Low temperature (LT) is an important abiotic factor affecting plant survival, growth and distribution. The response of Cryptomeria fortunei (Chinese cedar) to LT is not well known, limiting its application in production and ornamental value. In this study, we first screened ten clones (#3, #25, #32, #42, #54, #57, #68, #66, #74, #X1), originating from five different locations in China, for their degrees of cold resistance. We then selected the two showing the highest (#32) and lowest (#42) cold resistance to see the physiological and morphological response of different cold-resistant C. fortunei clones to LT. We found that the electrolyte leakage of all ten clones increased strongly between 0 and -8 degrees C, while below -8 or between 4 and 0 degrees C did not yield additional increases. Under cold stress, clones #32 and #42 showed different degrees of needle browning. From 25 to -20 degrees C, maximum and effective quantum yields of photosystem II (F-v/F-m and YII) and photochemical and non-photochemical quenching (qP and NPQ) decreased continuously in two clones with decreasing temperature, where #42 was more strongly affected compared with #32. The chlorophyll content first decreased significantly to the lowest from 25 to -12 degrees C, then increased significantly at -16/-20 degrees C compared with -12 degrees C. We observed changes in needle cellular ultrastructure at -8 degrees C, with chloroplasts of #32 swelling, while those of #42 were destroyed. Correlation analysis indicated that needle browning and chlorophyll fluorescence were closely related to temperature, and cellular ultrastructure changed notably around semi-lethal temperature (LT50), which can be used as physiological indicators for the identification of cold resistance. We found a clear difference in cold tolerance between clones of #32 and #42, with #32 being more tolerant, which can be exploited in breeding programs. We conclude that strongly cold-resistant clones have more stable physiological states and a wider adaptability to LT compared with weak ones.
