Antioxidant response of zinc-efficient and zinc-inefficient sunflower (Helianthus annus L.) cultivars to zinc deficiency conditions

Introduction

“Zinc (Zn) deficiency is a widespread nutritional constraint in agricultural soils worldwide, including Iran.”. It particularly disrupts plant growth, photosynthesis, and yield, especially in calcareous and saline-alkaline soils with high pH. “Sunflower provides a useful system for investigating genotypic variation in responses to Zn deficiency.”due to its high sensitivity to zinc deficiency. This study investigated the physiological and biochemical responses of two groups of sunflower genotypes, Zn-efficient and Zn-inefficient to zinc deficiency conditions.



Materials and Methods

The experiment was conducted as a factorial design in a completely randomized design (CRD) with three replications in spring and summer 2023 in the greenhouse of the Research Center for Natural Resources and Agriculture of West Azarbaijan Province. The first factor was two zinc levels — no Zn application, and application of Zn at 5 mg kg-1 soil (elemental Zn), supplied as 21.94 mg ZnSO4.7H2O per kg soil based on the nutrient solution composition — and the other factor was Zn-efficient (Diamantis and Suzuka) and Zn-inefficient (Shams and Master) sunflower cultivars. The planting substrate was prepared from the “Khaneh Arkh” River sand, sieved through a 2 mm sieve after preparation, and then washed five times with tap water. Subsequently, it was rinsed with deionized water to remove soluble salts, air-dried, and supplemented with the necessary nutrients for plant growth. It was then air-dried again. In addition to the aforementioned nutrients, half of the treatments received Zn from (ZnSO₄.7H₂O). Irrigation was performed daily to field capacity. Thirty days after planting, root and leaf samples were collected, and biochemical indices including hydrogen peroxide (H2O2), malondialdehyde (MDA), total phenol, flavonoid, total chlorophyll, and the activity of the enzymes catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX) were measured.



Results and Discussion

The results showed that zinc deficiency conditions increased the amount of hydrogen peroxide. This increase was significantly higher in the Zn-inefficient cultivar Shams than in the Zn-efficient cultivar Suzuka. No significant difference was observed between cultivars with different zinc efficiencies in the amount of phenol, either under sufficient zinc conditions or under zinc deficiency conditions. Zinc deficiency significantly increased flavonoid levels; however, cultivars were not differentially affected by zinc conditions. Total chlorophyll levels were severely reduced under zinc deficiency conditions. However, this reduction was much lower in Zn-efficient cultivars than in Zn-inefficient cultivars.

Variance analysis of antioxidant enzyme activity results showed that, except for guaiacol peroxidase, there was no interaction effect between cultivars and zinc conditions for any of the enzymes. Comparison of means revealed that zinc deficiency significantly increased the activities of catalase, guaiacol peroxidase, and ascorbate peroxidase, whereas superoxide dismutase activity significantly decreased under zinc-deficient conditions. Among the antioxidant enzymes, guaiacol peroxidase showed the greatest increase in activity (approximately 388%), followed by catalase (150%) and ascorbate peroxidase (31%). In contrast, superoxide dismutase activity decreased by 11.11% under zinc deficiency.



Conclusion

These findings suggest that Zn-efficient cultivars can respond more effectively to oxidative stress caused by zinc deficiency by utilizing a more effective antioxidant system. Therefore, selecting and cultivating Zn-efficient genotypes in soils with zinc deficiency, especially in arid and semi-arid regions of Iran, or utilizing these genotypes genomes in modern and advanced breeding programs such as genetic engineering, can play a key role.