Lecture 2: Seed Germination, dormancy, and the environment Flashcards
Plant Developmental Stages
- Seed
- Seedling (stage when cotyledons still attached)
- Vegetative (juvenile)
- Adult reproductive
Germination
The transition between seed and seedling
- see seed coat rupture and embryonic root start to come up
Factors affecting germination
Many factors, relative importance varies by species
- physical attributes
- chemical signals including plant hormones
- light
- temperature
Dormancy
- State of seed in which germination does not occur although conditions are generally favorable
- influences composition of seed bank
- preventing germination
Enforced dormancy (quiescent seeds)
Germination occurs easily when conditions are right
- example: seeds you buy at the store - you don’t have to break dormancy
Primary or innate dormancy
Seeds dormant on release from mother plant
Secondary or induced dormancy
Occurs if conditions are unfavorable for germination
- then must break the dormancy
Seed bank
Seeds being stored underground in the soil
Imbibition
- 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
Scarification
- rubbing seeds between sandpaper or soaking them in acid
- Mechanical (sanding) and chemical treatments (sulfuric acid)
Strophiole
Preformed weak site where imbibition begins
Testa
- Hard seed coats
- thickness varies among species
Chemical factors
- 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
Nitrates
- 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
Gap-detection mechanism
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
Ruderal species
Weedy species
Abscisic Acid (ABA)
high levels tend to prevent germination
- Phytohormone
Gibberellic Acid
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
Stratification
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
Temperature
- temp fluctuations felt more acutely near top of soil
- temp fluctuations dampened at deep soil depth or water depth, and under neighbors
Temperate Climate
summer annuals set seed in fall and germinate in spring
- **Require long exposure to low temps
- i.e. Hairy bittercress, Cardamine hirsuta
Mediterranean Climates
Winter annuals set seed in late spring and germinate in fall
- **Require long exposure to high temps
- i.e. Dwarf plantain, Plantago virginica
pigment Phytochrome
- 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
Chromophore
- the light detecting portion of the pigment Phytochrome
Pr
Phytochrome red
- red light absorbing form
- is the cis form isomer
Pfr
Phytochrome far-red
- far-red light absorbing form
- is the trans form isomer
Cuticle
helps control seed moisture; waxy
subcuticular layer
protection
lumen of Malphghian cell
- palisade cell layer - long, cylindrical cells
Ostesclerid cells
tough, protect seed
Structure of the Pr and Pfr
- 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
A light requirement:
- 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
the Pr form
- 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
the Pfr form
- 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
Phytochrome example
- 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
What is fluence?
- fluence = amount of light over a given area
- i.e. micromoles of quanta per meter squared
- fluence rate, (Irradiance/light intensity)
Very low fluence response
- VLFR: reciprocity (reverts) not FR (far-red) reversible -> stays in Pfr form
- tiny bit of red light
Low fluence response
- LFR: reciprocity FR reversible -> far-red reversible
High-irradiance response
- HIR: needs long exposure, no reciprocity
- not FR reversible
Law of Reciprocity
- 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
Photon Flux Density
- how many packages of light are illuminating a given area over a given time
