Reproduced from an article by
Private Bag 11 222
From the New Zealand
Garden Journal (Journal of the Royal New Zealand Institute of Horticulture),
Volume 5, Number 2, December 2002, pp. 12-14.
There are 30-40 species
of Gunnera (family Gunneraceae) distributed naturally almost
exclusively throughout the southern hemisphere. The largest and
most famous are Gunnera manicata (giant ornamental rhubarb,
native to mountain swamps of Brazil) and G. tinctoria (Chilean
rhubarb) (Figure 1). They have large (1.0-2.4m diameter) leaves
with prickly petioles attached to a stout rhizome. A vertical section
through the stem of G. tinctoria (Figure 2A) reveals dark
green nodules. If you examine one of these nodules under a microscope
you will find beads or filaments of the blue green alga (cyanobacterium)
Nostoc. What is the significance of this visitor to the
life of Gunnera?
|Figure 1. The magnificent and
infamous Chilean rhubarb (Gunnera tinctoria). Chilean
rhubarb is a fully naturalised exotic plant in New Zealand,
but it has weedy potential and features in the National Pest
Plant Accord List.
A mutually beneficial
symbiotic relationship exists between the alga and its host. Gunnera
provides Nostoc with carbohydrate and an intracellular
living space. And, for providing shelter and three square meals
a day, the Gunnera gets nitrogen in exchange.There are
vast amounts of nitrogen gas (as the diatomic and highly stable
molecule N2) in the air that we breathe. In fact, 78% of the atmosphere
consists of N2. Plants cannot utilise nitrogen directly as N2. Rather,
they assimilate nitrogen as ammonium (NH4+) or nitrate (NO3-), generally
from the soil via the roots. But imagine if plants were able to
tap more directly into that atmospheric source of nitrogen. That's
exactly what Nostoc does for the Gunnera.
(Top) A vertical section through a
stem of G. tinctoria, showing a dark green nodule,
the site of the Gunnera-Nostoc symbiosis.
(Bottom) A filament of a free-living
form of Nostoc with predominantly vegetative cells
and a few nitrogenase-containing heterocysts.
There are two types of
cell on a Nostoc filament: vegetative cells and heterocysts.
The heterocysts contain the enzyme nitrogenase, which converts N2
gas into NH4+, a process known as nitrogen fixation. The ammonium
is then assimilated by Gunnera. There is a huge increase
in the frequency of heterocysts in symbiotic Nostoc (up
to 80% of cells) compared to free-living Nostoc forms (5%
of cells) (Figure 2B). How does this occur and what is its significance?
As stated earlier, Gunnera provides carbohydrate to Nostoc.
Therefore less cellular machinery is required for photosynthesis,
which is located in the green vegetative cells. The provision of
carbohydrate by the host allows the cyanobacterium to become in
a sense a nitrogen-fixing factory. This enables Gunnera to
colonise habitats with low soil nitrogen. For example, the native
Gunnera species are commonly used as ground cover in rock
The nitrogen-fixing symbiosis
between white clover (Trifolium repens) and the bacterium
Rhizobium has a high profile in New Zealand because of
its commercial significance to the pastoral industry. While the
Gunnera-Nostoc symbiosis does not have the same profile,
it is the subject of some attention as researchers continue to unlock
the mysteries of how symbioses involving nitrogen fixers operate.
|Figure 3. A strawberry-like runner
of the New Zealand Gunnera monoica: Ideal propagating
material and research subject.
I propagated Gunnera
monoica (Figure 3) for my research by cutting sections of a
runner (one or more nodes with leaves and roots attached), placing
them in a tray of coarse sand, and feeding them weekly with a liquid
contained no nitrogen. I grew mine in a greenhouse to get good growth
over the winter months and then in a shadehouse in summer. I grew
my cuttings in sand so that the plants could be removed intact for
analysis, and the nutrient regime could be controlled for optimum
nodule production and nitrogenase activity. The plants were easy
to grow. My gunnera had the same green nitrogenfixing nodules in
the stem of the leaf as G. tinctoria. Nodules in G.
tinctoria are a lot larger than those in G. monoica,
but relative to the size of the plant they would be comparable.
|Figure 4. A flowering specimen of Gunnera
prorepens (Courtesy Mark Dean of Naturally Native New Zealand
Plants Ltd., Tauranga).
Gunnera monoica provides
a beautiful model system to study nitrogen fixation. I would not
describe the plant itself as beautiful (especially when my research
was not going well), although I would not call it ugly either. This
species has a different habit from its larger cousins, looking more
like a strawberry plant, with relatively small leaves and stoloniferous
growth. It does not have the spiny leaf margins of G. tinctoria,
so it is less problematic to stroke! To be fair, I only ever saw
my plants growing in trays, and it could look good draped over a
rock in the garden, with a pond nearby. Some of the other Gunnera
species appear to be pleasing to the eye because of the colour
of their fruit, e.g. G. prorepens (Figure 4) (cf. G.
monoica, in which the fruit are white), and how they are used
in a garden.
The Landcare Research
New Zealand Plant Names Database [http://nzflora.landcareresearch.co.nz/default.aspx]
lists five endemic Gunnera species, including the diminutive
G. monoica (Figure 3). Other sources of information on
Gunnera are [http://www.naturallynative.co.nz/index.html]
A note from
the editor on replacements for Chilean rhubarb
None of the New Zealand
endemic Gunnera can match the magnificent stature of Chilean
rhubarb (G. tinctoria), which is a fully naturalised exotic
plant in New Zealand. Although this species grows wild in only a
few localities, it is judged to have weedy potential, especially
on coastal cliffs, wetlands, slips and roadsides. It appears on
the National Pest Plant
Accord List as a plant that is prohibited from sale, propagation
and distribution in New Zealand. Mike Harré, Biosecurity
Officer (Response) with Auckland Regional Council, reports that
G. tinctoria is a problem in Taranaki, and that this species
(or possibly G. manicata) is also establishing on Banks
excellent article, which describes and illustrates the differences
between G. tinctoria and G. manicata, was written
for our journal by Bill Sykes (Sykes, 1969).
|Figure 5. This is Darmera peltata,
a member of the saxifrage family that could be a streamside
replacement for Chilean rhubarb (Gunnera tinctoria) in
cool areas. See D. peltata growing in Wanaka
in this issue of The Garden Journal (p. 17). This species is
described in the literature as a native of Oregon, having clusters
of small pink flowers borne on erect stems before the leaves
appear. It is also known as Indian rhubarb, umbrella plant or
Peltiphyllum peltatum, and is reputed to have edible
Photo courtesy of Stonecrop Gardens, Cold Spring, New York,
Auckland Regional Council
recently published and released the Plant Me Instead booklet.
This encourages gardeners in the Northern Climatic Zone (broadly
Northland, Auckland, northern Waikato, coastal Bay of Plenty and
East Cape) to look for alternatives to their favourite, but environmentally
pesky plants. For Chilean rhubarb, recommended alternatives are
plantain lily (Hosta), golden rays (Farfugium japonicum,
formerly Ligularia tussilaginea) and ornamental rhubarb
(Rheum) cultivars. See also Darmera peltata (Figure
Sykes, W.R. (1969). Gunnera
tinctoria and Gunnera manicata. Journal of the
Royal NZ Institute of Horticulture 1:56-59.
became interested in nitrogen fixation during his MSc research
at Waikato University. He now works as a Tutor in Cell and
Plant Biology at Massey University.