A conductivity method for screening populations of eucalypts for frost damage and frost tolerance

The assessment of frost tolerance of Eucalyptus leaf tissue by an electrical conductivity method is described. Leaf disc samples were exposed to freezing treatments in test tubes within a liquid cold bath with precise temperature control, and the conductivity of the disc leachate measured. Leaf discs were then frozen at -20°C to obtain an absolute conductivity for the tissue. A modified relative conductivity value (RC*) is presented together with calibration against visual observations of leaf damage for seedlings of E. regnans and E. delegatensis. RC* was found to be a reliable indicator of leaf survival status and the RC*-damage relationship was constant across provenances of both species. Provenance and family variation for frost tolerance were examined for both species and significant differences in tolerance levels were found for unhardened E. delegatensis and for hardened seedlings of both species at several test temperatures. Provenance rankings were identical to results from field trials reported in the literature. The effect of the period between frost treatment and conductivity measurement was assessed together with variation between leaves of the same plant and the effect of storage of cut leaves for 24 h prior to exposure to frosting treatments. The absence of any significant effects from storing leaves for 24 h indicates the potential of using the method to screen samples collected in the field. Problems of relating RC*, leaf damage and survival of the whole plant are discussed and the potential use of RC* as a simple screening technique for identifying frost-tolerant families and provenances is examined