Potato (Solanum tuberosum L.) is one of the most important crops in the world. According to ^{Fernie and Willmitzer (2001)}, its annual world production ranks fourth after rice (Oryza sativa L.), wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.).

• ^{Fernie and Willmitzer (2001)}
  Molecular and biochemical triggers potato tuber development

  Plant Physiol., 2001
  Fernie, A. R., and L. Willmitzer. 2001. Molecular and biochemical triggers potato tuber development. Plant Physiol. 127: 1459-1465.

Strategies for improving potato production are then necessary. The potato tuber that is consumed as food is a shortened and engorged underground stem produced from cell division and elongation in the sub-apical stolon region. Stolons are stems produced by elongation and development of underground lateral buds at the base of the main plant stem (^{Xu et al., 1998}).

• ^{Xu et al., 1998}
  Cell division and cell enlargement during potato tuber formation

  J. Bot., 1998
  Xu, X., D. Vreugdenhil and L. A. Van. 1998. Cell division and cell enlargement during potato tuber formation. J. Bot. 49: 573-582.

Stolon buds derive from undifferentiated tissue sheltered in the axillary stem areas. These buds can be identified from other surrounding tissues by their large-nuclei cells. After formation, the stolon bud tends to produce and develop successive internodes. The stolon apex originates a pair of opposed leaf primordia followed by a pair of alternate leaves. The apical meristematic dome is located inside these leaves, and it continues differentiation in other tissues close to this organ (^{Salas et al., 2003}).

• ^{Salas et al., 2003}
  Anatomía de estructuras vegetativas y origen de los estolones, microtubérculos y raíces adventicias en plantas in vitro de papa (Solanum tuberosum L.) cv. Granola.

  Rev. Fac. Agron., 2003
  Salas, J. E., N. J. Mogollón, M. E. Sanabria. 2003. Anatomía de estructuras vegetativas y origen de los estolones, microtubérculos y raíces adventicias en plantas in vitro de papa (Solanum tuberosum L.) cv. Granola. Rev. Fac. Agron. 20: 131-143.

Stolon and tuber production varies with the potato variety (^{Hernández, 2012}). Agata is a commercial variety featuring a small size (30 to 40 cm) tuber, 90 days production cycle (after planting, DAP), 10 to 13 stolons per vegetative stem, and high rates of stolon induction and tuber formation during the vegetative stage (2 to 10 days after emergence, DAE). The Fianna variety has a contrasting phenotype: the plant is 50 to 80 cm tall; its production cycle lasts 120 to 150 DAP; it produces six to eight stolons per vegetative stem; shows a higher stolon induction rate and tuber formation in the second half of the vegetative stage (30 to 50 DAE). According to ^{Trindande et al. (2003)}, tubers initially develop from cells in the sub-apical region of the stolon, followed by quick cell division from most of the parenchyma cells. These divisions cease when the tuber reaches approximately 30 to 40 g.

• ^{Hernández, 2012}
  Caracterización de las variedades de papa (Solanum tuberosum L.) utilizadas en el Cofre de Perote, Xalapa, Veracruz, bajo regimen de temporal, 2012
  Hernández L., S. 2012. Caracterización de las variedades de papa (Solanum tuberosum L.) utilizadas en el Cofre de Perote, Xalapa, Veracruz, bajo regimen de temporal. Tesis de Licenciatura. Universidad Veracruzana. 82 p.
• ^{Trindande et al. (2003)}
  Isolation and functional characterization of a stolon specific promoter from potato (Solanum tuberosum L.)

  Gene, 2003
  Trindade M. L, B. Horvath, C. Bachen, E. Jacobsen, and R. G. F. Visser. 2003. Isolation and functional characterization of a stolon specific promoter from potato (Solanum tuberosum L.). Gene 303: 77-87.

