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Cold hardiness in windmill palms (Trachycarpus sp.)

Palms are beneficial for urban ornamental horticulture, since they symbolize the tropics, but only few are adapted to temperate climates. Our objective was to monitor responses of the leaves to cold reflected by the changes in chlorophyll fluorescence of leaves of Trachycarpus geminisectus, T. nanus, T. takil, T. ukhrulensis, T. wagnerianus species with the use of a chlorophyll fluorometer during the fall of 2018. Our results indicate that there was a gradual decline of maximum quantum efficiency of photosystem II (PSII) represented by Fv/Fm values from the end of September till the end of November 2018. This change corresponds well with the change in air temperature. Some species like T. ukhrulensis seemed to be more sensitive to cold since the rate of decline was the greatest in this species, also in most cases the difference was significant compared to the other species. The smallest decline in Fv/Fm values could be observed in T. wagnerianus. However, the difference compared to the other species was only significant until the middle of October after which it was not significantly better than the other well performing species such as T. takil and T. geminisectus. T. nanus was intermediate in its values, since it had significantly lower values until the beginning of October compared to T. takil and T. geminisectus, but during the remaining measurement period was not significantly different from these species. T. geminisectus seemed to perform on average with the other species. We found that there was a correlation in some cases between the changes in minimum air temperature and that of the maximum quantum efficiency of photosystem II (PSII). The correlation was strongest in T. nanus, T. wagnerianus and T. ukhrulensis. It was weaker in T. geminisectus and none in T. takil.