PHENOLOGY, MORPHOLOGY, PRODUCTION, AND PHYSIOLOGY OF PROSOPIS SEEDS

Background to the use of guidelines for the collection, handling, storage, and pre-treatment of Prosopis seed should include an understanding of the phenology, morphology, production, and physiology of seed from tree species of the genus. Briefly, and in a general sense, these important topics are reviewed below.

Phenology

Phenological observations indicate that, quite commonly, flowering and fruiting in Prosopis varies from year to year, and from tree to tree within a population during the same year. Some of the intra-populational variation in fruiting has been attributed to differences in the attractiveness of inflorescenses to insects during a particular flowering season. Fluctuations in the populations of sucking insects also play a role in determining the number of incipient fruits that abort in a given year.

Flowering in Prosopis occurs shortly after bud-bursting in the beginning of spring, when the weather becomes warmer. Inflorescences appear soon after trees flush. The inflorescences vary in form according to species from small globose balls to long, furry catkins. In general, Prosopis growing in sites in the southern latitudes of Chile and Peru flower in October and November, while Prosopis in Mexico flowers from March through May. The flowering pattern of Prosopis in Mexico (as well as in the southwestern United States) is often more irregular than that observed in South America.

Flowering patterns may change when a species is introduced into a different environment. For example, Prosopis juliflora, when introduced to India from Mexico, becomes a prolific seeder and flowers twice a year, i.e., February–March and August–September.

A low percentage of the flowers ultimately produce fruits and only a few may produce viable seeds. Inadequate pollination and insect damage are major reasons for this small fraction. In fact, fewer than three percent of the millions of flowers produced by mature, large Prosopis trees initiate fruit development, and only about one-third to one-half of these subsequently produce fruit. Fruits take approximately three months to mature and are subject to heavy predation by insects.

According to the Report on Phase I of the FAO/IBPGR project (FAO 1980), seed collection seasons are February to March or April in Chile; December to March, plus August, in Peru; and August onward in Mexico.

Morphology

The fruit of all of the species of the genus Prosopis are indehiscent pods which may be long and linear to short and coiled, depending upon the species. These pods, which vary in length from 3 to 30 centimeters, are produced hanging on small stalks in clusters of up to 12. An example of the morphological composition of Prosopis pods is presented in Table 3. The outer covering, the exocarp, of mature pods range in color from straw to reddish-brown, often being mottled. Unfortunately, the pods are frequently infested with weevils.

Table 3

Morphological Composition of Prosopis Fruit

Source: Becker and Grosjean 1980.

Within a pod, seeds are embedded in a pulpy matrix, the mesocarp. Each seed is separated from another by an enclosed compartment called a septum. A protective covering, the endocarp, which is impermeable to water, surrounds each seed. The endocarp must be broken for germination to occur; its presence is responsible for the observed long life of some of Prosopis seeds in soil.

The seeds may be oblong to almost square, depending upon the species and in some cases, uneven at the margin. In general, the seeds of most species are about 0.3 to 0.7 centimeters long, 0.3 to 0.4 centimeters wide, and 0.2 to 0.3 centimeters thick. They are usually shiny, light brown in color.

Production

As might be expected, there is considerable variation between individual trees and populations of trees in the production of fully developed seeds per fruit. However, from work by Solbrig and Cantino (1975) on Prosopis chilensis, a rough estimate can be made of the quantity of seeds produced by an average tree in one reproductive season.

An average tree of Prosopis chilensis produces several million flowers; for illustrative purposes, we will assume 10 million flowers as a rough estimate. One mature fruit is produced for every 10,000 flowers. From observations by Solbrig and Cantino, an average fruit contains 19 seeds. Multiplying these factors together yields a result of 19,000 seeds produced by a tree in a single reproductive season.

A similar exercise conducted for an average tree of Prosopis flexuosa, a tree species endemic to Argentina, resulted in an estimate of 80,000 seeds.

An actual count of the total seed crop of a tree of Prosopis velutina in the southwestern United States yielded 142,000 seeds.

Combining the above estimates and actual count, and taking into account a large margin of error for the gross approximation of the number of flowers on a tree, an estimate of between 10⁴ and 10⁵ seeds being produced by a Prosopis tree in one reproductive season seems appropriate.

From 20,000 to 35,000 seeds have been obtained from one kilogram of freshly collected pods. The average number of seeds per kilogram in four samples was 29,500, with a low of 20,600 and a high of 38,300.

Physiology

For Prosopis seed to germinate, the endocarp must be cracked or removed, and two processes must take place: water imbibition and gaseous exchange. With the onset of these processes, the embryo starts to grow, by cell enlargement, and then by cell division. Eventually, the radicle penetrates its protective covering.

With sufficient water, Prosopis seed can germinate in only six hours at approximately 35°C. The highest percentage of germination appears to occur at temperatures near 30°C, although seed will germinate at temperatures anywhere in the range of 20°C to 40°C. Apparently, the effect of temperature is to regulate the rate and extent of water uptake by germinating seed. Insufficient water is imbibed for germination at temperatures above or below the critical range.

The high temperature requirements for the germination of Prosopis seed is probably due to the fact that this genus evolved in regions that are characterized by summer rainfall. To some extent, the high temperature requirement can be interpreted as a mechanism to prevent seed from germinating during the occasional, and less reliable, winter rains.

Although Prosopis seed will germinate on the soil surface, survival of the resulting seedlings depends on the seed being covered by a thin (one to two centimeter) layer of soil. Since germination itself is not affected by light, the requirement of a soil cover for seedling survival appears more related to proper anchorage and maximum contact with a layer of moist soil. Moisture in the surface layers of soil is available in most arid and semi-arid regions for only a limited period of time. Emergent seedlings are totally dependent on the moisture surrounding seeds for a short period of time. Through natural selection, precise mechanisms that use a relatively narrow range of temperatures and moisture conditions have apparently been favored to ensure the germination of seed only during rainy seasons, when conditions are suitable.

Dormancy of seed, which is common in Prosopis, must be broken to achieve germination. Seed dormancy, as defined herein, is coat-imposed dormancy, i.e., inhibition of germination imposed by the embryonic envelopes. Removal of or injury to the appropriate envelopes enables a non-dormant embryo to germinate.

Under controlled environments, stored Prosopis seed can remain viable for a long period of time. For example, in the dry atmosphere of a herbarium in the southwestern United States, seeds of Prosopis juliflora were 60 percent viable after 50 years.