Stolon induction and tuber formation is directly influenced by the hormonal balance and substances with regulation effects (^{Van den Berg et al., 1996}). Substances that act as plant growth and development regulators are auxins, gibberellins, cytokinins, brassinosteroids, abscisic acid, ethylene, and jasmonic acid (^{Davies, 1988}; ^{Tanimoto, 2005}). Gibberellins seem to be the most active in regulating the process of tuber formation (^{Jackson, 1999}).

• ^{Van den Berg et al., 1996}
  QTL analysis of potato tuberization

  Theor. Appl. Genet., 1996
  Van den Berg, J. H., E. E. Ewing, R. L. Plaisted, S. McMurry, and M. W. Bonierbale. 1996. QTL analysis of potato tuberization. Theor. Appl. Genet. 93: 307-316.
• ^{Davies, 1988}
  Plant Hormones and their Role in Plant Growth and Development, 1988
  Davies, P. J. 1988. Plant Hormones and their Role in Plant Growth and Development. 2nd ed. Editorial Kluwer. New York, USA. 681 p.
• ^{Tanimoto, 2005}
  Regulation of root growth by plant hormones-roles for auxin and gibberellin

  Crit. Rev. Plant Sci., 2005
  Tanimoto, E. 2005. Regulation of root growth by plant hormones-roles for auxin and gibberellin. Crit. Rev. Plant Sci. 24: 249-265.
• ^{Jackson, 1999}
  Multiple signaling pathways control tuber induction in potato

  Plant Physiol., 1999
  Jackson, S. D. 1999. Multiple signaling pathways control tuber induction in potato. Plant Physiol. 119: 1-8.

^{Ortiz and Flórez (2008)} have detected the plant hormones trans-zeatin riboside (ZR), 6-dimethyl aminopurine riboside (iPA) and 6-dimethyl aminopurine (iP) during tuber formation in some potato varieties. ^{Suttle et al. (2011)} reported progressive decrease of tuberonic acid (TA) concentration in seed tubers as latency is lost, and the concentrations of jasmonic acid, N-jasmonoyl-L-isoleucine and abscisic acid increase.

• ^{Ortiz and Flórez (2008)}
  Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk

  Agron. Colomb., 2008
  Ortiz, L. Y., y V. J. Flórez. 2008. Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk. Agron. Colomb. 26: 32-39.
• ^{Suttle et al. (2011)}
  The effects of dormancy status on the endogenous contents and biological activities of jasmonic acid, N-(jasmonoyl)-isoleucine and tuberonic acid in potato tubers

  Am. J. Potato Res., 2011
  Suttle, J. C., L. L. Huckle, and E. C. Lulai. 2011. The effects of dormancy status on the endogenous contents and biological activities of jasmonic acid, N-(jasmonoyl)-isoleucine and tuberonic acid in potato tubers. Am. J. Potato Res. 88: 283-293.

^{Sonnewald and Sonnewald (2014)} stated the prerequisites for tuber sprouting which are enough available sucrose, inhibition of bud sprouting by ABA and ethylene, promotion of latency rupture by GA and CK and stimulation of vascular development.

• ^{Sonnewald and Sonnewald (2014)}
  Regulation of potato tuber sprouting

  Planta, 2014
  Sonnewald, S. and U. Sonnewald. 2014. Regulation of potato tuber sprouting. Planta 239: 27-38.

^{Dutt et al. (2017)} described potato tuber formation as a complex biological phenomenon that involves factors like environment, genetics and nutrition. The major metabolites that regulate potato tuber formation are the protein StSP6A, RNAs StBEL5 and miR172, and gibberellins. ^{Sevcikova et al. (2017)} concluded that the dominant signals for tuber formation seem to be carbohydrates and gibberellin availability. The results of in vitro and in vivo studies allowed ^{Teo et al. (2017)} to propose that flowering and potato tuber formation involve a signal, florigen and tuberigen, from the leaves. The activation complex for tuberigen includes proteins like StSP6A, St 14-3-3s, and StFDL1 regulates the tuber formation.

