Assessment of foliar frost damage: a comparison of in vivo chlorophyll fluorescence with other viability tests

Frost resistance of leaves was assessed by in vivo chlorophyll fluorescence and electrolyte leakage and compared to percentage damage determined by di gital image analysis. As the development of frost damage is a time dependen t process, fluorescence and visible injuries were measured repeatedly withi n one week after the frost treatment on Rhododendron,ferrugineum L, leaves. In summer on freezing sensitive leaves no significant difference (P < 0.01) was found between the frost resistances (LT50) obtained by the three tests . The LT50-values were also completely independent from the testing time wi thin one week (P < 0.01). In midwinter on freezing tolerant leaves the testing time had a significant (P < 0.01) influence on LT50-values. Only the electrolyte leakage test and digital image analysis yielded similar results. The fluorescence test sign ificantly (P < 0.01) overestimated frost resistance by 5 degrees C. This wa s not caused by natural winter depression of PS II, as we have referenced F -V/F-M, to controls but by an underestimation of frost damage on partially frost injured leaves.