Lecture 9 - Seedbanks and Dormancy Flashcards
Definning Seedbanks
Following the dispersal event, seeds find their way to the soil surface.
Germination can occur immediately or be delayed for some indefinite period. These seeds, on or in the soil are now part of the soild seed bank.
Seeds in the seed bank can ve describes as :
1. Transient - survive for < 1 year.
2. Short-term persistence (1-5years)
3. Long-term persistence (5 years)
Short-term persistence and long-term persistence applies to the seeds of herbaceous plants and some shrubs.
Intuitively, this make sense: an annual plant that fails to reproduce one year will go extinct if there are no seeds persisiting in the bank .
Tree seeds are limited to transient persistence.
Evidence of longevity
The presence of viable seeds in a seedbanck of a species that is no longer present in the community is evidence of persistence.
Soil seed banks: example from the literature:
Seed bank densities vary widely: from very low (in woodlands – both tropical and temperate) and artic/alpine environment to very high in disturbed habitats and wetlands.
Examples
Arctic tundra – 0/m2 (low end)
Forests – a few/m2
Pastures/meadows – 100s/m2
Weeds in agricultural fields – 1000s/m2
Wetlands –100 000s/m2 (high end)
The highest ever recorded for a single species:
480,000/m2 for Spergularia marina (seed mass= 0.05mg)
Morphological traits:
Easily determines upon an external or internal inspection of the seed. They are functionally related to the dispersal process.
But also gives us information about soil persistence:
e.g. seed coat thickness and shape.
Biophysical traits:
Reflect the physical-chemical properties of the seed coat, the endosperm or the embryo, an they have to be measured in the laboratory.
Biophysical traits that are important to soil persistence: type of nutrient reserve, water content.
Germination traits:
The main germination trait is seed dormancy. **
Seed dormancy is an innate property of seeds**, much like seed mass.
Seed dormancy is defined as a block to the completion of germination of an intact viable seed under favourable conditions
A plant trait (or functional marker)
is a heritable trait that can be measured at the individual level.
These traits can be categorical or continuous.
THREE types of Dormancy
- Physiological
- Morphological
- Physical (sometimes called mechanical)
Physiological dormancy
In physiological dormancy, the block is abscisic acid- a germination inhibiting agent within the embryo. It’s removal/transformation requires some exposure to LOW tempertaure. How cold and for how long depends on the species–and within a species can differ among populations. In addition to abscisic acid (the inhibiting agent), gibberllins induce germination.
It is therfore the relative concentrations of those two plant hormones.
This is by far the most typical dormancy type for North American Trees.
Physiological dormancy is the most abundant form and is found in seeds of gymnosperms and all major angiosperm clades.
It is also the most prevalent dormancy form in temperate seed banks.
Once physiological dormancy is broken, a seed may still require light, warm temperatures and water to induce germination.
Exposure to cold to break dormancy is called stratification
For example, the stratification of Douglas fir seeds:
* Stratified seeds for 4 months at very cold temps = 75% germination at 10 degrees
* Stratified seed for 3 weeks at very cold temps = 50% germination at 10 degrees.
In short, we can go from 0% germination to 100% germination following exposure to prolonged cold.
Morphological dormancy
A strange kind of dormancy: at the time of absicssion, the embryo is not finished growing. It is a mass of undifferentiated cells. At this point the embryo represents 0.1% of the volume of the seed. Maturation of the embryo occurs on the ground.
Physical Dormancy:
Occurs in about 15 plant families.
The seed coat (or pericarp) is impermeable to water.
Water imbibed into the seed enters trough the opening left by the micropyle or funiculus (now the hilum scar).
Seeds displaying physical dormancy are often covered with waxes or a hard seed coat that must be slowly abraded. (MOST persistent seeds can and do imbibe water: seeds with physical dormancy don’t-they are by definition turly dormant).
Once the see coat is breached and water is imbibed, germination will occur. Some species will also have a light requirement.
Phyiscal dormancy:
Typical ways to abrade waxes and seed coats:
- Saltation, e.g. blowing across a desert substrate. In the lab, dormancy of these seeds is broken by shaking them in glass bottle or simply by poking a hole trough the seed coat.
- Stomach Acids: HCL in the stomach degrades the seed coat. Once it is eliminated by the animal, the seed can imbibe water. In the lab, this type of dormancy is broken by soacking seeds in sulfuric acid.
- Some species seem to germinate better after very high temperature. (fire) The effects of temp. in breaching physical dormancy is not well undetsood.
From dormancy to quiescence
A nondormant seed will not germinate unless a certain combination of physical environmental factors (e.g. temp, light, moisture), is PRESENT.
The nondormant see that does not germinate because of the absence of one or more of these factors is said to be in a state of quiescence.
Quiescent seeds germinate when they are accorded with appropriate environmental conditions such as water, the normal composition of the atmosphere, and a physiologically favorable temperature, whereas dormant seeds require, in addition, specific environmental, hormonal, mechanical, or other cues to trigger germination.
Temperature
The percentage germinating goes from 0 at some minimum temperature to essentially 100% at some higher temperature.
The minimum temperature for northern trees can be quite low –sugar maple is remarkable for its capacity to germinate in very cold soil a few centimeters away from a lingering patch of melting snow.
Balsam poplar and willow in Alaska can start germinating at temperatures between 1 and 5 degrees C.
More commonly, the temperature for seed germination in North America must be 15-25 degrees Celsius.
