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Lung’aho C., M. Nyongesa, M.W. Mbiyu, N.M. Ng’ang’a, D.N. Kipkoech, P. Pwaipwai,
and J. Karinga
Kenya Agricultural Research Institute-Tigoni, P.O. Box 338-00217 Limuru, Kenya

Potato is the second most important staple food after maize in Kenya. Limited accessibility to high quality
potato seed is a perennial problem amongst many growers and is partly attributable to inefficiencies in various
stages in the seed production system. The objective of this paper is to review and assess the various options
available for basic seed tuber production and suggest the way forward with particular emphasis on the
applicability of aeroponics technology for minituber production in Kenya. Tissue culture derived greenhouse
and screenhouse grown minitubers are the standard method used in modern limited generation seed potato
programs. Aeroponics would appear to have a number of potential attributes to make seed potato production
more efficient. The technique has potential to eliminate all but one generation of seed potato multiplication in
the field, thus lowering costs and raising the plant health quality of the first field production generation.
Important considerations that should be addressed to ensure an efficiently functioning system are highlighted in
this paper. Possible areas of research to improve productivity include optimizing nutrient solutions, plant
density, number of harvests and harvesting intervals.

Key words: Aeroponics, clonal multiplication, hydroponics, minituber production, rapid multiplication
techniques, tissue culture

The potato is one of the most important food crop in Kenya. It is cultivated in high altitude areas between 1,500
and 3,000 metres above sea level. The major production areas are found in Rift Valley, Central and Eastern
provinces (MOA, 2008). The crop is mainly grown by small scale farmers who account for over 90% of the
production. Most of the production is mainly rainfed and carried out in scattered patches of intensive small-scale
agriculture (McArthur Crissman, 1989; Anonymous, 2009).

Timely availability of high quality planting material of suitable potato varieties is considered to be one of the
most limiting constraints of this crop in Kenya (Crissman et al., 1993). Currently, only 1-2% of the seed potato
demand is being met. An inefficient seed system is a bottleneck to further growth of the potato industry and
negatively impacts on the potato value chain. In the absence of continuous sources of good quality seed many
growers opt to use their own farm saved seed or those obtained from markets. Such practices contribute towards
sub-optimal yields as well as spread of diseases such as bacterial wilt (Ralstonia solanacearum), a serious
disease of potatoes, and viruses (Kaguongo et al., 2008). Properly functioning seed systems generally start with
healthy planting material which is multiplied several times with low rates of degeneration and at an affordable
price. The aim of a multiplication system for seed production should be to produce or maintain a stock of
nuclear seed and multiply this stock as rapidly as is economically feasible, and to feed the system of commercial
seed production with healthy material at a rate that is in accordance with the existing rate of seed degeneration
(Struik and Wiersema, 1999). The trend in seed potato production systems is to minimize the number of field
multiplications because of the inefficiency (and thus high costs) and continuous risk of diseases (mainly, viral,
bacterial and fungal) and pests (such as nematodes) associated with many field multiplications. The objective of
this paper is to review and assess the various options available for basic seed tuber production and suggests the
way forward with particular emphasis on the applicability of the aeroponics technology for minituber production
in Kenya.

Materials and Methods
Data on potato production was obtained from the FAO data base (FAOSTAT, 2010). Additional information
was obtained from secondary literature sources including annual reports, journal papers, seed reports and
specialized documents dealing with seed potato production. Seed potato requirements were computed using the
following formulae:

(i)      Total seed requirement (tonnes) = Area under potatoes (ha) x planting rate (t/ha)
(ii)     Effective seed demand (tonnes) = Proportion of farmers likely to purchase seed x rate of seed renewal

(iii)      Quantity of pre-basic seed required to sustain the effective seed demand (tonnes) = Effective seed
           demand x 0.6 x 0.1 x 0.1, where 0.6 is the average multiplication factor during the first multiplication
           factor of minitubers and 0.1 is the average multiplication factor for subsequent field multiplications
           based on experiences at KARI-Tigoni.
(iv)       Number of minitubers = Quantity of minitubers required/average weight of one minituber (assumed to
           be 10 g).

