Variation in frost tolerance and seedling morphology of the spotted gums Corymbia maculata, C. variegata, C. henryi and C. citriodora

Abstract

The frost tolerance of seedlings of spotted gums representing four closely related species; Corymbia maculata (seven provenances), C. variegata (six provenances), C. henryi (one provenance) and C. citriodora (two provenances) was examined. Each provenance of C. maculata, C. variegata and C. henryi was represented by 10 seedlings from each of eight open-pollinated families, while the two C. citriodora provenances were each represented by 40 seedlings from a provenance bulk seedlot. Excised leaf discs from seedlings hardened by exposure to the Canberra winter were subjected to test temperatures of –4.2, –6.1 and –8.0˚C. Damage to the discs was estimated by the electrical conductivity of leachate produced by soaking the disks in water after exposure to freezing temperatures.

Exposure to –6.1˚C for 1 h produced leachate conductivities corresponding to 37% of discs receiving lethal injury and gave good discrimination between tolerant and susceptible material. There were significant differences in frost tolerance between species, provenances within species, and between open-pollinated families within provenances. Corymbia variegata displayed significantly greater frost tolerance than the other three species. Species rankings were consistent across the three test temperatures. Inland, high altitude provenances of C. variegata and C. maculata had greater frost tolerance than those from lowland coastal locations, while there were no consistent trends of frost tolerance with latitude in these species.

Analysis of seedling morphological characters effectively separated the different species in a manner consistent with the taxonomic review by Hill and Johnson (1995). Characters which separated C. maculata and C. variegata included the leaf pair at which the changeover from juvenile to intermediate leaf morphology occurred and the degree of pubescence of the leaves and stem, leaf size and shape. Corymbia variegata and the southern provenance of C. citriodora were separated only by the citronella scent of C. citriodora, the northern C. citriodora provenance had distinctly different leaf size and shape.Corymbia henryi seedlings had greater stem and leaf pubescence than those of C. maculata as well as distinctly shaped leaves, characterised by having the greatest length from the widest point to the leaf base.