Hydroponic Minituber Production in Growth Room Conditions and Carry-Over Effects of the Technique on Produced Minitubers

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 ms; 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.


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 UK) and humidity levels were predominantly below 60%.Temperature and humidity readings were followed with data loggers (Netafim, Climate Box, Netafim, Israel).

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.

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.

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.

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).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.

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.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

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 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.

Figure 2 .
Figure 2. Hydroponically produced minitubers foliage was developed slower than by minitubers produced in peat

Table 1 .
Comparison of hydroponic and peat minituber production methods on tuber number and tuber weight in cultivars Desiree, Van Gogh and Asterix Note.Values denote mean±standard deviation and different letters indicate significant differences between means by Duncan's multiple range test, p = 0.05.