Hydroponic minituber production in growth room conditions and carry-over effects of the technique on produced minitubers

Journal of Agricultural Science; Vol. 9, No. 1; 2017 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 41 Hydroponic Minituber Production in Growth Room Conditions and Carry-Over Effects of the Technique on Produced Minitubers Elina Virtanen 1 & Jussi Tuomisto 2 1 Natural Resources Institute Finland, Oulu, Finland 2 Potato Research Institute, Ylistaro, Finland Correspondence: Elina Virtanen, Natural Resources Institute Finland, P.O. Box 413, 90014 University of Oulu, Oulu, Finland. Tel: 358-29-5326-646. E-mail: [email protected] Received: September 27, 2016 Accepted: November 7, 2016 Online Published: December 15, 2016 doi:10.5539/jas.v9n1p41 URL: http://dx.doi.org/10.5539/jas.v9n1p41 Abstract The production of minitubers was implemented with a hydroponic technique in growth rooms and the carry-over effects of the technique on the characteristics of minitubers were studied. As a comparison, minitubers from in vitro plantlets were grown in a peat-based growing medium. The results show that hydroponic production of minitubers is successful in indoor conditions with the cultivars Desiree, Van Gogh and Asterix, when day-time growing temperatures of 19.4 °C-26.0 °C and night-time temperatures of 17.5 °C-22.6 °C were used. Photosynthetically active illumination was adequate at 2383-2509 μmol m -2 s -1 ; lighting conditions consisted of 14/10-hour day/night cycles. The cultivars Desiree and Van Gogh developed their first tuber three weeks faster than Asterix, and the minituber yield was 4.5 per plant for Desiree, 7.5 for Van Gogh and 4.0 for Asterix. When a peat-based growing medium was used, minituber yields were almost the same but the size of the minitubers was smaller than that of hydroponically produced minitubers. The results of the carry-over experiments showed that conventionally produced minitubers emerged faster, and in terms of foliage development and yielding capacity performed better than hydroponically produced minitubers. Keywords: Solanum tuberosum L., minituber production, hydroponics 1. Introduction Faster, more cost-effective technical solutions with a higher production capacity are needed for minituber and pre-basic seed potato (Solanum tuberosum L.) production. Production efficiency is often measured in terms of the rate of minituber production and the number of minitubers produced. It has been proposed that soilless minituber production techniques increase production volume (Rolot & Seutin, 1999). Various soilless, hydroponic and aeroponic (Corrêa et al., 2008; Ritter et al., 2001) and bioreactor-type (Kämäräinen-Karppinen et al., 2010; Akita & Ohta, 1998) production methods have already been developed and are in use. The production capacity of these new techniques is typically compared to the minituber yields achieved by conventional in vitro propagation techniques. Even though it is possible to increase conventional minituber production (Milinkovic et al., 2012; Veeken & van der Lommen, 2009) from in vitro plantlets, but it involves clearly higher production costs and relatively low tuber yield (Rolot & Seutin, 1999). In hydroponic production, plant roots are freely suspended in nutrient solution from which they derive the necessary nutrients and trace elements. It has been shown to be possible to increase the number of minitubers produced by using the hydroponic production technique (Rolot & Seutin, 1999). The hydroponic technique has also been used with sand substrates (Novella et al., 2008). The introduction of new minituber production techniques requires the optimization of production conditions. A specific requirement for hydroponic production is to achieve a balance between the nutrient ratios within the nutrient solution, electrical conductivity (EC) value, and pH as needed for potato production (Chang et al., 2011; Novella et al., 2008; Ritter et al., 2001). Although only few studies have been conducted on the effect of EC on potato, EC values from 1.2 to 1.7 dS m -1 are considered normal (Chang et al., 2011; Novella et al., 2008). In the studies of Chang et al. (2011), pH fluctuations between 5.0 and 7.2 in hydroponic production correlated with the EC levels of the nutrient solutions. In addition, temperature and illumination levels should be taken into consideration in minituber production jas.ccsenet. (Levy & V lighting ( R Accordin g and, corre tuberizati o p hotosynt h during the In norther n conventio n environm e alternativ e simulatio n alternativ e p roductio n minituber s 2. 