Results and Discussion
Area under potato and current seed potato requirements
In Kenya, the area under potato has expanded from 87,846 ha in 1990 to 120,000 ha in 2008 (Table 1). At a
planting rate of 2 t ha-1, the total annual certified seed requirement is estimated to have risen from 175,692 in
1990 to 240,000 t in 2008. The effective seed potato demand is much less than this because of two major
factors. First, according to previous work no more than 30% of the farmers purchase improved seed including
potato seed (Venkatesan, 1994) and secondly, because of seed degeneration, the gradual fall in yields due to
accumulation of diseases, farmers are generally advised to renew their seed stocks after 4 seasons of planting i.e.
once in every two years. Thus, the effective potato seed demand was estimated to be 36,000 t of certified seed
per year in 2008 (i.e. 240,000 t x 0.30 x ½). The quantity of pre-basic seed required to sustain the effective seed
demand is approximately 60.0 t. The major assumptions taken in calculating the basic seed requirement are that
there will be three field multiplications at a multiplication rate of 1:6 for the first field multiplication and 1:10
for the second and third field multiplications. Based on the assumption that one minituber weighs 10 g then 60.0
t of pre-basic seed are equivalent to 6.0 million minitubers.

        Table 1: Seed potato requirements derived from area under potatoes
         Year      Area       Total seed    Effective     Quantity of pre-basic           No. of minitubers
                   (ha)       requirement seed            required to sustain             required to sustain
                              (tonnes)      demand        effective seed demand           the pre-basic seed
                                            (tonnes)      (tonnes)                        requirement
         1990      87,846     175,692       26,354        43.9                            4,390,000
         1991      87,110     174,220       26,133        43.6                            4,360,000
         1992      68,018     136,036       20,405        34.0                            3,400,000
         1993      55,670     111,340       16,701        27.8                            2,780,000
         1994      83,007     166,014       24,902        41.5                            4,150,000
         1995      96,143     192,286       28,843        48.1                            4,810,000
         1996      98,024     196,048       29,407        49.0                            4,900,000
         1997      118,638    237,276       35,591        59.3                            5,930,000
         1998      90,391     180,782       27,117        45.2                            4,520,000
         1999      114,614    229,228       34,384        57.3                            5,730,000
         2000      108,463    216,926       32,539        54.2                            5,420,000
         2001      121,490    242,980       36,447        60.7                            6,070,000
         2002      111,747    223,494       33,524        55.9                            5,590,000
         2003      126,529    253,058       37,959        63.3                            6,330,000
         2004      128,485    256,970       38,546        64.2                            6,420,000
         2005      120,859    241,718       36,258        60.4                            6,040,000
         2006      116,348    232,696       34,904        58.2                            5,820,000
         2007      120,000    240,000       36,000        60.0                            6,000,000
         2008      120,000    240,000       36,000        60.0                            6,000,000
         2009*     153,114    30,6228       45,934        76.6                            7,660,000

        Table 2: Comparison of different methods of pre-basic seed production
                                                                   Minituber production method
        Characteristic                              Conventional Conventional
                                   mutliplication                                     Aeroponics
                                                    stem cuttings    tissue culture
        Mutliplication rate        1:6-10           1:3-5            1:6-10           1:50-100
        Risk of disease infection  Low              Modest           Modest           High

