Introduction
Sunflower (Helianthus annuus L.) is one of the most significant oilseed crops, valued for its high content of unsaturated fatty acids, which contributes to the production of high-quality oil. Although this plant exhibits relative adaptability to Iran's climatic conditions, it demonstrates moderate sensitivity to drought stress. Drought stress poses a major challenge to crop production by directly affecting plant growth and yield. While sunflowers can tolerate mild drought conditions, severe drought significantly disrupts their physiological responses. Reduced regulation of transpiration and limited leaf expansion are key factors contributing to the plant's vulnerability to severe drought. This stress condition results in excessive production of reactive oxygen species (ROS), intensifying oxidative stress. Consequently, oxidative stress leads to chloroplast degradation, decreased photosynthetic capacity, leaf shrinkage and thickening, reduced leaf area, and morphological alterations such as impaired stomatal movement. Brassinolides, which belong to the class of plant steroid hormones, play a crucial role in regulating plant growth and development, including cell division and elongation, organogenesis, delayed leaf senescence, and enhanced resistance to environmental stress. By modulating plant responses to drought, brassinolides can mitigate the adverse effects of oxidative stress and improve overall plant performance.
Materials and Methods
To investigate the effect of brassinolide under water deficit conditions on antioxidant enzyme activity and certain agronomic traits of the sunflower cultivar Oscar, an experiment was conducted in a split-plot design based on a randomized complete block design (RCBD) with three replications. The main factor consisted of irrigation regimes at three levels (full irrigation at 100% field capacity, moderate drought at 75% field capacity, and severe drought at 50% field capacity based on Class A pan evaporation). The sub-factor included brassinolide foliar application at three concentrations (0 (distilled water), 0.1 and 0.5 mg/L). In this study, traits such as relative water content (RWC) of leaves, antioxidant enzyme activities (catalase, peroxidase, and superoxide dismutase), leaf area index (LAI), filled and unfilled capitulum weight, seed yield, and oil yield were evaluated.
Results and Discussion
The results indicated that drought stress and brassinolide foliar application significantly affected the morphophysiological indices and agronomic performance of sunflower. Severe stress led to a reduction in relative leaf water content (by 18.79%) and an increase in antioxidant enzyme activity. At this stress level, leaf area index, grain yield, and oil yield decreased by 57.48%, 61.56%, and 70.01%, respectively, compared to full irrigation. This reduction highlights disruptions in water uptake and retention as well as diminished grain-filling capacity under drought conditions. On the other hand, the simple effect of foliar application of 0.5 mg/L brassinolide increased catalase and peroxidase activities by 18.58% and 36.40%, respectively. This increase underscores the role of brassinolide in mitigating oxidative stress and improving the physiological status of the plant. Additionally, the highest leaf area index was observed in the treatment combining full irrigation with 0.5 mg/L brassinolide foliar application. Grain yield and oil yield under the highest level of brassinolide foliar application in severe stress conditions increased by 83.57% and 66.66%, respectively, compared to the treatment without foliar application under severe stress.
Conclusion
The findings of this study demonstrate that brassinolide foliar application, particularly under drought stress conditions, can be an effective strategy for enhancing both the quantitative and qualitative performance of sunflower. Brassinolide enhances antioxidant enzyme activities, mitigates the negative effects of oxidative stress, and improves plant physiological traits, thereby preserving photosynthetic potential and promoting vegetative and reproductive growth. Based on the results, foliar application of brassinolide at 0.5 mg/L yielded the most significant improvements in leaf area index, seed yield, and oil yield, underscoring its effectiveness in optimizing plant growth even under limited water availability. Therefore, the application of brassinolide under optimal irrigation regimes, particularly during moderate to severe drought stress, is recommended as a viable approach to enhancing sunflower productivity and sustainability. |
