Seeds and Seed Germination

1. Seeds and Seed Germination

2. Seeds and seed germination Pollination Fertilization Embryo development Mature seed Seeds are normally the product of sexual reproduction

3. Seeds are for propagation The biological function of seeds is for propagation of the species How does seeds help in propagation of the species?

4. Diversity among seeds Thousands of Begonia seeds • Seeds are very diverse in term of size: • Begonia seed weigh 10-20 micrograms • Coconuts weigh more than a kilogram

5. Diversity among seeds In terms of adaptation to survive various environmentsuntil conditions are favorable for germination In terms of method of dispersal

6. Seeds are for propagation Protect the zygote against physically injury Store foods for seedling-- before it becomes independent Remaindormant to survive harsh environment Disperse the seeds to capitalize on their genetic variability Respondto environment cues and germinate at the right time and place

7. Seeds are alive! • To function in propagation, seeds must be alive • Seeds respire, albeit slowly • consume O2, produce CO2 and H2O • Seeds have a finite lifespan • they cannot be stored indefinitely • Which kind of seeds can remain viable longer? Seeds of tropical plant or temperate plant?

8. Seeds are alive! • Seeds of many tropical plants remain viable for only a short time, a few days • Tropical plants grow in environments that do not have a winter season through which seeds must survive before the favorable growing conditions of spring arrive

9. General features of seeds Embryo • Root and shoot, in a miniature form Food reserves • Allow seedling to grow before it is capable of performing photosynthesis Seed coat • Provides protection from the environment

10. Monocots and Dicots Dicots 200,000 species includes beans, roses, cacti, melons, citrus Angiosperms flowering plants Monocots 50,000 species includes grasses, lilies, orchids, palms Flowering plants (angiosperms): Dicotyledonous plants with two seed leaves Monocotyledonous plants with one seed leaf

11. Seed structure Cotyledon Plumule Seed coat or testa Radicle Micropyle

12. Endospermous and non-Endospermous seeds Takes place in the fruit on the parent plant Endospermous seeds: Retain the endosperm tissue, and is surrounded by a layer of living cells, the aleurone layer. Non-endospermous seeds: The endosperm tissue is absorbed by the cotyledons. The cotyledons then become the food reserve for the seed.

13. Endospermous or non-Endospermous seeds?

14. Endospermous and non-Endospermous seeds A LS of maize grain / fruit

15. SEED DORMANCY • It is a phenomenon in certain seeds in which they would not germinate if given an optimal condition ( water, oxygen, optimum temperature ). • Dormancy can be seen in seeds ( eg: legumes ), buds, spore & food storage organs ( tubers ). • Due to many factors include: • Lack of oxygen • Dryness • Presence of substances that inhibit germination

16. GROWTH UNDER EXTREME CONDITION DORMANCY • A period in the life cycle of many animals & plants when their metabolic activities become minimum & growth stop. • Is a resting stage • It can occur in the adult, egg, pupa, spore or seed stage. • A way of protecting an organism against unfavourable conditions such as insufficient food, cold ( winter ) & dry ( drought ). • It is controlled by hormones that • ~ response to physiological in plants & animals • ~ affecting the behaviour in animals CHAPTER : GROWTH

17. What physiological changes lead to dormancy Metabolism falls Number of organelles per cell _____ Dehydration – water content _____ Vacuoles in cells _____ Food reserves become _____ _____ _____

18. What physiological changes lead to dormancy Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense crystalline bodies

20. SEED Germination • Dormancy of these seeds may be broken by one or more of the following: • light, sunlight being the most effective; • low temperatures (1 to 5 degrees Celsius [33.8 to 41 degrees Fahrenheit]) for several weeks; • day/night fluctuating temperatures of 1 to 10 degrees Celsius (41 to 50 degrees Fahrenheit); • chemicals, such as nitrate in the soil, or applied hormones (gibberellins) in the laboratory; and • fire. CHAPTER : GROWTH

21. Dormancy mechanism is related to the seeds’ natural environment is particularly important for small, wind-dispersed weed seeds. Seeds that require light involves a receptor protein, phytochrome the cold winter may cause the parent plant to die, and thus remove competition for space in the spring. Seeds that need a period of low temperature The requirement for alternating temperatures will prevent germination of seeds beneath dense vegetation because the latter dampens the day/night temperature fluctuations; these seeds will germinate only when there is little vegetation cover, again reducing competition with established plants. The requirement for fire CHAPTER : GROWTH

