Comparing fractal analysis, electrical impedance and electrolyte leakage for the assessment of cold tolerance in Callistemon and Grevillea spp.

Lethal low temperatures were examined in two species of Grevillea and four species of Callistemon, two taxa originating from Australia. The demand in Europe and America for Australian plants is increasing both as cut flowers and as potted or landscape plants, but there is little information available on the cold hardiness of these species. LT50 (lethal temperature at which 50% of damage occurs) was determined by measuring electrical conductivity (electrolyte leakage), impedance spectroscopy and fractal spectrum of leaf colour. All the experiments were conducted on non-acclimated and cold-acclimated plants. LT50 calculated on non-acclimated plants were about 2-3degreesC higher than in acclimated plants, except for G. olivacea. In every case, the LT50 estimated by the three methods were very similar and ranged from -4.6degreesC for G. olivacea, the species most sensitive to freezing, to about -9.5degreesC for C. salignus, the most resistant to freezing among the species tested.