Chlorophyll fluorescence as a parameter for frost hardiness in winter wheat. A comparison with other hardiness parameters.

Frost hardiness of winter wheat leaves (Triticum aestivum L. cv. Urban) was measured during an eight weeks hardening period using chlorophyll fluorescence. Determination of frost induced damage after freezing, measured as the decrease of photochemical capacity of photosystem II (F-V/F-M = (F-M-F-O)F-M), was compared with three conventional methods to determine the degree of frost damage: electrolyte leakage, TriphenylTetrazoliumChloridereduction, and visual assessment. There was a good correlation between frost hardiness measured by chlorophyll fluorescence and the other methods. Winter wheat leaves reached a hardiness of -12 degrees C within eight weeks of frost hardening. In unhardened leaves, electrolyte leakage and TTC-reduction measurements were influenced by supercooling of the tissue, which lowered the lethal temperature about 4 degrees C. Supercooling could be prevented by addition of small ice crystals. After one week of hardening the effect of supercooling was no longer observed, because the plants were hardened to a lower temperature as could be achieved by supercooling. It is concluded that the determination of the ratio of variable to the maximal chlorophyll fluorescence after freezing is a reliable and rapid method to determine the frost hardiness of wheat leaves.