New Zealand Plants and their Story
Development in Pohutukawa
Clemens, Lekha Sreekantan, Robert Henriod and Paula Jameson
Institute of Molecular BioSciences, Massey University, Palmerston
Selections of New Zealand
Metrosideros make beautiful tub specimens and cut flowers,
as well as garden plants. We have been working on a number of projects
related to the horticultural development of these floricultural
products. Topics studied include the acceleration and maturation
in young plants rejuvenated by micropropagation (Clemens et al.
1999a), a description of flower initiation and development in the
field (Clemens et al. 1999b), controlling flowering in cultivation
(Henriod et al. 2000), and prolonging the life of flowers.
In this paper we outline
progress on two of these topics, describing the microscopic changes
taking place in vegetative and floral buds of pohutukawa (M.
excelsa) as they develop in garden plants over winter, and
how flowering in container plants can be manipulated by temperature
and daylength signals.
The birth of pohutukawa
Our first objective was
to describe when and how the flowers are first formed in the buds
of pohutukawa before they break in spring. We wanted to gain an
insight into how environmental factors, such as temperature and
daylength, might affect floral initiation and the development of
flowers. With this information we hoped to be able to have some
control over flowering in plants grown in the nursery.
The pohutukawa flower
buds we see breaking in spring each have 1-7 pairs of fully developed
three-flowered cymules, which are borne on a compressed shoot. The
cymules arise from the axillary buds of this shoot or inflorescence,
and each is protected by a bud scale or bract, rather than a leaf.
So when are these cymules formed in the bud scale axils? To answer
that question, we needed to study the pohutukawa buds through the
preceding autumn and winter.
A pohutukawa bud is extremely
and microscopically hairy; and the apical meristem at the heart
of the bud is protected by numerous pairs of scales. The inside
of the bud desiccates very quickly once the scales are cut away,
making them impossible to study with a conventional dissecting microscope.
We quickly moved to a protocol that enabled us to preserve the buds
so that they could be sectioned and stained before microscopic study.
The earliest evidence
of floral primordia was seen as a swelling of the axillary meristems
in May. These continue to swell through June and July, but it is
not until August that distinct cymule development occurs (Figure
lA). Between that time and bud break, further development of floral
organs occurs. Therefore, floral bud initiation is evident 1-2 months
after the period when daylength decreases at its fastest rate at
the autumn equinox. The apical meristems of buds that are going
to break in spring as vegetative shoots, produce leaves during winter,
and no swelling comparable to that seen in floral buds occurs in
the leaf axils (Figure lB).
Microscopic images of Metrosideros excelsa buds in
August showing (A) a floral bud with cymule development in bud
scale axils, and (B) a vegetative bud with leaf formation by
apical meristem and little axillary development. Magnification
By studying plants in
the field under natural conditions, we believed that a shortening
daylength was the key environmental factor leading to floral development.
We did not know at that stage whether the lowered temperature that
buds experience in winter was important for subsequent flowering.
To answer this question, we grew pohutukawa plants over winter in
greenhouses that were artificially heated, or had daylength artificially
extended. As expected, plants grown under long days did not flower
well; they did not flower at all if they were kept warm over winter
while receiving long days. Plants flowered when allowed to grow
under short daylengths, and flowering was better when the plants
were allowed to experience cool winter temperatures.
With these pieces of
information we are drafting a protocol for the control of flowering
in pohutukawa, paving the way for their development as flowering
pot plants that can be produced to schedule. Since then we have
extended our knowledge of the light environment required to enhance
flowering. We have also tested ways of accelerating or slowing flowering
by modifying the temperature regime in which the plants are grown
after bud break. There are numerous Metrosideros. We are
progressing our research with several M. excelsa cultivars,
including M. excelsa cv. Vibrance, and with those from
other parts of the Pacific.
Clemens, J., Henriod,
R. E., Bailey, D. G., and Jameson, P. E. 1999a: Vegetative phase
change in Metrosideros. Shoot and root restriction.
Plant Growth Regulation 28: 207-214.
Clemens, J., Jameson,
P. E., and Henriod, R. E. 1999b: Hastening and controlling flowering
in Metrosideros. International Plant Propagators Society Combined
Proceedings 48: 80-83.
Henriod, R. E., Jameson,
P. E. and Clemens, J. 2000: Effects of photoperiod, temperature
and bud size on flowering in Metrosideros excelsa (Myrtaceae).
Journal of Horticultural Science and Biotechnology 75: