Chapter 7 Principles of Propagation by Seed Principles

Chapter 7 Principles of Propagation by Seed

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

Phases of germination

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

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)

Fresh Weight Imbibition & Lag Phase

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

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

Radicle emergence

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

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!

aleurone layer

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

Canola Seed Oil

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

Germination curve

Standard seed germination curve

Seed vigor

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)

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)

Seeds primed and pregerminated in aerated PEG

Effects of seed priming on germination

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 midtemperature range and low on either end

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

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

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

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

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

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

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

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

Damping-off in tomato & soybean

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

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

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 (O 2/CO 2) • Preventing leaching of internal inhibitors • Supplies inhibitors to the embryo

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

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

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 KNO 3 or GA

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)

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)

Lettuce seed is light sensitive Dark Light

Tanada effect Exposed to red light (phytochrome active) Exposed to far- red light (phytochrome inactive)

Principles of Propagation by Seed • Endogenous dormancy – Physiological dormancy • Intermediate/deep physiological dormancy – – Stratification (moist-chilling) Must be aerated Moisture should be constant Temperature should reflect the plant’s native habitat for the winter/spring (≈ 35 - 45ºF) with a minimum of 23ºF – Time for seed-chilling requirements are related to budchilling requirements!

Effects of stratification on germination

Seeds hormone levels during stratification

Effects of vernalization/stratification on seedling development

Principles of Propagation by Seed • Endogenous dormancy – Physiological dormancy • Intermediate/deep physiological dormancy (continued) – For intermediate dormancy, if embryo is removed from the seed, it will readily germinate (dormancy is mostly seedcoat) and moist-chilling time required is short – For deep dormancy, an excised embryo will not germinate readily nor will it form normal plants (physiological dwarfs) and these seeds require a long moist-chilling period (> 2 mo. )

Principles of Propagation by Seed • Double dormancy – Seed has two kinds of dormancy – Ex: a rudimentary embryo & seed coat dormancy = morphological dormancy & physical (exogenous) dormancy • Thermodormancy – Different from thermal inhibition – Once exposed to high temperatures, will not germinate when temperatures return to the optimal germination range • Ex: lettuce in summer, therefore often primed (in PEG) at cool temperatures to allow germination prior to sowing

Principles of Propagation by Seed • Advantages of seed dormancy – Seedling survival - permits germination only when the environmental conditions are favorable – Creates a “seed bank” - not all seeds for a species germinate in a single year. Spreads germination out over time – Synchronize germination - for a particular time of year – Seed dispersal - especially after being carried in the digestive tract of animals

Summary of types of germination

Epigenous seed germination

Epigenous seed germination

Epigenous seed germination

Hypogenous seed germination