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(200 7 i gh illuminati o V eilleux (200 7 p ean potato c u t emperatures w 64-65°N), en v production i n n s are utilize d e nt study we e e rature, humi d n al method. T and the carr y n studied. e r Production t ion system c o r al processing s.nl/). The pr o 10 hydroponi c plantlets wer e m icroplantlets p 00 microplan t p lantlets of D e 1962) were w c losed nutrien g rowth rooms fabric at the b e tray. The sh o t ween the nyl o r oduction tray s d ed, with the n at the other e used in the ci r h ase and con t 3 0 kg/ha as c o . 7:1.0:0.5) wa n 2 was replac e Journal of A l et al., 2007) . o difying natur a 7 ), long perio d o n levels, low 7 ) consider +2 u ltivars. In th e w ere 24±4 °C a v ironmental f a n greenhouses , d . Soilless in d e xamined wh e d ity) of the p he hydroponi c y -over effects o nsisted of a m unit, nutrient s o duction syste m c growing tra y e planted in e a p er cultivar. T t lets/cultivar, t o e siree, Van G o w ashed with 3 t solution cy c (Figure 1). In b ottom of the t r o ots of the mi c o n fabric and t h s after transfe r V n utrient soluti o e nd, and then r culation syst e t inuing until t ompared to f i s replaced wi t e d by 3 (Ca: K A gricultural Sc i 42 . Hydroponic a l light with s u d s of daylight temperatures a 0 °C as the o p e studies of C a nd 12±2 °C b a ctors limit th e , and only o n d oor producti o e ther an indo o p roduction en v c production t e of the tech n m icroplantlet h a s olutions, and m s were plac e y s, each with a a ch tray 15 c m T he study was o tal 800 plant s o gh and Aste r 3 8 °C water a c le. After 13 d the hydropo n r ay, which en s c roplantlets gr o h e plastic shee r of the micro p V irtanen) o n fed to the t r recirculated t o e m depending t he initiation i eld nutrient l t h nutrient so l K :Mg–1.5:1.0: 0 i ence production ca n u pplemental i l and high tem p a nd relatively p timal temper a C hang et al. ( 2 y day and nig h e utilization o f n e production o n techniques o r hydroponic v ironment en a e chnique is n o n ique on the a rdening tabl e tanks with pu e d in two gro w a width of 22 5 m apart, with a conducted tw i s /cultivar). r ix grown on a a nd transferr e d ays, the mic r n ic system, th e s ures optimal d o w above a p l t. p lantlets and b e r ay via a hose o the tray vi a on the plant of tuber for m evel. At the i l ution 2 (Ca: K 0 .4) during tu b n be implem e l lumination ( C p eratures inhi b scarce nutrie n a ture for tube r 2 012) on hyd r h t, respectivel y f illumination period is pos therefore pr o production s y a ble minitube r o t, at present, i characteristic s e , cultivation t a mps (Living F w th rooms 8 m 5 mm, height a total of 300 i ce, in 2012 a n a regular sem i e d to hardeni n r oplantlets w e e plants are cu l d istribution of l astic sheet an e fore harvesti n at one end an d a a filter syst e developmenta m ation, nutrie n i nitial phase o K :Mg–0.4:1.0: 0 b erization. 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I n measure m com). 2.2 Conve n Hydropon i microplan t p lanted (4 White 42 0 (NPK 144 nitrogen, 5 were irrig a 2.3 Growt h It was use d plastic-co a p er growt h used and growth r o radiation ( growth ro o am) day/n i for both g OHM SR L and the P A were me a 754-O-P M Figure 2 Growth r o automatio n b etween 2 rooms. In in growth org furthermore P, w as 0.5 l min - o lution pH le v controller (N e n addition, th e m ents (Bluela b n tional M init u i c production t lets were als o 0 cm × 60 c m 0 W) was use d 4 -20) with a p H 5 00 mg kg -1 s o a ted from bel o h Room Cond i d two growth r a ted brick. Th e h room) (Osr a the lamp spe c o oms between ( PAR) in the f o o ms were imp i ght illuminat i g rowth rooms. L , Italy) equi p A R sensor me a a sured using M T, 754-C, Op t . Highp ressu r o om temperat u n (Ilmateollis u 2 0.9 and 26.0 ° growth room room 2. 