Introduction:

Peanut (Arachis hypogaea L.) belongs to the pea family, Fabaceae (formerly Leguminosae), subfamily, Papilionoidae, in the more diverged basal clade, Dalbergioid (Wojciechowski et al., 2004; Weller and Ortega, 2015). The plants are usually compact from 30-45 cm tall and 35-75 cm wide with a deep taproot. The roots harbor a symbiotic relationship with the soil bacterium belonging to genus Rhizobium and have the unique ability to convert soil nitrogen into a plant available form. Thus, peanut plants improve soil fertility by reducing their dependence on nitrogen fertilizers, which also results in improved water quality. Peanut is self-fertilizing and exhibits a unique mode of reproduction where flowers are produced on the plant and following fertilization, the pods containing seeds are developed underground. Because of this unique feature, peanut is also known as groundnut in many parts of the world.

Arachis hypogaea is generally recognized as the only domesticated species in the genus and is cultivated for human consumption. The seeds (kernels) contain about 45-55% oil and 20-25% protein (Davis and Dean, 2016; Wang et al., 2022; Dean and Eickholt, 2025). In many parts of the world, they are eaten raw, roasted or salted, or crushed for vegetable oil. However, in the U.S.A., the seeds are mostly crushed for peanut butter or used in the snack industry as roasted/salted nuts and in candies. Although peanut is not a traditional tree nut, compared to all other protein-rich tree nuts, it offers the cheapest and most affordable source of protein, particularly for many in the developing countries who cannot afford animal protein in their daily diets. Additionally, peanuts are a source of several vitamins, minerals and the antioxidant, resveratrol (Dean and Eickholt, 2025). A few other Arachis species have also been reported to have uses for nutrition, forage and ornamental value (Krapovickas and Gregory, 1994; Stalker and Simpson, 1995; Galgaro et al., 1997; Gimenes et al., 2000; Simpson et al., 2001; Krapovickas and Gregory, 2007; Stalker et al., 2013; Stalker et al., 2016; Stalker, 2017; Shahid et al., 2023). For example, seeds of A. villosulicarpa Hoehne and A. stenosperma Krapov. & W. C. Greg., are consumed by the indigenous people in Brazil; the Rhizomatous perennial peanut, A. glabrata, provides high protein forage to ruminants and A. repens and A. pintoi are commonly seen as ornamental ground cover in residential areas and roadsides from S. America to west Africa and China (Mathews et al., 2000; Simpson et al., 2001; Hernandez-Garay et al., 2004; Shahid et al., 2023). Arachis kempff-mercadoi grows in the avenue medians in Santa Cruz, Bolivia and Recife, Brazil as an ornamental (C. Simpson, personal observation). Cultivars derived from A. glabrata are also promoted as a groundcover in orchard farms for ease of maintenance, aesthetics, biological nitrogen fixation ability and as an ecosystem for beneficial insect pollinators (Shahid et al., 2023). Thus, peanut is one of the rare crops to be considered a food, oilseed, forage, and an ornamental crop.

History of Arachis:

Genus Arachis is native to South America with geographical distribution in Argentina, Bolivia, Brazil, Paraguay and Uruguay (Valls et al., 1985). Currently, 84 species have been named and described (Krapovickas and Gregory, 1994 and 2007; Valls and Simpson, 2005; Valls and Simpson, 2017; Cason et al., 2022; Leal-Bertioli et al., 2024; Seijo et al., 2025) and additional descriptions of new species are being compiled (G. J. Seijo, personal communication). Morphologically, the two most ancient species of the genus, A. guaranitica Chodat. and Hassl. and A. tuberosa Bong. Ex Benth., are still found growing in the eroded highlands of southwestern Mato Grosso do Sul, Brazil (Gregory et al., 1980; Simpson and Faries, 2001) suggesting that the genus Arachis likely originated in this region. Ecologically, many Arachis species also grow in deep friable sand to thick, gummy clay and on schist rocks with virtually no soil, to waterlogged conditions, suggesting that they have adapted to highly diverse and harsh environments (Simpson et al., 2001). The genus likely originated in tropical wetland areas, subsequently spread and adapted for survival in dry environments (Gregory and Gregory, 1979; Stalker and Simpson, 1995; Simpson et al., 2001). Consequently, species have evolved to accumulate biotic as well as abiotic stress resistances for survival. These properties make them valuable sources for use in the genetic improvement of A. hypogaea (Stalker, 2017).

All peanut species produce underground pods, botanically known as geocarpy. The geocarpic reproduction is likely an adaptive survival mechanism against adverse environmental stresses (Tan et al., 2010) and likely helped in sustained survivability and distribution of the genus in South America. Further, the different root modifications (e.g., rhizomes, stolons, tuberous roots) likely helped the species to adapt and spread to new habitats. Conversely, the geocarpic fruit also impeded rapid spread into new environments as Simpson et al. (2001) estimated that the species moved about one meter/year across the continent.

