X-ray photoelectron spectroscopy surface analysis of fluoride stress in tea (Camellia sinensis (L.) O. Kuntze) leaves

X-ray photoelectron spectroscopy (XPS) was applied to analyze the chemical form of fluoride and tea leaf surface changes. It was found that the leaf surface was mainly composed of C and O with some Si, N, and Al, besides a small amount of P and Ca in the adaxial surface. There was a tendency that Al was transferred from the abaxial surface to the adaxial surface with fluoride treatment. Under fluoride treatment, the content of C increased and O decreased in abaxial surface, while the opposite performance occurred in the adaxial surface. Meanwhile, the content of N of both the adaxial and abaxial surfaces decreased, suggesting a decreased nitrogen-fixation capacity. The results showed that fluoride treatment made cuticle thinner, and cutinized layer thicker and more active with a higher organic acid concentration for the adaxial surface, while both cuticle and cutinized layer got thicker and less active with a lower organic acid concentration for the abaxial surface. The content of protein decreased with fluoride treatment, causing tea leaf surface moisture loss. The ratio of O2/O1 was greater than 1 for untreated leaves, and the ratio dropped under treatment. AlF3 and trace amounts of MgF2 or CaF2 existed in the adaxial surface, however, only MgF2 for the abaxial surface.