Seed Dormancy & Germination PDF

Seed Dormancy & Germination Chapter 18 (part 1) The triploid central cell of the ovule develops into a nutrient-rich, multicellular mass: endosperm Embryonic development begins when the zygote divides into two cells The seed becomes dormant when the cotyledons have formed and germination usually...

Seed Dormancy & Germination Chapter 18 (part 1) The triploid central cell of the ovule develops into a nutrient-rich, multicellular mass: endosperm Embryonic development begins when the zygote divides into two cells The seed becomes dormant when the cotyledons have formed and germination usually requires some environmental triggers, e.g., a period of cold The ovule develops into a seed Seed Structure • Seed coat provides protection • Endosperm = food (STARCH) • Aleurone cells = store abundant protein • • • • • Cotyledon à leaves Epicotyl à shoot Hypocotyl à cotyledon attached Radicle à root Plumuleà leaf primordia 1 cotyledon (scutellum); Coleptile protects first leaves. Coleorhiza: protects radicle Capsella Seed: Embryo Seed Coat Endosperm: Food storage. Starch, oils, protein Shoot Apex Cotyledons - dicot Hypocotyl Radicle Root Apex Micropyle No Two Seeds Are Alike Seed Dormancy Seeds enter a quiescent phase. Intrinsic temporal block on germination • • • • • Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense crystalline bodies ADVANTAGES • Maximizes seedling survival • Creates a seed bank • Seed dispersal (birds) • Synchronizes germination with seasons Maintaining dormancy • Physical barriers The seed coat (testa) is waxy = waterproof and impermeable to oxygen • Physical state – dehydrated • Chemical inhibitors present e.g. salts, mustard oils, organic acids, alkaloids • Growth promoters absent Types of Dormancy in Seed Quiescent – The seeds are able to germinate upon imbibition of water at permissive temperatures. Primary Dormancy – Seeds cannot germinate even if immediate conditions are right. This form of dormancy delays germination until season, or other macroenvironmental issues are right for survival. Induced by ABA. Secondary Dormancy – An additional level of protection to prevent germination. Can be induced under very unfavorable conditions such as drought or cold, etc. Secondary Dormancy CAUSES 1- Permeability of seed coats 2- Temperature requirements 3- Light requirements and interaction with temperature 4- Germination inhibitors Exogenous Dormancy - Imposed by factors outside the embryo. Seed coat (impermeable / inhibitors). Endogenous Dormancy – Imposed by factors within the embryo. Embryo is not fully mature. Dormancy in Annuals Vivipary • Seeds that germinate while attached to the mother plant – Rare in Angiosperms (but mangrove) – Preharvest sprouting • $$ losses • Reduces grain quality ABA:GA • Determine seed dormancy – ABA => inhibits – GA => promotes Ethylene and brassinosteroids: reduce the ability of ABA to inhibit germination HORMONAL NETWORK regulating germination The balance between both biosynthetic and deactivation pathways is regulated at the gene level by action of transcription factors Seed viability • Viability: When a seed is capable of germinating after all the necessary environmental conditions are met. – Average life span of a seed: 10 to 15 years. • Some are very short-lived e.g. willow (< 1 week) • Some are very long-lived e.g. mimosa 221 years – Conditions are very important for longevity: cold, dry, anaerobic conditions • These are the conditions which are maintained in seed banks Factors Influencing the Life Span of Seeds 1- Internal Factors 2- Relative Humidity and Temperature 3- Seed Moisture Life expectancy of selected seeds Sugar Maple 2 weeks English Elm 26 weeks White Clover 90 years Sensitive Plant 200 years Indian Lotus 1040 years Arctic Lupine 10000 years Germination: The breaking of dormancy • The growth of the embryo and its penetration of the seed coat. • It’s NOT reversible! • Germination per se is the process up to the appearance of the radicle Factors affecting germination: 1. Seed must be viable 2. Environmental factors • Water**: most essential factor (turgor pressure à cell expansion) • O2: activates oxidative reactions (cellular respiration) • Temperature: some seeds need chilling (stratification) • Light: photoblasty. Phytochrome (R:FR) • Nitrate Germination: The breaking of dormancy • Germination starts when a seed absorbs water – Ends when the primary root emerges • After germination, the seedling goes through establishment – Until it is independent & photosynthesizing Forest seeds will not open until hole in canopy GERMINATION • • • • • Hydration (or imbibition) - Seeds must take up water A seed will absorb water only if the seed coat and/or other coverings are permeable. Water is absorbed by osmosis, driven by the high solute concentration in seed cells Metabolism processes – transcription and translation are reinitiated