Comparison of hydroponic and aeroponic c

SAN J O S E NEIKER - Instituto Vasco de Investigaci6n y Desarrollo Agrario, Apartado 46, 01080 Vitoria- Gasteiz, Spain Accepted for publication: 26 October 2000 Additional keywords: Sola

Potato Research 44 (2001) 1 2 7 - 1 3 5

Comparison of hydroponic and aeroponic cultivation systems for the production of potato minitubers

E R I T T E R , B A N G U L O , P R I G A , C H E R R A N , J R E L L O S O and M SAN J O S E NEIKER - Instituto Vasco de Investigaci6n y Desarrollo Agrario, Apartado 46, 01080 Vitoria- Gasteiz, Spain

Accepted for publication: 26 October 2000

Additional keywords: Solalmm mberosum L., in vitro culture, micropopagation, seed potato, repeated harvesting

Summary

Two different cultivation systems, aeroponics and hydroponics in greenhouse beds, were compared for the production of potato minitubers Plants in the aeroponic system showed increased vegetative growth, delayed tuber formation and an extended vegetative cycle of about seven months after transplanting Therefore in 1999, two production cycles were obtained with the hydroponic system, but only one with the aeroponic system However, compared with total production in hydroponics, the tuber yield per plant in the aeroponic system was almost 70% higher and tuber number more than 2.5 fold higher Average tuber weight was reduced by 33%

in the aeroponic system Advantages and possible problems with the aeroponic system for minituber production are discussed

Introduction

Seed potato production is mostly based on in vitro plantlets or microtubers, and on the subsequent production of minitubers as first ex vitro generation (Ranalli, 1997) Minitubers can be produced after acclimatisation from plantlets which are planted

at high densities in the greenhouse in beds (Wiersema et al., 1987) or in containers (Jones, 1988) using different substrate mixtures, or even in hydroponic culture (Muro

et al., 1997) L o m m e n (1995) presented alternative production techniques for mini- tubers using very high plant densities and non-destructive, r e p e a t e d harvesting of minitubers by lifting plants carefully from the soil mixture and replanting them after harvest These techniques allowed minitubers of ideal size to be produced, the n u m b e r

of tubers could be increased considerably, while total yield was reduced

In m o d e r n horticulture, different soil-less production techniques such as Nutrient Film Techniques (NFT, Cooper, 1979) and aeroponics (Peterson et al., 1968) have been developed The technique of aeroponic culture is an optional device of the soil- less culture methods in growth-controlled environments such as greenhouses This

m e t h o d consists of enclosing the root system in a dark c h a m b e r and supplying a solution of water and mineral nutrients with a mist device This technique has been applied successfully for the production of different horticultural species including

lettuce (Cho et al., 1996; Gysi & v o n Allmen, 1997; He & Lee, 1998), tomato (Biddinger et al., 1998), cucumber (Park et al., 1997) and ornamental plants such as chrysanthemum (Molitor et al., 1999) or poinsettia (Scoggins & Mills, 1998)

Despite increasing interest in soil-less culture methods in commercial horticultural production, little information is available for potatoes Earlier work has shown good results with NFT for potato tuber production (Wheeler et al., 1990; Wan et al., 1994) However, tuber initiation was poorer in nutrient solution without solid media than

in porous media (e.g perlite or vermiculite) The tuberization inhibition of stolons immersed in a solution could be the consequence of the lack of mechanical resistance (Vreugdenhil & Struik, 1989) Recently, an aeroponic system for seed potato pro- duction was established successfully in Korea (Kang, 1996a,b; Kim et al., 1997, 1999) under tropical and subtropical conditions

In view of the favourable results obtained by L o m m e n (1995) with respect to re- peated harvesting and considering the characteristics of aeroponic culture, the com- bination of both techniques seems to be particularly useful for minituber production

in potato In this paper we describe a method for the production of minitubers using aeroponics, and present first comparative results with respect to traditional produc- tion schemes in greenhouse beds using hydroponics