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During th e The amount p ped with RA D a sures the pho t the OL754 t ronic Laborat r e sodium lam p u res were adj u u us Oy, Finla n ° C in growth 1 daytime te m u midity in the Journal of A Z n, Fe and Na n w cycle was o n and EC 1.3 ± M C-Pro-Irrig a n utrient solut i e r, Bluelab C o o n e d to a conv e p ea t - b ased gro w m ) at a distanc e w ing medium. e lectrical cond u o rus and 2600 s olution as in t e search; 8 m 2 m ade of acryl i e rmany) with t h e same in bo t 18,000 lx re g 8 3-2509 μmol t ime light con d e production p of illuminati o D and PAR s e t osyntheticall y Portable Hi g ories, Inc., Or l p light spectr u u sted to 20±2 n d). Temperat u room 2. Nigh t m peratures wer e growth room s A gricultural Sc i 43 n u t rients in th e n e hour after t h ± 0.3 were adj u a tion, Israel) t o i on, EC and o rporation Li m e ntional minit u w ing medium . e of 10 cm fr o The peat was u ctivity 27 m mg kg -1 solu b t he hydroponi c and 9 m 2 in a r i c covered co n t ime control ( t h growth ro o g ardless of th e m -2 s -1 (sensor d itions, with 1 hase, the spec t o n was meas u e nsors. The R A y active radiati o g h Accuracy l ando, FL, US A u m in the gro w °C for dayti m u res varied in t -time temper a e 0.5 °C and n s was adjusted i ence eir different li q h e fluid flow, u sted accordi n o guide and c o temperature w m ited, New Z u ber producti o . A total of 16 0 o m the polypr o fer t ilized wi t S m -1 and cont b le potassium c technique. r ea. The walls n crete. High p r Schneider El e o ms (Figure 2 ) e measuring p measurement 4 h (07:00 a m t rum and inte n u red with a H D A D sensor m e o n in photons p U V -Visible s A ). w th rooms, me a m e and 14±2 ° C growth room a tures were b e n igh t -time tem p to 60-80% us i q uid forms. T h following a 5 - n g to the tar g o ntrol the hyd r w ere monitor e Z ealand, http:/ / o n method, a n 0 microplantle t o pylene boxes , t h Kekkilä sta r aining 1600 m (dry matter). T of the rooms w r essure sodiu m e ctric TAC X e ) . Light inten s p oints. Photo s wavelength 4 m -09:00 pm)/1 0 n sity of the lig h D 9021 phot o e asures radiati o p er square me s pectroradiom e a sured with a s C for the nig h 1 between 19 . e tween 15.5 a n p eratures wer e i ng a steam g e Vol. 9, No. 1; h e nutrient sol - minute pause g et values usi n r oponic prod u e d daily by s e / wwww.getbl u n d for this re t s per cultivar , and peat (K e r ter fertilizer n m g/kg wate r -s o T he growing b w ere construc t m lights (4 × 4 0 e nta, Sweden) s ity varied in s ynthetically a 4 00-700 nm). I 0 h (9:00 pm0 h t were deter m o -radiometer ( D o n efficiency i tre. Light spe c e ter (IS-670L s pectroradiom e h t period usin g . 4 and 26.0 ° C n d 22.6 °C in e 0.7 °C lowe r e nerator (Vapa c 2017 u tion . The n g an u ction e nsor u elab. a son, were kkilä n o. 1 luble b oxes ed of 0 0 W were both a ctive n the 0 7:00 m ined D elta i n lx, c trum L ED, e te r g AC C and both r than c LE, jas.ccsenet.org Journal of Agricultural Science Vol. 9, No. 1; 2017 44 UK) and humidity levels were predominantly below 60%. Temperature and humidity readings were followed with data loggers (Netafim, Climate Box, Netafim, Israel). 2.4 Observations and Samplings Plant development and tuber formation were observed according to Hack et al. (1993) using pre-marked individual plants (10 plants per growing tray), and leaf samples were taken from the same individual plants four times during the production phase. The leaf samples were taken in hydroponic production always before changing the nutrient solutions in circulation, i.e. on 24 May, 30 May, 14 June, and on 7 August during the harvesting phase, at which time tuber samples were also collected. Dry matter and nutrient analyses of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), sodium (Na), boron (B), copper (Cu), manganese (Mn), zinc (Zn) and iron (Fe) were conducted by laboratory of Suomen ympäristöpalvelu Oy. When plants were grown in a peat-based growing medium, samples were taken during the same developmental phases as in hydroponic production, and harvesting was conducted on 7 August and sampling and analyses were conducted as in the case of hydroponic production. 