New Zealand Journal of Botany abstract
Floral biology and breeding system of pohutukawa (Metrosideros
excelsa, Myrtaceae)
GABRIELE SCHMIDT-ADAM
KEVIN S. GOULD
BRIAN G. MURRAY
School of Biological Sciences
The University of Auckland
Private Bag 92019
Auckland, New Zealand
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 ul 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.
Keywords breeding system; floral biology; dichogamy;
geitonogamy; herkogamy; nectar production; pohutukawa; pollen/ovule ratio;
pollen tubes; Metrosideros excelsa; New Zealand; seed/ovule ratio;
self-incompatibility; stigma receptivity
B99002
Received 22 January 1999; accepted 28 July 1999
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