Quality Seed Production of Potato through Micropropagation

Introduction

The potato

is a major world food crop, next in production only to maize, rice and wheat. Its botanical name is Solanum tuberosum and belong to family named Solanaceae. It is basic vegetable of every kitchen, that’s why it is considered as king of vegetable. It is also used as fourth most important staple food crop in Nepal. Although it is cultivated in almost all part of the Nepal its production and productivity is not satisfactory and demand matching. The average annual productivity is 16.5 Mt/ha and cultivated in the area of 193997 ha in the fiscal year 2075/76. The major reason behind its low productivity is lack of quality seed and improved varieties.

QUALITY SEED

Quality seed is defined as varietal purity with high germination percentage, free from disease and pest and with proper moisture content and weight.

Quality seed ensure good germination rapid emergence and vigorous growth. These aspects translate to a good plant stand in field and consequently enhance the yield. Quality seed reduce the cost of production and make opportunity for farmers having low capital for initiating agro farm.

MICROPROPAGATION

Micropropagation is a tissue culture (in vitro) method used for rapid and true to type multiplication of plants on artificial nutrient media under controlled environment. Micropropagation is the most commercially exploited area of plant tissue culture, having been widely used for production of quality planting material in vegetative propagated species. This method can be used in Nepal to overcome the lack of quality potato seed. This technology can be used throughout the nation to produce demanded amount of seed for every year. Nepal relies mainly on supply of seed from India and spends millions of rupees for importing seed only.

Importance of Micropropagation

1. Large numbers of disease-free propagules can be obtained from a single plant in a short period.
2. Propagation can be carried out throughout the year.
3. The propagating material can be accommodated in a small space.
4. It can be made available for every farmer at correct time of planting .
5. Promote farmer for potato cultivation due to higher yield from this seed.

Steps involved in producing seed through Micropropagation:

The in vitro introduction of a potato variety

Main conditions:

• the candidate tuber: to verify the variety
• to get sprouts
• to cut the sprouts, disinfect them with alcohol and cut them
• between nodes (=explant to be introduced in in vitro culture)
• disinfection of the sprouts nodes = in Na hypochlorite, 5 à 8’
• + and rising them in 3 successive bads of sterile H2O, 5’ – 10’ and 5’
• culture media = MS + Sugar (20g/l) + Agar (6g/l), pH 5,9
• culture conditions: 16h/8h , 18 to 22 C, 4000 to 6000 lux

Phytosanitary control operated when introducing potato explant in in vitro collection

• Departure point: one or several tubers , each of them furnishing several sprout nodes: 1 node = 1 clone, then several clones / tuber, each of them having his own code (reference number).
• Each clone (node) is introduced in vitro and multiplied separately. Each clone will be fully tested.
• After being tested, only one clone among the diseases free clones will be chosen and introduced in the in vitro collection.
• A system of tracability must be operational in the micropropagation lab: each work and each analyse and results on the selected clone must be listed in a appropriate system of documentation.

Pathogens to be tested on the vegetal material entering in the in vitro collection

BACTERIA

Clavibacter michiganensis subsp sepedonicus
Ralstonia solanacearum
Dickeya sp and Pectobacterium sp.

VIRUSES
Potato virus X
Potato virus S
Potato virus Y
Potato virus A
Potato virus M

VIROID

Potato spindle tuber viroid

Maintenance of the selected clones in the lab

– Medium: MS + 20g/l sucre + 6g/l agar + (max 3% mannitol),pH 5,9
– Temperature: 18-20 C
– Photoperiod: 16h/8h
– Light intensity: 4000-6000 lux
– Maintenance (reniewing): 3 à 4 x/an
– True to type control: 1x/2 ans (au champ)
– System of tracability and documentation operational.

Fast multiplication in the lab (production).

Phytosanitary control on the material selected for the micropropagation:

viruses (main: PLRV, PVY, PVA, PVS, PVX, PVM)
bacteria (quarantine: Ralstonia, Clavibacter – common ones: Dickeya, Pectbact.)

Conditions of multiplication:

• Media: MS + 20g/l sugar + 6g/l agar ,pH 5,9
• Temperature: 20-22 C
• Photoperiod: 16h/8h
• Light intensity: 4000-6000 lux

Field transfer of the in vitro micropropagated material

– Intermediate steps are necessary

3 possible ways:

I. in vitro microtubers production (vitrotubers)
II. minitubers production
III. acclimatization and rooted vitroplants production

I. Microtubers production:

• in the lab , then in a dark room (6 weeks + 4 months + 1 month)

 first step: in vitro vegetative multiplication,

• Medium MS normal
• Photoperiod 16h/8h
• Temperature: 20-22 C

 second step: microtuberization (6 weeks)

• Medium: MS/2 + sugar (80 g/l) + coumarine (50 mg/l)+ kinetin (4 mg/l)
• Photoperiod: in the dark 24h/24h
• Temperature: 20-22 C
• production of 1 to 1,5 microtubers size 5/10mm / vitroplant
• manually harvested, rinsing in water, disinfection thiabendazol
• solution, drying, sorting (<5, 5/7, 7/10mm)
• cold storage (minimum 3 months, 2 to 3 C)
• presprouting (1 month at 18/20 C, 16h/8h photoperiod)
• sowing directly in field or minituber production in greenhouse

Advantages:

• Easy to produce, no costly (no greenhouse)
• No risks of contamination during the production cycle (in the lab)
• Easy to sow (mechanically)
• Easy to send (by airplane)

Disadvantages:

• In field vigour due to the small size: need hot and humid soil,
• and need to be presprouted
• before sowing,
• long production process
• (6 weeks + 3 months + 1 month):
• not flexible

II.Minitubers production:

• in greenhouses or screenhouses (insectproof), by cultivation of the microplantlets or microtubers
• classical way: by transfer of vitroplantlets in peat soil
• other ways: without soil, in hydroponic or aeroponic production method
• length of production: 3 to 4 months in green or screenhouses+ 3 to 6 months of cold storage, + 1 month of pre sprouting
• harvest made by hand, sorting (10 to 50 mm)
• needs to be checked on viruses and bacterias after production and before use.

Advantages:

• Size near the usual size of the potato seeds
• Good productivity in field (vigour and multiplication rate)

Disadvantages:

• long production process (3-4 months + 3 months + 1 month) flexibility!
• expensive (infrastructures – greenhouses)
• sanitary risks through the contamination of the substrate, or
• introduction of diseases vectors in the greenhouse

III.Acclimatized and rooted vitroplants production:

• in greenhouse by transfer of vitroplants in cubes of peat
• 4 weeks of acclimatization and rooting in greenhouse
• transfer in open field mechanically or manually
• virus and bacteria control before the transfer to the open field

Advantages:

• flexibility, rapid production (4 weeks of acclimatization)
• can be directly protected against diseases in field as soon
• they have been transferred

Disadvantages:

• needs rainfall/irrigations for a good recovery in field
• expensive (greenhouse) but less than minituber
• risk of infection before being transferred to the field.

Writer: Shiv Shankar Loniya (Address : Lumbini, Nepal, College: Gauradaha Agriculture Campus)