2.5 Carry-Over Study After three months of cold storage (4 °C) minitubers sized 10-20 mm of the cultivars Van Gogh and Desiree were transferred to a greenhouse and planted in plastic growing boxes (45 cm × 60 cm) in a peat (Biolan, Novagrow) growing medium. The cultivar Asterix was not included in the carry-over study due to insufficient amount of over 10-20 mm sized minitubers. A total of 3,200 minitubers were studied (100 tubers × 2 techniques × 4 replications × 2 cultivars × 2 times). The plants were irrigated mechanically with a liquid fertilizer (Nutri S-A 0.5 ‰ solution). The greenhouse temperature was recorded by an automatic weather station (a-Weather, AWS -1.04B, Alab, Finland). The emergence dates of each individual plant were noted and the developmental stages observed according to Hack et al. (1993) with potential foliage symptoms recorded at one-week intervals. Harvesting was carried out 77 days after planting (DAP). At harvest, the number of stems, the number and weight of the tubers and the external quality of the tubers were assessed individually. 2.6 Statistical Analyses The statistical analyses were conducted using the Mixed procedure of the SAS 9.2/SAS Enterprise Guide 4.3 (SAS Institute Inc., Cary, NC, USA) program, using a variance analysis model in compliance with the split plot study design. Furthermore minitubers mean±standard deviation was conducted by Duncan’s multiple range test. 3. Results 3.1 Minituber Production The foliage of Desiree and Van Gogh developed faster during hydroponic production than that of Asterix. Within 37 days from planting the microplantlets on the hardening table and transferring the plants to the trays, Desiree and Van Gogh foliage covered the entire tray. Tuber formation (i.e. the first stolon ends swollen to double their size) started after 41 days of minituber production in both cultivars. Asterix foliage development was almost 3 weeks behind Van Gogh and Desiree. Asterix tuber formation started 62 days after the microplantlet phase. When plants were grown in growth rooms in a peat-based growing medium, foliage development occurred almost 3 weeks later and also tuber formation started approximately 18 days later in all cultivars compared to hydroponic production. Hydroponically-grown tubers were harvested 96 days from planting the microplantlets on the hardening table. Minituber yield was 4.5 per plant for the cultivar Desiree, 7.5 for Van Gogh and 4.0 for Asterix. The minitubers were graded in size categories < 20 mm and > 20 mm; 56.1-58.4% of the Desiree and Van Gogh cultivars and 95.3% of the Asterix cultivar were in the size category < 20 mm. The average weight of the minitubers was 11 g for the cultivar Desiree, 13 g for Van Gogh and 6 g for Asterix. In peat-based production, Desiree produced 4.4 minitubers per plant, Van Gogh 4.6 and Asterix 3.3. Peat-produced mini-tubers were all < 20 mm in size and weighed 5-9 g (Table 1). jas.ccsenet.org Journal of Agricultural Science Vol. 9, No. 1; 2017 45 Table 1. Comparison of hydroponic and peat minituber production methods on tuber number and tuber weight in cultivars Desiree, Van Gogh and Asterix Method Tuber number per plant Tuber weight per plant Minitubers grading (%) < 20 mm > 20 mm Hydroponic 4.5 ± 0.2 b 11.3 ± 1.5 a 58.4 41.6 Conventional 4.4 ± 0.7 b 7.5 ± 0.8 b 98.9 1.1 Hydroponic 7.5 ± 2.1 a 13.1 ± 3.6 a 56.1 43.9 Conventional 4.6 ± 1.2 b 9.2 ± 2.7 b 97.2 2.8 Hydroponic 4.0 ± 0.9 b 6.1 ± 2.9 b 95.3 4.7 Conventional 3.3 ± 2.0 b 5.0 ± 2.2 c 100.0 0.0 Note. Values denote mean±standard deviation and different letters indicate significant differences between means by Duncan’s multiple range test, p = 0.05. There were no significant differences in the nutrient concentrations of leaf or tuber samples between hydroponic potato production and potatoes cultivated in a peat-based growing medium. Calcium concentrations were slightly (not significantly) higher in hydroponically-produced tubers compared to tubers produced using a peat-based growing medium. Correspondingly, manganese and iron concentrations were slightly (not significantly) higher in peat-produced tubers (results not shown). EC values, pH levels and temperatures in hydroponic production varied throughout the production season: EC 1.3-1.6, pH 5.8-6.2 and temperatures 19-23 °C. The pH levels in peat-based production varied pH 6.2-6.4. 