As previously mentioned, two processes must take place in orderthat 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, mechanicalbarrier to radicle protusion, etc., can result in dormancy of the seeds.Repeated attempts to germinate Prosopis seed with intact endocarps hasyielded, according to the literature, less than five percent germination.To overcome this dormancy, the seed must be scarified, i.e., ascratching of the seed coat to aid germination, or otherwise treated tomake the seed permeable to water and hasten germination.
A possible exception to the above is a fresh collection ofProsopis seed that has not yet been dried. It has been observed thatsuch seed may germinate promptly without pre-treatment.
Pre-treatments of Prosopis seed before sowing can be classifiedinto the following categories: mechanical treatment, water treatment,dry heat treatment, chemical treatment, and electrical treatment.
Mechanical treatment
Small numbers of seed can be effectively scarified by making asmall scratch on each seed with sand paper, by cutting each seed with aknife, or by sand papering the end of each seed that is opposite theradicle until the cotyledon is seen. However, as the seeds have to beindividually treated, these treatments may not be practical for largeoperations.
For large quantities of seed, mechanical scarification can beachieved by pounding the seeds with sand, or by rubbing the seeds overan 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 thetechniques can further improve germination. The simplest way of effectinga light impaction is by shaking the seeds in a metal or glass containerfor about 15 minutes at the rate of twice per second.
Mechanical scarification can also be accomplished through theuse of the thresher described by Flynt and Morton (1969) by replacingthe threshing screen with solid sheet metal and an adhesive-back sandpaper. Germination of Prosopis seed scarified in this manner approaches95 percent (see Figure 1, p. 17; and Appendix 2).
Scarification, if improperly done, is likely to reduce thelongevity of seed in storage and produce a varying percentage ofdamaged seedlings. Therefore, extreme care must be exercised.
Water Treatment
Seed dormancy in Prosopis can be overcome by covering the seedwith boiling water, and then allowing it to soak for 24 hours as thewater cools. This technique, which can be effective in enhancingimbibition and improving germination rates, is not useful for all ofthe tree species of genus Prosopis, however.
Soaking Prosopis seeds in tap water at ordinary temperatures isgenerally ineffective in breaking dormancy.
Dry heat treatment
Application of dry heat at 60°C to 80°C for 24 hours has beensuccessful in increasing the imbibition and germination of Prosopisseed. However, when exposed to temperatures of 90°C and higher forperiods of three hours or more, the seeds can be seriously damaged.
In a laboratory experiment conducted in India, the exposure ofProsopis juliflora seed to a constant temperature of 35°C for 24 hoursincreased germination rates, while exposure to low temperatures, i.e.,3°C to 10°C had little effect.
Chemical treatment
Small samples of Prosopis seed have been successfully scarifiedby immersion in absolute ethyl alcohol for 12 hours.
For large seedlots, a concentrated (98 percent) sulfuric acidtreatment is frequently recommended, if the soaking time in the acid ispredetermined. Most commonly, soaking times vary from 15 to 30 minutes.The increase in germination due to treatment with sulfuric acid is generallyattributed to a softening of the seed coat by oxidation, increasingthe permeability of air and water through the seed coat. The seedmust be meticulously rinsed several times in large quantities of waterafter soaking in the acid.
Electrical Treatment
Accoring to work done by Nelson et al. (1978), through theexposure 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
