An assessment of the freezing tolerance of urban trees using chlorophyll fluorescence

The purpose of this research was to determine whether chlorophyll fluorescence values obtained from excised leaves of urban trees subjected to freezing under laboratory conditions provided a measurable indicator of whole-tree freezing tolerance under glasshouse conditions. Laboratory tests consisted of measurements of the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) performed on excised leaves taken from four year old stock of 15 urban tree species following freezing from 0°C to -10°C at 2 K intervals. Based on reductions in Fv/Fm of excised leaves following freezing trees were ranked in order of tolerance. Four year old trees of seven of the 15 species tested were then subjected to a freezing temperature of 28° C and placed under glasshouse conditions for 14 weeks. Recovery of whole trees from freezing damage as measured by chlorophyll fluorescence, photosynthetic rates and leaf chlorophyll content mirrored tolerance ranking of excised leaves under laboratory conditions. In addition, based on mortality at the cessation of the experiment, freezing tolerance followed a similar order as that obtained from Fv/Fm values of excised leaves. Irrespective of species, freezing damage significantly reduced most growth parameters of whole trees measured at the cessation of the experiment. Results indicate that screening of excised leaf material under laboratory conditions using chlorophyll fluorescence as a quick, reliable and inexpensive procedure can provide a useful means of estimating the freezing tolerance of young trees.