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Solanum acaule

Common Name(s) Apharu papa, Apharuma, Arakk papa, Atokk papa, Aya papa, Papa de Perdiz, Jupaypa papan, Khapharu ch’ogue, Machocc papa, Shiri, Shoco papa
Tubers of the wild potato species Solanum acaule (average size)
Solanum acaule tubers
 
Code acl
Synonyms S. depexum, S. punae, S.schreiteri, S. uyunense
Clade Mixed
Series Acaulia
Ploidy Tetraploid (4x)
EBN 2
Tuberization Photoperiod
Short day
Self-compatible Yes
Genome
AAAaAa
Cytoplasm Type M (Hosaka 2009, as C)
Citation
Bitter: Repert. Spec. Nov. Regni Veg. 11: 391. 1912.

Description

Solanum acaule distribution map
Solanum acaule distribution map

Solanum acaule (acaule means “without stem”) is a wild potato species that ranges from central Peru through Bolivia and into northern Argentina.  There may also be a population in Ecuador although the identity of this population is contested.  This is the greatest natural distribution of any of the wild potato species (Ochoa 1990).  This species grows largely at high altitudes, ranging from about 7900 to 15400 feet (2400 to 4700 m). A large part of its natural range is in the Puna, a highland region of Peru and Bolivia where the annual average temperature is only about 45 F (7 C).  In the wild, it acts as a ground-cover plant, growing in large mats, and survives at higher elevations than any other potato species (Ochoa 2004).

The growth habit of S. acaule can change significantly depending on climate (Ochoa 2004).  The plants have a low growing habit, usually not exceeding eight inches in height in cold, high elevation conditions.  They may form much longer and taller stems in warmer climates.   Plants occasionally have no noticeable aerial stem at all, just a flat rosette growing against the ground.  Flower color ranges from blue to violet or is occasionally white. Unlike most potatoes, S. acaule lacks a floral abcission zone (or flower joint). While this is not a trait of much importance to the breeder, it may be helpful in identifying mixed progeny. Stolons are typically two to three feet long.  Tubers are typically round, white, and 1/2 to 1 inch in diameter.

S. acaule is most likely an allotetraploid (more specifically, an amphidiploid) as it has disomic inheritance.  This means that the chromosomes segregate in pairs, dividing two ways as if the plant were a diploid. This is an indication that it probably originated as a hybrid of two different diploid species and has retained their independently segregating genomes. This probably explains why it is self-compatible when most potato species are not.

S. acaule has historically been assigned a number of subspecies, including ssp. acaule, ssp. aemulans, ssp. albicans, and ssp. punae. Some of these are no longer recognized or have been raised to species.  Most populations of this species are tetraploid (4x).  Hexaploid (6x) populations of S. acaule have been reported, but there is some doubt about whether they are actually S. albicans, a similar and closely related species that is normally hexaploid.

It is often noted that S. acaule bears both morphological and genetic similarities to S. demissum, which is a North American species that appears to have arisen from a similar evolutionary process.  They are both allotetraploids, although from a different combination of species.

Subramanian (2017) found that at least some accessions of this species have unusually high dry matter content.

Resistances

S. acaule can have impressive frost tolerance; resistance to viruses PVX, PVY, PLRV, and PSTV; and potato cyst and root knot nematode resistance.  A resistance gene for Potato Virus X, Rx2, has been identified in this species.

This species can resist frosts down to 22 degrees F (-5.5 C) (Sukumaran 1972) or perhaps even down to 20 degrees (-6.5 C) (Li 1977). Vega (1995) found that 100% of 351 accessions of this species survived through two light frosts with no damage.  The rosette-like growing habit of this species keeps the plant close to the ground and probably aids in tolerance to low temperatures, but this is not a sufficient explanation. Chen (1977) found three structural differences between S. acaule and S. tuberosum that may account for its frost resistance. It has significantly thicker cell walls than S. tuberosum. Its chloroplasts have a much larger number of osmiophylic globuli and these globuli increase in number following frost exposure. The chloroplasts also become depleted of starch following frost exposure. This suggests the conversion of starch to soluble sugars, a common adaption to reduce the freezing point of plant cells. Taken together, these features suggest that S. acaule may be able to more effectively delay, survive, and recover from frost induced dehydration.

