Light Flashcards
What are the 3 main ecological roles of light?
providing energy for photoautotrophs (basis of food webs)
determines climate and weather, esp temp
involved in determining physiology, morphology, behaviour and life history for most organisms
What is the main source of energy for life on earth?
solar radiation
What is the visible spectrum of light and the photosynthetically active spectrum?
visible: 390-760 nm
PAR: 380-710 nm
Which wavelengths are the major component of solar radiation hitting the earth? what happens to the other wavelengths?
visible wavelengths (390-760 nm)
UV is absorbed by ozone
IR is absorbed by CO2 and H2O
What are the 3 trajectories of light as it passes through the atmosphere?
reflected
scattered
absorbed
What physiographical factors influence solar radiation intensity?
latitude
aspect
slope
all three can interact and have compounding effects
How does latitude influence solar radiation intensity?
the angle of incidence of solar radiation is different depending on the latitude
ex. at the equator, light hits more directly and spreads little across the surface = higher intensity
vs.
closer to the north pole, light hits at more of an angle and the light spreads across more distance = lower intensity
At which latitudes is the role of aspect more important? less?
kind of important around equator
less important at equator and at poles
most important between mid latitudes and poles
How does aspect of slopes influence solar intensity?
eastern aspects will have higher intensity in the mornings as the sun is rising
western aspects will have higher intensity in the afternoon as the sun is setting
southern and northern aspects receive max intensity at midday but south»_space;» north
How does slope interact with aspect and latitude to alter solar intensity?
increasing slope increases solar intensity when it is at maximum at any aspect
What biological factors influence solar intensity?
height and density of vegetation
How does slope influence the role that vegetation plays on influencing radiation intensity?
the effects of the height and density of vegetation will be more pronounced on flat ground v sloping ground (more light penetration)
Use the bryophyte-rich field layer vs. lichen-rich field layer Scots pine forest example to discuss the differences in tree height and solar radiation
lichen rich Scots pine forests are being replaced by mossy forests as trees are increasing in height and density
the larger trees reduce the amount of solar radiation that hits the ground
- thickening OM layer on soil
- higher soil water content
- better conditions for moss than lichen
in the lichen rich forests with small less dense Scots pine, the soil is mostly mineral substrate and holds less water
define photoperiod
the duration of daily light period
it varies in different locations because of the earth’s rotation on its axis
What determines the variation in photoperiod?
latitude - the length of photoperiod changes with latitude and season
How is solar radiation influenced by season?
photoperiod changes with seasons, especially in latitudes away from the equator
ex. days longer in northern hemisphere summer than in winter
Define albedo
the proportion of incident solar radiation that is reflected by the earth’s surface
What is the albedo of fresh snow vs. forests?
fresh snow - 75-95% albedo (reflection)
forests reflect 3-10% of incident radiation - much more efficient at absorbing
What else absorbs radiation to regulate earth’s temperature?
atmospheric gases like CO2 and H2O
What is the greenhouse effect?
when solar energy absorbed by earth’s surface is converted to thermal energy and re-radiated from the surface into the atmosphere - gets trapped by the atmosphere
What are the 3 major characteristics of solar radiation?
intensity
photoperiod
spectral quality
What is spectral quality?
The wavelengths of light within different ranges
ex. visible spectrum, PAR spectrum, IR, UV
Which spectral quality do plant pigments absorb?
the full PAR spectrum (380-710 nm) but within this, cholorophyll absorbs best in violet-blue and orange-red wavelengths (reflects green)
Which wavelengths do chlorophyll absorb most efficiently? reflect?
chlorophyll b absorbs violet-blue wavelengths most efficiently (~480-510)
chlorophyll a absorbs orange-red wavelengths (~650-710 nm)
they reflect green (~510-650)
How do some animals take advantage of other wavelengths outside of the visible spectrum?
animals like snakes have IR detectors which sense heat of prey to hunt in the dark
some beetles can detect heat via IR from forest fires at great distances
many insects can ‘see’ UV wavelengths
UV converts sterols into vitamin D in human skin
T or F: UV plays an essential role in plant physiology
false
but high energy UV can cause damage to plant
How do plants protect themselves from UV damage?
some develop protective pigments on their cells
What is an example of plant adaptations to prevent UV damage?
anthocyanin activation in plant leaves turn leaves red/purple to protect against UV
What 2 factors influence the NPP of a forest stand in regard to light capture?
leaf area * leaf photosynthetic efficiency
What is Leaf Area Index (LAI)?
the amount of leaf area/ground area
What is the equation for NPP that includes light capture by leaves? What does each component mean?
NPP = LAI * E(I) * c
LAI = leaf area index = leaf area/ground area
E(I) = photosynthetic efficiency (can be measured in many ways)
c = ground area of interest (ex. 1m^2, 10 m^2)
How does leaf area effect NPP?
more leaves (higher LAI) = higher E(I) = higher NPP
Why do forests have high NPP?
because trees have high LAI compared to other plants
What happens to NPP as canopies become more dense?
