New Zealand Garden Journal, 2002, Vol.5 (2) 2A

Fixation with Gunnera Paul Stock1 There are 30-40 species of Gunnera There are two types of cell on a example, the native Gunnera species (family Gunneraceae) distributed Nostoc filament: vegetative cells and are commonly used as ground cover naturally almost exclusively heterocysts. The heterocysts contain in rock gardens. throughout the southern hemisphere. the enzyme nitrogenase, which The nitrogen-fixing symbiosis + The largest and most famous are converts N2 gas into NH4 , a process between white clover (Trifolium repens) Gunnera manicata (giant ornamental known as nitrogen fixation. The and the bacterium Rhizobium has a rhubarb, native to mountain swamps ammonium is then assimilated by high profile in New Zealand because of Brazil) and G. tinctoria (Chilean Gunnera. There is a huge increase in of its commercial significance to the rhubarb) (Figure 1). They have large the frequency of heterocysts in pastoral industry. While the Gunnera- (1.0-2.4m diameter) leaves with prickly symbiotic Nostoc (up to 80% of cells) Nostoc symbiosis does not have the petioles attached to a stout rhizome. compared to free-living Nostoc forms same profile, it is the subject of some A vertical section through the stem of (5% of cells) (Figure 2B). How does attention as researchers continue to G. tinctoria (Figure 2A) reveals dark this occur and what is its significance? unlock the mysteries of how green nodules. If you examine one of As stated earlier, Gunnera provides symbioses involving nitrogen fixers these nodules under a microscope carbohydrate to Nostoc. Therefore operate. you will find beads or filaments of the less cellular machinery is required for blue green alga (cyanobacterium) photosynthesis, which is located in Growing gunneras Nostoc. What is the significance of the green vegetative cells. The I propagated Gunnera monoica this visitor to the life of Gunnera? provision of carbohydrate by the host (Figure 3) for my research by cutting A mutually beneficial symbiotic allows the cyanobacterium to become sections of a runner (one or more relationship exists between the alga in a sense a nitrogen-fixing factory. nodes with leaves and roots attached), and its host. Gunnera provides Nostoc This enables Gunnera to colonise placing them in a tray of coarse sand, with carbohydrate and an intracellular habitats with low soil nitrogen. For and feeding them weekly with a liquid 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 Figure 1. The magnificent and infamous Chilean rhubarb (Gunnera tinctoria). Chilean rhubarb nitrogen. That’s exactly what Nostoc is a fully naturalised exotic plant in New Zealand, but it has weedy potential and features in the National Pest Plant Accord List. does for the Gunnera. 1Institute of Molecular BioSciences, Massey University, Private Bag 11 222, Palmerston North. 12 New Zealand Garden Journal, 2002, Vol.5 (2) 2A Figure 3. A strawberry-like runner of the New Zealand Gunnera monoica: Ideal propagating material and research subject. 2B Figure 2. (A) A vertical section through a stem of G. tinctoria, showing a dark green nodule, the site of the Gunnera-Nostoc symbiosis. (B) A filament of a free-living form of Nostoc with predominantly vegetative cells and a few nitrogenase-containing heterocysts. fertilizer that 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 Figure 4. A flowering specimen of Gunnera prorepens (Courtesy Mark Dean of Naturally Native gunnera had the same green nitrogen- New Zealand Plants Ltd., Tauranga). fixing nodules in the stem of the leaf as G. tinctoria. Nodules in G. tinctoria does not have the spiny leaf margins [http://nzflora.landcareresearch.co.n are a lot larger than those in G. of G. tinctoria, so it is less problematic z/default.aspx] lists five endemic monoica, but relative to the size of to stroke! To be fair, I only ever saw Gunnera species, including the the plant they would be comparable. my plants growing in trays, and it diminutive G. monoica (Figure 3). Gunnera monoica provides a could look good draped over a rock Other sources of information on beautiful model system to study in the garden, with a pond nearby. Gunnera are nitrogen fixation. I would not describe Some of the other Gunnera species [http://www.naturallynative.co.nz/in the plant itself as beautiful (especially appear to be pleasing to the eye dex.html] and when my research was not going well), because of the colour of their fruit, [http://web.kadel.cz/flora/r/kvResult although I would not call it ugly either. e.g. G. prorepens (Figure 4) (cf. G.3 .asp?Rod=Gunnera]. This species has a different habit from monoica, in which the fruit are white), its larger cousins, looking more like a and how they are used in a garden. strawberry plant, with relatively small The Landcare Research New leaves and stoloniferous growth. It Zealand Plant Names Database New Zealand Garden Journal, 2002, Vol.5 (2) 13 A note from the editor Officer (Response) with Auckland Chilean rhubarb, recommended Regional Council, reports that G. alternatives are plantain lily (Hosta), on replacements for tinctoria is a problem in Taranaki, and golden rays (Farfugium japonicum, Chilean rhubarb that this species [or possibly G. formerly Ligularia tussilaginea) and manicata] is also establishing on ornamental rhubarb (Rheum) cultivars. None of the New Zealand endemic Banks Peninsula. See also Darmera peltata (Figure 5). Gunnera can match the magnificent An excellent article, which describes stature of Chilean rhubarb (G. and illustrates the differences between Reference tinctoria), which is a fully naturalised G. tinctoria and G. manicata, was Sykes, W.R. (1969). Gunnera tinctoria exotic plant in New Zealand. Although written for our journal by Bill Skyes and Gunnera manicata. Journal of this species grows wild in only a few (Skyes, 1969). the Royal NZ Institute of Horticulture localities, it is judged to have weedy Auckland Regional Council recently 1:56-59. potential, especially on coastal cliffs, published and released the Plant Me wetlands, slips and roadsides. It Instead booklet. This encourages appears on the National Pest Plant gardeners in the Northern Climatic Paul Stock became interested in Accord List Zone (broadly Northland, Auckland, nitrogen fixation during his MSc northern Waikato, coastal Bay of [http://www.protectnz.org.nz] as a research at Waikato University. He plant that is prohibited from sale, Plenty and East Cape) to look for now works as a Tutor in Cell and propagation and distribution in New alternatives to their favourite, but Plant Biology at Massey University. Zealand. Mike Harré, Biosecurity environmentally pesky plants. For 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 stems [www.backcountryrangers.com/edibles/Saxifragaceae.html]. Photo courtesy of Stonecrop Gardens, Cold Spring, New York, USA. 14 New Zealand Garden Journal, 2002, Vol.5 (2).

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