User:TbC32rbog/Soil Seed Bank

= Soil Seed Bank =

Seed dormancy
Seed banks (seed pools) are found in stratified soil layers consisting of many overlapping generations of seeds deposited over a number of years. Germination depends on seed viability and the process of breaking dormancy. Generations of seeds can remain dormant until environmental conditions favor germination. Hormonal signaling and environmental cues such as soil moisture, temperature, and photoperiodism all influence the transition from dormancy to germination and are balanced by two hormones, abscisic acid (ABA) that represses germination, and giberellins (GAs) which break dormancy and promote germination. Instances of frost heaving can act as a form of natural scarification, increasing the chances for successful germination. Cycles of freezing and thawing can assist with the temporal and spatial movement of seeds deeper into the soil. The degree of latency within seed populations is found to introduce resistance to change in the genetic composition of plant populations, with more dormancy slowing the changes in allele frequencies in comparison to seeds that lack dormancy.

Population genetics and evolutionary perspectives
Seed banks are a strong determinant of demographic success because they may act as a source of “genetic memory”. Seed pools provide a genetic reservoir, leading to increased levels of genetic variation. Instances of harsh climatic and stochastic events contribute to genetic bottlenecks, but the loss of genetic diversity in plant populations can be buffered by seed banks. Such instances are shown in studies examining rare species, as with the annual species of Clarkia springvillensis, a rare California grassland.

Temporal and spatial models account for historical genetic changes, suggesting that higher levels of genetic diversity may be found in seed pools rather than the above ground vegetative populations. Studies also show higher heterozygote deficiencies present in upper soil layers, likely stem from increased self-pollination and habitat fragmentation. Soil seed banks may work as an evolutionary filter by selecting for traits that influence fitness and limit levels of inbreeding depression by introducing historical genotypes. It is suggested that seed banks may be critical for the continued maintenance of genetic variation within plant populations.