Rapid Multiplication of Banana and Plantain Plants

At ECHO’s November 2007 Agriculture Conference, Darren demonstrated a technique for rapid multiplication of banana and plantain plants. He learned the technique at the Centre Africain de Recherches sur Bananiers et Plantains (CARBAP). CARBAP’s website is www.carbapafrica.org

The technique described in this article is very practical and applicable in certain situations. For example, it is becoming very useful for us here in Gamboula, Central African Republic (CAR), to produce planting material to distribute to our agroforestry cooperatives. It would also be helpful as a method to rapidly multiply disease resistant cultivars that could take years to establish and multiply in an area if using traditional means of multiplication.

The technique requires no specialized equipment and does not require aseptic conditions. In fact, the only tool needed for this technique is a knife. (You will also need a bucket or other container to hold the banana or plantain sucker while it sprouts.) Thus the technique can be reproduced by small-scale farmers with locally available materials. (Note that juice from banana plants stains permanently, so wear clothes that are dark in color or can otherwise be stained.)

In Fruits of Warm Climates, Julia Morton makes reference to “A greenhouse technique [involving] cleaning and injuring a corm to induce callus formation from which many new plants will develop. As many as 180 plantlets have been derived from one corm in this manner.” This is a good, condensed description of the technique Development I learned at CARBAP. It is not unusual for CARBAP to produce more than 100 plantlets from a single corm, but each cultivar responds differently. To date, they have successfully used this technique to propagate over 40 cultivars of bananas and plantains.

In the CAR there is one potentially serious problem that may limit the usefulness of the technique for small-scale farmers. Marketing the product could be difficult, particularly if a farmer’s production exceeds the local demand. The infrastructure in western CAR is such that marketing the product outside one’s own village would pose a great challenge. In a slightly more developed area or country, such as Cameroon, access to other markets is possible because of the improved transportation infrastructure.

Below I describe the technique in terms of seven steps: pup selection; phase I conditioning; phase II conditioning; placement of conditioned pups (explants) in the germination bed; root and plantlet development; excision, reactivation and replanting; and planting and hardening of plantlets.

1. Pup Selection

Bananas are propagated from rhizomes called pups. The ideal pup is a sword sucker that has 5 to 40 cm of stem (actually it is a pseudostem—not a true stem—but we will call it a stem for simplicity’s sake) extending above the surface of the soil. [Sword suckers have narrow sword-shaped leaves (Figure 1), in contrast to water suckers that have broad leaves]. I prefer to harvest pups in the 30 to 40 cm range, especially if the banana mat is no more than one to two years old. Generally speaking, the larger the pup’s corm, the greater the number of plantlets that pup is capable of producing—although there are other factors involved that influence a pup’s potential to produce plantlets. Furthermore, in my limited experience, dessert banana pups seem to have a smaller corm than plantain pups. Pups that have a bulbous/spherical corm are desired over those with a thin, elongated corm, although both forms are acceptable.

When removing the pup from the mother plant, dig around the pup while being careful not to injure its corm. Sever the pup as close to the corm of the mother plant as possible. If possible, sever the pup in a single motion to minimize damage. Wash excess soil from the pup. Remove any dead, dried leaves.

2. Phase I Conditioning

Using a sharp knife, peel away 2 to 5 mm of the corm’s cortex (the outer layer of tissue), including the roots. Begin peeling at the point where the outermost leaf sheath connects to the corm. If necessary, you can peel deeper, but try to leave at least a thin layer of cortex surrounding the central cylinder. Cut away any obvious signs of weevil or nematode damage. The most obvious sign of nematode damage is a red discoloration in and around the root channels. If this red discoloration is found, trim the roots right down to the level of the central cylinder. Dip the corms in a nematicide if one is available, but this is not necessary. Reduce the length of the corm to 10 cm. The corm of the pup should be completely white (Figure 2), although it will quickly turn brown on contact with air (due to oxidation). From this point on, do not allow the pup to contact soil that could potentially be infected with nematodes.

