Seed Viability and Dormancy Dr Margaret Johnston Centre

Seed Viability and Dormancy Dr Margaret Johnston Centre for Native Floriculture The University of Queensland Gatton 1 Dr Margaret Johnston

Seed viability • Seed viability – A seed that is capable of germinating to give a “normal” seedling – The degree to which the seed is alive (metabolically active) – Viability is highest at the point of physiological maturity and then gradually declines 2

Tests for seed viability • Germination test (under defined laboratory conditions) • Tetrazolium test – May need to disrupt seed coverings – Imbibe seed for 24 h at ambient temperature – 1. 0% (w/v) 2, 3, 5 -triphenyltetrazolium chloride (TZ) solution and held at 40°C in the dark for 24 h – Cut seed (embryo) longitudinally – Interpretation can be difficult 3

Tests for seed viability • Excised embryo test (good for dormant seeds) • Cut seed test • X-Ray test (allows evaluation of normal morphology 4

Seed dormancy • • Survival mechanism Genetically inherited trait Wild plants show more dormancy than crop plants A degree of dormancy is desirable as it prevents premature sprouting on the parent plant • Dormancy is defined as a state in which seeds are prevented from germination even when environmental conditions are favourable 5

Seed dormancy • Several physical and physiological mechanisms of dormancy, including primary and secondary dormancy occur in seeds • Primary dormancy – Exogenous dormancy • Essential germination components (water light temperature) are not available so the seed fails to germinate • Usually related to properties of the seed coverings or light 6

Seed dormancy • Causes of exogenous dormancy – Lack of water due to impermeability of seed coat – Impermeability of the seed coverings to gases (oxygen) • Mucilaginous material surrounding seed • Phenolic compounds – Mechanical restriction of embryo growth – Removal of seed coverings may also remove inhibitors 7

Methods of breaking exogenous dormancy • In nature this type of dormancy is overcome by – Ingestion by animals – Microorganisms – Fires (smoke, heat shock) – Temperature fluctuations – Natural soil acidity – Freeze-thawing 8

Methods of breaking exogenous dormancy • Mechanical scarification – Abrasion – Brief immersion in boiling water – Piercing seed coats – Duration of treatment is critical – Prolonged treatment may cause damage 9

Methods of breaking exogenous dormancy • Chemical scarification – Sulfuric acid – Sodium hypochlorite – Hydrogen peroxide – Cellulase and pectinase – Hazardous – Seeds must be washed and dried – Reduction in seed germination 10

Endogenous dormancy • Due to inherent properties of the seed Causes • Environmental conditions during seed maturation influence the duration of endogenous dormancy – – – 11 Daylength Moisture status Position on the parent plant Age of the mother plant Environmental conditions during seed development and maturation

Endogenous dormancy • Only physiological changes are able to relieve endogenous changes – Rudimentary embryo dormancy • Seeds are shed before the embryo is mature • Continued development can take from a few days to several months • After-ripening 12

Endogenous dormancy – Physiological dormancy • Seed dormancy is believed to be regulated by a balance of endogenous growth inhibitors and promoters – Level of the substances is controlled by certain environmental stimuli, These substances may be » Cyanide » Phenolics » Abscisic acid (ABA) – Promoters » Gibberellic acid » Cytokinins 13

Endogenous dormancy • Osmotic inhibition – Fleshy fruit may contain such substances • Chemical inhibitors Methods of breaking endogenous dormancy • Leaching inhibitor (chemical or osmotic) 14

Endogenous dormancy • Temperature – Seeds with a specific temperature requirement for germination often contain inhibitors and promotors – Stratification (3 to 10 ˚C) • Physical and physiological changes may occur in imbibed fruit – Development of the embryo – Inhibitor-promoter balance – Alternating temperatures – After-ripening 1 to 2 months at 15 to 20 ˚C 15

Endogenous dormancy • Light – Dormancy is broken by exposure to red light (670 nm) – Continuous light may inhibit germination of some species • Circadian rhythms – Time measuring capacity – Appears to influence the pattern of seed germination • Interaction of primary dormancy mechanisms – Seed may show a hard seed coat and physiological dormancy • Embryo excision can be used to overcome dormancy 16

Secondary dormancy • Non-dormant seed encounter conditions that cause them to become dormant – Spring wheat and winter barley • Exposure of dry barley seed to temperatures between 50 to 90 ˚C • Seven days storage of winter barley at high moisture contents at 20 ˚C • One day storage of spring wheat at air-tight container at 50 ˚C • Placement of seed under water and in darkness for 1 -3 days at 20 ˚C • Secondary dormancy is temperature, light or darkness imposed 17

Germination factors Centre for Native Floriculture The University of Queensland Gatton 18

Chemical promotion of seed germination • Gibberellins usually GA 3 – Promotes germination in many species – Can substitute for light and temperature – Important role in the regulation of seed dormancy – 50 to 500 mg/L 19

Chemical promotion of seed germination • Cytokinins – Kinetin – Known to break primary dormancy in some seeds but appear to be more effective at overcoming secondary dormancy – Exact role is unclear – Overcomes chilling requirement of sugar maple, Proteaceae (Leucodendron and Protea) – Overcomes light requirement (Rumex, lettuce, celery) 20

Chemical promotion of seed germination • Ethylene – Stimulates germination of some species – Regulates auxin levels in dormant seeds – Peanuts and sunflowers 21

Chemical promotion of seed germination • Hydrogen peroxide – Stimulates germination of several species – Conifers, legumes, tomatoes and barley – Respiration stimulant accelerates breakdown of food reserves – Disinfectant 22

Chemical promotion of seed germination • Auxins – Auxins and other plant growth regulators are universal components of plants and common constituents of seeds – IAA reported to increase lettuce seed germination – Effect is temperature dependant – High concentrations inhibit while low concentrations promote or a ineffective – May interact with light in influencing germination 23

Chemical promotion of seed germination • Potassium nitrate (KNO 3) – Widely used to promote seed germination – Concentrations used 0. 1 to 0. 2% – Most seeds that are sensitive to KNO 3 are also sensitive to light – May interact with temperature and light – May act cooperative with plant growth regulators (GA 3 and kinetin) – There are reports that KNO 3 can inhibit germination – KNO 3 may influence the respiratory system, it may stimulate O 2 uptake or serve as a co-factor to phytochrome 24

Chemical promotion of seed germination • Thiourea – Like KNO 3 thiourea promotes germination of many species – Perhaps replacing temperature and light requirements 25

Chemical promotion of seed germination • Other chemicals – Plants can produce substance that promote or inhibit germination – Scopotelin (phenolic) 26

Other factors • These factors may affect germination – Osmotic pressure (high osmotic pressure make imbibition more difficult and usually retards germination) • Halophytes germinate better in saline environments – p. H • Germination of most species occurs readily between p. H 4 to 7. 6. 27

Other factors • Presoaking – Can speed germination – Presoaking at 20 ˚C protects seeds from chilling injury during subsequent germination at lower temperatures – Prolonged soaking can be damaging. – Need to avoid low O 2 levels • Osmoconditioning 28

Other factors • Frost and cold nights prior to seed harvest – May injure seed – Depend on • • • 29 Temperature Exposure Moisture content of seed Physiological maturity of seed Husk protection Variety

Other factors • Radiation – Exposure to gamma radiation above 10 Krad may retard seed germination – Effects more pronounced at high temperatures and high seed moisture content 30

Other factors • Mechanical damage – Harvesting, processing and handling – Susceptibility to mechanical damage increases as moisture content decreases – Varieties can vary in their susceptibility to mechanical damage 31