Resistance of spinach plants to seawater stress is correlated with higher activity of xanthophyll cycle and better maintenance of chlorophyll metabolism

Abstract

The relationship between the activity of xanthophyll cycle and chlorophyll (Chl) metabolism was investigated using two cultivars, Helan No. 3 (seawater-tolerant cultivar) and Yuanye (seawater-sensitive cultivar), of spinach (Spinacia oleracea L.) plants cultured in Hoagland's nutrient solution, with or without seawater (40%). The results showed that, in plants of two cultivars with seawater, the xanthophyll cycle seems to show a principal protection mechanism against photoinhibition under seawater stress. Furthermore, accumulation of reactive oxygen species (ROS) in chloroplasts of two cultivars was enhanced by seawater to lower the activity of porphobilinogen deaminase. Namely, the conversion of porphobilinogen into uroporphyrinogen III involved in Chl biosynthetic processes was inhibited by seawater. In Helan No. 3 spinach plants with seawater, higher activity of xanthophyll cycle in the leaves dissipated more excess light energy, which appeared to lower the levels of ROS in chloroplasts. As a consequence, the Chl biosynthesis in Helan No. 3 leaves with seawater showed only a weak inhibition and the activity of chlorophyllase (Chlase) was not affected by seawater stress. In contrast, a more pronounced accumulation of ROS in chloroplasts of Yuanye leaves, which possess lower xanthophyll cycle activity, severely inhibited Chl biosynthesis and remarkably enhanced the activity of Chlase, which aggravates the decomposition of Chl. These results suggest that higher activity of xanthophyll cycle in seawater-tolerant spinach plays a role in maintaining Chl metabolic processes, probably by decreasing the levels of ROS, when the plants are cultured in the nutrient solution with seawater (40%).