Physiological and cellular responses to fluoride stress in tea (Camellia sinensis) leaves

Abstract

Tea (Camellia sinensis (L.) O. Kuntze) hyper-accumulates fluoride (F), mainly in the leaves. To understand how tea copes with the stress caused by F, we tracked photosynthesis, antioxidant defense, and cell ultrastructure under different F concentrations (0–50 mg L−1). High F (≥5 mg L−1) caused decreases in photosynthetic and chlorophyll fluorescence parameters. Activated oxygen metabolism was altered by F, as manifested in increasing lipid peroxidation, electrolyte leakage (EL), and accumulation of H2O2. The activities of ascorbate peroxidase (APX, EC 1.11.1.1) and catalase (CAT, EC 1.11.1.6) increased at 0–5 mg L−1 F, but sharply decreased less than 10–50 mg L−1 F. The activity of manganese superoxide dismutase (Mn-SOD, EC 1.15.1.1) decreased with increasing F concentration. Expression of genes encoding antioxidant enzymes were in accordance with their measured activities. The results suggest that the antioxidant enzymes in the tea plant can eliminate reactive oxygen species (ROS) at <5 mg L−1 F, but not at 20–50 mg L−1 F. High F increased the number of epidermal hairs on tea leaves and decreased the stomatal aperture, reducing water loss. The leaf cellular structure appeared normal under 1–50 mg L−1 F, although starch grains in chloroplast increased with increasing F. Proline and betaine play important roles in osmotic regulation in tea plant tolerating F stress. ROS scavenging and greater number of epidermal hairs are likely parts of the tea plant F-tolerance mechanism.