• ^{Dutt et al. (2017)}
  Key players associated with tuberization in potato: potential candidates for genetic engineering

  Crit. Rev. Biotechnol., 2017
  Dutt, S., A. Sharma M., P. Raigond, B. Singh, S. Siddappa, V. Bhardwaj, P. G. Kawar, V. U. Patil, and H. B. Kardile. 2017. Key players associated with tuberization in potato: potential candidates for genetic engineering. Crit. Rev. Biotechnol. 37: 2017.
• ^{Sevcikova et al. (2017)}
  Carbohydrates and gibberellins relationship in potato tuberization

  J. Plant Physiol., 2017
  Sevcikova, H., P. Maskova, D. Tarkowska, T. Masek, and H. Lipavska. 2017. Carbohydrates and gibberellins relationship in potato tuberization. J. Plant Physiol. 214: 53-63.
• ^{Teo et al. (2017)}
  Potato tuber induction is regulated by interactions between components of a tuberigen complex

  Plant Cell Physiol., 2017
  Teo C.-T., K. Takahashi, K. Shimizu, K. Shimamoto, and K. Taoka. 2017. Potato tuber induction is regulated by interactions between components of a tuberigen complex. Plant Cell Physiol. 58: 365-374.

^{Kolachevskaya et al. (2017)} proposed a multihormonal model that controls tuber formation. In this model, gibberellins inhibit tuber induction and initiation, while cytokinins promote both processes, and auxins stimulate future growth and development of the formed tubers. In 2015, Kolachevskaya et al. reported increased expression of the tms1 gene in tubers compared to stems. Also, they found that auxin synthesis increases during tuber formation and documented a positive correlation between expression of this gene, IAA content, and tuber growth. GA3 mixed with zinc sulfate and sprayed after tuber sprouting increased tuber yield (38 %) and expression of crude protein (8.37 %) (^{Javanmardi and Rasuli, 2017}). This treatment promoted stolon and tuber growth, but not its induction.

• ^{Kolachevskaya et al. (2017)}
  Auxin synthesis gene tms1 driven by tuber-specific promoter alters hormonal status of transgenic potato plants and their responses to exogenous phytohormones

  Plant Cell Rep., 2017
  Kolachevskaya, O. O. , L. I. Sergeeva , K. Flokova, I. A. Getman , S. N. Lomin, V. V. Alekseeva, E. B. Rukavtsova, Y. I. Buryanov , and G. A. Romanov . 2017. Auxin synthesis gene tms1 driven by tuber-specific promoter alters hormonal status of transgenic potato plants and their responses to exogenous phytohormones. Plant Cell Rep. 36: 419-435.
• ^{Javanmardi and Rasuli, 2017}
  Potato yield and tuber quality as affected by gibberellic acid and zinc sulphate

  Iran Agricultural Research, 2017
  Javanmardi, J., and F. Rasuli. 2017. Potato yield and tuber quality as affected by gibberellic acid and zinc sulphate. Iran Agricultural Research 36: 1-5.

In the medicinal plant Tulipa edulis, high soluble sugar availability is essential for the initial development of the stolon (^{Miao et al., 2016}). This behavior confirmed the high activity of sucrose synthase (SS) and soluble starch synthase (SSS), and the expression of the SS, SSSI and SSSII genes. Additionally, gibberellin and zeatin riboside content reached their highest concentration at the beginning of stolon formation, while IAA and levels remained high from the beginning through the middle of this period, and decreased significantly at the end of stolon formation. In tubers of Helianthus tuberosus L., biomass and sugar accumulation correlated positively with endogenous contents of zeatin and negatively with GA3, GA3/ ABA and IAA/ABA contents. GA3/ABA and ABA/IAA ratios decreased, while endogenous zeatin and ABA content increased during tuber growth, and GA3 and AIA content decreased during the study after two weeks (^{Li et al., 2017}).