Condition Type Level of Resistance Source
Alternaria solani (Early Blight) Fungus Resistant Prasad 1980
Alternaria solani (Early Blight) Fungus Somewhat resistant Jansky 2008
Clavibacter michiganensis (Ring Rot) Bacteria Somewhat resistant Laurilla 2003
Drought Abiotic Somewhat resistant Machida-Hirano 2015
Frost Abiotic 21 to 23 F Chen 1977
Globodera pallida (Pale Cyst Nematode) Invertebrate Somewhat resistant Jackson 1988, Bachmann-Pfabe 2019, Castelli 2003, Machida-Hirano 2015
Globodera rostochiensis (Potato Cyst/Golden Nematode) Invertebrate Somewhat resistant Castelli 2003
Heat Abiotic Somewhat resistant Machida-Hirano 2015
Leptinotarsa decemlineata (Colorado Potato Beetle) Invertebrate Somewhat resistant Machida-Hirano 2015
Meloidogyne spp. (Root Knot Nematode) Invertebrate Somewhat resistant Machida-Hirano 2015
Pectobacterium carotovorum (Blackleg/Soft Rot) Bacteria Some resistance Chung 2011
Phytophthora infestans (Late Blight) Fungus Somewhat resistant Bachmann-Pfabe 2019
Potato Leaf Roll Virus (PLRV) Virus Resistant Machida-Hirano 2015
Potato Spindle Tuber Viroid (PSTVd) Virus Somewhat resistant Machida-Hirano 2015
Potato Virus X (PVX) Virus Resistant Machida-Hirano 2015
Potato Virus Y (PVY) Virus Somewhat resistant Machida-Hirano 2015
Potato Virus Y (PVY) Virus Not resistant Cai 2011
Ralstonia solanacearum (Bacterial Wilt) Bacteria Somewhat resistant Machida-Hirano 2015
Synchytrium endobioticum (Wart) Fungus Resistant Machida-Hirano 2015

Glykoalkaloid content

Osman (1986) determined that the primary glycoalkaloids in Solanum acaule are demmissine and tomatine and total glycoalkaloids ranged from 35 to 126mg/kg (Osman 1978).  Van Gelder (1988) agreed that demissine and tomatine were the primary glycoalkaloids, along with a smaller amount of solanidine in one accession and TGA ranged from 12 to 85 mg / 100 g.  Some varieties fall below the 20 mg / 100 g safety limit and this agrees with my taste buds. 

I have tasted tubers from many different plants of different accessions.  Few of these were bitter.  Many of them have a sort of sour aftertaste at the back of the tongue and throat.  I would guess that this is the flavor of tomatine.  I have made several selections of this species that are palatable and taste indistinguishable from domesticated varieties, lacking any aftertaste.

Images

Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plants
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of Solanum acaule
Solanum acaule plant
Berry of the wild potato species Solanum acaule
Solanum acaule berry
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Above ground plant of the wild potato species Solanum acaule
Solanum acaule plant
Inflorescence of the wild potato species Solanum acaule
Solanum acaule inflorescence
Flower of the wild potato species Solanum acaule
Solanum acaule flower
Flower of the wild potato species Solanum acaule
Solanum acaule flower
Flower of the wild potato species Solanum acaule.
Solanum acaule flower
Flower of the wild potato species Solanum acaule
Solanum acaule flower
Flower of the wild potato species Solanum acaule
Solanum acaule flower
Flower of the wild potato species Solanum acaule
Solanum acaule flower
Ground cover of Solanum acaule
Ground cover of Solanum acaule
Solanum acaule berries
Solanum acaule berries
Tubers of the wild potato species Solanum acaule
Solanum acaule tubers
Berries of the wild potato species Solanum acaule
Solanum acaule berries
Tubers of the wild potato species Solanum acaule (average size)
Solanum acaule tubers
Tubers of the wild potato species Solanum acaule (large size)
Solanum acaule tubers (large selection)
Tubers of the wild potato species Solanum acaule
Solanum acaule tubers, mixed accessions
Flowering tuber of the wild potato species Solanum acaule
Solanum acaule flowering tuber
 
