Seeds and Seed Germination © 2008 Paul Billiet ODWSODWS

Seed structure Cotyledon Plumule Radicle Micropyle Seed coat or testa © 2008 Paul Billiet ODWSODWS

Seed maturation  Takes place in the fruit on the parent plant  Endospermous seeds: Retain the endosperm tissue, which eventually dies but it is surrounded by a layer of living cells, the aleurone layer.  Non-endospermous seeds: The endosperm tissue is absorbed by the cotyledons. These then become the food reserve for the seed. © 2008 Paul Billiet ODWSODWS

Dormancy  Metabolism falls  Number of organelles per cell falls  Dehydration – water content falls  Vacuoles in cells deflate  Food reserves become dense crystalline bodies © 2008 Paul Billiet ODWSODWS

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 © 2008 Paul Billiet ODWSODWS

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 © 2008 Paul Billiet ODWSODWS

Germination: The breaking of dormancy The growth of the embryo and its penetration of the seed coat Break down of barriers Abrasion of seed coat (soil particles) Decomposition of seed coat (soil microbes, gut enzymes) Cracking of seed coat (fire) Change in physical state - rehydration Destruction and dilution of inhibitors Light, temperature, water Production of growth promoters © 2008 Paul Billiet ODWSODWS

Seed Germination  The first stage of seed germination is the absorption of water or imbibment. Some seeds are capable of absorbing twice their own mass in water.  The absorption of water causes the seed to expand and rupture its seed coat.  The water stimulates the embryo to release giberillins which in turn stimulate enzymes such as amylase to begin to breakdown stored starch to form energy.  Other enzymes are stimulated to form amino acids and build proteins, others form cellulose, lipids, and nucleic acids.

Germination STAGEEVENTS PREGERMINATION(a)Rehydration – imbibition of water. (b)RNA & protein synthesis stimulated. (c)Increased metabolism – increased respiration. (d)Hydrolysis (digestion) of food reserves by enzymes. (e)Changes in cell ultrastructure. (f)Induction of cell division & cell growth. GERMINATION(a)Rupture of seed coat. (b)Emergence of seedling, usually radicle first. POST GERMINATION(a)Controlled growth of root and shoot axis. (b)Controlled transport of materials from food stores to growing axis. (c)Senescence (aging) of food storage tissues. © 2008 Paul Billiet ODWSODWS

Seed germination germinati on Physiology germWater resorptionFood reserve hydrolysisMobilisation of foodRespiration and asimilationEmbryo growth Morphology germ Radicle growthPlumule growth

1. Water Resorption * osmosis or imbibition * through seed coat or micropyl * mechanical strength decreasing of cellulose of seed coat : seed coat soften

2. Food reserve hydrolises  Hydrolyses of macromolecules of food reserve become monomere  Need enzymes: α-amylase, β-amylase, protease, peptidase, lipase  Enzymes : a. pre-exist : β-amylase b. de novo : α-amylase  The forming of de novo enzyme inducted by GA

The control of food reserve hydrolysis  Control by growth promotors such as gibberellin and growth inhibitors such as abscisic acid  These directly affect the genes for enzyme synthesis or the activity of the enzymes themselves  The growth substances are affected by environmental factors (e.g. light, temperature, humidity) © 2008 Paul Billiet ODWSODWS

The control of food reserve hydrolysis  Negative feedback control of enzymes  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 Starch + H 2 0 Maltose  - amylase Negative feedback © 2008 Paul Billiet ODWSODWS

CarbohydratesStarches (amylopectin & amylose) Amylases Maltose and glucose Proteinse.g. ZeinProteasesAmino acids LipidsOilsLipasesFatty acids & glycerol 3. The mobilisation of food reserves  The food reserves are stored as large insoluble macromolecules  They are hydrolysed using enzymes to smaller soluble molecules for transport © 2008 Paul Billiet ODWSODWS

4. Asimilation and respiration of food  The micromolecule synthesize become new macromolecule : construct cell substantion Respiration of some of micromolecule: to get energy for cell division and growth of radicle and plumule

Stages leading to cell division Mitchondria reconstituted Soluble sugars Respiration Initially anaerobic Later aerobic ATP RNA activated Polysomes Protein synthesis (0.5h) Enzymes (proteins) Mitosis (70h) DNA synthesis (45h) © 2008 Paul Billiet ODWSODWS

Conditions for seed germination Successful seed germination requires  Viable, vigorous seed  Adequate moisture  Supply of oxygen  Appropriate temperature  Protection from pathogens and pests

Moisture is required for seed germination  Seed survive at very low moisture  Germination starts with imbibition,uptake of water into the seed  Once seed are imbibed and development is initiated, the seedling is very sensitive to dehydration

Moisture for germination Maintaining an adequate supply of water is essential for good germination In greenhouse (indirect seeding)  thorough watering after seeding  monitoring of soil moisture and regular watering or misting  using plastic wrap to retain water Beware! Many pathogens thrive under high humidity conditions

 Oxygen is readily available if seed is planted in a well drained, porous medium - soil or greenhouse mix  Oxygen deprivation results from waterlogging, flooding or other soil drainage problems Germination requires oxygen

Temperature Requirement varies between species  Temperature can be adjusted in greenhouse use of “bottom heat”  In the field, planting depends on environmental conditions and weather, not necessarily the calendar

Protection from pathogens  Developing seedlings are a rich source of food for microbes  Seedlings are relatively unprotected cuticle and other defenses are not fully developed in young seedlings  Many fungal pathogens cause “damping off” of seedlings Phytophthora, Pythium, Rhizoctonia

Protection from pathogens Preventive measures include:  soil pasteurization (180˚F, 30 min)  using purchased sterilized soil mix  clean containers  controlling moisture  using clean seed  use of fungicides, seed treatments