Gunnera is a genus comprising approximately 63 species of large, herbaceous, perennial flowering plants in the monogeneric family Gunneraceae, characterized by their often massive, palmately lobed leaves that can exceed 2 meters in diameter in some species, and a unique symbiotic association with nitrogen-fixing cyanobacteria of the genusNostoc that enables growth in nutrient-poor, wet environments.[1][2][3]Native predominantly to the Southern Hemisphere, including regions from Mexico through Central and South America (such as Brazil, Colombia, and Chile), as well as southern Africa, Madagascar, the Hawaiian Islands, New Zealand, and parts of Southeast Asia and Australia, species of Gunnera typically inhabit moist, boggy soils near streams, wetlands, and forest margins, where their rhizomatous growth forms dense clumps.[1][4] The plants are dioecious or polygamomonoecious, producing small, unisexual or bisexual flowers in large, terminal panicles or spikes that develop into drupaceous fruits, with leaves featuring stiff prickles on the petioles and undersides for protection.[2][4]Notable for their dramatic ornamental value, several Gunnera species, such as G. manicata and G. tinctoria, are cultivated in temperate gardens for their architectural foliage, though they can become invasive in introduced regions like western Europe and the Pacific Northwest due to prolific seeding and rhizomatous spread.[5][6] The cyanobacterial symbiosis, established via specialized stem glands, provides fixed nitrogen to the host plant, making Gunnera the only known angiosperm capable of hosting such intracellular partners, a trait that underscores its evolutionary distinctiveness within the core eudicots.[3][2]
Taxonomy and Evolution
Classification and Etymology
Gunnera is the sole genus within the family Gunneraceae, which is placed in the order Gunnerales among the core eudicots according to the APG IV classification system.[7] This monogeneric family highlights the unique position of Gunnera in angiosperm taxonomy, distinguished by its herbaceous habit and symbiotic associations.[8]The genus Gunnera was established by Carl Linnaeus in 1767, named in honor of the Norwegian botanist and bishop Johann Ernst Gunnerus (1718–1773), who contributed to early studies of Scandinavian flora.[9] Initially described based on material from southern Chile, the name reflects Linnaeus's practice of honoring contemporary naturalists.[10]Within Gunnera, species are organized into several subgenera based on morphological and geographic distinctions: Gunnera, comprising rhizomatous species primarily from South America; Ostenigunnera, comprising small-statured species from South America; Pseudogunnera, found in New Zealand and Australia; Milligania, restricted to Australia; Panke, featuring the giant-leaved species primarily from South America; and Misandra, encompassing the African taxa.[11] These divisions, proposed in the late 19th and early 20th centuries, aid in understanding the genus's diversity.[12]Historically, Gunnera's taxonomic placement underwent revisions, with the family Gunneraceae formally recognized as distinct in the 19th century, notably by Carl Friedrich Meisner in 1842, separating it from earlier associations with families like Haloragaceae.[7] This elevation to family status was supported by anatomical and floral differences, solidifying its independent lineage.[13]
Phylogenetic Relationships
Gunnera belongs to the order Gunnerales, which comprises two monotypic families: Gunneraceae (Gunnera) and Myrothamnaceae (Myrothamnus), recognized as sister taxa based on molecular and morphological evidence.[14] In the APG IV classification, Gunnerales occupies a basal position within the core eudicots, sister to the remaining core eudicot lineages, including the Pentapetalae clade that encompasses superrosids and superasterids.[14] This placement highlights Gunnerales as one of the earliest diverging orders in the core eudicots, distinct from more derived groups like rosids and asterids.Early molecular phylogenies established this basal eudicot position through analyses of chloroplast rbcL and nuclear 18S rDNA sequences, which resolved Gunnerales as sister to other core eudicots with strong support. These studies, incorporating multiple genes such as atpB alongside rbcL and 18S, confirmed the order's divergence near the base of the eudicot radiation, separate from earlier proposals linking it to rosid or other lineages. Prior to molecular data, Gunnera was often classified near Saxifragaceae due to shared herbaceous habits and leaf features, or tentatively allied with Rosaceae in broader rosid assemblies, reflecting uncertainties in pre-cladistic taxonomy.[15]Within Gunneraceae, Gunnera is monophyletic, with its traditional subgenera (such as subg. Gunnera, subg. Panke, and subg. Milligania) supported as distinct clades by sequence data from the nuclear ribosomal ITS region, chloroplast rbcL, and rps16 intron. Subsequent analyses incorporating matK chloroplast gene sequences have reinforced these subgeneric relationships, showing consistent resolution of intraspecific diversification across the genus's 63 species.[16][1] This molecular framework underscores Gunnera's cohesive evolution as a single genus, with subgenera reflecting adaptations to diverse habitats from temperate to tropical regions.
