1.2 Flashcards
Primary (innate) dormancy
Internal/ embryo-imposed dormancy
Dormancy is inherent in the embryo, not due to any influence of the seed coat or other surrounding tissues.
The seed remains dormant even if the seed coat is removed.
Embryonic axis is unable to elongate, even when it is removed from seed coat.
Reasons for primary (innate) dormancy - embryo imposed dormancy
Immature embryo
May be due to an immature embryo.
Does not show full development at the point of ripening.
Further growth of embryo is needed after the seed becomes separated from the plant at maturity.
Eg Ilex aquifolium
Reasons for primary innate dormancy - embryo imposed dormancy.
Presence of inhibitors
Presence of inhibitors, especially ABA.
Abscence of growth promoters eg. Gibberellic acid.
Loss of embryo dormancy is often associated with a sharp drop in ration of ABA to GA.
Eg. Fraxinus excelsior which has internal germination inhibitors present in embryo.
Seed-coat enhanced dormancy.
Seed remains dormant even though the seed coat is removed.
Seed coat-enhanced dormancy.
Causes
Interruption of water uptake.
Mechanical restraint.
Interference with gas exchange.
Supply of inhibitors to embryo or promotion of the synthesis of inhibitors within the embryo.
Prevention of the movement of inhibitors from the embryo.
For many seeds, more than one of these factors operates to maintain coat-enhanced dormancy
Reduction in water uptake.
Example
Lathyrus odoratus
Reduced water uptake by seed is due to impermeable seed coat.
Reduced gaseous exchange.
Examples
Seed coat reduces gaseous exchange, oxygen is limited, respiration rate is reduced to ensure embryo does not deteriorate but prevents germination. eg. Beta vulgaris and Paeonia delavayi
Primary (innate) dormancy
Definition
Present when seed is shed.
Most plants have a period of innate dormancy to prevent germination on parent plant.
Mechanical restraint
Embryo can be mechanically restrained by tough testa.
Mechanical restraint
Example
Prunus spp.
Germination inhibitors in surrounding fruit.
Examples
Abscisic acid in Rosa spp.
Hydrogen cyanide Malus spp. And Prunus spp.
Seed coat can also prevent exit of inhibitors eg. Avena sativa
Secondary (induced) dormancy
Definition
Develops from conditions that delay germination
Seeds are able to germinate immediately before or just after being shed or after primary dormancy has been broken but if conditions cause delay in germination, dormancy can occur, resulting in difficulties in subsequent germination.
Eg Can develop in seeds during dry storagek
Example of secondary (induced) dormancy.
Acre spp. Eg Acer palmatum and Acer campestre
Overcoming secondary (induced) dormancy
Sowing ‘green’ seed - seed that is collected when it is mature but before it is dispersed.
Sow this seed immediately, before dormancy mechanisms become established.