The point at which the banana leaf sheath connects to the corm is called a ‘node’ or the ‘transition zone’ (Figure 3). It is usually an obvious line, although it may be difficult to identify on the outermost leaf sheath due to the paring process described above. Once you identify the transition zone of the outermost leaf sheath, locate the “V” formed by the two edges of the outermost leaf sheath (Figure 3). Using a sharp knife and beginning at one edge of the “V,” make a cut around the circumference of the stem, through the width of the outermost leaf sheath at a point 2 mm above the transition zone. Remove and discard the cut leaf sheath.

Locate the transition zone and “V” of the next leaf sheath and repeat the above process. If the transition zone is obscured and you are unable to identify a point 2 mm above it, make your cut 2 mm above the point at which you removed the first leaf sheath.

Remove 2 to 5 leaf sheaths in total, following the above procedure. The number of leaf sheaths you can remove will be dependent on the size of the pup. A rule of thumb is to stop removing leaf sheaths when it becomes difficult or impossible to visualize the “V” formed by the edges of the leaf sheaths.

Next, reduce the height of the remaining stem to approximately 2 cm above the point at which you removed the last leaf sheath (Figure 4). The net effect of this process is that you have a small “staircase” with each step being 2 mm in height, with the last step being 2 cm in height.

Finally, place the pup in a location that receives filtered sun (shade cloth that lets through 50% or less of the light is recommended). Typically, pups are left to dry in the shade for about 48 hours, although leaving the pups for an extra 24 hours is acceptable. This step stresses the plant and also makes the pup easier to work with because it dries a bit.

3. Phase II Conditioning

Take your pup that was air-drying for the past two to three days (Figure 5) and begin reducing the height of the remaining stem in very small increments (i.e. 1 to 2 mm or less).

You will eventually come to a point where a small dot at the center of the surface you are cutting becomes slightly translucent (clear). Stop cutting at this point, which will usually be within 1 to 5 mm of the last leaf sheath you removed during phase 1 of the conditioning process (Figure 6).

Locate the “axis of growth” of the pup; that is, find the orientation or side of the pup from which it was growing from the mother. Make a crosswise incision, 3 cm deep, across the width of the pup along the axis of growth that transects the translucent point described above. Make another crosswise incision perpendicular to the first. The point of these incisions is to try to damage the growing tip (apical meristem), not to destroy it. It may be worthwhile to make a third crosswise incision to make sure you cut the apical meristem (Figure 7). Damaging the apical meristem will break apical dominance (hormonal control over the other buds) and will allow the lateral buds to push and form new plantlets. The conditioned pup is now called an “explant.” Set the explant aside for 2 to 3 hours in the shade.

4. Placement of Explants in the Germination Bed

After 2 to 3 hours, place the explant (conditioned pup) in a germination bed where it will have time and space for banana plantlets to develop. The germination bed can be any type of container, as long as it is deep enough; it could be a simple bucket, or it could be a cement or brick structure. It needs to be well drained and deep enough to contain 5 to 10 cm of substrate (planting material) below the explant, plus the height of the explant, plus an additional 4 to 5 cm above the explant. The preferred planting material is fine sawdust. If you try using soil or compost, sterilize the material first.

The germination bed must be covered with clear plastic so as to create a greenhouse effect (uniform higher than ambient temperatures and humidity). The germination bed needs to be located in approximately 50% shade. The ideal temperature within the germination bed is 30 to 34ºC (86 to 93ºF). If temperatures exceed 34ºC, you may have problems with burning the leaves of the small plantlets. If temperatures are likely to exceed 34ºC, you can remove the plastic cover during the hottest part of the day.

When the explants are first introduced into the germination bed, the substrate should be completely dry. As stated above, you should have approximately 5 to 10 cm of substrate below the explant. The top of all the explants should be at the same level in the germination bed. The explants can be packed close together with no more than 2 to 3 cm separating one from another. The explants are then completely covered with substrate to a depth of 4 to 5 cm. Leave the explants in the dry substrate for 24 hours; then thoroughly water the germination bed. After the first watering, the substrate will settle and should cover the explants to a depth of 2 to 3 cm.

The germination bed should be periodically watered to maintain a constant level of moisture. The substrate should not be saturated, nor should it be allowed to dry out. When a handful of substrate is squeezed in your hand, you should not be able to squeeze out more than a few drops of water.

