Autumn Frost Resistance on Several Walnut Species: Methods Comparison and Impact of Leaf Fall

Four walnut tree species, of value for their wood production, were evaluated for autumn cold resistance: Juglans regia L., Juglans nigra L., Juglans major Ton. A. Heller, and Juglans Xintermedia. Early frost events damage and reduce wood quality and lengthen timber harvesting rotation. The frost vulnerability of these species could be an even more important problem in the future, depending on which Mediterranean climatic scenario develops. The work is based on the use of the chlorophyll fluorescence parameter Fv/Fm measured in cortical bark chlorenchyma as a reliable estimator of freezing injuries in Juglans. Two methods traditionally used to evaluate the effects of frost on deciduous trees were also analyzed: visual damage and relative electrolyte leakage. The study was carried out using annual woody shoots collected in November 2009, which were subjected to low temperatures (from 5 C to 16.5 degrees C) in a controlled cold chamber. The relationship between leaf fall stages and autumn frost damage has also been discussed. The results obtained using the different methods studied were comparable. No injuries were detected in any species until the temperature dropped below 8.5 degrees C. In the study, black walnut clones were the most resistant to subzero temperatures, J. regia were the least, and J. Xintermedia showed an intermediate behavior with respect to its genitors. The assessment of cold damage using the Fv/Fm parameter was easy and quick. It seems that this would be particularly useful for screening a large number of samples.