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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
Code acl
Synonyms S. depexum, S. punae, S.schreiteri, S. uyunense
Clade Mixed
Series Acaulia
Ploidy Tetraploid (4x)
EBN 2
Self-compatible Yes
Cytoplasm Type M (Hosaka 2009, as C)

Description

Solanum acaule distribution Map note

Solanum acaule (acaule means “without stem”) is a South American species that grows largely at high altitudes, ranging from about 9700 to 15000 feet (2950 to 4560 m). It is found 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). S. acaule can have impressive frost tolerance; resistance to viruses PVX, PVY, PLRV, and PSTV; and potato cyst and root knot nematode resistance.  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. That is, 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.

S. acaule 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.

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.

Condition Level of Resistance Source
Frost 21 to 23 F Chen 1977
Wart Resistant Machida-Hirano 2015
Bacterial Wilt Somewhat resistant Machida-Hirano 2015
Potato Virus X Resistant Machida-Hirano 2015
Potato Virus Y Somewhat resistant Machida-Hirano 2015
Potato Leaf Roll Virus Resistant Machida-Hirano 2015
Potato Spindle Tuber Viroid Somewhat resistant Machida-Hirano 2015
Bacterial Ring Rot (C. michiganensis) Somewhat resistant Laurilla 2003
Colorado Potato Beetle Somewhat resistant Machida-Hirano 2015
Potato Cyst Nematode Somewhat resistant Machida-Hirano 2015
Root Knot Nematode Somewhat resistant Machida-Hirano 2015
Heat Somewhat resistant Machida-Hirano 2015
Drought Somewhat resistant Machida-Hirano 2015
Early Blight Resistant Prasad 1980

Glykoalkaloid content

The primary glycoalkaloids in Solanum acaule are demmissine and tomatine (Osman 1986).  Total glycoalkaloids ranged from 35 to 126mg/kg (Osman 1978).  Van Gelder (1988) found that demissine and tomatine were the primary glycoalkaloids, along with a smaller amount of solanidine in one accession.  TGA ranged from 12 to 85 mg / 100 g.

Images

   

Cultivation

I have often found seeds of Solanum acaule difficult to germinate.  They are usually fairly slow 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 significant 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.

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).

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 Germination Ploidy Source
S. acaule S. tuberosum Low Very Low Low Hexaploid Camadro 1990
S. tuberosum S. acaule None       Camadro 1990
S. tuberosum S. acaule Minimal None     Jackson (1999)
S. acaule S. tuberosum Moderate Moderate     Jackson (1999)
S. acaule 2x S. tuberosum Moderate High   Tetraploid Ochoa (2004)
S. acaule 4x S. tuberosum Low Low     Ochoa (2004)
4x S. tuberosum 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 Germination Ploidy Source
S. acaule 2x S. brevicaule (as S. gourlayi) High High High Triploid Camadro 1995
S. acaule 4x S. brevicaule (as S. gourlayi) Low Very Low Very Low Tetraploid Camadro 1995
2x S. brevicaule (as S. gourlayi) S. acaule None       Camadro 1995
4x S. brevicaule (as S. gourlayi) S. acaule None       Camadro 1995
S. acaule S. commersonii Low Very Low   Triploid Camadro 1995
S. acaule S. bulbocastanum Low Very Low Low   Hermsen 1966
S. acaule S. boliviense (as S. megistacrolobum) Low Low Low Triploid Okada 1982
S. acaule S. boliviense (as S. megistacrolobum) Some Some     Ochoa (1990)
S. acaule S. chomatophilum High Moderate     Ochoa (2004)
S. chomatophilum S. acaule High Moderate     Ochoa (2004)
S. acaule S. boliviense (as S. megistacrolobum) High None     Ochoa (2004)
S. boliviense (as S. megistacrolobum) S. acaule High None     Ochoa (2004)
S. acaule S. raphanifolium High High     Ochoa (2004)
S. acaule S. sogarandinum High Moderate     Ochoa (2004)
S. sogarandinum S. acaule High Moderate     Ochoa (2004)
S. acaule S. candolleanum (as S. bukasovii) High Moderate   Triploid Ochoa (1990), Ochoa (2004)
S. candolleanum (as S. bukasovii) S. acaule High Moderate     Ochoa (2004)
S. acaule S. albicans High Low     Ochoa (2004)
S. albicans S. acaule High Low     Ochoa (2004)
S. acaule S. berthaultii High High     Ochoa (1990)
S. berthaultii S. acaule Low None     Ochoa (1990)

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