Materials and m e t h o d s

Plandet preparation Plantlets of the local cultivar Nagore were produced in vitro according to Espinoza et al (1984) from replicated nodes on standard Murashige- Skoog medium (Murashige & Skoog, 1962) After the last multiplication step, nodes

of the plantlets were cultivated under non-sterile conditions on filter paper soaked with the same medium but without agar and sugar in order to induce root formation Rooted nodes were transferred after 6 to 7 days to seedling trays containing a mixture

of 60% white and 40% black turf with added fertilizers as substrate Plantlets were acclimatised for one week under the humid conditions of a plastic cover and rege- nerated for three weeks to small potato plants of 4 to 5 cm height This plant material constituted the starting material for the assays Two cultivation systems (hydroponics

in greenhouse beds and aeroponics) were assayed and compared

Hydroponic system Two production cycles were obtained during 1999 near Vitoria in Northern Spain One was from March 3 to June 29 and the second from July 15 to November 10 Temperature was maintained between 18 and 22 ~ by a cooling/ heating system The plants grew under natural light conditions without additional light supply The small potato plants were planted without washing the roots into a greenhouse bed containing perlite (Agroperl) Substrate depth was approximately 30

cm The plant density was 10x10 cm Nutrient solution (pH 6.5, EC=mS cm -t, NO3-, SO42-, H2PO 4", CI-, K +, Ca 2+, Mg 2+, NH4 +, Na + and micronutrients) were supplied when necessary using drop irrigation Fungicide and insecticide treatments were applied when appropriate After 16 weeks, the plants had nearly completed their vegetative cycle, haulms were destroyed by a total herbicide (Paraquat) and tubers

were harvested one week later F o u r representative areas of 1 m 2 (100 plants) were selected within the greenhouse bed and harvested separately T u b e r numbers, and tuber yield were r e c o r d e d for each replicate Shortly after harvest a new set of small

p o t a t o plants were planted in the same greenhouse bed The plants were cultivated under identical conditions and harvested as described before

Aeroponics A e r o p o n i c culture was p e r f o r m e d in the same greenhouse in six closed

containers (60 cm height, 45 cm width, 85 cm depth), which had a r e m o v a b l e front panel for control and harvesting Small acclimatised plantlets of cv Nagore were planted on 3 May by fixing them on top of the boxes with the help of a gum grid Perlite was used to seal the grid In this way the foliage of the small plants grew in the light while roots, stolons and tubers developed in total darkness in the containers

T h e plant density was 10xl0 cm with a total of 36 plants per container Water and nutritive elements were supplied with the same nutrition solution as in the hydro- ponic culture For this purpose four fog nozzles (4 l/h) located at the b o t t o m of the containers sprayed nutrient solution every 10 min for 3 sec into the inner part of the box in order to k e e p the roots wet Residual nutrition solution was recycled Fig 1 shows the aeroponic device with plants Since the plants reached a considerable height during their d e v e l o p m e n t it was necessary to stake them using ribb.ons The same phytosanitary treatments were applied as in the hydroponic system Tubers were harvested repeatedly and in total eight times (Table 1) The harvest criteria were to r e m o v e all tubers which were larger than 15 to 20 m m in length A t the last harvest all tubers larger than 5 m m were considered The first harvest was on 5 July when the first tubers of sufficient size were available The following harvest intervals were around one m o n t h and were later shortened due to increased production and tuber d e v e l o p m e n t (Table 1) The n u m b e r of tubers and total yield were recorded each time in each box A f t e r each harvest the plants were re-fixed in the gum grid approximately 2 to 4 cm lower, in order to allow new stolon and subsequent tuber formation

Data processing and statistical analyses Analyses of variance were p e r f o r m e d using

SAS, Proc G L M (SAS Institute, 1989)

Results

The potato plants in the greenhouse bed reached a final height of 90 to 110 cm, while those of the aeroponic system reached a total height of 150 to 180 cm Increased vegetative growth of plants in the aeroponic system was also observed for the root system and for stolon length (data not shown)

