| Summary: | To find out the critical damage temperature of tea leaf, a new method of subzero treatment was provided by fitting the air temperature data from six heavy frost events. Furthermore, the study explored the characteristics of Fuding Dabai tea plant response to low temperature stress of 2, 0, −2, −4, −8, −10 and −15 °C by observing the microstructure and ultrastructure changes of the leaves. All samples were collected in an ambient temperature of 8.6 °C which served as control. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microstructure and ultrastructure of stomata and mesophyll. SEM observation results indicated that stomata of tea leaves have an obvious low temperature stress when the temperature was lower than −2 °C. The extent of opening of the stomata increased and enhanced guard cell protection of tea leaves against cold injury. However, dehydration, shrinkage and deformation of cells occurred as the temperature decreased from −2 °C to −15 °C. TEM observations showed that the cell nucleus, cell walls, chloroplasts and mitochondria all had normal structure at a temperature of 8.6 °C where the membrane and granum lamella were clearly visible. As the temperature decreased to −2 °C, the membrane system of tea leaf was the first to be damaged and the cell walls became fuzzy. Therefore, the leaf microstructure and ultrastructure showed obvious changes at −2 °C, which might define the critical damage temperature for freeze damage of Fuding Dabai tea. Control strategy based this critical damage temperature is useful for wind machine frost protection in tea fields within the Yangtze River region. Keywords: Frost damage, Freeze injury, Chill injury, Frost protection
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