Enzyme activation - after seed hydration: respiratory enzymes are activated and food reserves (starch) are metabolized to produce fuel (mostly ATP) for synthesis of other enzymes needed for such growth. Carbohydrate, fat and protein reserves in the cotyledons or endosperm are mobilized to support the renewed development of the embryo. Seed Germination 1. Imbibition - water uptake, softens inner tissues causes swelling and seed coat rupture more water uptake increase in the cellular respiration rate activates GA synthesis Seed Germination 1. Imbibition - water uptake, softens inner tissues causes swelling and seed coat rupture more water uptake increase in the cellular respiration rate activates GA synthesis 2. Gibberellic Acid (GA) - dissolved & distributed by water (diffusion) stimulates the synthesis of enzymes arrives at aleurone cells activates certain genes Seed Germination 3. Transcriptionà transportationà translation à amylase 4. Amylase accelerates hydrolysis of starch 5. Starch is converted into maltose and then into glucose: moves to the cotyledon and radicle to initiate growth 1. Imbibed water stimulates Gibberellin synthesis. 2-3. Gibberellins diffuse to the aleurone layer and stimulate the synthesis of enzymes. 4-5. Enzymes break down the starch and the sugars are transported to the developing embryo. a-amylase ns GA DNA tra growth Embryo cri p ti on RNA shoot apex radicle apex H2O imbibition Endosperm Aleurone Layer Storage Protein s cotyledon Fruit+Seed Coat ysi maltose is s y starch ol r d hy sugar exocytosis hy dro l monocot tran sl a tion Barley Seed Germination Amino Acids Lettuce Seed Germination Eudicot is ys l o dr shoot apex hy starch Seed Coat sugar cotyledons cri p ti on ns DNA tra Embryo grow th RNA tra nsl atio n a-amylase radicle apex phytochrome photoreversibility red and H2O imbibition photoactivation Pfr 660 nm 730 nm dark white light Pr stimulate germination The control of food reserve hydrolysis • Negative feedback control of enzymes Negative feedback Starch + H20 a - amylase Maltose • The action of the enzyme also limited by substrate • Once all the starch in an amyloplast is hydrolysed the enzyme stops work Therefore the release of the stored food is adjusted to suite the demand The mobilisation of food reserves Carbohydrates Proteins Lipids Starches (amylopectin & amylose) Amylases Maltose and glucose e.g. Zein Proteases Amino acids Lipases Fatty acids & glycerol Oils • The food reserves are stored as large insoluble macromolecules • They are hydrolysed using enzymes to smaller soluble molecules for transport • Vacuole stores has K+, Mg2+, Ca2+, phytin (phosphates) Stages leading to cell division Mitchondria reconstituted Respiration Initially anaerobic Later aerobic Soluble sugars ATP RNA activated Polysomes Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) http://www.rbgsyd.nsw.gov.au/ Mitosis (70h) Postgermination Establishment • Critical for plant survival, growth & development • Highly susceptible to biotic/abiotic factors • Sequence: Radicle emergence and exhaustion of seed reserve à appearance of first leaf à seedling is capable of self-sustained growth Seedlings photosynthesize, assimilate water & nutrients, cellular and tissue differentiation and maturation, respond to environmental stimuli Germination Stimulators and Inhibitors Although hormones have been shown to promote germination, it is by no means clear-cut. Endogenous and exogenous levels of hormones are known to affect the germination of seeds. The role of endogenous levels or exogenously applied hormones on germination has been widely investigated. Auxins Gibberellins Citokinins [Auxins] are known to control development of different plant parts. Rapid and sharp increase in the endogenous IAA content of lupin, bean, maize, wheat and pine seeds in response to water imbibition, especially during the time of radical emergence. Stimulates the germination of seeds. Actively metabolized in germinating seeds: promoters. The levels of endogenous cytokinins in the endosperm decrease during imbibition and clearly germination. Abscisic acid (ABA) Ethylene ABA is detected just prior to germination. It soon disappeared after the emergence of radicle. Exogenously applied: stimulates germination. Germinating seeds are able to produce ethylene. Some ethylene effects are gene activation and formation of some mRNA and membrane integrity, changes in the level of hydrolytic enzymes, endogenous auxins. AUXIN Production: Shoot tips & developing seeds Auxin is synthesized in the shoot apex => moves down Required for elongation of cells In the root: minimum [auxin] is required but if it’s too high -> inhibits This is due to the effect auxin has on ethylene Stimulation of growth requires energy Acid growth hypothesis: cell wall elongation is mediated by low pH. Expansins loosen the cell wall by weakening hydrogen bonds when pH is acidic