Morphological traits and grain yield of safflower (Carthamus tinctorius L.) in response to waterlogging stress at different growth stages under field conditions

Introduction Waterlogging stress is a common agricultural problem in many regions across the globe. Crop productivity is threatened primarily by waterlogging in rainfed areas. Plant growth and crop production is constrained severely by waterlogging in many regions around the world. Waterlogging significantly reduces the grain yield. Moreover, oil yield reduces mainly due to a significant decrese in grain yield under waterlogging stress condition. Waterlogging at different growth stages has different effects on crop growth and yield. In most crop plants, it was shown the different adverse effects of waterlogging on yield determination at the vegetative and reproductive stages. However, the effects of waterlogging stress at different growth stages on safflower under field conditions is not yet known.   Materials and methods In the current study, in order to study of the influence of waterlogging stress at different growth stages on the morphological traits and grain yield of safflower under field conditions, a field experiment was carried out during 2020-2021 growing season in a split plot arrangement in randomized complete blocks design using three replicates per treatment. The study site was located at the research farm of Shahid Chamran University of Ahvaz, Iran. The plants were grown in the filed to ensure the environmental conditions found in flooded soils. Three waterlogging treatments were applied to plants as main plots: Well-drained controls watered weekly, mild stress (waterlogged for 24 h) and severe stress (waterlogged for 48 h). Waterlogging treatments implemented at the vegetative and reproductive stages as sub plots. After the waterlogging period, waterlogging treatments were watered with sufficient water (80% field capacity) until the end of the experiment.   Results and discussion The waterlogging treatments at vegetative and reproductive stages significantly decreased biological yield, grain yield, number of sub-branches, branche number, capitulum number per plant, grain number per capitulum, grain number per plant, 1000-grain weight and oil yield. There was no significant difference between growth stages in terms of all traits except for capitulum number per plant and the number of sub-branches. The mild and severe waterlogging stress at both growth stages was led to a significant reduction in grain yield by 34 and 39%, respectively, compared to control. The waterlogging treatments at different growth stages differently reduced the grain yield. The maximum decrease was found when waterlogging occurred for 48 h at the reproductive stage. The reduction in the number of sub-branches was associated with capitulum number per plant and grain number per plant. Under waterlogged conditions, the reduction in capitulum number per plant was mainly due to the decrease in the number of sub-branches.   Conclusion These findings indicate that safflower is more sensitive to waterlogging at the reproductive stage. The effect of waterlogging stress on grain and oil yield varied between two growth stages and waterlogging durations. Under waterlogging condition, grain yield loss occurred mainly due to a decrease in the number of sub-branches, branch number, capitulum number per plant, grain number per head and grain number per plant, which led to a significant decrease in grain yield and ultimately reducing the oil