Also as expected, plants in the bed completed their biological cycle approximately four months after transplanting, while the aeroponic plants continued to grow and

f o r m new shoots f r o m lateral buds Occasionally secondary growth was observed which sometimes led to the f o r m a t i o n of new stolons with additional tubers Plants in aeroponic culture finished their cycle approximately seven months after transplanting

Fig 1 Aeroponic system: A) empty aeroponic containers with fog nozzles and connecting tubes, B) acclimatised plantlets growing in aeroponic system, C) fully developed potato plants,

D) minitubers obtained

Results of the measured yield parameters are shown in Table 1 A total of 4.9 tubers per plant was obtained in both cycles with the hydroponic plants Total yield was approximately 65.3 g per plant and the corresponding average tuber weight was 13.3

g This weight corresponds to a grade of 40-45 mm in cv Nagore which has long tubers with a form index of 1.8 to 2.2 (length/width) In the first production cycle a higher yield with slightly reduced tuber numbers but considerably larger tubers were obtained

Although plants of the hydroponic system were planted two months earlier, tuberization and tuber development were delayed in the aeroponic system as can be seen in Table 1, with respect to the first three harvesting dates The development of aerial parts of the plants was initially favoured Production between consecutive harvests increased during development and was reduced as the plants senesced After

a slow initial phase of tuber formation, fewer but larger tubers were produced A large peak in total production due to an increasing n u m b e r of smaller sized tubers was observed after five months of development (6 th harvest date) A total tuber yield

Table 1 Yield parameters per plant obtained with different minituber production systems

Aeroponic system

(Cycle from May 3 to December 3)

TN/P= average number of tubers per plant,

Y/P=yield per plant (g),

ATW=average tuber weight (g),

Pr>F=probability for F-value from Analyses of variance

of 109.9 g was obtained per plant which was c o m p o s e d on average of 12.4 tubers with

an average tuber weight of 8.9 g (grade 30-35 mm) C o m p a r e d with each single harvest in the hydroponic system, its n u m b e r of tubers and total yield was three to four times higher

In order to c o m p a r e the two systems, we considered the whole year as a productive period Therefore, tuber numbers and yields were summed over both production cycles

in the hydroponic system and c o m p a r e d with total tuber n u m b e r and yield of the aeroponic system by means of analyses of variance The influence of the cultivation system was highly significant for all analysed yield p a r a m e t e r s (Table 1) C o m p a r e d with the total production in hydroponics, the tuber yield per plant in the aeroponic system was almost 70% higher and tuber n u m b e r was over 2.5 fold higher However, average tuber weight was 33% less in the aeroponic system

Previous work showed that the post-harvest behaviour of tubers seem to be similar

in both production systems (unpublished data) Tubers grown in aeroponics showed

an increased n u m b e r of o p e n e d lenticels, due to high humidity conditions in the system H o w e v e r , when stored under appropriate conditions, no differences were found with respect to storage (% rotten tubers; velocity and intensity of sprouting) When planted in the greenhouse or field, no apparent differences in development and tuber yield was observed c o m p a r e d with hydroponic seed tubers (results not shown)

Discussion

Soil-less cultivation systems frequently used in horticulture present several advantages compared with soil production Hydroponics lead to higher production (Resh, 1978; FAO, 1990) Gysi & v o n Allmen (1997) found higher yield for tomatoes grown in NFT and aeroponics than grown in soil In previous experiments the hydroponic system proved more convenient than the traditional propagation system using peat/ sand mixtures and mineral fertilisers (unpublished data) These findings confirmed those of Muro et al (1997), who obtained increased yield and tuber numbers with hydroponics Moreover, hydroponics gave higher quality and were from one week

to one month earlier (Morard, 1995), avoided several soil pathogens or unsuitable physical and chemical soil properties, and optimised crop production by the enhancement of water and mineral availibility in the root environment However, aeroponic techniques optimise root aeration which is certainly the major factor leading

to a yield increase compared with classical hydroponic systems (Softer & Burger, 1988) Cho et al (1996) observed in cherry tomatoes a growth and yield increase in aeroponic compared with classical hydroponics