Genetic Resources:

The world’s largest collection of peanut germplasm resources is at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), India with about 15,000 cultivated peanut accessions and also 470 accessions of Arachis wild species. In the U.S., the National Plant Germplasm System (NPGS) maintains the peanut germplasm collection at the USDA-ARS Plant Genetic Resources Conservation Unit (PGRCU) in Griffin, GA. It consists of about 9,000 A. hypogaea accessions and an additional 600 accessions of Arachis wild species. Additionally, Argentina, Bolivia, Brazil, China, and India maintain large peanut germplasm collections at their national germplasm centers with smaller collections maintained in many countries in Africa, and Asia. Further, a core and mini core set of accessions have been assembled for ease of working with a smaller, representative sample of lines from the original collections of ICRISAT and USDA. The ICRISAT collection contains 1704 and 184 lines in the core and mini core, respectively (Upadhyaya et al., 2002 and 2003). Similarly, the USDA collection has 831 lines as part of the core set (Holbrook et al., 1993) and the mini core contains 112 lines (Holbrook and Dong, 2005). The world’s largest Arachis wild species collection of about 1200 accessions is housed at EMBRAPA in Brazil under the direction of Dr. Jose Valls (Stalker et al., 2016). A large collection of wild Arachis is also maintained by Dr. Charles Simpson at the Texas A&M AgriLife Research Station in Stephenville, Texas.

Conservation and characterization of germplasm is the most critical step for proper maintenance and utilization of genetic resources. ICRISAT and USDA have developed different peanut descriptors for characterizing the morphological genetic variability observed within the cultivated peanut (IBPGR-ICRISAT, 1992; Pittman, 1995). They include a standard set of several plant, pod and seed traits and occasional data on resistances or other quality traits to help classify the cultivated germplasm into related groups (Wang et al., 2022). The characterization data provides valuable information to researchers to select suitable germplasm for effective and efficient use in peanut improvement. The U. S. peanut collection characterization data including digital images of the various plant, pod and seed features are publicly available to researchers around the world on the Germplasm Resources Information Network Global at https://npgsweb.ars-grin.gov/gringlobal (GRIN Global).

According to Krapovickas and Gregory (1994; 2007), genus Arachis is defined by the morphological features of plants as well as its underground structures of pods, rhizomatous stems, stolons, root systems, and hypocotyls. These defining characters grouped the Arachis collections into different geographic areas and ecological features which, along with crossabilities of species, led them to assemble the collections into nine different taxonomic sections (Krapovickas and Gregory, 1994; 2007). Further, evolution of different genomes occurred independently in different sections such as A, B, D, F, G and K, in section Arachis; C, Caulorhizae; E, Erectoides; EX, Extranervosae; H, Heteranthae; P, Procumbentes; R1 and R2, Rhizomatosae; TR Trierectoides, and T, Triseminatae (Smartt et al., 1978a and Smartt et al., 1978b, Smartt and Stalker, 1982; Seijo et al., 2007; Robledo and Seijo, 2010; Seijo et al., 2014; Stalker, 2017). It is believed that different species originated sympatrically in different sections where isolation barriers developed leading to the geographical separation of the species (Stalker and Simpson, 1995; Seijo et al., 2014). Often two, and as many as five species from four different sections growing sympatrically were observed and collected and in several locations, species of the same section were found growing sympatrically (C. Simpson, personal observations). Section Arachis is the largest in the genus and contains about 40% of the species. The cultivated species, A. hypogaea is a self-fertilizing allotetraploid (2n = 4x = 40; AABB), and belongs to section Arachis. In addition to A. hypogaea, section Arachis also contains another tetraploid species, A. monticola (2n = 4x = 40), 28 diploid (2n = 2x = 20) and three aneuploid (2n = 2x = 18) species (Krapovickas and Gregory, 1994 and 2007; Lavia et al., 2008; Stalker et al., 2013; Stalker et al., 2016; Stalker, 2017). Brazil contains the greatest number of species from all nine sections followed by Bolivia, Paraguay, Argentina and Uruguay.