• ^{Miao et al., 2016}
  Dynamic changes in starch metabolism and endogenous hormones during stolon formation in Tulipa edulis

  J. Amer. Soc. Hortic. Sci., 2016
  Miao, Y., Q. Guo, Z. Zhu, X. Yang, C. Wang, Y. Sun, and L. Liu. 2016. Dynamic changes in starch metabolism and endogenous hormones during stolon formation in Tulipa edulis. J. Amer. Soc. Hortic. Sci. 141: 211-221.
• ^{Li et al., 2017}
  The endogenous plant hormones and ratios regulate sugar and dry matter accumulation in Jerusalem artichoke in salt-soil

  Sci. Total Environ., 2017
  Li, L., T. Shao, H. Yang, M. Chen, X. Gao, X. Long, H. Shao, Z. Liu, and Z. Rengel. 2017. The endogenous plant hormones and ratios regulate sugar and dry matter accumulation in Jerusalem artichoke in salt-soil. Sci. Total Environ. 578: 40-46.

Currently, the hormones that regulate stolon initiation in potato have not been identified with certainty. It is then convenient to determine the hormones that promote stolon initiation since tuber formation depends on it. These compounds should be present at low concentrations and their characterization requires purification, quantification, and identification, which can be performed via chromatography. Among the methods used for detection of plant growth regulators is high pressure liquid chromatography (HPLC) (^{Arteca, 1996}) coupled to a UV-VIS detector (^{Olivella et al., 2001}; ^{Oliveros et al., 2011}).

• ^{Arteca, 1996}
  Plant Growth Substances, Principles and Applications, 1996
  Arteca, R. N. 1996. Plant Growth Substances, Principles and Applications. Editorial Chapman & Hall. New York, USA. 332 p.
• ^{Olivella et al., 2001}
  Determinación de ácido abscísico, ácido indolacético, zeatina y ribósido de zeatina en hojas desarrolladas de Gerbera jamesonii cv. Bolus y su variación con la edad

  Invest. Agr. Prod. Prot. Veg., 2001
  Olivella, C., M. Vendrell y R. Sauvé. 2001. Determinación de ácido abscísico, ácido indolacético, zeatina y ribósido de zeatina en hojas desarrolladas de Gerbera jamesonii cv. Bolus y su variación con la edad. Invest. Agr. Prod. Prot. Veg. 16: 333-342.
• ^{Oliveros et al., 2011}
  Extracción y cuantificación de cumarina mediante HPLC-UV en extractos hidroetanólicos de semillas de Dipteryx odorata

  Rev. Latinoam. Quím., 2011
  Oliveros B., A., I. Cordero, D. Paredes, D. Buendía, y F. A. Macías D. 2011. Extracción y cuantificación de cumarina mediante HPLC-UV en extractos hidroetanólicos de semillas de Dipteryx odorata. Rev. Latinoam. Quím. 39: 17-31.

This study analyzed plant growth regulators present in the stolon apex during early development stages of the latent phase of stolon differentiation using HPLC. The methodology proposed might identify plant growth regulators responsible for stolon differentiation.

Commercial varieties of Agata and Fianna potatoes were tested in this study. These varieties contrast in growth habit and tuber quantity. Seed tubers for both varieties, healthy and apparently pathogen and pest free were obtained from the municipality of Zaragoza, Puebla, México.

Seeds were sown in 4-L capacity black polyethylene bags and kept in a greenhouse. The bags were covered with anti-aphid mesh to control pests and diseases until production. Three seeds were placed into perlite at 10 cm depth. Bags were kept in a controlled-environment chamber (GCA model 815 Freas; USA) at 20±1 °C, 50 % relative humidity, and in complete darkness. Ten days after sowing (DAS), tubers with vegetative shoots (Figure 1) were sampled. Sampled shoots were placed in a cooler (6±1 °C) for dissection in the laboratory. Sampling continued until 30 DAS. Mother tubers were washed three times with distilled water to remove substrate particles. From each tuber, vegetative buds were extracted with a size 11 scalpel and a stereoscopic microscope (OLYMPUS model SZ-CTV; USA).

