Many of the problems of obtaining good Prosopis seed arise from the difficulties of fruit collection. Often, this work is arduous, especially if the pods must be gathered from standing trees. Additionally, the period of time during which pods can be collected is relatively short. It is not surprising, therefore, that the problems of procurement frequently involve undesirable compromises on quality and provenance of seed.
Considerations to be made relative to the collection of Prosopis fruit include the selection of parent trees, a forecasting of potential crops, and using an appropriate technique of collection.
In general, the best seed producers, i.e., the trees that produce abundant crops with the lowest proportion of sterile seed, are dominant or co-dominant trees, usually of middle age.
Sampling within each population can be done either randomly or selectively. Although the latter option is often used in within-population sampling for evaluation purposes, it should be remembered that phenotypic superiority is no guarantee for genetic superiority, especially in cases where the past history of the population is unknown. It should also be remembered that because of genotype environmental interactions commonly present in arboreal species, selection for superiority on one site does not necessarily mean that the material will perform well when grown on a different site.
Figure 1. Tasks in the collection, handling, storage, and pre-treatment of Prosopis seeds.
Collection for conservation purposes should ideally be done only after the amount and patterns of intra-specific variation present have been determined. Such collections, aimed at maintaining maximum allelic diversity in the samples, should be random, not selective.
Because of pressures on existing populations, collection for evaluation and collection for conservation must often be made simultaneously, requiring a compromise in sampling techniques. This is the case in the present FAO/IBPGR project, where the conservation in situ of many of the populations sampled cannot be guaranteed for the duration of the evaluation phase. In this case, cooperating countries have been requested to collect seed from a minimum of 25 trees, spaced at least 50 meters apart to minimize the possibilities of the mother trees being related. As many of the stands are limited in extent, these guidelines will generally preclude any selection for superior phenotypes within the populations.
Another consideration when selecting trees for the collection of seed is the common observation that isolated individuals often do not produce desirable progeny, probably due to difficulties of cross-pollination and resultant, inbred seed. Such trees should, therefore, be avoided.
Before fruits are collected, the parent tree must be correctly identified. Identification can be easy in some instances, but where several different species of the genus Prosopis that do not differ much morphologically grow together, errors of identification are likely to occur unless care in identification is taken.
Unfortunately, if incorrectly labelled seed is sown, the error in identification may not be detected until years later, and the possible reasons for a failure in the planting may never be known.
Trained personnel should be responsible for identifying parent trees. Also, it is generally advisable to collect herbarium samples or form a reference collection of fruits and seeds kept in small glass bottles together with a few leaves, flowers, and a sample of bark for each tree species. Seed should be collected from a minimum of 25 mother trees and each of these should be permanently marked. A detailed record should be kept of the locality of the collection, which should be recorded on a special collection form filled in at each site (see example of collection form in Appendix 1).
An estimate of the potential seed crops should be made prior to scheduling the actual collection of fruit whenever possible. A month or two before collection, fruit should be counted on a number of sample trees growing in several locations within the collection zone. Using the results of these counts as a basis, trees from which fruit will be collected are marked. Subsequently, fruit should be collected from marked trees only.
In general, species of the genus Prosopis produce seed each year. A year in which the seed production is abundant is known as a “seed year.” A seed year is seldom a regional occurrence for a tree species, as it is usually limited to a few locations within the region. Often, more seed is collected in a seed year than is required for sowing during the next planting; in these instances, the excess should be stored for use in poor years.
Ripe Prosopis fruit, i.e., pods which are generally yellowish in color and partly dry, can be picked directly from a standing tree or collected from the ground.
When picked directly from standing trees, and if the trees and their branches are low to the ground, fruit attached to the branches can be picked manually by collectors standing on the ground. For larger trees, sticks can be used, but carefully, for beating the branches to dislodge the fruit. Also, poles with shears, saws, or hooks of various designs attached to one end are frequently used for detaching fruit on higher branches. These poles vary in length, with bamboo, plastic, or aluminum poles four to six meters in length commonly favored. Using these poles, the collector standing on the ground can bring down fruit from branches seven to eight meters overhead. Trees that are too tall to be harvested by using poles must be climbed. However, trained personnel and suitable equipment, i.e., ladders, climbing irons, and safety belts, will be necessary for this job.
When collected from the ground, fruit should be gathered up at least daily as it falls. Such collections can be made more efficient by spreading large pieces of cloth, mats, plastic sheets, or tarpaulins under the trees to catch the fruit as it falls. If this is not possible, the ground should be cleared of leaves, branches, and undergrowth to make collecting easy and to minimize impurities in the fruit collected. The fruit needs to be collected as soon as it falls, otherwise, many of the fruits may be attacked by insects.
An advantage of picking fruit directly from standing trees over collecting fruit from the ground is the assurance that the seeds collected all come from the desired parent trees.
Each sample collected should be meticulously labelled, both outside and inside the bag or container. The following minimum information should be provided on the labels:
Seedlot Number:
Species and Provenance: Name
Collection: Date and Location
Name of Collector:
For more detailed information, see Appendix 1.
