A subtropical plant with optimum temperature requirement ranging from 18 to 30 C0 should be protected when night time temperature fall below the range. Very high and very low temperature may stress transplant in addition to affecting leaf stevioside contents. In colder areas, Stevia should be planted after the last frost or should be covered with plastic sheet. Stevia plant requires 12 hours sunlight to remain green and growing so longer summer days at higher latitudes favor leaf yield and Stevioside content.
Soil type and land preparation
Stevia grows best in rich well drained soil preferably a sandy loam or loam soil. It requires fine field with pH ranging from 4 to 8 but does not tolerate salinity. Raised beds (60-90 cm wide and 10-15 cm high) are ideal for stevia cultivation as beds facilitate additional water drainage as standing water is harmful for the crop. Beds should be mulched with organic matter in extreme weathers to conserve moisture and save the plants from injury.
It is best to propagate stevia plants from cuttings or tissue culture from a plant that has proven to be successful.
Growing stevia from seed normally has a very low germination success; sometimes only 10%. Only black or dark brown seeds are viable. A tan or clear color suggests they are empty shells, lacking an embryo. Good seeds will be solid and white inside. Even firm black seeds tend to lose viability rapidly.
Seedlings grow slowly, so allow 7 to 8 weeks from seed to transplanting. Seeds sown in December and January will be ready to transplant in February and March. For Rabi (winter) planting, nursery may be raised during May/June under controlled environment which will be ready for transplanting in September/October. A plastic flat covered by a clear plastic dome makes a good germination chamber when placed beneath a growing light. Place a thermometer inside and maintain a 21 to 23oC temperature by adjusting the level of the light. Use small containers (with drainage holes) or plastic cell packs filled with standard potting soil. Place 3 or 4 seeds on the soil surface in each container and cover with a thin layer (about 1/8 inch) of horticultural vermiculite. Water from below as needed by pouring water into the tray. Seedlings should emerge in 1 to 2 weeks. Thin to one plant per container and extra seedlings may be transplanted to empty containers.
Cuttings should be planted in winter months, e.g. December, January and February. For Rabi planting, cuttings should be planted during May/June under controlled environment which will be ready for transplanting in September/October. For cuttings, use a sharp blade or pruning tool, make soft cuttings (terminal portion) 2 to 4 inches long. Each cutting should have 2 or 3 nodes. A node is where leaves attach to the stem. Cut between, rather than at the nodes. Plunge the proximal end (closest to the roots on the mother plant) of the cutting into the rooting medium far enough so that at least one node is buried and at least one node remains above the surface. Remove all leaves from buried nodes.
Above the surface, remove large leaves by cutting or pinching leaf stems, taking care not to damage the tiny axillary leaves emerging behind large leaves. These axillary leaves are the growing points of your new plant. Keep cuttings at 15 to 21oC. Stevia plants require high humidity to grow, therefore plant the cuttings in plastic cups and place the cups in plastic trays/bins. The height of these trays should be 20-30 cm. Cover the trays with plastic sheet after placing the cuttings in it and tighten it well. After about a week, growth should be evident if rooting was successful. After 3 to 4 weeks, transfer plants in low tunnel (2-4 feet high) and sprinkle water to maintain high humidity. Transplant these to the field in another 2 to 4 weeks or keep as a container plant.
Collect the twigs (about 5-6 cm) of shoots Stevia rebaudiana plants, wash the twig with node explants under tap water and then washed again thoroughly by adding a few drops of Tween-20 to remove the superficial dust particles as well as fungal and bacterial spores. Then surface sterilize with 0.1% mercuric chloride for 5 min followed by rinsing them five times with double distilled water inside the Laminar Air flow chamber. Prepare nodal segments (with a single axillary bud) about 0.5-0.8 cm aseptically and implant vertically on MS medium fortified with specific concentrations of growth regulators (BA, KIN and NAA) singly or in combination adding 30 g lG1 sugar (market sugar) and 0.7% Difco Bacto-agar.
Adjust the pH of the medium to 5.7 with 0.1 NaOH before autoclaving at 1.06 kg cmG2 and 121°C for 20 min. Then incubate the cultures at a constant temperature of 26±1°C with 16 h photoperiod (2000 lux). Subculture should be done every 21 days interval and nodal segments from the proliferated shoots should be sub-cultured again for further multiple shoot induction. Cut the regenerated multiple shoots and place individual shoots in MS medium containing 0.1 mg lG1 IAA. Then subject the rooted plantlets for hardening and establishment in soil.
Stevia nursery prepared from the either mentioned methods requires to be acclimatized under controlled environment. The plantlets then require to be transferred to plastic pots containing peat moss and sand (1:1) in the greenhouse (28 ± 2°C, RH 70-80%). The potted plants need to be irrigated and initially covered with plastic bags, which may gradually be eliminated within four weeks time for completing their acclimatization.
