Chapter 7

1. Chapter 7 Principles of Propagation by Seed

2. Principles of Propagation by Seed • Germination process- • Seed must be viable = embryo alive and capable of germination • Seed must receive: • Water • Proper temperature • Oxygen • Light (depending on species) • Primary dormancy must be overcome = after-ripening. Often removed by environmental conditions

3. Phases of germination

4. Principles of Propagation by Seed • Phases of Germination: • I.) Water uptake • By imbibition = a physical process in seeds with a permeable seed coat • Occurs whether seed is alive, dead, dormant or non-dormant • First 10 - 30 minutes = rapid uptake • Followed by 1 - 3 hours of slow uptake • Seeds generally do not wet uniformly • Volume of seed increases

5. Principles of Propagation by Seed • Phases of Germination: • I.) Water uptake (continued) • Leakage: amino acids, sugars, proteins,... Since cell membranes are not fully functional yet • Quantity leaked is proportional to seed quality • High leakage means the seed is susceptible to attack by insects, fungi, and bacteria (can be measured by an electrical conductivity meter)

6. Imbibition & Lag Phase Fresh Weight

7. Principles of Propagation by Seed • II.) Lag phase • Mitochondria mature • Proteins are synthesized (enzymes are activated) • Food reserves are metabolized • Enzymes loosen cell walls

8. Principles of Propagation by Seed • III.) Radicle emergence • Result of cell enlargement • Food reserves continue to be used • Enzymes degrade certain cell walls to permit exit of the radicle • GA promotes enzymatic cell wall hydrolysis and radicle emergence • ABA inhibits enzymatic cell wall hydrolysis

9. Radicle emergence

10. Principles of Propagation by Seed • Use of storage reserves • Proteins in protein bodies • In cotyledons and endosperm • Enzymes (proteinases) are required to break down proteins into amino acids • Proteinases synthesized during imbibition

13. Principles of Propagation by Seed • Starch • In endosperm (in grain crops) • Order of events: • Imbibition • GA in embryo & scutellum (protective sheath around cotyledons in monocots) translocated to the aleurone layer • Aleurone layer = a secretory cell layer that surrounds the endosperm • Enzymes are synthesized (-amylase) • Enzymes convert starch to glucose & maltose sugars and then transported to the embryo for use in development and growth • Note: these sugars are important in beer making!

14. aleurone layer

18. Principles of Propagation by Seed • Lipids • Oil bodies in endosperm & cotyledons • Oils = triacylglycerides (glycerol & fatty acids) • Glyoxysomes are organelles found ONLY in seeds! They process stored oils. • Fatty acids are high energy compounds used in the glyoxylate cycle to produce sucrose • Sucrose is then transported to the embryo for use in development and growth

19. Canola Seed Oil

22. Principles of Propagation by Seed • Measures of germination: • Germination percentage (%) = number of seedlings produced in a specified time • Germination rate - T50 value = # of days required to achieve 50% germination of the seed lot

23. Germination curve

24. Standard seed germination curve

25. Seed vigor

26. Principles of Propagation by Seed • Environmental factors influencing germination: • 1.) Water - threshold water potential = amount of water needed by the seed for radicle emergence • Rate of water movement in soil depends on: • Texture (pore space) • Packing (pore space) • Closeness of seed/soil contact • Water with high salt content can counter-balance the effects of water imbibition (this is a problem in California with subirrigated fields and high water evaporation)

27. Principles of Propagation by Seed • Seed priming • Regulates water imbibition of seeds • Charles Darwin suggested this possibility in 1855! • Polyethylene glycol (PEG) used today (aerated) • Starts metabolic processes without radicle emergence • Seed is re-dried for short-term storage at cool temperatures • Results in uniform germination • Used on bedding plant plug production (annuals)

28. Seeds primed and pregerminated in aerated PEG

29. Effects of seed priming on germination

30. Principles of Propagation by Seed • 2.) Temperature • The MOST important environmental factor that regulates TIMING of germination • Boil-treat seeds to control disease. This won’t damage the seed as long as the seed is DRY • Store seed at low temperatures to prolong viability • Temperature affects germination percentage and germination rate • Germination rate increases with an increase in temperature (up to a point) • Germination percentage is constant in the mid-temperature range and low on either end

31. Principles of Propagation by Seed • Temperature ranges: • Minimum - lowest temperature for germination • Maximum - highest temperature for germination • Optimum - a range where the greatest percentage of seedlings are produced at the highest rate

32. Principles of Propagation by Seed • Temperature categories • Cool-temperature tolerant - native to temperate zones prefer 40 - 86°F • Ex: broccoli, cabbage, carrot, peas, alyssum • Cool-temperature requiring - native to a Mediterranean climate. No germination if > 77 °F • Ex: celery, lettuce, onion, delphinium