Evolutionary History
The genus Gunnera is estimated to have originated in Gondwana during the Late Cretaceous, approximately 90 million years ago, marking the beginning of its diversification across southern landmasses. This timeline aligns with the early fragmentation of Gondwana, which facilitated the initial spread of the lineage through vicariance and subsequent dispersal events. Phylogenetic analyses calibrated with fossil data place the crown age of Gunnera at around 90 Ma, with a 95% highest posterior density interval spanning 165–40 Ma, underscoring its deep evolutionary roots in a period of continental reconfiguration.[17]Fossil records provide key evidence of Gunnera's ancient Southern Hemisphere distribution, predominantly through pollen grains that reveal its presence in diverse paleoenvironments. The earliest confirmed fossils include Tricolpites reticulatus pollen from the Turonian stage (~90 Ma) in Peru, representing the genus's foothold in South America during the Cretaceous. Additional Cretaceous pollen and macrofossils occur in Antarctica, Australia, and broader North and South American deposits, illustrating a once-widespread range before climatic shifts restricted it. In Patagonia, Gunnera pollen appears in the Paleocene Salamanca Formation (~66–61 Ma), signaling continuity into the early Cenozoic amid humid coastal lowlands. Further north in South America, Middle Pliocene records from Colombia document later persistence. In New Zealand, Gunnera pollen is documented in Miocene assemblages (~23–5 Ma), associated with temperate, high-rainfall forests during a period of climatic variability.[17][18][19]Post-Gondwana breakup, Gunnera experienced adaptive radiations tied to the emergence of wet, temperate climates across isolated southern continents, enabling ecological specialization in moist habitats. These events, spanning the Paleogene and Neogene, allowed the genus to exploit niches in fragmented landscapes, though its overall distribution contracted due to increasing aridity and seasonality. Relictual species, such as G. perpensa in Madagascar, exemplify this history, serving as living remnants of Gondwanan lineages isolated by continental drift and long-distance dispersal limitations.[17][20]
Morphology and Description
Vegetative Characteristics
Gunnera species exhibit perennial herbaceous growth, forming either rhizomatous clumps with creeping underground stems or rosulate rosettes of basal leaves emerging directly from the crown. These plants range dramatically in stature across the genus, with diminutive species such as G. magellanica reaching heights of approximately 10 cm, while giants like G. manicata can attain 2.5 m or more in optimal conditions.[21][5][22]The leaves are predominantly basal and arise in a tufted arrangement near the apex of the rhizome, displaying orbicular to reniform shapes with palmate venation. Laminae are simple to deeply lobed, with crenate to dentate margins, and can vary from a few centimeters in smaller species to exceptionally large dimensions, up to 3 m in diameter in members of subgenus Panke. Veins on the leaf undersurface and petioles often bear reddish prickles or stiff hairs that provide protection against herbivores.[21][15][23][24][25]Stems are typically absent or greatly reduced, with the plant's architecture supported by short, succulent petioles that equal or exceed the length of the lamina; these petioles frequently feature prickly margins in larger species and bear specialized glands at their bases that facilitate entry of symbiotic cyanobacteria.[21][15][26]Root systems are fibrous, with numerous root hairs aiding anchorage and nutrient uptake in consistently moist environments.[15][27]
Reproductive Structures
Gunnera species produce large, conspicuous inflorescences that are typically terminal panicles of spikes arising from the rhizome or axils of distal leaves, reaching heights of 0.5 to 2 meters in larger species such as G. manicata.[2][28] These inflorescences are scapiflorous, ebracteate, and pseudo-terminal or axillary, with flowers aggregated in simple or compound spikes or racemes.[28] Most species are monoecious, bearing unisexual flowers on the same inflorescence, with pistillate flowers proximally, staminate distally, and sometimes bisexual in between; however, some species exhibit dioecy with separate male and female inflorescences.[2][29]The flowers are small, inconspicuous, and sessile or pedicellate, lacking petals (apetalous) and typically featuring a perigynous perianth with two valvate sepals that may be vestigial.[2][28]Male flowers possess two (rarely one) stamens with short filaments, basifixed extrorse anthers, and 3–5-aperturate, colpate, binucleate pollen grains.