 
 

Cultivation

I have often found seeds of Solanum acaule slow and difficult to germinate, taking two to three times as long as seeds of domesticated varieties.  Germination may require treatment with giberellic acid (GA3). The US Potato Genebank found that some accessions had no germination without a 24 hour soak in 2000PPM GA3 (Bamberg 1999).  Even with GA3 treatment, many seeds seem slow to germinate.  S. acaule might require different conditions than are generally used for S. tuberosum.

Bamberg (1995) found that at least some accessions of S. acaule are able to produce a large amount of viable pollen even when temperatures exceed 100 degrees F for several hours during the day.  However, flowering was reduced to a very low level under these conditions.

Many varieties produce berries at ground level, even becoming partly covered in soil (Ochoa 2004). This can be a disadvantage when growing at lower elevation where the berries may be eaten, particularly by slugs.  Because this species lacks a flower joint, the berries are more difficult to harvest.  You might need a knife or scissors to collect berries that ripen before the plant dies back.

Chen (1976) found that this species reaches its maximum frost resistance (about 15 degrees F, -9.3 C) following a three week period of shortening photoperiod and reducing day and night temperatures.

Bamberg (2017) found an 80% increase in seed set in this species with supplemental applications of liquid fertilizer at four and seven weeks after potting.

Towill (1983) summarized seed germination data from the USDA Potato Introduction Station and found that S. acaule retains excellent germinability over long storage, with almost no reduction after periods as long as 27 years at 1 to 3 C (about 34 to 37 F).

Tuber dormancy of five to six months is common in this species.  Tubers will remain dormant for at least 16 months when stored at near-freezing temperatures.

Breeding

Frost resistance is the primary interest of modern potato breeders in Solanum acaule.  The plant grows in very challenging environments where frosts can occur throughout the growing season and drought is also frequently a problem.

Ochoa (2004) describes S. acaule as being essentially cleistogamous (exclusively self-pollinating) as a consequence of its self-compatibility, short style, and early anthesis. Because of this, it should be emasculated when it will be used as a female parent in crosses.

Crosses with S. tuberosum

There are five common methods for making crosses between Solanum acaule and tetraploid S. tuberosum: (1) Pollinating S. acaule with a diploid S. tuberosum and then chemically chromosome doubling the progeny; (2) Chemically doubling S. acaule and then pollinating with tetraploid S. tuberosum; (3) Pollinating S. acaule with a diploid, 2EBN wild species, chemically chromosome doubling the progeny, and then crossing to tetraploid S. tuberosum; (4) Directly pollinating S. acaule with 4x S. tuberosum pollen and hoping that S. acaule produces some unreduced ovules; (5) Artificially combining S. acaule and tetraploid S. tuberosum through protoplast fusion.

S. tuberosum can be crossed to S. acaule when S. acaule produces unreduced ovules. The unreduced ovule of S. acaule is 4x and 4 EBN and the normal pollen of S. tuberosum is 2x and 4 EBN. This results in a hexaploid (6x), 4EBN hybrid.  S. acaule cannot be crossed to S. tuberosum with much probability of success because it rarely produces unreduced pollen.  Camadro (1990) reported success in crossing 4x S. tuberosum to the hexaploid progeny, but no success in the reciprocal cross, even though the plants appeared to produce viable pollen.

In at least one case mentioned by Ochoa (2004), crossing the diploid S. tuberosum (as S. goniocalyx) cultivar Tumbay to S. acaule resulted in self-fertile tetraploid progeny. There might be unrealized potential in crosses between diploid andigena varieties and S. acaule, which can be easily performed by amateurs.

The most commonly used method for crossing with S. tuberosum is to cross S. acaule x 4x S. tuberosum, chemically double the triploid hybrid to 6x/4EBN, and then cross again to 4x S. tuberosum, yielding mostly tetraploids. Because this method involves at least two crosses with S. tuberosum, the progeny are only 1/4 S. acaule. In practice, it often requires more than one backcross to S. tuberosum to get a true tetraploid, further reducing to the contribution of S. acaule to 1/8 or 1/16.

