Freezing stress deteriorates tea quality of new flush by inducing photosynthetic inhibition and oxidative stress in mature leaves

The unprecedented early spring frost that appears as a freezing stress can cause significant yield losses in tea [Camellia sinensis (L.) O. Kuntze]; however, its effect on the quality of newly emerged tea leaves and the underlying physiological mechanisms still remain unclear. In the present study, two years-old tea seedlings were exposed to freezing temperature (−5 °C) for 3, 6 and 12 h, designated as mild, moderate and severe freezing stress, respectively; and gas exchange, chlorophyll fluorescence, water use efficiency and reactive oxygen species accumulation were analyzed after a period of recovery. The results showed that net photosynthetic rate, stomatal conductance, transpiration rate, water use efficiency and maximal photochemical efficiency of photosystem II significantly decreased but accumulation of hydrogen peroxide increased in mature leaves with increasing freezing duration after 2 day recovery. Even after 15 day recovery, water content and dry weight of one bud and one leaf in newly emerged tea flush significantly decreased, showing the lowest decrease (2.7% and 23.7%) by mild and the highest decrease (10.6% and 66.4%) by severe freezing stress, respectively. Meanwhile, tea polyphenol concentration increased but amino acid concentration decreased in young leaves, resulting in a gradual increase (48.9-83.1%) in polyphenol to amino acid ratio with increasing freezing duration. These results suggest that freezing-induced photosynthetic inhibition and oxidative stress in mature leaves eventually decrease both yield and quality of young leaves and thus indicating a profound effect of source leaves that experienced stress on the quality of newly emerged young tea leaves.