[2]Female flowers have a syncarpous gynoecium of two carpels forming an inferior, unilocular ovary with apical placentation, two free terminal styles, and a single anatropous, bitegmic, crassinucellate ovule.[2][28]Pollination in Gunnera is primarily anemophilous (wind-mediated), facilitated by the exposed, elongated inflorescences and lightweight pollen, though occasional insect visitation has been observed in some New Zealand populations.[30][31][24]Following pollination, the ovary develops into a small, indehiscent drupe, typically 1.5–2 mm in diameter and orange when mature, containing a single seed with oily endosperm and a straight embryo.[2][28][24] Seed dispersal occurs mainly via hydrochory (water) or ornithochory (birds), with fruits capable of floating and persisting in soil seed banks; a single plant can produce up to 250,000 seeds annually.[24]Seeds exhibit high viability, with germination rates exceeding 70% within the first year under natural conditions, forming large persistent seed banks of up to 30,000 seedlings per square meter.[24][32] Optimal germination requires moist, shaded environments, often in disturbed or riparian soils, with viability declining after one to two years but remaining viable in buried banks for longer periods.[24][33]
Distribution and Ecology
Geographic Range
Gunnera species exhibit a predominantly Southern Hemisphere distribution, with the vast majority concentrated in Latin America, where approximately 45 species occur from the Andes through to Patagonia. This region serves as the primary center of diversity for the genus, encompassing a wide latitudinal range from Mexico and Central America southward to the southern tip of South America. Native species also occur in tropical and subtropical regions of the Northern Hemisphere, including Mexico, the Hawaiian Islands (two species), and parts of Southeast Asia (Malesia, several species). The disjunct patterns across southern continents and into these northern tropical areas underscore the Gondwanan heritage of Gunnera, originating from ancient continental connections.[1][34]In addition to Latin America, smaller numbers of species are native to other Southern Hemisphere landmasses, including three in New Zealand, one in Tasmania, Australia, and seven in Africa, spanning South Africa and Madagascar. These isolated occurrences highlight the relictual nature of the genus's range, shaped by vicariance following the breakup of Gondwana. Endemism is notably high in hotspots such as Chile, where many species are confined to montane and coastal areas of the Andes, and New Zealand, with all three species endemic to the archipelago.[1][34]Beyond native ranges, certain species have been introduced to new areas, primarily through horticultural trade. Gunnera manicata, native to Brazil, and G. tinctoria, from southern Chile and Argentina, have established populations in Europe—including the United Kingdom and Ireland—and in New Zealand, where they sometimes form invasive stands. These introductions do not alter the genus's fundamentally Southern Hemisphere-centric native distribution.[35][36][37]
Habitat Preferences and Ecological Role
Gunnera species typically occupy wet, boggy, or streamside habitats, with many exhibiting semi-aquatic or amphibious adaptations, thriving on the banks of rivers, streams, coastal cliffs, and wetland margins, particularly in their South American ranges. Species in temperate regions favor cool climates with high rainfall, often exceeding 2000 mm annually, and temperatures suited to USDA hardiness zones 7-9, with tolerance to elevations from sea level to 2000 m; however, preferences vary across the genus, including subtropical conditions in areas like Hawaii and Southeast Asia. As shade-loving understory perennials, Gunnera species are commonly found in canopy gaps and forest edges of temperate-humid rainforests, particularly on nutrient-poor, alluvial, or volcanic soils.[24][34][6]Ecologically, Gunnera plays a key role as a pioneer in wetlandsuccession, aggressively colonizing disturbed montane sites such as landslides and glacial forelands to initiate ecosystem recovery in moist environments.[34] Its robust root systems and expansive foliage may contribute to soil stabilization on unstable, wet terrains like colluvial slopes and riverbanks, though evidence is limited.[24][34] Furthermore, the plant's large rhizomes create sheltered microhabitats that support invertebrate communities, including mollusks and annelids, enhancing local biodiversity in boggy areas.[24]In native interactions, Gunnera demonstrates herbivory resistance through dense coverings of glandular trichomes and prickles on leaves and petioles, which deter browsing by insects and larger herbivores.[24] It also engages in competition with ferns, notably tree ferns, for light and nutrients within the shaded understories of Andean Yungas forests and similar habitats.[24]