In the present study, increased vegetative growth of potato plants and delayed tuberization was observed in aeroponic systems compared with hydroponics, which might be due to the unlimited nitrogen supply in our aeroponic system Kang et al, (1996a) presented similar observations when the N supply was increased in the nutrient solution of aeroponically-grown potato plants Furthermore, Vreugdenhil & Struik (1989) found that tuber initiation needs the cessation of stolon growth, which

is linked to ethylene synthesis A delay of tuberization has been observed when the stolon environment did not provide mechanical stress (Lugt et al., 1964) This is so in aeroponic cultures, where roots grow in air without mechanical resistance Moreover, the increased stolon length obtained in the present study supports this hypothesis Previous observations showed that absolute darkness is necessary for tuber formation, otherwise with a minimum of light stolon tips developed small bleached leaves and no tuber formation occurred However, hand pulling down the plants after each harvest was necessary to increase the formation of new stolons with tubers The same beha- viour was also observed in the repeated harvest assays of Lommen (1995) who replanted the plants deeper after a previous harvest

The number of tubers and yield in our hydroponic system was favourable and within the expected range Compared with the results obtained by Muro et al (1997), production in terms of tuber number, yield and average tuber weight was higher, possibly because a higher plant density was used in our study Nevertheless, with the aeroponic system for minituber production with repeated harvesting we were able to increase yield and especially tuber number although there were two productive cycles per year with hydroponics (Table 1) This can be attributed to an improved availa- bility of nutrients, mainly calcium, that is required in stolon tips for tuber initiation (Balamani et al., 1986), and due to the removal of the dominant large tubers, which allow initiation of new tubers as well as the development of existing tubers (Lommen, 1995) Moreover, tuber number is positively correlated with leaf area and number of

young leaves (Kahn et al., 1983), and this is so in aeroponic culture, where potato plants showed greater vegetative growth Using repeated harvesting systems Lommen (1995) obtained up to 3500 tubers of small size (>5 mm) using high plant densities of

up to 800 plants/m 2 However, our local agroclimatical conditions require larger minitubers to obtain an acceptable yield when planted in the heavy soils of the local fields This is particularly important under dry weather conditions if irrigation is not available Furthermore, compared with L o m m e n (1995), total tuber yield could be increased in our aeroponic system, since root damage due to lifting and replanting can be avoided

Similar post-harvest behaviour of tubers from both production systems is consistent with the results of Kim et al (1999), who also did not find differences in growth and tuberization behaviour of seed tubers obtained from aeroponics compared with normal seed tubers

The harvest technique in aeroponics is convenient and clean, and repeated harvesting offers the possibility of obtaining tubers of a desired size However, many aspects of this technique have to be investigated in order to optimise the system These include, for example, studies on appropriate nutrient solutions, plant densities and number of harvests and harvest intervals, as well as all possible interactions between production factors With respect to harvesting intervals Kim et al (1997) found that highest yields were obtained with shorter intervals of 10 days Furthermore, Kim et al (1999) improved production by increasing light intensity and by CO 2 enrichment Other aspects which need to be considered include possibilities of automatization Labour and cost intensive processes in our system represented, for example, the staking of plants, manual harvesting and pulling down of the plants after each harvest Furthermore, in order to introduce the aeroponic system for large-scale minituber production an economic evaluation is necessary because methods of soil-less culture, and particularly aeroponics, present some disadvantages for industrial production: lack of water and chemical buffering capacities that must be compensated by security systems (alarms, pumps), high infrastructure costs, high technology and a specialised organisation of growers The results presented here lead to the conclusion that aeroponics can be an appropriate system for producing potato minitubers under temperate climatical conditions as it is in tropical areas (Kang et al., 1996a)

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