33. Principles of Propagation by Seed • Temperature categories (continued) • Warm-temperature requiring - native to subtropical and tropical regions • Must be > 50°F for sweet corn & tomato • Must be > 60°F for beans, pepper, cucumbers, cotton

34. Principles of Propagation by Seed • Temperature categories (continued) • Alternating temperatures • Day/night temperature fluxes are better than constant temperatures • Used in seed testing labs • 18°F (10°C) difference often used • Imbibed weed seeds deep in soil do not germinate since there is little temperature flux, however, they will germinate if the soil is cultivated and seeds are brought to the surface where there is temperature flux

35. Principles of Propagation by Seed • Aeration effects on germination • Oxygen uptake is proportional to the amount of metabolic activity • Oxygen diffuses through water slowly therefore waterlogged soils slow/inhibit germination

36. Principles of Propagation by Seed • Light effects on germination • Involves quality (wavelength) and photoperiod (duration) • Light-sensitive seed are generally small in size or are epiphytes (grow on other plants) • Ex: alyssum, begonia, coleus, orchids

37. Principles of Propagation by Seed • Light effects on germination • A few plants have germination inhibited by light • Ex: amaranthus, allium, phlox • Some require dark to germinate • Ex: calendula (pot marigold), delphinium, pansy • Some require a specific daylength • Ex: birch, hemlock

38. Principles of Propagation by Seed • Disease during germination • Damping-off • Pythium ultimum • Rhizoctonia solani • Botrytis cinerea • Phytophthora spp. • Drying, salts and excess heat at the soil surface can also look like damping off Optimum growth between 68-86°F Pb. on warm-requiring seeds Secondary pathogens

39. Damping-off in tomato & soybean

40. Principles of Propagation by Seed • Dormancy - regulation of germination • Quiescent seeds - only need to be imbibed and incubated @ an appropriate temperature for germination. NO dormancy! • Primary dormancy - a type of dormancy where seeds will not germinate despite adequate environmental conditions • Secondary dormancy - induced under unfavorable environmental conditions

41. Principles of Propagation by Seed • Dormancy is important to propagators because it allows storage, transport and handling of seed • After-ripening - changes in the dry seed during storage that allow the seed to germinate following favorable conditions

42. Principles of Propagation by Seed • Types of Primary Dormancy • Exogenous dormancy - factors outside the embryo (seed coat or parts of the fruit) • Inhibits water uptake • Physical restriction on embryo expansion or radicle emergence • Controlling gas exchange (O2/CO2) • Preventing leaching of internal inhibitors • Supplies inhibitors to the embryo

43. Principles of Propagation by Seed • Exogenous physical dormancy (“seed coat”) • Outer integument becomes hard or fibrous during dehydration and ripening (Ex: coconut, honeylocust, Kentucky coffeetree) • In drupes (cherry, peach, etc.). Have a hardened endocarp (“pit” or “stone”) • In nature, hard seed coats are softened by: • Microorganisms • Passage through an animal’s digestive tract • Abrasion -freeze/thaw • Fire

44. Principles of Propagation by Seed • Exogenous chemical dormancy • In fleshy fruits • Contain chemical inhibitors such as ABA (Ex: citrus, cucumbers, apples, pears, grapes, etc.) • Desert plant fruits have chemical inhibitors that must be leached away by rains that then provide enough water for germination and seedling development

45. Principles of Propagation by Seed • Endogenous dormancy • Morphological dormancy • Rudimentary embryo = araliaceae (ginseng), papaveraceae (poppy), ranunculaceae (anemone) • Linear embryo = ericaceae (rhododendron), annonaceae (pawpaw) • Overcome by: • Alternating temperatures • Treat with KNO3 or GA

47. Principles of Propagation by Seed • Endogenous dormancy (continued) • Physiological dormancy • Non-deep = after-ripening. Fresh seeds of herbaceous plants (annuals and many perennials) lose dormancy during standard storage • Photodormancy • Seeds require either light or dark conditions • Involves phytochrome (in most plants) which is photoreversible • There is often an interaction between light and temperature • Light requirement can sometimes be offset by cool temperatures or alternating temperatures (Ex: lettuce seed, can germ. in dark if temp. below 73°F)

48. Principles of Propagation by Seed • Endogenous dormancy • Physiological dormancy • Photodormancy (continued) • Seed coat or underlying endosperm act as light sensors (if removed, light control disappears) • Hormones (GA) can overcome a light requirement • Red > far-red in natural sunlight (2:1) therefore phytochrome is active (PFr form) and seed stimulated to germinate • Under foliage, far-red light penetrates more than red light, therefore phytochrome is inactive (Pr form) and seeds fail to germinate • Red light does not penetrate soil as deeply as far-red light, therefore light-sensitive seeds stay dormant until they get closer to surface (Ex: weed seeds)

49. Lettuce seed is light sensitive Dark Light