After reading this article you will learn about:- 1. Seed Size and Number 2. Soil Seed Banks 3. Seed Viability.
Seed Size and Number:
Seeds are admirably suited to their triple role as a means of multiplication, dispersal and stress avoidance. But equally important for the long-term survival of the species involved is the fact that in most cases each of the seeds is genetically unique.
Seed size is at least partly a function of the size of the parent plant. On a world-wide basis it has been calculated that trees, shrubs and herbaceous plants have mean seed weights of 328, 69 and 7 mg, respectively. Mean seed weight tends to be a fixed characteristic of each species.
Amongst the angiosperms it ranges over ten orders of magnitude, from the dust-like seeds of orchids which weigh about 10-6 g, to he enormous seeds of the double coconut (Lodoicea maldivica) which weigh up to 27 kg.
Seed size has been shown to be correlated with a number of environmental factors. Studies have shown that larger seeds are associated with drier habitats. Even within an individual species ecotypes from drier regions may have larger seeds.
The large seed size in plants exposed to drought is thought to be due to selection in favour of seedlings which can establish an extensive root system quietly by drawing on their own food reserves.
Very small seeds are characteristic of plants which are parasitic or saprophytic, at least in the early stages of growth. Since the nutrition of such seedlings is provided externally, the seeds have no need for stored reserves of food. Small seeds are also characteristic of species which have persistent dormant seed banks in the soil.
The small size may facilitate burial because of the ease with which such seeds would filter into cracks in the soil. A reduction in seed size has also been shown to be associated with predator avoidance, a large number of small seeds being more likely to escape predation than a small number of large ones. The disadvantages of the large seed is further compounded by the fact that larger seeds have relatively much heavier seed coats.
Soil Seed Banks:
The seed bank imparts a knowledge of the numbers of seeds present at one time, but also a knowledge of its dynamics; the rates of input and the rates at which seeds are lost through germination, predation and death (Fig. 181).
After dispersal most seeds undergo a period of dormancy. Depending on the species and the prevailing conditions, dormancy may last from a few days to many decades (or longer). In some species it is normal for a proportion of the seeds to become incorporated into the soil and become part of a store or ‘bank’ of seeds which can be drawn upon intermittently over a long period.
As long as these seeds remain buried they maintain their dormant state. If some disturbance brings them to the surface they will normally germinate, giving rise to plants whose parents may have existed many generations before. The seed bank partly reflects the history of the vegetation and is likely to contribute to its future.
The viability of seed is the life span of particular seed during which it is able to germinate. An important feature of the seed bank strategy is the length of time which the seeds can remain viable in the soil. Evidence of extreme longevity in seeds comes from many sources.
It usually takes the form of a viability test on specimens of known age. These may be from archaeological sites, or from dated herbarium sheets, or from experiments mi buried seeds.
No claims for long-term viability can compare with that made for the arctic lupin (Lupinus arcticus). Viable seeds of this species were found in lemming burrows in Yukon Territory, Canada and dated (by comparison with C14 analysis of remains found in similar circumstances) at 10.000 years.
They are thought to have been buried in a frozen state by a landslide which insulated the layers in which they were trapped.
The physiological processes which enable seeds to remain in suspended animation for so long are of considerable interest in themselves. Viability tests of seeds which have been subjected to the moisture, temperature and gaseous composition of soil under field conditions will clearly be a more accurate guide to ‘ecological longevity’.
The indication of the normal period of viability for a species under field conditions can be obtained by burying a known number of seeds and monitoring their germ inability over a period of years. It has been found that the numbers remaining viable tend to decline exponentially with time.
The rate of loss of viability is greatly increased if the soil is subjected to cultivation because many seeds will be lost from the seed bank by germinating.
The viability of seeds can of course best be determined by direct germination tests. These may give misleading results if the seeds are dormant and can germinate only after scarification, stratification, leaching, after ripening, heat treatment or other means. In most cases the germination restraints are located in the seed-coat, and removal of the seed-coat causes immediate development of the embryos.
With the seeds of desert plants, which have so many mechanism to prevent premature germination seed coat removal is probably the simplest method of establishing viability.
A frequently-used test for seed viability is the tetrazolium method, in which the viable embryos are stained when placed in a dilute solution of 2, 3, 5-triphenyl-tetrazolium chloride (TTC). If the seed is viable 1% solution of TTC is turned pink by chemical reaction due to respiration of the viable seed.