Samples of buds in four stages of stolon development were extracted from buds at the basal zone of the stem with a stereoscopic microscope (Carl Zeiss model SZ60; Germany): E1) flattened and latent stolon bud at initial stage; E2) swollen and still latent stolon bud; E3) sprouted stolon bud, also called stolon primordia, in which it starts its elongation and development; and E4) stolon in early formation stage.

Two hundred mg of buds (containing almost no meristematic tissue) were placed in 2 mL Eppendorf tubes. This quantity was obtained with 50 buds per variety and in each development event. Microtubes containing the buds were kept at -20 °C (Puffer Hubbard model IUF1821, USA) until processing.

Buds at stages E1, E2, E3 and E4 were extracted by longitudinal dissection (with a metal blade; Guillette^®) of axillary buds from underground stems from both varieties. Buds were stained with fuschine acid (0.05 %)

Before extraction, samples were lyophilized for 12 hours (Labconco model Freezone 4.5; USA). Ten mg from each lyophilized bud type were placed in a plastic microtube and 500 μL of extraction solution added as per Pan’s protocol (2010). The extraction solution contained 2-propanol HPLC grade (J.T. Baker^®), deionized water and reagent grade concentrated HCl (J.T. Baker^®) in a 2:1:0.002 ratio. Microtubes were agitated at 100 rpm for 30 minutes using a vortex (Scientific Industries model G560; USA) and kept at 6±1 °C. After agitation, 1000 μL of HPLC-grade dichloromethane were added to each microtube. The mixture was agitated for 30 min at 100 rpm and then centrifuged at 716 g for 10 min (Hettich, model EBA 21; USA). The lower organic phase was extracted with an Eppendorf micropipette (USA) and placed in a clean microtube. Using nitrogen gas (high purity grade; Infra^®), the solvent was completely evaporated, and 500 μL of HPLC-grade filtered methanol were added (J.T. Baker^®) to each microtube. These microtubes were stored until chromatographic analysis.

Quantitative standards for this research were 3-gibberellic acid (GA3, 90 % purity), indole-3-acetic acid (IAA, 98 % purity), abscisic acid (ABA, 98.5 % purity), kinetin (KIN, 98 % purity), 6-(^γ,^γ-dimethylalylamino)-purine (2-iP, 98% purity) and 6-benzylaminopurine (BAP, 99 % purity) (Sigma-Aldrich^®). For each standard, dilutions at 0.01, 0.1 and 1.0 mg mL^-1 were prepared in HPLC-grade methanol (J.T. Baker^®).

An Agilent HPLC model 1100 coupled to an UV-Vis detector was used for detection. The column used for separation was a 4.6(75 mm Rx/SB-C8 column (model 866953-906; Agilent). The mobile phase contained an 80:20 mixture of solutions A and B. Solution A included HPLC-grade acetonitrile (J.T. Baker^®) and triflouroacetic acid spectrophotometric grade (Sigma-Aldrich^®) in a 1:0.001 ratio. Solution B was a 1:0.001 mixture of deionized water and spectrophotometric-grade trifluoroacetic acid (Sigma-Aldrich^®). Both solutions were filtered using 0.45 μm HV-type Millipore^® membranes (adapted from ^{Pan et al., 2010}).

• ^{Pan et al., 2010}
  Quantitative analysis of major plant hormones in crude plant extracts by high-performance liquid chromatography-mass spectrometry

  Nat. Protoc., 2010
  Pan, X., R. Welti and X. Wang. 2010. Quantitative analysis of major plant hormones in crude plant extracts by high-performance liquid chromatography-mass spectrometry. Nat. Protoc. 5: 986-992.