Species Diversity
Number and Subgeneric Classification
The genus Gunnera comprises 63 accepted species as recognized in the most recent taxonomic assessments.[1] This count reflects ongoing revisions, with potential for additional species as molecular and morphological studies refine boundaries, particularly in undercollected regions of South America.[1]Traditionally, Gunnera is divided into six subgenera—Gunnera, Milligania, Misandra, Panke, Pseudogunnera, and Ostenigunnera—delimited by combinations of morphological traits such as leaf size, habit, and reproductive structures, corroborated by DNA sequence data from nuclear and chloroplast markers.[11][38] Subgenus Panke is the largest, encompassing over 40 species with characteristically large, peltate leaves and a robust rhizomatous habit, primarily distributed in the Americas.[34] In contrast, subgenus Milligania includes smaller-leaved, stoloniferous species, numbering six, confined to Australasia including Australia, New Zealand, and Tasmania.[39] The remaining subgenera (Misandra, Pseudogunnera, Ostenigunnera, and nominate Gunnera) each contain fewer species, often with specialized habits like diminutive rosettes or insular endemism, and are defined by features such as pollen morphology and inflorescence structure.[11]Taxonomic history includes the merger of former genera into Gunnera, resolving earlier synonymy debates; for instance, Panke Molina and Milligania Hook.f. were elevated as subgenera following phylogenetic analyses that demonstrated monophyly within the broader genus.[1] These reclassifications, supported by evidence from the rbcL gene and ITS regions, highlight ongoing debates over species limits in morphologically variable groups like Panke.[38]Speciation patterns in Gunnera are predominantly allopatric, arising from geographic isolation via vicariance during the Gondwanan breakup, which fragmented ancestral populations across southern continents, supplemented by long-distance dispersal events to isolated sites like Hawaii.[40] This vicariance-driven diversification accounts for the genus's disjunct distribution and subgeneric radiations.[40]
Notable and Endemic Species
Gunnera manicata, native to the Serra do Mar mountains in southeastern Brazil, stands out as one of the most striking species in the genus due to its enormous, orbicular to reniform leaves, which can attain diameters of up to 2 meters with rounded lobes and an overlapping basal sinus. This pachycaul perennial produces diffuse inflorescences with branches measuring 11-20 cm, and it belongs to subgenus Panke, characterized by its robust growth in humid, montane forests. Its colossal foliage provides significant ecological cover in native habitats, supporting understorybiodiversity through shading and moisture retention.[41]Gunnera tinctoria, also in subgenus Panke, is endemic to the Andean regions of southern Chile and southwestern Argentina, including Chiloé Island, where it thrives in moist, shaded valleys and stream banks. The species features large, irregularly outlined leaves up to 2 meters in diameter, supported by prickly petioles, and compact, cylindrical inflorescences with branches typically under 8 cm long. Known historically as G. chilensis, it plays a key role in stabilizing wetland soils and fostering symbiotic nitrogen fixation in nutrient-poor Andean environments.[42]Gunnera perpensa represents the sole African member of the genus and the smallest in stature among its prominent species, forming erect, rhizomatous colonies up to 1 meter tall in wetland margins.[43] Native to eastern tropical Africa and South Africa, it bears robust, reniform to circular leaves, 40-250 mm wide, with irregularly dentate margins and palmate venation, adapted to muddy, waterlogged soils.[43] Its compact spikes, 2-7 cm long, emerge on scapes from November to February, highlighting its adaptation to high-altitude, seasonally wet habitats above 1500 meters.[44]Among endemic species, Gunnera magellanica is a diminutive, dioecious perennial restricted to Patagonia in Chile and Argentina, as well as the Falkland Islands and higher Andean slopes in Peru and Ecuador.[45] This creeping herb, reaching only 5-20 cm in height, produces spineless, kidney-shaped leaves on branching rhizomes and small, red fruits in late summer, occupying alpine bogs and aquatic edges above the timberline.[46]Hybrids such as Gunnera × cryptica (G. manicata × G. tinctoria), first noted in Europeancultivation around 1873, exhibit intermediate morphology with leaves up to 2 meters across and inflorescence branches around 11 cm, demonstrating the close genetic affinity within subgenus Panke. This hybrid's prevalence underscores the potential for interspecific gene flow in shared ornamental settings, blending traits from its Brazilian and Chilean progenitors.
