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

Abstract

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 p1 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 crosspollen 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 selfcompatible and incompatible individuals. Self-in- B99002Received 22 January 1999; accepted 28 July 1999 compatibility 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.

Biological flora of New Zealand 14:Metrosideros excelsa, pōhutukawa, New Zealand Christmas tree

“…1a). Inflorescences are composed of broad compound cymes terminating in groups of three flowers (Dawson 1968); a single inflorescence is composed of an average of 14.3 flowers (Schmidt-Adam et al 1999). Pedicels are stout and tomentose (Allan 1961).…”

“…In earlier phases, individual flowers produce an average of 50 µL nectar d −1 . Pollen has high levels of viability (93.6%), however, pollen tube growth is slow compared with other angiosperms; this, coupled with relatively long styles, results in a 10-15 d delay before the pollen reaches the ovary (Schmidt-Adam et al 1999).…”

“…All ovules (c. 933) within the ovaries appear to be potentially fertile, although the number of ovules that develop to form fertile seeds is low (c. 10.2%), irrespective of pollen availability (Schmidt-Adam et al 1999). Once ovules are successfully fertilised, seed develops and is later released in March-April (Bergin & Hosking 2006).…”

“…Once ovules are successfully fertilised, seed develops and is later released in March-April (Bergin & Hosking 2006). Despite low seed viability, overall seed production per tree is still sufficiently high (Schmidt-Adam et al 1999). Metrosideros excelsa seed is readily dispersed by wind.…”

“…However, mass flowering episodes coupled with a hermaphroditic phase and an absence of evolutionary traits to avoid pollen and stigma interference result in a high rates of self-fertilisation (Schmidt-Adam et al 1999). Approximately 60% of seeds produced are self-pollinated.…”