3.2 Carry-Over Effects of Techniques on Minitubers Hydroponically produced minitubers emerged slower than peat-based produced minitubers by a difference of 5 days (p = 0.000) (Figure 2), but no carry-over effect of either production method was found in foliage development, number of stems (p = 0.22) or number of tubers (p = 0.56) (Table 2). The number of stems varied from 1.6-1.7 and the number of tubers between 4.7-5.2 per plant. Although no carry-over effect of the methods was found in terms of the number of tubers, conventional peat production had a positive effect on crop yield (g/plant) (p = 0.005) (Figure 3), in cultivar Desiree. Correspondingly, the yield of hydroponically produced minitubers was 87 g/plant and conventionally peat-based 104 g/plant. 0 5 10 15 20 25 30 Hydroponic Conventional Hydroponic Conventional Van Gogh Desiree DAP Figure 2. Hydroponically produced minitubers foliage was developed slower than by minitubers produced in peat jas.ccsenet.org Journal of Agricultural Science Vol. 9, No. 1; 2017 46 Table 2. The production of minitubers was implemented with a hydroponic technique in growth-rooms and the carry-over effects of technique on characteristics of minitubers were studied Method Desiree Van Gogh Hydroponic Conventional Hydroponic Conventional Hydroponic Conventional Emergence (days after planting) 22.5*** 17.2*** 24.6*** 17.4*** 20.4*** 16.8*** Development (foliage) 6.7. 20.0 18.0 20.0 18.0 20.0 18.0 19.7. 49.0 47.5 49.0 48.0 49.0 47.0 2.8. 60.5* 55.5* 59.0* 55.0* 62.0* 56.0* 19.8. 67.5 67.0 69.0 69.0 66.0 65.0 No. of stems 1.6 1.7 1.5 1.7 1.7 1.8 No. of tubers 4.8 5.2 4.7 5.1 4.8 5.2 Fresh wt. of tubers (g/plant) 174 208** 154 207*** 194 209 Dry mattercontent of tubers (%) 22.8 24.0 20.5 22.0 25.0 26.0 Note. As a comparison, minitubers from in vitro plantlets (conventional) were grown in a peat-based growing medium. The cultivars were Desiree and Van Gogh. Statistically significant differences between methods or cultivars at *P = 0.05, **P = 0.01 and ***P = 0.001. There were no significant interactions between other production methods or cultivars for any other of the variables measured. 0 20 40 60 80 100 120 Hydroponic Conventional Hydroponic Conventional Desiree Van Gogh >50mm 40-50 mm 30-40 mm 15-30 mm g Figure 3. A carry-over effect of the production methods was found in yielding, with conventional peat production having a positive effect on yield (g/plant) (p = 0.005) and with a greater effect in cultivar Desiree than in cultivar Van Gogh 4. Discussion It is clearly important that the methods used in minituber production are cost effective, the production characteristics of the minitubers, such as fast rate of foliage development and high yielding capacity, are also desirable. In our study Desiree and Van Gogh foliage covered the entire trays in the hydroponic system within 37 days after planting and tuber formation of the Desiree and Van Gogh cultivars started within 41 days from planting, similarly to the studies of Chang et al. (2012) (within 30-65 days, depending on the cultivar and the production technique). Yields (4.5-7.5 per plant) in this study are comparable to the hydroponic production levels obtained by Ritter et al. (2001); also with regard to tuber weight (11-13 g). Tuber formation of the late-maturing cultivar Asterix (63 days) was considerably slower than that of Desiree or Van Gogh, and also the average weight of the tubers was lower (6 g). According to Chang et al. (2008), hydroponic production techniques may not be favourable with late-maturing cultivars because the nutrient solutions may