Seed germination Flashcards
What is imbibition?
Water absorption and resuming of respiration at the start of germination
What is germination characterised by?
Explain the regulation of germination within a seed
the rupture of the seed coat by the radicle.
first there is testa rupture then endosperm rupture
When the seed is not germinating ABA is synthesised in the endosperm and is transported to the embryo.
At some point when the conditions are right there will be less ABA which allows for the production of GA that trigger the production of enzymes that loosen the cell wall in the endosperm as well as cell wall loosening (cell expansion) and cell division in the embryo. This allows for the growth of the radicle to break down the endosperm, it usually breaks at the place it was attached to the mother plant called the micropyle, this is where there is more influx of water when the seed coat is broken.
Regulation of germination
What are the two hormones involved?
Germination is triggered in non-dormant seeds by imbibition under favourable environmental conditions (temperature, light, oxygen).
The processes involved in germination are controlled mainly by two hormones: abscisic acid (ABA) and gibberellins (GA).
What does ABA (Abscisic acid) do?
What does Gibberellic acid (GA3) do?
ABA inhibits germination.
GA promotes germination.
Reserve mobilisation
What is a glyoxysome for?
Seed lipids are oxidised in a specialised organelle called glyoxysome. (part of the Peroxisomes) this is where glyocisms are broken down.
Starch is hydrolysed by amylases and glucosidases.
Proteases cut seed storage proteins into free amino acids.
These are all produced during germination and are triggered by the Gibberellins.
What is the role of the aleurone layer in cereals (during andafter germination)?
The aleurone layer (outside layer of the endosperm) GA travels from the embryo to the aleurone layer.
uses its stored reserves to produce hydrolytic enzymes to remobilize storage compounds in the starchy endosperm.
The scutellum also produces enzymes and is essential for transport between tissues.
What is the Scutellum (tissue)
Scutellum = thin cotyledon in monocots
Why could it be advantageous to have a dormancy period?
Dormancy prevents untimely germination
Dormancy is the temporary failure of a seed to germinate under favourable conditions. Germination is blocked by one or several internal factors.
Dormancy allows seeds to germinate during the appropriate season/conditions and can favour distribution of germination over space and time.
Why could it be advantageous not to germinate under favourable conditions?
*To avoid too much competition and have more resources for the seeds
*to make sure you germinate in favourable conditions that will last and that are more stable instead of just two weeks in autumn.
What are the types of dormancy?
*Embryo dormancy:
-Undifferentiated or immature embryo
-Physiological dormancy: hormonal inhibition of germination
*Coat-imposed dormancy:
-Waterproof seed coat preventing water uptake
-Seed coat limiting entry of O2
-Mechanical restraint to embryo growth
Describe physiological dormancy
Physiological dormancy is established during seed maturation.
Physiological dormancy defines the conditions in which a seed can germinate.
When there is less and less dormancy the window for germination increases
Establishment of physiological dormancy
Dormancy is induced by high levels of ABA in the seed, which depend on the level of several environmental factors during maturation:
-Temperature
-Light
-Nitrate
Release of physiological dormancy
Physiological dormancy can be released over time: this is called after-ripening, or quickly during imbibition under favourable conditions.
What is Stratification?
Stratification is a pre-germination short treatment that reduces dormancy.
Seed crops
Seed crops include the most important food crops, with nearly 75% of human food eaten as seeds:
Cereals
Legumes
And also rapeseed, sunflower, nuts, coffee, cocoa…