The symbiotic association between Gunnera species and cyanobacteria of the genus Nostoc, particularly N. punctiforme, is initiated when motile hormogonia from environmental Nostoc filaments are attracted to mucilage-secreting glands located at the bases of the leaf petioles. These glands, which develop on the stems near petiole attachments, exude a viscous mucilage containing hormogonium-inducing factors (HIFs), small heat-labile proteins that stimulate hormogonia differentiation and positive chemotaxis in compatible Nostoc strains. The infection process occurs primarily during early developmental stages of the plant, such as in young leaf primordia or buds, where the glands are most receptive.[47][48]Once attracted, the hormogonia glide along extracellular channels lined by the gland cells, penetrating the plant tissue and establishing residence intracellularly within specialized gland cells in the stem. This intracellular housing confines the cyanobacteria to membrane-bound compartments surrounded by a protective, carbohydrate-rich mucilage matrix, preventing dispersal while allowing controlled exchange. The symbiosis is transmitted horizontally, with Nostoc acquired anew from the surrounding environment by each host generation, rather than being inherited vertically through seeds or propagules.[47][49]Maintenance of the association relies on reciprocal provisions: the Gunnera host supplies carbohydrates, including sucrose and glucose derived from photosynthesis, to fuel cyanobacterial metabolism, while the intracellular gland environment shields the oxygen-sensitive symbionts from atmospheric exposure that could inhibit their processes. This selective housing mechanism ensures long-term stability within the gland tissues, with Nostoc filaments persisting for the lifespan of the host cells.[47][50]
Nitrogen Fixation and Benefits
In the Gunnera-Nostoc symbiosis, nitrogen-fixing cyanobacteria of the genusNostoc utilize the enzymenitrogenase to convert atmospheric dinitrogen (N₂) into ammonia (NH₃), which is then available for assimilation by the host plant.[51] This process occurs primarily within specialized cells called heterocysts, which Nostoc differentiates into at a high frequency—up to 60% of cyanobacterial cells in the symbiotic state, compared to about 10% in free-living conditions—providing protection from oxygen inactivation of the oxygen-sensitive nitrogenase.[52] These Nostoc strains are specifically adapted to the Gunnera environment, enabling efficient intracellular nitrogen fixation following entry through stem glands.[3]Experimental evidence for nitrogen fixation activity in this symbiosis has been demonstrated through acetylene reduction assays, which measure nitrogenase function by quantifying the conversion of acetylene (C₂H₂) to ethylene (C₂H₄) as a proxy, indicating robust nitrogenase activity under symbiotic conditions.[51] In field studies, the symbiosis can contribute substantial fixed nitrogen to the ecosystem, with species such as Gunnera magellanica achieving potential maximal rates of 300 kg N per hectare per year, though contributions vary by species and habitat.[53]The primary benefits of this nitrogen fixation accrue to the Gunnera host, enabling enhanced growth and development in nitrogen-poor environments such as wetlands and infertile soils where combined nitrogen is scarce. Nostoc also produces bioactive compounds, including phytohormones, that further promote host growth and productivity. By rendering the plant independent of external nitrogen sources, the symbiosis facilitates Gunnera's colonization of otherwise unproductive habitats, supporting vigorous vegetative expansion and overall productivity.[3][51][47]
Cultivation and Uses
Ornamental and Culinary Applications
