) and one dehiscent (Schewin) sesame cultivars. 5 th June sowing date was considered as early sowing date, and 5 th July sowing date was considered as late sowing date. Data were collected on various parameters, including plant heigh, height of the first capsule, length of the capsule-bearing zone, days to the first capsule emergence, days to flowering, stomatal conductance, photosynthetic rate, transpiration, chlorophylle index, seed yield, biological yield, harvest index, oil content and oil yield.
Results and Discussion
Significant genotypic differences were observed in phenological traits, photosynthetic parameters, and seed and oil yields across cultivars and sowing dates. Different sowing dates influenced photosynthetic properties, phenological traits, and yields of sesame cultivars in distinct ways. Changes in plant height, height of the first capsule, length of the capsule-bearing zone,  days to the first capsule emergence, days to flowering, stomatal conductance, photosynthetic rate, transpiration rate, chlorophylle index, seed yield, biological yield, harvest index, oil content and oil yield were significant across both years. In 2021, the June 5 sowing date resulted in a significant decrease in the seed yield of Chamran, Schewin, Mohajer and, Barkat cultivars by 1%, 25%, 11 and 6%, respectively, compared to the June 20 sowing date. In contrast, the July 5 sowing date led to significant reductions in the seed yield of Mohajer and Barkat by 21% and 37%, respectively, but increased the seed yields of Chamran and Schewin by 12% and 28%, respectively. There was also genetic variation in seed and oil yield across cultivars in response to sowing dates. The Barkat and Mohajer cultivars, which had higher seed yields, showed greater yield stability across the early sowing date compared to the Schwin cultivar.
 Conclusion
Overall, the results of this experiment suggest that the optimal sowing window for improving sesame production in humid tropical regions is between June 5 and June 20. Moreover, the Barkat cultivar, which exhibited the highest seed and oil yields, is recommended for both early and late sowing conditions in the Khuzestan region, alongside the Mohajer cultivar.