Growing Potatoes

Comparative production of in vitro mini-tuber potatoes grown in sawdust and specially blended media. The experiment aimed to analyse the cost effectiveness and viability of using a specifically designed blended growing media in comparison to commonly available sawdust.

Report by Ben Safronovitz

Potatoes are the world’s fourth largest food crop. Production of in vitro mini-tubers of potatoes is a rapidly growing industry due to the need for a constant supply of potato plant material, which is free of pathogens and of a high quality. Production of in vitro tissue culture plantlets in hydroponics culture is highly effective and controllable, ensuring ‘clean’ conditions and mass production.

Using hydroponics culture effectively means that employing soilless culture specifically for the type of plant such as the potato required a specific growing media that can support the characteristics of the potato plant, initiation of tubers and tuber development.

In vitro tissue culture plantlets development

Plantlets are mini-plants at a very early stage of development. Contrary to potato tubers used as plant material in potato cultivation, plantlets do not have food storage in the form of a tuber, which contains enough nutrients to build up root formation, leaves etc.

Plantlets therefore depend entirely on growing conditions, supporting the root zone, precise nutrient supply and balanced moisture at the root environment.

When a plant is established, it will develop a strong vegetative stage to build up enough carbohydrates to eventually transform to food storage in the form of tubers. The process of shifting from vegetative stage to production is triggered by certain hormones, growing conditions and other factors, which promotes tuber initiation (elongation of stem underground [stolon] that transforms from stem to tuber).

The final result of the growing process is tubers that are genetically identical to the mother plant from which the tissue culture plantlets were taken. These tubers are considered ‘potato ‘seeds’ and are ready for planting as tuber plant material following treatment in cool dark storage conditions, which promotes the formation of buds. The ready tubers are then sold to growers as plant material and are planted in open field cultivation.

In vitro potato seed production requirements

Due to the extreme vulnerability of the plant, it is grown in a clean environment in greenhouses under controlled climatic conditions and strict nutrient and irrigation protocols.

The commercial viability of potato tuber production is based on the following criteria:

•  Targeting highest yield possible of marketable tubers per sq/m

•  Growing varieties, which are disease resistant, producing high yield. Shape, size and weight, which justifies its economics

•  All the above in considerably short growing cycle, which is achievable in hydroponics greenhouses.

There are certain factors that directly influence the final results when it comes to in vitro potato cultivation:

•  The variety genetics (prolific producer, large size tubers, small tubers, nutritional values etc.)

•  Time of the year

•  The geographic location of the growing site and its climate

•  The growing technique

•  The nutrients and irrigation program

•  The growing media

•  Management.

Varieties

Usually, varieties, which are newly developed and proven to fit the commercial criteria, are grown massively to feed the market need. Some varieties are more suitable to certain profiles of the market than others; some are more successful due to climate and location.

Time of the year

The potato is adaptable to a certain degree to various climates and times of the year. Its nature of growing better in rising spring temperatures and increasing day length can be easily manipulated to successful growth in short day and at considerably lower temperatures , even if results are less impressive in comparison to the perfect season.

Geographic location

Most successful in vitro growing structures are located geographically in mild temperature zones in summer, which suit the genetics of the potato better.

The growing technique

There are various techniques (all in hydroponics culture), which are used (i.e. bag culture, open beds, containersaeroponics, etc.).

Nutrient and irrigation program

Since plantlets are sensitive and delicate, it is extremely vulnerable to the amount of nutrients, its mineral content and the level of moisture around the root zone. Therefore, the irrigation and nutrient program has been carefully designed to support the plantlet development gradually to ensure prolific and healthy vegetative growth. Without proper vegetative growth the plant’s shift to tuber production depends entirely on the amount of carbohydrate accumulated in the vegetative parts to be used by the developing stolons, which is then converted to tuber storage of carbohydrates.

The growing media – from the above, it’s quite clear that a growing media, which better supports the dynamics of nutrients in water, is preferred by the plant (i.e. not too wet/soggy, not too dry, good aeration etc,) but in particular, enough space should be allowed in a soft texture to support the elongation and formation of stolons.