Gunnera manicata is highly prized in ornamental gardening for its bold, dramatic foliage, which can reach up to 8 feet (2.4 meters) across, creating a striking tropical or prehistoric effect in landscape designs.[54] It is commonly planted in bog gardens, pond margins, or moist borders where its large, umbrella-like leaves provide architectural interest and help control erosion on slopes.[55][56]Cultivation of Gunnera species requires consistently moist, fertile, humus-rich soil in full sun to partial shade, with propagation typically achieved through division of the crown in early spring or, less commonly, by sowing seeds in warm conditions.[54][56] These plants thrive in USDA hardiness zones 7 to 10, where summers are cool and moist, but they demand protection from frost by mulching the crowns with leaves or dry material in autumn to prevent winter damage.[54][55]In Chilean cuisine, the young petioles of Gunnera tinctoria, known locally as nalca or pangue, are harvested and consumed raw in salads, dipped in salt or chili for flavor, or cooked as a vegetable with a refreshing, slightly acidic taste due to their tannin content.[57][58] The fruits of G. tinctoria have been traditionally eaten fresh by indigenous communities such as the Yaganes and Araucano peoples, and both stems and fruits are occasionally processed into juices, syrups, or desserts.Historically, the roots of G. tinctoria, rich in tannins, have been used by South American indigenous groups to produce a high-quality black dye for wool and leather, as noted in early accounts from the region.[6][58] In traditional South American remedies, decoctions of the roots or stems serve as an astringent for treating diarrhea, uterine pain, and inflammation, while the plant's hemostatic and febrifuge properties have been applied for wound care and fever reduction.[57][58]
Invasiveness and Conservation
Certain species within the genusGunnera, particularly G. tinctoria and G. manicata, have become invasive in regions outside their native South American ranges, including parts of the United Kingdom, Ireland, New Zealand, Australia, and the western United States.[59] These plants form dense colonies through vegetative spread via rhizomes and seed dispersal by birds and water, outcompeting native vegetation by shading and suppressing growth.[60] In the UK, G. tinctoria has been banned from sale and cultivation since 2017, and following a 2023 genetic study revealing that cultivated G. manicata is actually the fertile hybridG. × cryptica (G. manicata × G. tinctoria), it was added to the ban in 2024 under the EU Invasive Alien Species Regulation, prohibiting sale, propagation, and planting in the wild.[61][55] Management efforts often involve manual removal, herbicide application, and public awareness campaigns, as the plants' large size and persistence make eradication challenging.[62]In contrast, several Gunnera species face conservation challenges in their native habitats due to habitat loss, overharvesting, and limited distributions. Gunnera hamiltonii, endemic to New Zealand's Stewart Island, is classified as Nationally Critical under the New Zealand Threat Classification System, with threats including predation by introduced animals and habitat alteration in coastal dunes.[63] Conservation actions for this species include population monitoring by the Department of Conservation and propagation in community nurseries to support reintroduction.[64] Similarly, G. aequatoriensis in Ecuador is assessed as Vulnerable on the IUCN Red List, primarily due to its restriction to three known populations in montane forests vulnerable to deforestation and agricultural expansion.[65]Hawaiian endemics such as G. kauaiensis and G. petaloidea are considered imperiled, with NatureServe global ranks of G2 (imperiled) and G2 (imperiled), respectively, owing to their confinement to specific wetland and bog habitats on limited islands, exacerbated by invasive species and development.[66][67] Efforts to conserve these species involve habitat protection within national parks and ex situ propagation by botanical gardens, though ongoing threats from climate change and habitat fragmentation persist.[68] In South Africa, G. perpensa populations are declining due to overharvesting for traditional medicine, despite an overall Least Concern status on the national Red List, prompting calls for sustainable harvesting guidelines.[69][70]