Freezing mechanisms, acclimation processes and cold injury in Eucalyptus species planted in the Mediterranean region

Abstract

Eucalyptus plantations of the Mediterranean countries are frequently jeopardized by the occurrence of relatively severe frosts every 5–10 years. In order to examine the mechanisms of frost injury in the species generally utilized in the region, whole seedlings and samples of tree tissues of different species ( Eucalyptus occidentalis Endl., E. × trabutii Vilm., E. globulus Labill., E. gunnii Hook. f.) were tested by artificial frost regimes. Non-hardened vs. cold-hardened plants, woody vs. leaf tissues and stem pieces of different sizes were compared for their supercooling ability, exotherm characteristics and frost tolerance. The species most often utilized for planting in the Mediterranean region, E. × trabutii, E. occidentalis and E. globulus did not show any capability to respond to artificial or natural hardening regimes; however, the first two species were irreversibly damaged as soon as ice formed in the tissues, probably because of ‘non-equilibrium freezing’. Eucalyptus globulus showed a modest capability to withstand extracellular ice and, therefore, ‘equilibrium freezing’. Eucalyptus gunnii, a subalpine species utilized for interspecific crosses, was able to respond to hardening pretreatments and to withstand, in the hardened state, extracellular ice formation and temperatures as low as −14°C.