There are two major constraints to the germination of Prosopis africana in the field. First the pods do not dehisce like the papillionaceous ones. It means that after dropping from the tree, the pods do not release the seeds except by some accidental or mechanical force. Even then the seeds do not scatter as in the case of those with explosive mechanism and therefore their dispersal is restricted. The second constraint to germination of this seed is the waxiness and hardness of its seed coat which therefore imbibes water slowly. These difficulties may also partly explain the scarcity and isolated occurrence of this plant in the field.
Acid scarification has been used by a lot of workers to break dormancy in seeds. Somade and Ekeke (in prep) found concentrated sulphuric acid to be very effective in seed coat dormancy in Terminalia superba. Adeola and Dada (1985) worked on the germination of Acacia albida and Acacia senegal and also found that treatment with concentrated sulphuric acid for 10 min. significantly gave better germination percentage of 99% and 57.3% respectively than other treatments. Germination trials carried out by Nwankiti (1982) showed that concentrated sulphuric acid treatment to scarify the seed coat considerably increased the percentage of germination.
Treatment with Chemicals
Embryo dormancy of seeds has often been broken by various chemicals such as gibberellic acid and the cytokinins. In species with relatively mild embryo dormancy oxidising agents such as hydrogen peroxide stimulate respiration and accelerate germination. However, hydrogen peroxide has been shown to have practical limitations including germination of some seeds with embryo dormancy.
The property of the seed to remain in a state of apparent inactivity (dormant) after separation from its mother renders it the most efficient mechanism for perpetuation and propagation of the parent tree. Under favourable conditions the seed resumes active living evidenced by germination and growth. The process requires that the seed must be viable and must be subjected to favourable environmental conditions of water, temperature and oxygen. Any internal condition which prevents germination even when the environmental conditions are favourable needs to be overcome through pre-germination treatment.
A good quality seed is true to species and variety and has capacity for high germination. It is also free from mixture with other crop seeds, weed seeds and extraneous materials (impurities). Seed test on a small representative sample of the seed determines the purity and germination capacity of the seeds.
The purity or percentage of the pure seed present in the sample is carried out by sorting and expressing it in weight or numbers. The information is important and can be used in determining the rate of sowing to obtain a given stand of seedlings per unit area.
Reduced seed viability may result from several factors such as improper seed development on the tree, injuries during harvest, improper handling procedure during processing, storage, and ageing. Methods of determining viability include visual examination, cutting, floatation, colour, weight, chemical stain test, and germination. Viability can be represented by the germination percentage, which expresses the number of seedlings which can be produced by a given amount of seed. Viability of seeds can therefore be tested by seed sample germination of whole seed or carefully excised embryo.
Tetrazolium test is a biochemical seed testing method in which viability is ascertained by the red colour appearing when the seed is soaked in 2,3, triphenyl tetrazolium, chloride (TTC). Living tissues become red while dead ones remain uncoloured. The TTC is effective in both dormant and non dormant seeds, yield quick results and indicates weakness in seed before germination is actually impaired. However difficulty may occur in judgment where both stained and unstained area occur in the same seed. The difficulty and time required in preparation for the test may be greater than in the excised embryo test for most seeds. The TTC solution deteriorates with exposure to light and therefore seed has to be soaked in darkness.
Measurement of germination involves the germination percentage and germination rate. The rate of germination is dependent upon the degree of dormancy still present in the seed and the environmental conditions influencing the seed. Germination therefore takes place over a period of time, and the rate at which it occurs is measured by seed vitality germinated within a specific number conditions of days and is usually estimated by laboratory germination, under optimum conditions, of 4 replications each of about 100 randomly picked seeds and averaging the results.