Although genus Arachis originated in the eroded highlands of Brazil, the primary center of origin of the cultivated species, A. hypogaea, is believed to be southern Bolivia to northwestern Argentina. This observation was based on the presence of the ancestral diploid wild species of A. hypogaea in this region, the wide range of variation observed in pod and seed morphologies and that the germplasm collected in this area exhibited primitive characters associated with wild species, thus supporting the likely origin of A. hypogaea in this region (Hammons,1982; Stalker and Simpson, 1995; Ferguson et al., 2004). Arachis hypogaea is presumed to have originated as a natural hybrid of two section Arachis diploid species, the A genome donor, A. duranensis Krapov. & W.C. Greg.and A. ipaёnsis Krapov. & W.C. Greg. contributing the B genome (Kochert et al., 1996; Seijo et al., 2004 and 2007; Bertioli et al., 2016). Following the hybridization, a single polyploidization event of the sterile diploid hybrid led to the fertile allotetraploid (AABB). Then, the early humans selected desirable types from this and later populations, possibly for compact plant habit, and increased pod and seed sizes leading to the present day cultivated species of A. hypogaea. Further domestication in varied geographical environments led to the different subspecies, botanical varieties and market types of the cultivated taxon. Pod size, color, number and size of seeds per pod vary in different market types of A. hypogaea (Upadhyaya, 2003; Stalker et al., 2016; Wang et al., 2022). Consequently, the vast amount of morphological variability observed in the cultivated taxon likely resulted from natural and/or artificial selection rather than from the introgression of genes from different species (Seijo et al., 2007). Further, Krapovickas (1968) and Gregory and Gregory (1976) recognized six other regions in South America as the secondary centers of diversity for the cultivated species based on morphological variability of the landraces. Additionally, Africa, China and India are considered as tertiary centers of diversity for A. hypogaea because of the large number of landraces and other local germplasm displaying different pod and seed traits (Gibbons et al., 1972). Interestingly, Simpson et al. (2001) suggested possible alternate regions for the origin of A. hypogaea on the west coast of Peru and/or the eastern slopes of Cordillera in the Andes, based on archaeological evidence and the favorable environmental conditions for survival of plant tissue for long periods.

The Spanish and Portuguese explorations to South America led to the geographical spread of cultivated peanut to Europe, then to Africa and Asia via trade voyages. There was no substantiated evidence for the occurrence of cultivated peanut in North America before the Spanish arrival on the continent. It was suggested that peanut was introduced into the U.S.A. on slave trade ships from Africa via the coast of northeastern Brazil, where peanut was gathered as food source to complete the journey, strongly suggesting that the first peanut introductions into the U.S.A. were from Brazil rather than from Africa (Stalker and Simpson, 1995; Tallury, 2017). Further, Williams (2022) provided a detailed history and dissemination of peanut from its centers of origin and diversity in South America to Europe, Africa and Asia.

Botanical classification of A. hypogaea:

Krapovickas and Rigoni (1960) classified A. hypogaea into two subspecies, subsp. hypogaea and subsp. fastigiata, mainly, on the presence or absence of flowers on the main stem and the sequence of floral and leaf nodes on the lateral branches (Figure 1). The subsp. hypogaea contains no flowers on the main stem with alternate vegetative and floral nodes (two vegetative nodes alternate with two floral nodes) on the lateral branches and a long-life cycle. The subsp. fastigiata is characterized by the presence of flowers on main stem with sequential order of vegetative and floral nodes on the lateral branches and shorter life cycle. They also proposed two botanical varieties of subsp. fastigiata, var. fastigiata and var. vulgaris based on pod traits. Later, Krapovickas (1968) proposed that subsp. hypogaea should also be divided into var. hypogaea and var. hirsuta based on pod reticulation. With additional collections of A. hypogaea, Krapovickas and Gregory (1994; 2007) not only confirmed the two subspecies of A. hypogaea (subsp. hypogaea and subsp. fastigiata) but also expanded botanical varietal groups to six (vars. hypogaea, hirsuta, fastigiata, peruviana, aequatoriana and vulgaris) based on plant growth habit, leaf color and branching patterns, which also includes the four major market types grown in the U.S.A. (Table 1; Figure 2).

Figure 1. A) A. hypogaea subsp. hypogaea mainstem flower absent; B) A. hypogaea subsp. fastigiata mainstem flower present.

Figure 2. Pod and seed characteristics of A. hypogaea botanical varietal groups (cm). Subsp. hypogaea var. hypogaea: A) Market type: Virginia (PI 536122), B) Market type: Runner (PI 497455); C) var. hirsuta, Market type: Peruviana runner (PI 576638). Subsp. fastigiata var. fastigiata: D) Market type: Valencia (PI 493324); E) var. aequatoriana (PI 690056); F) var. peruviana (PI 502053); G) var. vulgaris, Market type: Spanish (PI 537448).

Table 1

Arachis hypogaea taxonomic classification.

