Abstract
Freezing tolerant tea cultivars are urgently needed. The tea cultivars with highly freezing tolerance showed resistance to freezing stress induced photoinhibition. Freezing sensitivity index (H) of 47 tea clonal cultivars was investigated after severe freezing winter in 2016. To develop instrumental methods for freezing tolerance selection, the maximum photochemical efficiency of photosystem II (PSII) (Fv/Fm) and leaf color indicator a on the Hunter color scale were determined on control group (non-frozen) and frozen group (being frozen at −15 °C for 2 h and then stood at 20 °C for 5 h) of the cultivars. When the two indicators were expressed as the ratios (RFv/Fm and Ra) of frozen group to control group, linear regression of the freezing sensitivity index (H) upon the RFv/Fm and Ra produced significant relationship respectively, i.e., H = 60.31 − 50.09 RFv/Fm (p < 0.01) and H = 30.03 − 10.82 Ra (p < 0.01). Expression of gene psbA encoding D1 protein and gene psbD encoding D2 protein in PSII showed that the frezzing tolerant tea cultivars maintained a high expression level of psbA after freezing stress, which is considered to be beneficial to de novo synthesis of D1 protein and sustaining PSII activity. These findings can provide instrumental tools for assessing freezing tolerance of tea cultivars in tea breeding program.
Highlights
Low temperature affects the growth, development, and productivity of tea plants, and severe freezing winter conditions at high latitudes limit the distribution of tea plants and the expansion of tea production areas
The present paper reveals a linear regressive relationship between freezing sensitivity index (H) and Fv/Fm ratio of frozen group to control group (HFv/Fm ): H = 60.31 − 50.09 RFv/Fm (p < 0.01) (Table S1)
The freezing tolerances of tea cultivars are differentiated as normal rule distribution, in which a few are highly freezing-tolerant and freezing sensitive respectively, with most of them being moderate freezing-sensitive
Summary
Low temperature affects the growth, development, and productivity of tea plants, and severe freezing winter conditions at high latitudes limit the distribution of tea plants and the expansion of tea production areas. Tea cultivar ‘Yinghong 1’, bred from descendants of Camellia assamica could not survive in the lower reaches of Yangtze River around 30◦ N latitude where the lowest winter temperature is about. Breeding and cultivating freezing tolerant tea cultivars is an effective way to conquer freezing stress in winter at high latitude tea areas. To accelerate the freezing tolerance breeding, various indicators were introduced to evaluate the freezing tolerance potential of the tested tea cultivar. These indicators included the layers of palisade cell and the ratio of the palisade to spongy parenchyma in leaf transverse section [2], leaf electrolyte leakage after freezing treatment [3]
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