retard root and stolon growth. In the present study minituber yield was higher in hydroponic production compared to peat-based production. In conventional minituber production (a peat-based growing medium), foliage development occurred almost 3 jas.ccsenet.org Journal of Agricultural Science Vol. 9, No. 1; 2017 47 weeks later and also tuber formation started approximately 18 days later in all cultivars compared to hydroponic production. One aim of this study was also to investigate whether hydroponic production technology effects on growth and yield of produced minitubers as carry-over effect compared to peat produced minitubers. In the carry-over study conventionally produced minitubers emerged and developed faster and produced higher yields than hydroponically produced minitubers. The carry-over effects indicate that more research is needed to clarify which cultivars are genetically the best suited into hydroponic production and especially comparison between early and late maturing cultivars. In our hydroponic system, the pH and EC levels of the nutrient solutions remained at the automatically regulated levels. At its lowest, the EC was 1.3 dS m -1 at the time of vigorous root growth, when the pH level was a maximum of 6.2. The results are consistent with the studies of Chang et al. (2012) in which decreased EC levels and increased pH levels were found to be indicative of active root growth and rapid intake of nutrients. There were no differences in the nutrient concentrations of leaf or tuber samples between hydroponic minituber production and minitubers cultivated in conventional peat-based growing medium. Potato does not require large amounts of illumination to photosynthesize effectively (Degamante & van der Zaag, 1988), but the amount of light and the relative proportions of wavelengths should be suitable for photosynthesis (Mathews, 2006; Yanovsky et al., 1998). In the present study, the amount of photosynthetically active illumination, 2383-2509 μmol m -2 s -1 , provided artificially by the growth room minituber production environment proved adequate. In addition to illumination, temperature is another key factor affecting tuber formation. According to Levy and Veilleux (2007), high night-time temperatures are more damaging than high daytime temperatures. In the present study the amount of heat produced by the high-pressure sodium lamps (4 × 400 W per growth room) could not be controlled effectively enough with the air conditioning equipment in use. This was evident, in particular, as inadequate differences between daytime and night-time temperatures in growth room temperatures of ca. 19-26 °C by day and 15-23 °C by night. Conclusion Our study provides significant information regarding indoor hydroponic minituber production system and artificial simulation of the production environment. In northern conditions (64-65°N), if a hydroponic production system is located in an environment, where natural light is utilized, makes it more difficult to achieve a combination of light intensity that is optimal for plant growth and tuber formation. Based on the present study, indoor hydroponic minituber production needs more research, because the carry-over effects on the characteristics of minitubers were not clear and the total carry-over yield needs to be improved to achieve at least the conventional peat-based production level. Also more studies are needed for choosing cultivars which are optimal to be used in hydroponic minituber production. Acknowledgements Our sincere thanks to Pirjo Riikola, Timo ter Voort, Tapio Uotila and Anu Kankaala for their technical assistance. The financial support received from the Potato Research Institute and the Ministry of Agriculture and Forestry of Finland is acknowledged. References Akita, M., & Ohta, Y. (1998). A simple method for mass propagation of potato (Solanum tuberosum L.) using a bioreactor without forced aeration. Plant Cell Reports, 18(3-4), 284-287. http://dx.doi.org/10.1007/s002990 050572 Chang, D. C., Cho, I. C., Suh, J.