S. acaule can be crossed in either direction with diploid S. tuberosum. The gametes of S. acaule are 2x and 2EBN and the gametes of diploid S. tuberosum are 1x and 2EBN, resulting in hybrids that are triploid (3x) and 2EBN.  The triploid progeny can be chemically doubled, yielding 4EBN hexaploids than can be crossed with 4x S. tuberosum.

Although out of the reach of most amateurs, it is also possible to hybridize S. acaule with S. tuberosum via embryo rescue and protoplast fusion.  Direct 4x S. tuberosum x S. acaule crosses can be made with the assistance of embryo rescue.  You can find the details in Iwanaga (1991).

Female Male Berry Set
Seed Set Germ Ploidy Source
S. acaule S. tuberosum Low Very Low Low Hexaploid Camadro 1990
S. acaule S. tuberosum Moderate Moderate     Jackson 1999
S. acaule S. tuberosum 2x Moderate High   Tetraploid Ochoa 2004
S. acaule S. tuberosum 4x Low Low     Ochoa 2004
S. tuberosum S. acaule None       Camadro 1990
S. tuberosum S. acaule Minimal None     Jackson 1999
S. tuberosum 4x S. acaule Low Moderate     Ochoa 2004

Crosses with other species

Solanum acaule crosses fairly easily with South American diploid species with an EBN of 2.

Watanabe (1991) found that 1.3% (only one plant) of varieties of this species produced 2n pollen and Jackson (1999) found 5-20%, which would be effectively octaploid and 4EBN.

According to Dionne (1963), S. acaule has been used as a bridge species to bring traits from 1EBN Central American diploid species into compatibility with S. tuberosum. The 1EBN diploids are used to pollinate S. acaule and the resulting hybrids can then be used to pollinate tetraploid S. tuberosum. The hybrids were triploid and sterile, but became fertile after chromosome doubling with colchicine.

Female Male Berry Set
Seed Set Germ Ploidy Source
S. acaule S. albicans High Low     Ochoa 2004
S. acaule S. berthaultii High High     Ochoa 1990
S. acaule S. boliviense (as S. megistacrolobum) Some Some     Ochoa 1990
S. acaule S. boliviense (as S. megistacrolobum) High None     Ochoa 2004
S. acaule S. boliviense (as S. megistacrolobum) Low Low Low Triploid Okada 1982
S. acaule S. brevicaule 2x (as S. gourlayi) High High High Triploid Camadro 1995
S. acaule S. brevicaule 4x (as S. gourlayi) Low Very Low Very Low Tetraploid Camadro 1995
S. acaule S. bulbocastanum Low Very Low Low   Hermsen 1966
S. acaule S. candolleanum (as S. bukasovii) High Moderate   Triploid Ochoa 1990, Ochoa 2004
S. acaule S. chomatophilum High Moderate     Ochoa 2004
S. acaule S. commersonii Low Very Low   Triploid Camadro 1995
S. acaule S. raphanifolium High High     Ochoa 2004
S. acaule S. sogarandinum High Moderate     Ochoa 2004
S. albicans S. acaule High Low     Ochoa 2004
S. berthaultii S. acaule Low None     Ochoa 1990
S. boliviense (as S. megistacrolobum) S. acaule High None     Ochoa 2004
S. brevicaule 2x (as S. gourlayi) S. acaule None       Camadro 1995
S. brevicaule 4x (as S. gourlayi) S. acaule None       Camadro 1995
S. candolleanum (as S. bukasovii) S. acaule High Moderate     Ochoa 2004
S. chomatophilum S. acaule High Moderate     Ochoa 2004
S. sogarandinum S. acaule High Moderate     Ochoa 2004

References

Solanum acaule at Solanaceae Source

Solanum acaule at GRIN Taxonomy

Solanum acaule at CIP

Solanum acaule at Plants of the World Online

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