    and contamination of an
    entire production unit
    Labour required                High               Modest             Modest              Modest
    Land requirement               High               Medium             Medium              Low
    Cost investment                Relatively low     Modest             Modest              investment is
    Energy dependence
                                   Not required       Required           Required            Not required
    (Sterilization of substrate)
    Ease of harvesting             Not that easy      Easy               Easy                Relatively easy
    Potential for using
    Rhizobial inoculation to       ?                  ?                  ?                   High
    increase yields
    Possibility of increasing                                                                period can be
                                   Nil                Nil                Nil
    the vegetative period                                                                    longer by 2-3
                                                      Possible but       Possible but not
    Sequential harvests            Not possible                                              Possible
                                                      not convenient     convenient
    Level of management
                                   Average            High               High                Very high
    level required
    Possibility for tuber size
                                   Minimal            Moderate           Moderate            High
                                   No sterilization   Substrate          Substrate           No sterilization
                                   of substrate       sterilization      sterilization       of substrate
                                   required           required           required            required
    Water requirements             High               Modest             Modest              low
    Nutrient recirculation         Nor possible       Nor possible       Nor possible        Possible
    Potential for reducing the
    number of field                Nil                Medium             Medium              High
    Potential for automation       Nil                Medium             Medium              High
                                                      Relatively                             Relatively
                                                                         Relatively cheap
                                                      cheap and                              cheap and
    Transportability of pre-       Costly due to                         and convenient
                                                      convenient due                         convenient due
    basic seed                     bulkiness                             due to small size
                                                      to small size of                       to small size of
                                                                         of tubers
                                                      tubers                                 tubers

Current strategies for basic seed tuber production
Clonal multiplication
The conventional method of seed potato production is to repeatedly propagate a sample of tubers which have
been proved to be free of pathogens in a system called clonal multiplication (Bryan, 1981; Struik and Wiersema,
1999). The clonal multiplication system has been been practised effectively in the Netherlands, South Africa
(Haverkort, 2007) and Kenya (Crissman et al., 1993).

The clonal multiplication system as practiced in Kenya is based on the row and plot technique (Figure 1). The
first generation (clone A) consists of minitubers produced either in pots or from an individual

motherplant. Clone A is planted in a single row (the row unit). If any plant tests positive for diseases the entire
row is discraded. The second and third generations (Clone B and C) continue to be seaprated to distinguish the
original motherplant source and are planted in plot units where roguing is done during certication inspections.
This system of seed increase is laborious, expensive and time consuming. Because of a low multiplication rate
(usually 1:6), it takes many years to build significant quantities of seed to meet the demand of the potato
industry. Another constraint is the degeneration of seed stocks from one generation to another due to
accumulation of bacteria, fungi, viruses and viroids. Modern seed programmes tend to minimize use of clonal
systems or use a combination of clonal and rapid multiplication systems.

Rapid multiplication techniques
There are several in vitro and semi-in vivo techniques that multiply healthy planting material rapidly. These
include sprout cuttings, stem cuttings, leaf-bud cuttings, single node cuttings and tissue culture (Ranalli, 1997;
Struik and Wiersema, 1999). The major disdavantage of the first four techniques are that they are heavily
dependent on fuel energy which is used to sterilize the propagation media. Many seed programmes prefer to use
minitubers, defined as the small tubers, (usually 5-25 mm), that can be produced through out the year under
semi-in vivo conditions in glasshouses and screenhouses using in vitro propageted plantlets, planted at high
density. Production of minitubers is a resource and labour intensive operation. Several techniques have been
evaluated and proved suitable for minituber production including aeroponic culture (Kang et al., 1996; Kim et
al., 1999; Nugaliyadde, et al., 2005); hydroponic culture in inert aerated substrate (such as wood, perlite,
vermiculite), in which case irrigation takes place by percolation, drip irrigation and sub-irrigation (Correa et al.,
2009) and nutrient film cultures, i.e. hydroponics systems in which roots grow directly in either a pure
circulating nutrient solution or in a
circulating nutrient solution system with very little substrate (Struik and Wiersema, 1999). In Kenya, techniques
for minituber production that have been used include various types of cuttings and in vitro plantlets planted
either in pots or beds in sreenhouses and glasshouses (Lung’aho et al., 1997).