-T., Kim, S. J., & Lee, Y. B. (2011). Growth and yield response of three aeroponically grown potato cultivars (Solanum tuberosum L.) to different electrical conductivities of nutrient solution. American Journal of Potato Research, 88(6), 450-458. http://dx.doi.org/10.1007/s12230-011-92116 Chang, D. C., Par, C. S., Kim, S. Y., & Lee, Y. B. (2012). Growth and tuberization of hydroponically grown potatoes. Potato Research, 55(1), 69-81. http://dx.doi.org/10.1007/s11540-012-9208-7 Chang, D., Park, C. S., Kim, S. Y., Kim, S. J., & Lee, Y. B. (2008). Physiological growth responses by nutrient interruption in aeroponically grown potatoes. American Journal of Potato Research, 85(5), 315-323. http://dx.doi.org/10.1007/s12230-008-9024-4 Corrêa, R. M., Pinto, J. E. B. P., Faquin, V., Pinto, C. A. B. P., & Reis, É. S. (2007). The production of seed potatoes by hydroponic methods in Brazil. Fruit Veg Cereal Sci Biotech, 3(Special Issue 1), 133-139. jas.ccsenet.org Journal of Agricultural Science Vol. 9, No. 1; 2017 48 Demagante, A., & Van der Zaag, P. (1988). The response of potato (Solanum spp.) to photoperiod and light intensity under high temperatures. Potato Research, 31(1), 73-83. http://dx.doi.org/10.1007/BF02360023 Hack, H., Gall, H., Klemke, T., Klose, R., Meier, U., Stauss, R., & Witzenberger, A. (1993). The BBCH-scale for phonological growth stages of potato (Solanum tuberosum L.). Proc the 12th Annual Congress of the European Association for Potato Research (pp. 153-154). Paris. Kämäräinen-Karppinen, T., Virtanen, E., Rokka, V. M., & Pirttilä, A. M. (2010). Novel bioreactor technology for mass propagation of potato microtubers. Plant Cell, Tissue and Organ Culture, 101(2), 245-249. http://dx.doi.org/10.1007/s11240-010-9679-7 Levy, D., & Veilleux, R. E. (2007). Adaptation of potato to high temperatures and salinity―A review. American Journal of Potato Research, 84, 487-506. http://dx.doi.org/10.1007/BF02987885 Mathews, S. (2006). Invited review: Phytochrome-mediated development in land plants: Red light sensing evolves to meet the challenges of changing light environments. Molecular Ecology, 15(12), 3483-3503. http://dx.doi.org/10.1111/j.1365-294X.2006.03051.x Milinkovic, M., Horstra, C. B., Rodoni, B. C., & Nicolas, M. E. (2012). Effects of age and pretreatment of tissue-cultured potato plants on subsequent minituber production. Potato Research, 55, 15-25. http://dx.doi.org/10.1007/s11540-011-9203-4 Murashige, T., & Skoog, F. (1962). A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiologia Plantarum, 15(3), 473-497. http://dx.doi.org/10.1111/j.1399-3054.1962.tb08052.x Novella, M. B., Andriolo, J. L., Bisognin, D. A., Cogo, C. M., & Bandinelli, M. G. (2008). Concentration of nutrient solution in the hydroponic production of potato minitubers. Ciência Rural, 38(6), 1529-1533. http://dx.doi.org/10.1590/S0103-84782008000600006 Ritter, E., Angulo, B., Riga, P., Herran, C., Relloso, J., & San Jose, M. (2001). Comparison of hydroponic and aeroponic cultivation systems for the production of potato minitubers. Potato Research, 44(2), 127-135. http://dx.doi.org/10.1007/BF02410099 Rolot, J. L., & Seutin, H. (1999). Soilless production of potato minitubers using a hydroponic technique. Potato Research, 42(3-4), 457-469. http://dx.doi.org/ 10.1007/BF02358162 Veeken, A. J. H., & van der Lommen, W. J. M. (2009). How planting density affects number and yield of potato minitubers in a commercial glasshouse production system. Potato Research, 52(2), 105-119. http://dx.doi.org/10.1007/s11540-008-9124-z Vreugdenhil, D., Bradshaw, J., Gebhardt, C., Govers, F., Taylor, M. A., MacKerron, D. K. L., & Ross, H. A. (2007). Potato Biology and Biotechnology. Advances and Perspectives. Elsevier, Oxford, Amsterdam. http://dx.doi.org/10.1017/S0014479708006832 Yanovsky, M., Alconada-Magliano, T., Mazzella, M., Gatz, C., Thomas, B., & Casal, J. (1998). Phytochrome A affects stem growth, anthocyanin synthesis, sucrose-phosphate-synthase activity and neighbour detection in sunlight-grown potato. Planta, 205(2), 235-241. http://dx.doi.org/10.1007/s004250050316 Copyrights Copyright for this article is retained by the author(s), with first publication rights granted to the journal. This is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).