5. Root and Plantlet Development

New roots will begin growing within 8 to 15 days and may be visible on the surface of your substrate. Within 16 to 22 days, internodal buds start pushing and some growth may also be visible in the region of the apical meristem (growing tip). After 4 to 6 weeks you should see plantlets growing through the surface of your substrate.

6. Excision, Reactivation and Replanting

Somewhere between 6 and 8 weeks you should have a number of plantlets with 2 to 5 leaves. At this point, carefully remove the entire explant from the germination bed and gently remove or wash away any substrate that is attached to it

If you have plantlets with 2 to 5 leaves that are less than 1.5 cm in diameter at their base, you can cut them from the explant with a small, sharp knife or scalpel. When cutting out a plantlet, be careful not to damage neighboring plantlets or developing buds. The plantlet you are removing should be cut out with a very small amount of the explant’s corm still attached to it. The plantlet that you cut out need not have any roots

Once you have cut out all plantlets with 2-5 leaves and reactivated all large plantlets (see next paragraph), you place the explant back into the germination bed at the same depth as before. After 24 hours you may water the germination bed if necessary.

If a plantlet’s stem has a diameter of 1.5 cm or larger, it is a candidate for reactivation. Basically this means that while the plantlet remains attached to the original explant, you remove several of the sucker’s leaf sheaths and cut it again as you did the explant (as shown in Figure 7), to disorganize its apical meristem. This in turn allows the lateral buds that are present on the suckers to begin growing, because apical dominance has once again been broken.

Here are more detailed instructions for reactivation. Remove 2 to 4 leaf sheaths from the plantlet with a small, sharp knife, razor blade or scalpel while the plantlet remains attached to the explant. The point at which you begin removing leaf sheaths is important. When looking at the plantlet to be reactivated you will notice that the plantlet did not initially grow straight up, there will be a slight curve at its base. Follow the inside of the curve and identify the point at which the curve ends and vertical growth begins. Measure 2 cm up from the point at which vertical growth begins and decapitate the stem of the plantlet (that is still attached to the explant). Beginning at the top edge of the curve, remove the first leaf sheath. Remove another 1 to 3 leaf sheaths if possible. The remaining stem is then reduced to a few millimetres above the point at which you removed the last leaf sheath. You will not likely see the translucent centre portion referred to in the section on phase II conditioning. Crosswise incisions are made as described in that section (and as shown in Figure 7), but only to a depth of 2 to 2.5 cm. Make sure that the first crosswise incision is made parallel to the axis of growth. Dry substrate is then sprinkled over the freshly cut tissue.

You can continue the process of removing explants from the germination bed, excising and reactivating plantlets, and placing the explant back into the germination bed for 4 to 10 cycles. Eventually, the energy stores of the explant will be depleted and it will stop producing pups. At this point the explant will become noticeably soft and may begin to rot.

7. Planting and Hardening of Plantlets

The recommended media for planting the plantlets is a 40/60 mixture of sand and organic matter such as coffee, cacao or rice hulls. If using compost or soil as your organic matter, the ratio is 50/50, but the compost or soil should be sterilized to destroy any nematodes that may be present.

When looking at your banana plantlets you will see that a portion of the stem is green and a portion is white. The white portion is the part that was under the substrate in the germination bed and the green portion was growing above the surface and exposed to light. Plant the plantlets in a one-gallon pot or sac to the depth of the white portion of the pseudostem. Place the sacks/pots in 50% shade and keep them well watered. After 6 to 8 weeks in 50% shade, the plantlets should be large enough and sufficiently hardened off to move them into direct sunlight.

Notes
Single Plantlet Developing from the Explant’s Apical Meristem

During phase II conditioning, if you successfully wounded the explant’s apical meristem, many plantlets should develop from the damaged tissue. If you only see one large plantlet developing, you probably missed cutting the apical meristem with your crosswise incisions. You need to remove a few leaf sheaths from the developing plantlet, reduce the stem and make fresh crosswise incisions. Basically, you follow the steps for reactivation, except you begin removing leaf sheaths 0.5 to 1 cm up the plantlet’s stem. There is no need to identify the curve, because the plantlet should already be growing vertically. Make your 0.5 to 1 cm measurement from the surface of the explant.