Genetic improvement of Prosopis juliflora - Ashwani Kumar, Padmini Shivkumar, A.M. Rai and A.C. Banerjee

Forest Research Institute, Utter Pradesh Forestry Department, Kanpur 208024, India

Introduction

Prosopis juliflora is a highly esteemed fuelwood source and is also valued for the shade, wood and forage it can provide (Ffolliot and Thames, 1983). P. juliflora can also be profitably cultivated in different types of waste lands, and has been called a “miracle tree” for dry zones, saline and saline-alkaline (usar) soils, where no other vegetation exists. Many of the species of the genus Prosopis are useful as multi-purpose trees. They are highly adapted for survival in arid, semi-arid and usar soils. Prosopis fruits make excellent food for livestock and wild animals, and the flowers are a source of large quantities of nectar for bees. The wood is strong and durable and is excellent for round wood constructions and posts, and has a high calorific value and hence is excellent as a fuelwood. The heartwood is hard and has a good grain and colour that makes it very suitable for furniture, wood-block flooring and small decorative products (Rawat, 1984).

Despite all the advantages of Prosopis, rural people with marginal lands are against the plantation of this species because of its stiff spines and its bushy and invasive nature. Large scale plantations of P. juliflora established along roadsides and on community land, has lead to the social unacceptance of this species by the rural people, because it grows ever-closer to the edge of rural roads, restricting the path for animal driven carts which has created congestion on the main roads leading to frequent accidents. In the arid zones of Uttar Pradesh, where this species grows in abundance, the stiff thorns often get in the soft foot pads of camels resulting in wounds and subsequently septic conditions or even the death of the animal. A solution to these problems is to develop tree varieties which have small spines or which are thornless, in which considerable success has been achieved by the Forest Research Institute, Kanpur.

Plus tree selection for genetic improvement

The first step in making genetic improvement in P. juliflora is to select a large number of quality plus trees from selected superior stands, qualitatively selected by good tree form and fewer and smaller spines. There is considerable variation in the population with respect to the shape and form of the stem, and also with respect to the size and stiffness of the spines, with emphasis laid on these characters in the plus tree selection programme conducted in Uttar Pradesh, where it has become naturalised. The stands identified for seed collection were Fisher Forest (Etawah), Allen Forest (Kanpur), Bovine Area (Etawah), Varindavan Block (Mathura) and Kukrail (Lucknow). Plus trees were selected based on their phenotype, hence it was essential to progeny test the plus trees to find out their breeding value and then identity the superior genotypes. Progeny testing of plus trees is an essential component of tree improvement programmes. Under this programme, two half-sib progeny trials were conducted in the FRI, Kanpur, with seeds collected from plus trees in Allen forest (Kanpur) and in Mathura.

The first trial was planted in usar soil (pH 10.5) at Unnao in 1985, with no soil amendments, to select progenies well adapted to usar soils. Initial survival after two years was 60% and selection is in progress for well adapted progenies exhibiting good tree form with smaller spines. The second, half-sib progeny trial was conducted to develop lines with less, or preferably no spines at all. The seed material for this trial was obtained from plus trees from Mathura, which had smaller spines. A lot of variation in the size, number and distribution of spines throughout the whole plant was observed in the nursery stock of this trial, and further selection of this material is in progress.

Vegetative propagation by stem cuttings

With the growing trend towards plantation forestry and genetic improvement of economic forest trees, it has become necessary to develop rapid and economical methods of producing improved planting material. Reproduction of tree species by cuttings is one of the methods by which forest trees can be vegetatively propagated, and appears to be very useful for the clonal multiplication of species. Clonal planting material is of considerable importance in forestry practice because it offers the advantage of uniformity of growth and development by eliminating genetic differences between trees and by making immediately available superior individuals for seed orchards and plantations. Stem or shoot portions are generally good material for rooting purposes because they usually have undifferentiated tissues to permit easy differentiation of root primordia and preformed buds. Self incompatibility in Prosopis species causes out-crossing, so that trees propagated from seed are extremely variable. Vegetative propagation techniques are necessary to reduce genetic variability in controlled greenhouse and field experiments. Successful rooting of Prosopis in growth chambers with the application of various hormones has been reported by Felker et al. (1981). A study on the vegetative propagation of P. juliflora through stem cuttings was also conducted at FRI Nursery, Kanpur in 1986.

Table 1. P. juliflora plus trees selected for vegetative propagation

Stem cuttings for the trial were taken from plus trees of Allen Forest, Kanpur and Fisher Forest, Etawah. The selected plus trees were vigorous, healthy, with good tree form, small spines and very good seed bearing capacity. Data for the plus trees are given in Table 1. Cuttings of approximately 1 cm in diameter and 18 cm in length were taken from one year old branches of each P. juliflora plus tree, at monthly intervals in the middle of each month. Each cutting contained at least 2 nodes, with the leaves removed from the lower node. Two sets of cuttings were taken for the experiment, the first treated with rooting hormone and the second an untreated control. The cuttings were planted initially in polythene bags (30 x 13.5 cm) filled with sterile river sand. Observations of callous formation and root differentiation were recorded after one month.

Experimental results (data not presented) indicated that there was pronounced seasonal variation in the rooting response, and that hormonal application to the cuttings is not essential for root formation. Seasonality appears to be the main factor influencing rooting of cuttings, with stem cuttings planted with the onset of rains in July having successful rooting while cuttings taken in February sprouted but did not root, possibly due to lower air temperatures. Successfully rooted cuttings were planted out in the field in Kanpur after one year, and plant growth after 6 years is given in Table 2. This indicates the potential of propagating P. juliflora by stem cuttings from plus trees without the application of any hormones in July and August. Such clonal material will have many uses, i.e. the preservation of genotypes through the use of clonal banks, multiplication of desired genotypes for special uses, such as in seed orchards, and genetic evaluation of plant material including genotypic-environment interaction studies.

Table 2. Performance of P. juliflora cuttings 6 years after out-planting.

References

Felker, P. and P.R. Clark, 1981. Rooting of mesquite (Prosopis) cuttings. Journal of Range Management 34: 466-483.

Ffolliott, P.F. and J.L. Thames, 1983. Handbook on Taxonomy of Prosopis in Mexico, Peru and Chile. FAO, Rome.

Rawat, M.S., 1984. Guidelines for the improvement of Prosopis juliflora. Indo-Danish Project on Seed Procurement and Tree Improvement. Hyderabad, India.