Flow rate for the mobile phase was 2 mL min^-1. Three 20 μL replications of every extract were injected into the HPLC using a manual injector (Rheodyne^®, model 755). Readings were taken at three wavelengths, and the reading showing the highest absorption was chosen. The working wavelengths were 206 nm for gibberellins, 254 for auxins and abscisic acid (^{Harborne, 1994}), and 280 for cytokines. Peak area was recorded as milli-absorbance units per second (mAU s^-1) (^{Ortiz and Flórez, 2008}). The identity for each fraction was determined by comparison with retention time and maximum absorption wavelength for the standards. Calibration curves were also calculated for each standard.

• ^{Harborne, 1994}
  Phytochemical Methods, 1984
  Harborne, J. B. 1984. Phytochemical Methods. 2nd ed. Editorial Chapman & Hall. London, England. 228 p.
• ^{Ortiz and Flórez, 2008}
  Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk

  Agron. Colomb., 2008
  Ortiz, L. Y., y V. J. Flórez. 2008. Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk. Agron. Colomb. 26: 32-39.

Latency loss and sprouting in the stolon bud was detected at stage E3 (Figure 2).

The extraction and analysis method in this research was specifically designed to detect and quantify hormones. Thus, in each tested wavelength, each fraction should correspond to compounds with similar physical and chemical (size and charge) characteristics to the standards (Figure 3 and Table 1). In this research, no cromatographic fraction at any wavelength overlapped; thus, every peak from each of the eight fractions obtained was considered independent (Table 2).

Fractions F-1 and F-8 agreed with auxins, and their maximum absorption was at 254 nm. Fractions F-2, F-3 and F-4 agreed with cytokines, which absorb maximally at 280 nm. At 206 nm, fractions F-5, F-6 and F-7 matched gibberellins (Table 2).

The eight fractions changed concentration as stolon development stages progressed in both varieties (Figure 4). Four of the eight fractions appeared to be directly associated with stolon sprouting stages from E2 to E3, where E3 is the key event in the tuber formation. The fractions involved in this response were F-3, F-4 and F-8.

In both varieties, fraction F-7, a gibberellin-type compound, decreased sharply by E3 (less than 1000 mAU s^-1), and it accounted for 17 % of its content in the latent bud at E1 (Figure 4); F-8, an auxin-type compound, was expressed in E1 and E2 but not in E3. This behavior suggests that both growth regulators (F-7 and F-8) act as inhibitors of sprouting for stolon buds in the varieties Agata and Fianna, and probably in other potato varieties.

Detectable hormones during formation of stolons in H. tuberosus L. were GA3 and zeatin. The first hormone content increased initially and then decreased as tuber induction and growth continued. Zeatin content increased during all stages, and its lowest content occurred during stolon initiation (^{Li et al., 2017}).

• ^{Li et al., 2017}
  The endogenous plant hormones and ratios regulate sugar and dry matter accumulation in Jerusalem artichoke in salt-soil

  Sci. Total Environ., 2017
  Li, L., T. Shao, H. Yang, M. Chen, X. Gao, X. Long, H. Shao, Z. Liu, and Z. Rengel. 2017. The endogenous plant hormones and ratios regulate sugar and dry matter accumulation in Jerusalem artichoke in salt-soil. Sci. Total Environ. 578: 40-46.

According to ^{Taiz and Zeiger (2002)}, low auxin levels promote bud development, and an increase in its concentration inhibits sprouting. In contrast, GA1 at low concentrations promotes stem elongation stems and high concentrations tend to drastically reduce their growth rate.

• ^{Taiz and Zeiger (2002)}
  Plant Physiology, 2002
  Taiz, L. and E. Zeiger. 2002. Plant Physiology. 3rd ed. Sinauer Associates, Inc. USA. 1338 p.

