Comparing image (fractal analysis) and electrochemical (impedance spectroscopy and electrolyte leakage) techniques for the assessment of the freezing tolerance in olive

Olive growth and productivity are limited by low temperatures mainly during winter, but sometimes also in spring and fall. The most effective way to avoid these damages in areas subjected to these climatic conditions is to select least susceptible varieties, but the choice of the right method to determine cold hardiness is extremely difficult. The aims of the work were (1) to assess LT50 (lethal temperature at which 50% of damage in plants subjected to low temperatures occurs) of some olive varieties in two seasons (summer and winter) and (2) to assess the reliability of different methods to evaluate cold hardiness. LT50 was determined on 21 different olive (Olea europaea L.) Italian varieties by leaf and shoot electrolyte leakage, shoot impedance spectroscopy and leaf color determination of fractal spectrum. All the experiments were conducted on non-acclimated and cold-acclimated plants. Our results showed that all the three methods were able to detect damages on olive plants after exposure to low temperatures, with leaves appearing more sensitive to cold stress than shoots. Among these methods, fractal analysis could be very useful in assessing cold hardiness of plants on the basis of visible injury, without sophisticated or expensive instruments and in a reliable and cost-effective way, using only a scanning device, a personal computer and dedicated freeware software.