Management – growing in vitro potato seeds demands continuous attention to plant development, controlling pH and nutrient requirements, and scouting for any abnormalities and diseases. The planting program and records are kept regularly to secure healthy and disease free production, and is according to standards and inspection-body regulations.

The experiment

The purpose of the experiment was to determine by results the difference in production of marketable tubers from potato plantlets in two different types of growing media.

All the other aspects and factors were according to the above criteria and are as follows:

•  Variety: Avalanche, known to produce considerably large tubers.

•  Time of the year: Winter cycle – plantlets were planted on 14 June  and harvested on 31 October 2012.

•  Location: on the Long Tom Pass, in the Drakensberg Mountain slopes close to Lydenburg, in Mpumulanga, South Africa. The production unit is located 2154 metres above sea level in an outstanding micro climate for the production of seed potatoes. In addition to its altitude, it has cool summers and cold winters.

•  The growing technique: bag culture, 9L black plastic bags, organised on elevated beds, 168 bags per bed, three plants per bag.

•  Nutrients and irrigation program: not specified.

•  The growing media: two control beds filled with sawdust. One bed filled with a blend of 40% peat, 40% vermiculite (horticulture grade) and 20% medium-grade perlite. The blend encourages better root structure and keeps the media moist enough, but not waterlogged, which improves oxygen levels and uptake of nutrients. The blended media was supplied by Infigro (Pty) Ltd (http://infigro.co.za/).

•  Management: on-site team of experts at the growing facility.

Following the harvesting, weighing and sorting of the production, results were recorded as per the spreadsheet and graphs included in this article.

 

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Results and conclusions

The plantlets grown in the blended media developed impressively during the vegetative stage in comparison with the sawdust controls.

The total mini tuber production by numbers has shown higher tuber production (26%) in the sawdust media (1471) vs (1092) in the blended.

That higher number of tubers produced in the sawdust media was due to increased numbers of button-sized mini-tubers in comparison to the blended media.

The blended media has a definite impact on tuber size and weight, as demonstrated on the data sheet – blended media produced distinctively higher weight in total (47%) – 74,950g  vs 39,850g in sawdust.

The average weight per tuber in both productions (blend and control) was directly related to overall tuber sizes, and as a result,  the tuber average weight was 68.6g in the blended media vs 27.08g in the sawdust media (60%).

Marketing aspects

The definition of ‘marketable tuber size’ varies. In some parts of the world tuber sizes of <20 mm is considered marketable, while research has shown that the best yield is obtained with tubers sized 20-40 mm (Nepal Agric. Res. J. Vol. 6, 2005 ‘Performance of Different Size True Potato Seed Seedling Tubers’ – Ram C. Adhikari):

The significantly maximum total and marketable (>20g) tuber yield was obtained when larger seedling tubers were planted. Seedling tubers 20-40g size produced the highest total and marketable yield.

Comparing the end results of this experiment clearly shows that the growing media composition and origin have a direct effect on tuber size, weight and uniformity.

Considering the definition of marketable tubers, it can be concluded that if most tubers formed between 20-40mm , the preferred result, then the blended media performed better.

If overall quantity of tuber production, irrespective of size and weight, is considered marketable, then the sawdust media production is more economical.

Seed size

Large, whole seed pieces, between about 70-90g will produce more stems per plant than smaller seeds or cut seeds. Tuber set (number) is directly related to stem numbers, therefore the higher the stem count per plant, the higher the tuber set. Growers producing small, whole tubers under irrigation should aim to have at least four or five stems per linear 30cm of row.

About the author

Ben Safronovitz graduated from agriculture school in Israel and has practised horticulture in South Africa since 1992. He specialises in soilless culture and has undertaken several experiments with various growing media and cultivars in South Africa and Israel. Ben is heading the Hydroponics Academy in Krugersdorop and is the horticulture consultant of AECI Group of companies in South Africa.

PH&G March 2013 – Issue 129