Applications of Plant Tissue Culture in Forestry

The following points highlight the five applications of plant tissue culture in forestry.

They are: (1) Micro Propagation (2) Apical Meristem Culture (3) Embryo Culture (4) Endosperm Culture and (5) Haploid Plants.

Application # 1. Micro Propagation:

Efficient methods for micro-propagation has been used for many trees (Fig 15.1). So far much success has been achieved in micro-propagating some tree plants which are difficult to clonally multiply by the conventional methods. Success of micro-propagation is judged on the basis of rate of shoot multiplication and the success of trans­plantation to the field. A list of tree plants which have been micro-propagated in vitro is given in table 15.1.

The advantage of micro-propagation is that the shoot multiplication cycle is very short i.e. within 2-6 weeks, each cycle resulting in a log­arithmic increase in the number of shoots. AL AFOCEL (France) micro-propagation of Sequoia sempervirens are regularly applied for the pro­duction of 30,000 plantlets from approximately 200 selected clones. Similarly, a small produc­tion laboratory has been built at the Forest Re­search Institute at Rotorua (New Zealand) where tissue culture techniques are applied to produce 100,000 radiata pine plantlets annually.

Teak is one of the most important timber trees of India and in neighbouring countries. The wood of teak is highly superior for its strength, durability and insect resistance. Similarly Eu­calyptus citriodora is commercially valued for its wood and the essential oil of its leaves. The main component of oil is citronella. Suitable methods are not available for vegetative propagation of such elite trees by cutting.

Micro propagation of such plants have been developed by which about 500 plants of teak and 100,000 plants of Eucalyptus citriodora can be obtained from a single bud in a year. This preliminary experi­ment clearly demonstrates vast potentials of mi­cro-propagation techniques for clonally multiply­ing elite trees. The production of vast quantity of plant obtained through micro-propagation will fulfill the requirements of reforestation and in fu­ture provide the timber, oil, fuel-wood and the raw materials for paper industry.

Micro propagation has got some disadvan­tages such as:

1. In some cases multiple shoot formation takes place as usual. But the rooting of such shoot is very difficult. It depends upon so many factors viz. correct choice of explant, age of the plant, culture medium, hormones etc. As for example, in Feijoa terminal and sub-terminal cuttings taken from 4 year old plant developed multiple shoots in vitro but rooting takes place rarely. On the other hand, nodal cutting of the same plant de­veloped multiple shoots in vitro and gave 100% rooting under non-sterile conditions.

2. Browning of medium and browning of ex- plant in culture is a serious problem for the micro-propagation of tree plant. Browning takes place due to oxidation of phenolic sub­stances that come out from the cut surface of the explant which turns the medium dark brown and is toxic to the explant. It in­hibits the growth of explant in culture and sometimes the tissue dies. Several methods have currently been adopted to reduce the browning problems.

Application # 2. Apical Meristem Culture:

Manihot esculenta, commonly known as Cassava, is systematically infected with one or two viruses such as African Cassava Mosaic virus, Cassava Brown Streak virus. Virus attack does not always lead to death of the plant but it can reduce the yield and quality of the plant. Eradication of viruses is highly desirable to opti­mize the yield.

Apical meristem culture is widely used to eliminate virus and to raise a number of virus free plants using a single meristem tip. In Cassava, microscopic size of apical meristem (200-500 µm) has been cultured in nutrient me­dium supplemented with GA₃, NAA and BAP. By this technique, a number of virus free plants have been raised. This experiment demonstrates the great potentials of apical meristem culture for making disease free tree plants.

Application # 3. Embryo Culture:

Embryo culture is generally applied for ob­taining rare hybrids. Conventional interspecific or inter-generic breeding of forest plants has been done for improvement of timber quality, oil con­tent etc. and also for making disease resistant line. Sometimes fertilization occurs but germinable seeds are not obtained due to premature death of hybrid embryo.

If the hybrid embryos are cultured before the onset of abortion, it is easy to get a hybrid plant. Some Pinus sp. e.g. P. lambertiana is very susceptible to blister rust disease whereas P. armandi and P. koraiensis are resistant to such disease.

An attempt has been made to cross P. lambertina x P. armandi and P. lambertiaria x P. koraiensis to bring the disease resistant character in hybrid. In both cases, fertilization occurs, but seeds are not germinable. In such cases by successful rearing of excised premature hybrid embryos through tis­sue culture it is possible to grow a hybrid disease resistant Pinus plant.

Application # 4. Endosperm Culture:

It is well known that endosperm tissue of the seed of angiosperm is cytologically triploid in nature. Now, if the endosperm tissue is used as explant for tissue culture to raise a plant, the plant will be triploid. Triploid plants are seed sterile and usually undesirable. But some­times triploids are desirable for plant improve­ment. Quaking aspen (Populus tremuloides) is an important pulpwood yielding plant.

It has been seen that triploid plants raised through en­dosperm culture have better pulpwood qualities as compared to their diploid plants obtained from seeds. Again, seed sterility of the triploid plant is not a serious setback because seedless triploid quaking aspen plant can be propagated vegetatively. So it is commercially very useful.

Application # 5. Haploid Plants:

Haploid plants are obtained by isolated microspore and anther culture. Haploids are very important for mutation research on forest tree. Mutant varieties can be achieved very easily from homozygous diploid plants by treatment of mu­tagenic agent. Hevea brasiliensis is a rubber yielding plant. In China, haploid plants of Hevea brasiliensis have been obtained from microspores in culture.