Eucalyptus camaldulensis × globulus hybrids

Abstract

Controlled pollination techniques were used to produce hybrids between Eucalyptus camaldulensis and E. globulus, with E. camaldulensis as the female parent. There were substantial barriers to hybridisation, with a 28% reduction in the percentage capsule set, a 78% reduction in the number of seeds produced per capsule, a 99% increase in the number of inviable abnormal seedlings and overall a 92% reduction in the number of normal seedlings produced per pollinated flower compared with outcrossed E. camaldulensis. The number of normal seedlings per pollinated flower varied from 0.1 to 4.7 and was most strongly affected by the source of the pollen. Selfing E. camaldulensis showed it to be partially self-incompatible, with a 29% reduction in the percentage capsule set and an 86% reduction in the number of seeds produced per capsule compared with outcrosses. One reciprocal combination of E. camaldulensis clones originating from Erudina and Broken Hill gave a very low seed set.

Cotyledon shape of the E. camaldulensis × globulus hybrids was intermediate between the two parents and an excellent way of checking that seedlings were hybrids. Shape and size of juvenile leaves did not distinguish the hybrids, but hybrid leaves started to change from being opposite and sessile to alternate and petiolate at a lower node than in the E. globulus parent, although not as quickly as for E. camaldulensis. In adult trees, although the hybrid leaves were longer than leaves of the parent species, it was difficult to identify the hybrids in the field on this basis. Flowers of the hybrid were more similar to E. camaldulensis, being in pedicellate clusters of 3–6. Flower size was intermediate between the parents (operculum height and width, stamen and style length). Fruits were also intermediate in size but valves were exserted as in E. camaldulensis. Hybrids were fertile and overlapped in flowering time with E. camaldulensis, so that if planted adjacent to natural stands of E. camaldulensis, some introgression may occur.