Lecture 2: Seed Germination, dormancy, and the environment

Question 1

Plant Developmental Stages

Answer 1

1. Seed
2. Seedling (stage when cotyledons still attached)
3. Vegetative (juvenile)
4. Adult reproductive

Question 2

Germination

Answer 2

The transition between seed and seedling

- see seed coat rupture and embryonic root start to come up

Question 3

Factors affecting germination

Answer 3

Many factors, relative importance varies by species

• physical attributes
• chemical signals including plant hormones
• light
• temperature

Question 4

Dormancy

Answer 4

• State of seed in which germination does not occur although conditions are generally favorable
• influences composition of seed bank
• preventing germination

Question 5

Enforced dormancy (quiescent seeds)

Answer 5

Germination occurs easily when conditions are right

- example: seeds you buy at the store - you don’t have to break dormancy

Question 6

Primary or innate dormancy

Answer 6

Seeds dormant on release from mother plant

Question 7

Secondary or induced dormancy

Answer 7

Occurs if conditions are unfavorable for germination

- then must break the dormancy

Question 8

Seed bank

Answer 8

Seeds being stored underground in the soil

Question 9

Imbibition

Answer 9

• when the seed takes in water, which triggers the cellular growth and replication that allows the seed to germinate
• specifically with seeds
• absorption of H2O

Question 10

Scarification

Answer 10

• rubbing seeds between sandpaper or soaking them in acid

- Mechanical (sanding) and chemical treatments (sulfuric acid)

Question 11

Strophiole

Answer 11

Preformed weak site where imbibition begins

Question 12

Testa

Answer 12

• Hard seed coats

- thickness varies among species

Question 13

Chemical factors

Answer 13

• water soluble germination inhibitors in pericarp or testa
• nitrate (NO3-)
• smoke
• plant hormones - ABA & GA
• moderate rains wash away specific organic compound or accumulated salts

Question 14

Nitrates

Answer 14

• many ruderal species stimulated by nitrate
• gap-detection mechanism
• large plant biomass depletes soil of nitrate
• in the absence of vegetation mineralization & nitrification causes nitrate to accumulate breaking dormancy
• required for plant growth and survival

Question 15

Gap-detection mechanism

Answer 15

Suggests the available nitrate to take up, and if there are fewer plants in an area then it is ok to germinate
- I.e. Gap in understory, a break in the plant coverage so it is place to germinate

Question 16

Ruderal species

Answer 16

Weedy species

Question 17

Abscisic Acid (ABA)

Answer 17

high levels tend to prevent germination

- Phytohormone

Question 18

Gibberellic Acid

Answer 18

high levels tend to release dormancy by loosening cell walls and converting starch to sugar

• seed can then take advantage of sugar
• phytohormone
• Gibberellins are applied to large amount of barley seeds to induce germination at the same time
• Germinated seeds are then blasted with high heat to halt germination and caramelize sugars to create malted barley

Question 19

Stratification

Answer 19

breaking seed dormancy through exposure to low temperatures

• aka long exposure to cold temperatures
• mimicked by people placing seeds between moist layers of sand or paper towels in a refridgerator

Question 20

Temperature

Answer 20

• temp fluctuations felt more acutely near top of soil

- temp fluctuations dampened at deep soil depth or water depth, and under neighbors

Question 21

Temperate Climate

Answer 21

summer annuals set seed in fall and germinate in spring

• **Require long exposure to low temps
• i.e. Hairy bittercress, Cardamine hirsuta

Question 22

Mediterranean Climates

Answer 22

Winter annuals set seed in late spring and germinate in fall

• **Require long exposure to high temps
• i.e. Dwarf plantain, Plantago virginica

Question 23

pigment Phytochrome

Answer 23

• a small protein with light detecting portion, the chromophore
• detects presence or absence of red and far-red light
• absorbs light thereby changing protein folding that begins a series of signal transductions which end in a response
• if seeds are close to soil surface or not shaded by neighbors, the Red-Far red light ration increases and vice versa

Question 24

Chromophore

Answer 24

• the light detecting portion of the pigment Phytochrome

Question 25

Pr | Phytochrome red

Answer 25

- red light absorbing form | - is the cis form isomer

Question 26

Pfr | Phytochrome far-red

Answer 26

- far-red light absorbing form | - is the trans form isomer

Question 27

Cuticle

Answer 27

helps control seed moisture; waxy

Question 28

subcuticular layer

Answer 28

protection

Question 29

lumen of Malphghian cell

Answer 29

- palisade cell layer - long, cylindrical cells

Question 30

Ostesclerid cells

Answer 30

tough, protect seed

Question 31

Structure of the Pr and Pfr

Answer 31

- the chromophore undergoes a cis-trans isomerization at carbon 15 in response to red and far-red light - structure of the Pr and Pfr forms of the chromophore and the peptide region bound to the chromophore

Question 32

A light requirement:

Answer 32

- prevents fatal germination of seeds buried too deep - indicates absence of established vegetation or a soil disturbance - gap detection (detects the presence and location of neighbors) - indicates status of leaf canopy by effects on spectral composition. Light under leaf canopy has low R:FR, which can prevent seed germination

Question 33

the Pr form

Answer 33

- absorbs at a peak of 666 nm - is the form synthesized in dar-grown seedlings - when Pr absorbs red light, it is converted to the Pfr form

Question 34

the Pfr form

Answer 34

- absorbs at a peak of 730 nm - the Pfr form is the active form that initiates biological responses - when Pfr absorbs far red light, it is converted to the Pr form - Pfr can also spontaneously revert to the Pr form in the dark over time = dark reversion

Question 35

Phytochrome example

Answer 35

- lettuce seeds grown in the dark do no receive enough red light to switch the phytochrome to an active state and germination does not occur (a photomorphogenic response) - will germinate if exposed to 1 min. of red light - but, if red light treatment followed by few minutes of far-red light then no germination

Question 36

What is fluence?

Answer 36

- fluence = amount of light over a given area - i.e. micromoles of quanta per meter squared - fluence rate, (Irradiance/light intensity)

Question 37

Very low fluence response

Answer 37

- VLFR: reciprocity (reverts) not FR (far-red) reversible -> stays in Pfr form - tiny bit of red light

Question 38

Low fluence response

Answer 38

- LFR: reciprocity FR reversible -> far-red reversible

Question 39

High-irradiance response

Answer 39

- HIR: needs long exposure, no reciprocity | - not FR reversible

Question 40

Law of Reciprocity

Answer 40

- magnitude of response dependent on the product of the fluence rate and the time of irradiation - has the amount been met and has it been for a certain amount of time

Question 41

Photon Flux Density

Answer 41

- how many packages of light are illuminating a given area over a given time