Physiology of germination and dormancy Alison A Powell University of Aberdeen UK
Dry seed Normal seedling (ISTA germination) Physiological germination (radicle protrusion)
Dry seeds Low moisture content High matric potential (-350 to -50MPa) Low water potential
Water uptake Mobilisation of food reserves, decreased water potential Imbibition Radicle protrusion Water potential gradient (seed vs environment) results in rapid water uptake (physical process) Water potential of seed Priming and environment at equilibrium
Membrane changes during imbibition? Phosphatidyl head group Dry seed Imbibed seed Fatty acid Hydrated, liquid crystalline Dry gel Hydrated, liquid crystalline Rapid water uptake Imbibition damage Reduced germination
Respiration during imbibition and germination Embryo Storage tissue I: Activation of enzymes Active mitochondria II: Synthesis of enzymes / mitochondria Temporary anaerobiosis (CO 2 / O 2 ) Ethanol / lactate produced Taken from Bewley and Black, 1994 Seeds: Physiology of development and germination
Protein and RNA synthesis Resume minutes after hydration rrna, trna, some mrnas: retained in dry seed Initial synthesis: Translation of mrnas (mrna turnover) Polysomes Proteins synthesised
DNA synthesis DNA repair DNA replication DNA repair DNA damage drying/ rehydration; storage Single and double strand breaks endonuclease activity, free radicals, base loss Repair by DNA polymerases and ligases
Germination (%) Time course of germination 100 A 80 Deterioration B 60 40 20 Lag period Repair Longer lag period More repair needed F Germination H at 20 o C I 0 0 24 48 72 96 120 144 Time from set to germinate (h) Repair also during priming
Normal germination (%) (20 C).. From Khajeh Hosseini, Nasehzadeh and Matthews,2010 Seed Science and Technology,38, 602-611 Lag period, repair and normal vs abnormal seedlings (oil seed rape) 100 80 60 40 A: less deteriorated Less repair needed Fewer abnormals, more normals B: deteriorated lot More repair needed Repair incomplete Few normals 20 0 0 20 40 60 80 100 Germination (%) after 2 days at 20 C R 2 = 0.62 *** B Slow germination Low germination at 5d Long lag period A Faster germination High germination at 5d Short lag period
DNA synthesis Cell cycle G1 Normal cell growth 2C DNA S Phase 4C DNA Doubling of DNA Increased β tubulin Mitosis G2 growth phase
DNA synthesis 2C DNA Cell cycle G1 Normal cell growth Dry seed 2C DNA Imbibition S Phase Doubling of DNA Increased β tubulin 4C DNA G2 growth phase Mitosis After radicle protrusion Priming: 4C DNA produced when germination is advanced following repair Implications for seed storage( Powell et al, 2000 Journal of Experimental Botany, 51,2031-2043) High germination, high vigour (little deterioration). Advancement, 4C DNA synthesis, reduced longevity High germination, low vigour (deteriorated seed) Repair, little advancement, improved longevity
Final stages of germination Radicle extension and protrusion: cells expand increased turgor; cell walls yield causes unknown possible role for expansins proteins involved in cell wall relaxation in vegetative growth; loosen H bonds? Production of normal seedling Mobilisation of storage reserves Seedling growth
Dormancy Prevents germination even in conditions adequate for germination Evolutionary adaptation Bet-hedging Two types of dormancy Primary dormancy part of genetic programme of seed development and maturation Secondary dormancy Mature imbibed seed Induced by environment Occurs in non-dormant seeds + initially with primary dormancy
Dormancy : inability to germinate in apparently favorable conditions Primary dormant seeds Breaking of primary dormancy Induction of secondary dormancy Non germination Non-dormant seeds Secondary dormant seeds Germination Breaking of secondary dormancy Germination Courtesy of Françoise Corbineau
Factors that maintain dormancy Maternal Testa / pericarp/ endosperm/ megagametophyte (gymnosperms) Mechanical Natural chemical inhibitors Permeability (water, gases) Embryo Endosperm: Restraint of radicle growth Hormones: ABA/GA 3 antagonism; embryo sensitivity to ABA and GA 3 Genetics Interaction of dormancy promoting + germination repressing loci vs germination promoting loci Dormancy classification: Baskin and Baskin
Dormancy breaking 2 processes Dormancy breaking Dormancy breaking agent Threshold value; single event or incremental events >1 factor may be effective Increased range of conditions in which germination will occur Germination Requires right conditions even after dormancy broken e.g. Summer annuals: Dormancy broken by low temperature; Germination requires a higher temperature Dormancy cycling Primary dormancy decays Increased range of conditions in which germination will occur, until nondormant But if germination not triggered, dormancy re-established.
Environmental Temperature Factors breaking dormancy Dry after-ripening; alternating temperature; stratification Light Chemical Light / dark; single doses Inorganic CO 2, nitrate, nitrite Organic Varied; butenolides (KAR1) Hormonal ABA / GA balance
Role of hormones in breaking dormancy
Germination (%) Seed physiology Seed testing 100 80 60 40 20 F H I 0 0 24 48 72 96 120 144 Time from set to germinate (h) Thank you!