22. Dormancy mechanism is related to the seeds’ natural environment Seeds that require light involves a receptor protein, phytochrome Seeds that need a period of low temperature The requirement for alternating temperatures The requirement for fire CHAPTER : GROWTH

23. Maintaining dormancy Physical barriersThe 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

24. The breaking of dormancy 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

25. To break dormancy seeds need: Dormancy is caused by: Is overcome by: Example: or or Seed Germination: Emergence of Radicle through Seed Coat Water Warm Temperature So if you want to store seeds what are the conditions? Dry Cold Dormant seeds need more than moisture and warmth: nick digest scrub fire freeze-thaw cycles Thick Seed Coat Scarification Kentucky Coffee Tree Thin Seed Coat Light Lettuce Dark Pea Insufficient Development Soil Fungus Association Orchids Inhibitor: Abscisic Acid Stratification > Vernalization Most CT feral plants 20° C 6 weeks at 4° C Inhibitor: Phenolics Leaching by Repeated Rain Cacti

26. Germination

27. Stages leading to cell division http://www.rbgsyd.nsw.gov.au/ Respiration Initially anaerobic Later aerobic Mitchondria reconstituted Soluble sugars ATP RNA activated Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) Mitosis (70h)

28. Mobilization of food reserves 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)

29. The control of food reserve hydrolysis Negative feedback  - amylase Maltose Starch + H20 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

30. 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

31. Triggering factors for germination Light, chilling or water (rain) trigger the inactivation of ABA, which makes dormant seeds able to germinate. ABA ABA ABA GA

32. After seeds take up water, GA is released from the embryo to signal aleurone. GA

33. The aleurone responds by synthesizing and secreting digestive enzymes (a-amylase) to hydrolyze stored nutrients in the endosperm. a-amylase

34. Nutrients (ex. sugars) absorbed from the endosperm by the scutellum (cotyledon) are consumed by the seedling during germination.

35. The growth of seedling starting with the roots first, then shoot growth follows.

36. hydrolysis Fruit+Seed Coat starch Endosperm sugar Aleurone Layer cotyledon Storage Protein translation hydrolysis RNA shoot apex Amino Acids growth DNA transcription GA radicle apex water imbibition Barley Seed Germination maltose exocytosis -amylase monocot Embryo

37. Capsella Seed: Embryo Seed Coat Shoot Apex Endosperm Cotyledons - dicot Hypocotyl Radicle Root Apex Micropyle

38. shoot apex hydrolysis Seed Coat starch sugar cotyledons translation RNA transcription growth DNA radicle apex water Pfr Pr imbibition photoactivation dark Lettuce Seed Germination -amylase dicot Embryo phytochrome photoreversibility red and white light stimulate germination 660 nm 730 nm

39. Germinationofseeds 1. Utilization of stored reserves • In cotyledons or endosperm tissue • During germination, enzymes are made that convert stored reserves (large molecules) into compounds that can be used by the seedling (smaller molecules) • starches  sugars • lipids, fats  sugars • proteins  amino acids

40. Germination of seeds 2. Transport of compounds into growing seedling through vascular system • These compounds have two functions • Support respiration in the embryo • Provide a source of building blocks (carbon, nitrogen, etc.) for the seedling 3. Expansion and growth of seedling • Root radicle elongates down, hypocotyl expands up • Establishment of root system and emergence of shoot

42. Seedling establishment Shoot emerges and is exposed to light Chlorophyll is produced and seedling starts to perform photosynthesis Seedling is no longer dependent on reserves from the seed If stored reserves are consumed before photosynthesis is established, the seedling will die

43. Seedling establishment • Growth of the seedling can be measured in many ways • Length • Increases after seed imbibes • Fresh weight • Increases as seedling grows • Dry weight • Declines initially as stored reserves are consumed by respiration, increases once photosynthesis is established

44. Conclusions • Seeds are alive but dormant • Comprise an embryonic plant and stored reserves • Germination requires • Water - for imbibition • Oxygen - for respiration • Suitable temperature • Outcome of successful germination is a seedling capable of independent growth

46. TYPES GERMINATION • Radicle will emerge first from the seed • Next, the shoot tip breaks through the soil surface • Types of germination: • i. epigeal • - cotyledons appear above the ground • { hypocotyl forms a hook and pushes aboveground, raising the cotyledons } • ii. hypogeal • - cotyledons remain underground • { epicotyl forms a hook and shoot tip is lifted out of the soil }

47. …TYPES GERMINATION

48. …TYPES GERMINATION