An increase in the number of vegetative shoots in potato might be achieved faster by supplying GA3 at low concentrations; this behavior shows the important role gibberellins play in potato development (^{Alexopoulos et al., 2007}; ^{Salimi et al., 2010}). ^{Abdala et al. (2000)} showed by comparison of potato organs that gibberellins influence stolon development in stages following sprouting. These authors obtained increased levels of jasmonates, GA1 and GA3 in sprouted and developing stolons, and such levels were higher during tuber formation.

• ^{Alexopoulos et al., 2007}
  The effect of postharvest application of gibberellic acid and benzyl adenine on the duration of dormancy of potatoes produced by plants grown from TPS

  Postharvest Biol. Technol., 2007
  Alexopoulos, A. A., K. A. Akoumianakis, S. N. Vemmos, and H. C. Passam. 2007. The effect of postharvest application of gibberellic acid and benzyl adenine on the duration of dormancy of potatoes produced by plants grown from TPS. Postharvest Biol. Technol. 46: 54-62.
• ^{Salimi et al., 2010}
  Effects of gibberellic acid and carbon disulphide on sprouting of potato minitubers

  Sci. Hortic., 2010
  Salimi, Kh., R. Tavakkol A., M. B. Hosseini, and P. C. Struik. 2010. Effects of gibberellic acid and carbon disulphide on sprouting of potato minitubers. Sci. Hortic. 124: 14-18.
• ^{Abdala et al. (2000)}
  Changes in jasmonate and gibberellin levels during development of potato plants (Solanum tuberosum)

  Plant Growth Regul., 2002
  Abdala, G., G. Castro, O. Miersch, and D. Pearce. 2002. Changes in jasmonate and gibberellin levels during development of potato plants (Solanum tuberosum). Plant Growth Regul. 36: 121-126.

Contrary to reports by other researchers, the results in our study show that the gibberellin-type fraction (F-7) participated as a repressor of sprouting of stolon buds (Figure 4). Fraction F-7 did not match GA3, as the analytical standard for this hormone had different retention time (Table 2 and Figure 4).

In contrast, in Fianna potato fractions F-3 and F-4 of the cytokinin-type, were not present in the developmental stages E1 and E2 but did show in E3 and E4. These results may indicate that the two fractions promote stolon germination only in some genotypes. When the stolon broke latency during stage E3 and E4, fractions F-3 and F-4 did not appear, thus suggesting that in this variety these cytokines are unnecessary to induce stolon sprouting. These results coincide with those of ^{Ortiz and Flórez (2008)}; they pointed out that the interaction genotype(cytokinins was not significant between the potato varieties in the study after detection and quantification of different kinds and concentrations of hormones.

• ^{Ortiz and Flórez (2008)}
  Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk

  Agron. Colomb., 2008
  Ortiz, L. Y., y V. J. Flórez. 2008. Comparación cuantitativa de ácido abscísico y citoquininas de Solanum tuberosum L. y S. phureja Juz. et Buk. Agron. Colomb. 26: 32-39.

It is then possible to assume that stolon sprouting in both varieties of potatoes takes place when gibberellin F-7 decreases and auxin F-8 is absent, because these two growth regulators, inhibit or suppress stolon sprouting. It can also be inferred that cytokinins participation in stolon sprouting is required only in some potato genotypes, and that the interaction of cytokinin(genotype is associated with habits of expression of stolons. This can be appreciated in var. Agata: stolon induction was early and abundant compared to Fianna. In the latter variety, this process occurred gradually and at a later stage.

The results in this study contribute new knowledge on the process of stolon initiation in potato, although more research is necessary to identify the chemical structure of the compounds described here, as well as to carry out bio-essays to determine their role.

From the eight fractions described in this study, those identified as F-7 (gibberellin type) and F-8 (auxin type) inhibit stolon formation. For bud sprouting and stolon formation, it is necessary that the concentration of F-7 decreases and that F-8 is undetectable in the bud apex. Fractions F-3 and F-4 (cytokinin type) regulate the differentiation of the stolon in interaction with the potato genotype, since these fractions were detected only in var. Fianna.