Effect of Light Quality and Sapling Age on Antioxidants Activity and some Biochemical Characteristics of Grapevine (Vitis vinifera L., var, ‘Yaghooti’) Under Salinity Stress in Greenhouse

Introduction                                          
Light quality is a critical factor directly influencing normal growth and secondary metabolite biosynthesis in plants. Blue and red lights, with their key roles, are essential for leaf photosynthetic performance. Blue light, with a wavelength of 420-450 nm, plays a crucial role in chlorophyll synthesis, photomorphogenesis, and anthocyanin synthesis. Red light, with a wavelength of 600-675 nm, regulates the synthesis of phytochemical compounds such as phenolic compounds. The combination of red and blue lights, as an optimal light quality, regulates normal growth in most plants. Salinity, particularly in arid and semi-arid regions where the majority of grape cultivation occurs, significantly limits grape production globally, including in Iran. With diminishing water resources, the utilization of saline or low-quality water becomes unavoidable, necessitating the exploration of strategies to enhance tolerance to such water conditions. This study hypothesized that employing specific ratios of red and blue light for a limited period at the end of the day could effectively mitigate the adverse effects of salinity stress, address light limitations, improve plant nutritional quality, stimulate antioxidant synthesis, and maintain high yields.
 
Materials and Methods
This research investigated the impact of light quality on enzymatic and non-enzymatic antioxidant levels in two- and three-year-old ‘Yaghooti’ grape saplings subjected to salinity stress in a greenhouse. The study was conducted as a factorial experiment with three factors based on a completely randomized design with three replicates over two consecutive years. The first factor was light quality at five levels: 100% red light, 100% blue light, red/blue light combinations at 70/30 and 50/50 ratios provided as end-of-day lighting, and natural greenhouse light (control). The second factor was electrical conductivity of the nutrient solution induced by adding sodium chloride at four levels: 1.67 mS/cm (control), 3.63, 6.05, and 10.23 mS/cm. The third factor was sapling age at two levels: two and three years. Following the treatment period each year, leaf proline content, total phenol concentration (using the Folin-Ciocalteu method), total flavonoid concentration, anthocyanin content, catalase enzyme activity (by measuring the decrease in hydrogen peroxide absorbance), peroxidase enzyme activity (using guaiacol), superoxide dismutase enzyme activity, and soluble protein content were measured.
 
Results and Discussion
The results indicated that the 50/50 red/blue light combination and 100% blue light were effective in maintaining and increasing anthocyanin content, particularly in three-year-old saplings. However, other measured characteristics were generally higher in two-year-old saplings. Blue light significantly increased total phenol and flavonoid content and peroxidase enzyme activity at 10.23 mS/cm salinity. The 50/50 red/blue light combination resulted in the highest, and 100% red light the lowest, levels of proline and soluble proteins at 10.23 mS/cm salinity, whereas 100% red light showed the greatest impact on soluble carbohydrate content and catalase and superoxide dismutase enzyme activities. The red/blue light combination, especially at a 70/30 ratio, proved more effective in improving the vitality of ‘Yaghooti’ grapevines under greenhouse conditions compared to monochromatic lights, and also mitigated the negative effects of salinity, enhancing both quantitative and qualitative parameters, ultimately improving salinity tolerance in grape saplings. Further results of this study revealed the seedling age response to salinity stress, where the content of phenolic compounds (total phenols and flavonoids), proline, carbohydrates, soluble proteins, and antioxidant enzymes were higher in two-year-old seedlings compared to three-year-old seedlings. The increased accumulation of these compounds as plant defense mechanisms in younger seedlings indicates their higher sensitivity and greater requirement to cope with stress and ensure survival.
 
Conclusion
Overall, the study demonstrated that light quality treatments, contrary to salinity stress, reduced the activity of antioxidant enzymes, while significantly increasing the levels of soluble proteins, soluble carbohydrates, proline, anthocyanin, phenol, and leaf flavonoids (non-enzymatic antioxidants) in response to salinity. The application of artificial lighting with specific qualities, particularly the 70/30 red/blue combination, effectively improved light conditions and regulated the levels of beneficial compounds. It appears that by optimizing environmental conditions for ‘Yaghooti’ grapes and achieving adequate non-enzymatic antioxidant concentrations, the necessity for increased enzymatic antioxidant activity was diminished. Therefore, the use of red and blue light, either alone or in specific combinations via LED lamps, can serve as a general strategy to enhance beneficial compound production and improve stress tolerance in greenhouse-grown plants.