Crossability of Eucalyptus globulus

Eucalyptus globulus (subgenus Symphyomyrtus; section Maidenaria) is one of the faster growing eucalypts and is widely grown for pulpwood. However, the species is frost sensitive and there is interest in attempting to extend the environmental range of plantations through interspecific hybridisation (Orme, 1988). A study of the crossability of E. globulus was thus undertaken and we here report the success of self-outcross and interspecific pollinations of E. globulus females.

The controlled pollinations undertaken are indicated in Fig. 1. With the exception of E. camaldulensis, pollen species were all from the same section as E. globulus (Maidenaria) and, except for E. ovata and E. brookerana, from the same series (Viminales). Females were mainly ornamental trees growing near Hobart, Tasmania and while their origin is unknown, it is unlikely any females were derived from the same provenance as the E. globulus pollen. Self-pollinations comprised both assisted and unassisted treatments. For assisted self-pollination, flowers were emasculated and self pollen manually applied to the receptive stigmata as in other controlled pollinations. However, in the unassisted treatment flowers were isolated in a bag but not emasculated. lntraspecific outcrosses were either single-pair or polycrosses using a mix of pollen from five trees. Capsules were harvested approximately one year after pollination and the number of viable, inviable and insect eaten seeds were counted from all capsules collected. Seeds were classified as inviable if they were not full or if squashed did not yield a firm white embryo. Seeds eaten by insects were classified as viable. The total number of seed and capsules yielded per flower crossed was determined for each cross and cross-types were compared using individual cross values as replication. Successful interspecific hybridisation has been verified from the morphology of progenies.