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 inflore scences 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. Neit her dichogamy nor herkogamy is important in preventing pollen and stigma in terference. Pollen is highly viable (93.6%), and stigma receptivity extends for at least 9 days, as indicated by peroxidase activity, pollen germinati on, pollen tube length 24 h after pollination, and seed set. Stigmatic exud ate 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. Flora l design and display of pohutukawa are consistent with high levels of autog amous and geitonogamous self-pollination. Controlled pollination experiment s were used to assess the effect of self- and cross-pollen and a pollen mix ture from five unrelated parents on capsule and seed production, and on pol len tube growth in seven trees. Three trees in the experiment were self-inc ompatible, as quantified by the index of self-incompatibility (ISI), indica ting that natural populations may consist of a mosaic of self-compatible an d incompatible individuals. Self-incompatibility is late-acting as pollen t ubes from selfs and crosses reached the ovary simultaneously at 10-15 d aft er pollination. In common with other Myrtaceae, the seed/ovule ratio in poh utukawa is low and this is likely to be genetically determined rather than limited by stigmatic pollen load. Germination of fertile seeds from all pol lination treatments was equally high (98.4%), indicating that no inbreeding depression is acting at this stage of the life cycle. The pollen/ovule rat io of 462.5 (s.e. +/-43.3) places the breeding system of pohutukawa between facultative selfing and facultative outcrossing.