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

The frost tolerance of seedlings of spotted gums representing four closely related species; Corymbia maculata (seven provenances), C. variegata (six p rovenances), C. henryi (one provenance) and C. citriodora (two provenances) was examined. Each provenance of C. maculata, C. variegata and C. henryi w as represented by 10 seedlings from each of eight open-pollinated families, while the two C. citriodora provenances were each represented by 40 seedli ngs from a provenance bulk seedlot. Excised leaf discs from seedlings harde ned by exposure to the Canberra winter were subjected to test temperatures of 4.2, 6.1 and -8.0 degrees C. Damage to the discs was estimated by the el ectrical conductivity of leachate produced by soaking the disks in water af ter exposure to freezing temperatures. Exposure to -6.1 degrees C for 1 h produced leachate conductivities corresp onding to 37% of discs receiving lethal injury and gave good discrimination between tolerant and susceptible material. There were significant differen ces in frost tolerance between species, provenances within species, and bet ween open-pollinated families within provenances. Corymbia variegata displa yed significantly greater frost tolerance than the other three species. Spe cies rankings were consistent across the three test temperatures. Inland, h igh altitude provenances of C. variegata and C. maculata had greater frost tolerance than those from lowland coastal locations, while there were no co nsistent trends of frost tolerance with latitude in these species. Analysis of seedling morphological characters effectively separated the dif ferent species in a manner consistent with the taxonomic review by Hill and Johnson (1995). Characters which separated C. maculata and C. variegata in cluded the leaf pair at which the changeover from juvenile to intermediate leaf morphology occurred and the degree of pubescence of the leaves and ste m, 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 an d shape. Corymbia henryi seedlings had greater stem and leaf pubescence tha n those of C. maculata as well as distinctly shaped leaves, characterised b y having the greatest length from the widest point to the leaf base.