The case for aeroponics
Aeroponics is an alternative method of soil-less culture in growth-controlled environments (Figures 2-4). The
underground parts are enclosed in a dark chamber and supplied with a nutrient solution by way of a mist device
(Otazu, 2010). Aeroponics optimizes root aeration resulting in a high yield of minitubers compared to
hydroponics (Soffer and Burger, 1988). Additional advantages include limited water use, nutrient recirculation
and good monitoring of nutrients and pH (Figure 1). The aeroponics technique has been used in production of
different horticultural and ornamental seeds (Biddinger et. al., 1998, He and Lee, 1998, Molitor et al., 1999).
An aeroponic system for seed tuber production has been applied successfully in Korea (Kang et al., 1996; Kim
et al., 1999) and China. In Africa, the technology is being pilot tested in several countries including Kenya,
Uganda and Malawi. In Kenya, there are several pilot units at KARI-Tigoni, Genetics Technology International
Limited, Kisima Farm, Suera Farm and Agricultural Development Farm, Molo.

Fig. 2: An aeroponic system for quality seed potato production (Source: Otazu, 2010)

Fig. 3: Foliage development under aeroponics conditions (Source: Otazu, 2010)

Fig. 4: Tuber development under aeroponic conditions (Source: Otazu, 2010)

Although the inhibition of tuberization in immersed organs or organs subjected to continuous mist culture has
been reported (Tibbitts and Cao, 1994), tuberization under these procedures could be promoted under stress
conditions such as N deficiency (Kraus and Marschner, 1982) and short term reductions in nutrient solution pH
(Wan et al., 1994). Ritter et al. (2001) demonstrated that aeroponics minituber production under temperate
weather conditions substantially improved yields. Farran et al. (2006) reported a minituber yield of 800
tubers/m2 at a plant density of 60 plants m2 over a five month period in weekly harvests which translates to a
multiplication rate of 1:13. They also found the field performance of aeroponically produced tubers to be similar
to minitubers produced aeroponically. Research at the International Potato Centre shows that multiplication
rates of 1:100 can be achieved (Otazu, 2010). Harvesting minitubers in aeroponic systems is convenient, clean,
and permits a greater size control through sequential harvesting (Ritter et al., 2001). Lommen and Struik (1992)
found that the number and timing of non-destructive harvests were the key factors in the optimization of
minituber production. A major limitation of aeroponic systems is that they are dependent on the low volume
available to the root system and any losses of power to pumps can produce irreversible damages including
complete loss of plantlets. Additionally, any pathogen infecting the roots can lead to loss of all the plants in a
production unit (Otazu, 2010).

Growers who have planted minitubers indicated that the minitubers are quite delicate in comparison with
conventional seed tubers and that the incidence of rotting can be quite high. They also reported that the
technology of growing the minitubers is lacking.

Conclusion and future prospects
Aeroponics would appear to have a number of potential attributes to make potato production more efficient. The
aeroponics technique has the potential to eliminate all but one generation of seed potato multiplication in the
field thus lowering costs and raising the plant health quality of the first field production generation.
Furthermore, aeroponic systems offer the opportunity to determine the precise fertilizer requirements for the
crop, and thus increase productivity (using drip fertigation), reduce fertilizer use, and reduce the risk of
excessive fertilizer residues moving into the subterranean water table. Important considerations should,
however, be addressed before setting up a commercial system for minituber production. A fully automated
system monitoring both nutrient solution and dynamic parameters, such as concentration, pH, and flow rate
should be incorporated into the system. The system should also be equipped with suitable sensors and alarms to
control electric power, pumping, spraying, and chemical variables. Both the cost of these additional facilities
and such labour-intensive tasks as staking and manual harvesting of plants should be economically assessed
prior to switching to the system. Power generators, solar or wind systems are an absolute necessity especially in
areas with frequent power interruptions. To optimize an aeroponics system, appropriate nutrient solutions, plant
densities, number of harvest and harvesting intervals, as well as possible interactions between them require to be
studied. Agronomic packages for growing mintubers under field conditions need to be optimized.

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