Two processes must take place in order that Prosopis seed can germinate: water imbibition and gaseous exchange. However, various types of inhibition caused by seed envelopes, i.e., impermeability to water, impermeability to oxygen, mechanical barrier to radicle protusion, etc., can result in dormancy of the seeds. Repeated attempts to germinate Prosopis seed with intact endocarps has yielded, according to the literature, less than five percent germination. To overcome this dormancy, the seed must be scarified, i.e., a scratching of the seed coat to aid germination, or otherwise treated to make the seed permeable to water and hasten germination.
A possible exception to the above is a fresh collection of Prosopis seed that has not yet been dried. It has been observed that such seed may germinate promptly without pre-treatment.
Pre-treatments of Prosopis seed before sowing can be classified into the following categories: mechanical treatment, water treatment, dry heat treatment, chemical treatment, and electrical treatment.
Small numbers of seed can be effectively scarified by making a small scratch on each seed with sand paper, by cutting each seed with a knife, or by sand papering the end of each seed that is opposite the radicle until the cotyledon is seen. However, as the seeds have to be individually treated, these treatments may not be practical for large operations.
For large quantities of seed, mechanical scarification can be achieved by pounding the seeds with sand, or by rubbing the seeds over an abrasive slab. Both of these techniques are simple and inexpensive, and they have been found successful.
Light impacting of Prosopis seed that has been scarified by the techniques can further improve germination. The simplest way of effecting a light impaction is by shaking the seeds in a metal or glass container for about 15 minutes at the rate of twice per second.
Mechanical scarification can also be accomplished through the use of the thresher described by Flynt and Morton (1969) by replacing the threshing screen with solid sheet metal and an adhesive-back sand paper. Germination of Prosopis seed scarified in this manner approaches 95 percent (see Figure 1, p. 17; and Appendix 2).
Scarification, if improperly done, is likely to reduce the longevity of seed in storage and produce a varying percentage of damaged seedlings. Therefore, extreme care must be exercised.
Seed dormancy in Prosopis can be overcome by covering the seed with boiling water, and then allowing it to soak for 24 hours as the water cools. This technique, which can be effective in enhancing imbibition and improving germination rates, is not useful for all of the tree species of genus Prosopis, however.
Soaking Prosopis seeds in tap water at ordinary temperatures is generally ineffective in breaking dormancy.
Dry heat treatment
Application of dry heat at 60°C to 80°C for 24 hours has been successful in increasing the imbibition and germination of Prosopis seed. However, when exposed to temperatures of 90°C and higher for periods of three hours or more, the seeds can be seriously damaged.
In a laboratory experiment conducted in India, the exposure of Prosopis juliflora seed to a constant temperature of 35°C for 24 hours increased germination rates, while exposure to low temperatures, i.e., 3°C to 10°C had little effect.
Small samples of Prosopis seed have been successfully scarified by immersion in absolute ethyl alcohol for 12 hours.
For large seedlots, a concentrated (98 percent) sulfuric acid treatment is frequently recommended, if the soaking time in the acid is predetermined. Most commonly, soaking times vary from 15 to 30 minutes. The increase in germination due to treatment with sulfuric acid is generally attributed to a softening of the seed coat by oxidation, increasing the permeability of air and water through the seed coat. The seed must be meticulously rinsed several times in large quantities of water after soaking in the acid.
Accoring to work done by Nelson et al. (1978), through the exposure of Prosopis juliflora seed to various periods of radio-frequency (RF) dielectric heating by electromagnetic fields of 10 megahertz (MHz) and 39 MHz, germination rates were increased. However, further studies will be necessary to demonstrate the practicability of RF treatment of seeds to break dormancy.
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