Care and maintenance
When stems reach 7 to 10 inches in length, cut them back to promote branching and vigor. Over-wintered plants look devitalized by the end of the winter but regain vigor when transplanted outdoors. Do not use fertilizers with high nitrogen. Adding extra Boron will help keep the Stevioside level high. If soil could be mounded up into a raised bed, this would be even better. Apply a layer of mulch such as grass clippings, bark mulch, leaf manures, straws etc.
This will help keep roots cool, preserve water, keeps the leave clean from soil (prevents dirty taste in green powder) and hold down weeds. Avoid weeding around mature stevia plants as their brittle branches are easily broken.
Micro sprinkler/drip irrigation is the best methods of irrigating the Stevia plants to avoid damage by excessive level of moisture. Irrigate once or twice a week for consistent moisture supply depending upon season and soil type. Frequent light watering is recommended during the summer months while during winter months as the beds starts loosing moisture. Heavy irrigation in summer months cause suffocation, high temperature and high humidity may cause fungal diseases. Avoid overwatering after transplanting and in winter as houseplant. Keep evenly moist during summer heat; drip or soaker hose are very effective for summer watering.
Stevia plants respond well to fertilizers with lower nitrogen content. Low nitrogen is recommended as excess nitrogen promotes growth with poor flavor. Most organic fertilizers would work well since they release nitrogen slowly. Feeder roots of stevia plant tend to be very near to the soil surface; therefore FYM and compost should be added to supply extra nutrients in sandy soils and to conserve moisture. Nitrogen application is must for the production of dry matter and good harvest of leaves. A low nitrogen formula is recommended in split application at planting and in midsummer.
Removal of weeds can be done manually. Since the crop is grown on raised beds, intercultural operations are easier by manual labor.
The fungal diseases such as Septoria steviae, Seclerotinia sclerotiorum and phytophthra may occur. Dark brown lesions form on stem near soil line followed by wilting and collapse of plant in sclerotine. Inphytophthra attack, plant wilts and dies. Wet weather and standing water favor development of disease. Fungicides should be sprayed to control the diseases. Aphids, thrips, whitefly, loopers and mealy bug can cause damage in heavily infested green house. The relevant insecticides are required to be sprayed to control the insects.
Harvest entire plant in the morning as flower buds appear for highest glycoside /sugar content. Plants transplanted in February and March should be harvested till mid-May or as flower buds appear. It does not produce a single normal leaf once flowering has begun. Removing flower heads is not effective. Failure to harvest plants before several flowers have opened, will allow these flowers to impart a bitter/dirty flavor to the leaves. Harvesting is done by cutting the entire plant at the base. With a rubber band, tie loose branches together and hang upside down to dry under warm, dark, drafty conditions for 2-4 days. Avoid using food dehydrators or open oven doors as this will also tend to cause a bitter flavor. Remove any small branches and grind leaves into powder using an electric grinder for 25-30 seconds. Dried green stevia powder will last almost indefinitely or at least until the next harvest. Expected yield would be 2500-3000 kgs of dried leaves/acre.
The dried leaves can be ground and used as a sweetener or soaked in water and the liquid used in making preserves. The powdered leaves are also added to herb teas. The leaves are sometimes chewed by those wishing to reduce their sugar intake. The leaves can also be cooked and eaten as a vegetable. Many who hesitate to consume artificial food additives may prefer stevia because it is all natural. Production of Stevioside involves water extraction from the dried leaves, followed by clarification and crystallization processes. Most commercial processes consist of water extraction, de-coloration, and purification using ion-exchange resins, electrolytic techniques, or precipitating agents
Today, stevia is being used in more than 6,000 food, drinks and medicinal products all over the world. It is being used as sweetener in colas, ice cream and for value addition in soy sauce and pickling products. It is also used in chewing gum, rice wines, yogurts, soft drinks, fruit juices, candies and canned foods. The Coco Cola Company, Cargil and Coke and PepsiCo Inc are all competing on the market with their stevia products. Coke markets Sprite Green and Odwalla juice drinks that contain stevia.
A study by Zenith International, a food and drink consultancy, estimates that worldwide stevia sales reached 3,500 metric tons in 2010 taking its overall market value to $285 million. Zenith forecasts that the global market for stevia will reach 11,000 metric tons by 2014, equivalent to $825 million.
In Pakistan, National Engineering Laboratories is manufacturing Glucose-H (it contains glycoside extract of stevia) which is natural sugar replacement, low in calories, anti-fatigue and energy booster. Qarshi Industries Pakistan Pvt. Ltd. is also using stevia in its many food products.
AVAILABILITY OF SEED AND PROPAGATION MATERIAL
Seed and propagation material is available at Plant Physiology Section, Agronomic Research Institute, Ayub Agricultural Research Institute, Jhang Road, Faisalabad.
For further information please contact
Dr. Hafiz Muhammad Akram
Agronomic Research Institute
Ayub Agricultural Research Institute, Faisalabad
Ph # +092 41-9200655
Fax: +092 41-9201677
Cell: +092 300 7662117
E-mail: [email protected]
Dr. Hafiz Muhammad Akram, Dr. Muhammad Shoaib, Shahid Abbas