Floral biology and breeding system of pohutukawa (Metrosideros excelsa, Myrtaceae)

The floral biology and breeding system of pohutukawa (Metrosideros excelsa, Myrtaceae), a mass-flowering tree of northern New Zealand coastlines, were examined. Trees flower over a peak period of 2 weeks, and compound inflorescences contain an average of 14.3 showy, hermaphrodite, red brush flowers that remain open for 7 days. A brief female flower stage (mean duration 1.3 d) is followed by the main hermaphrodite phase that lasts for 4 days. Neither dichogamy nor herkogamy is important in preventing pollen and stigma interference. Pollen is highly viable (93.6%), and stigma receptivity extends for at least 9 days, as indicated by peroxidase activity, pollen germination, pollen tube length 24 h after pollination, and seed set. Stigmatic exudate production appears to increase up to 5 days post-anthesis. On average, flowers produce 46 mu l nectar per day, containing 18% (w/v) sucrose. Floral design and display of pohutukawa are consistent with high levels of autogamous and geitonogamous self-pollination. Controlled pollination experiments were used to assess the effect of self- and cross-pollen and a pollen mixture from five unrelated parents on capsule and seed production, and on pollen tube growth in seven trees. Three trees in the experiment were self-incompatible, as quantified by the index of self-incompatibility (ISI), indicating that natural populations may consist of a mosaic of self-compatible and incompatible individuals. Self-incompatibility is late-acting as pollen tubes from selfs and crosses reached the ovary simultaneously at 10-15 d after pollination. In common with other Myrtaceae, the seed/ovule ratio in pohutukawa is low and this is likely to be genetically determined rather than limited by stigmatic pollen load. Germination of fertile seeds from all pollination treatments was equally high (98.4%), indicating that no inbreeding depression is acting at this stage of the life cycle. The pollen/ovule ratio of 462.5 (s.e. +/-43.3) places the breeding system of pohutukawa between facultative selfing and facultative outcrossing.