Krapovickas (1968) suggested that A. hypogaea subsp. hypogaea var. hypogaea was the most ancient type as it was the most predominant type found in the chaco region between southern Bolivia and northwestern Argentina, which is where the ancestral species of A. duranensis and A. ipaёnsis were found and A. hypogaea was believed to have originated. Additionally, the plants found in this area exhibited many primitive traits such as the runner growth habit, a branching pattern similar to the wild Arachis species, small, two-seeded pods with marked constriction and slight reticulation, and seed dormancy. Further, the above observations led Krapovickas and Gregory (1994; 2007) to conclude that SE Bolivia is the center of origin as well as diversity for subsp. hypogaea, whereas subsp. fastigiata differentiated in NW Bolivia and possibly in Peru, along with vars. fastigiata, peruviana and aequatoriana. However, investigation of genetic diversity among botanical varieties using simple sequence repeat (SSR) markers by Ferguson et al. (2004) revealed the similarities of three botanical varieties of subsp. fastigiata, namely fastigiata, vulgaris and aequatoriana but did not support the inclusion of var. peruviana in subsp fastigiata. Further, they also found that the botanical varieties, hypogaea and hirsuta are not closely related and felt that they should not be grouped under subsp. hypogaea. Contrarily, He and Prakash (2001) demonstrated with AFLP markers that vars. aequatoriana and peruviana were closer to subsp. hypogaea than to subspecies fastigiata. Interestingly, Grabiele et al. (2012) suggested that the six botanical varieties originated from a single genetic origin and that A. monticola is the immediate ancestor of A. hypogaea. Thus, there still exists, confusion about the taxonomic classification of the cultivated species. Among the market types, Gregory et al. (1980) and Hammons (1982) suggested that the Bolivian and Amazonian geographic regions are the possible sites for the origin of the large-seeded Virginia types. Further, Hammons (1982) indicated that the Guarani area of northeastern Argentina, Paraguay and southern Brazil is the center of variation for the Spanish (var. vulgaris) market type whereas, the Valencia type (var. fastigiata) probably spread from Paraguay and central Brazil (Krapovickas, 1968; Hammons, 1982).

Morphology, Growth and Development:

Flower:

The flowers are formed on an inflorescence in leaf axils on the branches and also on the main stem in subspecies fastigiata types (Figure 1). The inflorescence is a raceme and usually contains three to five flowers, but as many as 13 have been observed on one inflorescence (C. Simpson, personal observation). Usually, plants start producing flowers about 30 days after seed germination and due to peanut’s indeterminate growth habit, flowers are produced throughout the growing season until harvest. Generally, only one flower opens on a given day in each inflorescence and the interval between the openings of flowers within the same inflorescence vary up to several days. However, it is not uncommon to see two flowers at a node. Because the flowers contain both male and female tissues, natural self-fertilization occurs, leading to the development of pods. The flowers are usually orange, reddish orange or yellow in color. Sometimes, white flowers have been seen in at least three A. hypogaea accessions and in seven different wild species (C. Simpson, personal observations.) The flower contains five petals (corolla) including a large standard (Banner), two wing petals and two fused keel petals. The calyx is green with five lobes with four fused to cover the back side of the standard and one lobe is opposite the keel (Figure 3). The standard is usually yellow or orange with red veins on the inner face. The wings are usually yellow surrounding the keel. The keel is almost colorless and encloses the stamens and style. The androecium is monadelphous with filaments of stamens fused into a bundle with eight functional stamens and two, small sterile ones. The stamens contain pollen to fertilize the egg cell. Although the flower is sessile, it is attached to the stem (at the leaf axil) by a long tubelike structure called a hypanthium or “calyx tube” and thus appears as pedicillate (Figure 3). The style is enclosed within the hypanthium and is connected to the ovary located at the base of the hypanthium in the leaf axil. The tip of the style, called stigma, is usually at the same height as the anthers so pollen reaches it easily (Figure 3). Differences in stigma morphology were noticed between A. hypogaea and the wild species. In A. hypogaea, the stigma is of dry papillate type (Lakshmi and Shivanna, 1986) with no surrounding hairs and probably accommodates about 15 pollen grains (Moss and Rao, 1995). On the other hand, the annual Arachis species have large, globular stigmatic surface whereas the perennial species have smaller, cuticularized stigmas with unicellular hairs accommodating a maximum of only three pollen grains (Lu et al.,1990; Akromah, 2001). In the wild species A. lignosa, Banks (1990), observed that natural self-pollination is restricted because of the truncated shape of the stigma and its elevated position relative to the anthers and suggested manual tripping of flowers for pollen to reach the stigma for fertilization and later pod development. Although self-pollination is the predominant mode of reproduction, outcrossing is possible with bees or other pollinators. It was reported that the outcrossing rate is limited to less than 10% under natural field conditions (Hammons,1973; Knauft et al., 1992).

Figure 3. A. hypogaea floral and reproductive structures.

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