LAI increases and NPP will increase until the canopy is too dense and NPP plateaus due to self-shading (leaves overlap)
How does an evergreen forest LAI compare to a temperate deciduous forest? why?
evergreen LAI < 18 (much higher)
TD LAI < 7
evergreens have higher LAI because the shape of their canopy (triangular) allows more leaves to be exposed to sunlight
What is the light compensation point (LCP)?
the balance between gross PS and respiration of an individual leaf
where Ps = R
What is the light saturation point (LSP)?
the balance between light harvest and CO2 capture of an individual leaf
Where maximum net Ps occurs
How does photosynthesis of an individual leaf increase in relation to the LCP and LSP?
increases from the LCP to the LSP
Why do LCP and LSP vary with species?
they differ depending on the species adaptations to light
shade vs. sun leaves
shade tolerant vs. shade intolerant species
nutrient and moisture availability, temperature, leaf age
Describe the trade off that exists between LCP and LSP
For example, shade intolerant species like Ponderosa Pine have a high LCP (need more PAR to balance Ps and R) and high LSP (higher net Ps rate - need more sunlight to reach)
vs. shade tolerant species like Norway spruce which have a low LCP (need less light to reach LCP) and lower LSP (lower net Ps rate - need less light to reach) - adapted to less sun so not as efficient
How does leaf age affect LCP and LSP?
the older a leaf is, the higher its respiration rate is and lower Ps = less efficient
high R = high LCP
lower Ps = low LSP
What happens to LCP as respiration increases?
LCP increases because LCP = Ps - R
if there’s higher R, then more Ps needed to reach the LCP
means more light is required
What happens to respiration rate as branch biomass increases?
respiration increases
Compare photosynthesis efficiency in conifers vs broad leaved trees and explain the differences
conifers have high leaf:branch biomass ratio and therefore, high Ps: respiration = more efficient
this is due to their apical dominance (conical crown) - require less branching and high leaf exposure
broadleaves require more branches to support their leaves - lower leaf:branch ratio would lower the ps:R and reduce efficiency per unit area
How does the leaf area in canopies relate to NPP?
high leaf area = high NPP
How does the LSP of individual leaves compare to canopies?
individual leaves have lower LSP (~30-50% of full sun) than canopies
Describe the graph (curve) and relationship of light intensity as it passes through the layers of canopy leaves
exponentially decreases through the layers according to the Beer-Lambert Law
What is the Beer Lambert Law?
t(x) = e^[-K * LAI(x)]
where t(x) = the proportion of PAR transmitted to a given point (x)
LAI(x) = the leaf area above point x
K = extinction coefficient (constant) - the extent of light absorption through the canopy
this law assumes random and even leaf distribution
Rate the LSP for a forest canopy, shoots, and individual leaves in order of lowest to highest
Individual leaves require less sunlight to reach max Ps = lower LSP
Shoots are middle = many leaves increases light exposure
canopy has very high LSP
What is the extinction coefficient (K)? What canopy attributes influences it?
K is the constant value at which the light hitting the canopy changes (decreases) through the layers of the canopy
this varies by species and depends on the canopy structure and distribution of species
How would the extinction coefficient change for a mixed canopy structure to a conifer to a broadleaved?
a canopy with mixed species and heights = light intensity decreases but not exponentially - does reach 0
a conifer canopy decreases exponentially - does not reach 0
a broadleaved canopy decreases rapidly (not exponentially) but does not reach 0. Usually only 50% of light extinguished
If a tree species dominates a canopy and has a high extinction coefficient (K), what does this mean for the leaf area?
high K = low LA to absorb the incident light
low K = high LA to absorb more of the incident light
How does the canopy composition influence the understorey composition?
because of extinction coefficient (K), the amount of incident sunlight that passes through the canopy dictates what species can grow under the canopy (shade tolerant vs. intolerant species)
T or F: once a canopy is established, both shade tolerant and intolerant species can regenerate underneath
false, only shade tolerant can regrow
How does shade tolerance vary?
with species (ex. western hemlock very shade tolerant vs. Doug fir, not shade tolerant)
environment (ex. coastal not shade tolerant vs. Interior Douglas fir shade tolerant)
age (ex. Norway spruce seedlings < 5% full sun, 10-15 yrs need 15-24%)
What adaptations do understorey plants have to complete their lifecycles in deciduous forests?
they grow and flower in early spring, before deciduous trees flush their leaves and block the sunlight
What are sunflecks? How important are they for understorey?
occur under a closed canopy which usually are short term (< 2 min majority, average 6 minutes)
an important source of radiation on the forest floor (47-68%)
How are shade tolerant species adapted to utilizing the short-lived sunflecks?
they have Ps machinery ready to go for when they can get access to the light, ie., faster Ps induction
What wavelength spectrum does most of the light that passes through the canopy have?
700-750 nm (Far Red)
What is a phytochrome? describe it
light sensing pigment in plants and green algae
structure: a dimer, where each monomer is a light-sensitive chromophore and a polypeptide tail
What are heliophytes?
helio = sun
grow well in high light intensity
What are sciophytes?
skia = shade
grow well in partial shade
How does the light wavelength spectrum that passes through the canopy have an effect on plants?
a morphological effect
more FR = phytochrome ratio changes which decreases the shoot:root ratio (less light increases shoot:root)
this effect varies by species
What happens when the phytochromes absorb red or FR light?
conformational change of the chromophore (monomers of the phytochrome dimer) from either Pr or Pfr form (changes ratio)
What are the 2 responses to changes in wavelength absorption between red and FR?
rapid, short term changes in membrane permeability
slow, long-term changes in gene expression
When red light is absorbed by the phytochrome, what happens to the chromophores?
the Pr form is converted to the Pfr form
When far red light is absorbed by the phytochrome, what happens to the chromophores?
the Pfr form is converted to the Pr form
What is the Pfr conformation?
when the ratio of Pfr:Pr is high and most of the the phytochromes in a plant are in the Pfr conformation
the physiologically active form
How is the Pfr form converted back to the Pr form?
during periods of darkness (less/no red light exposure)
WHat are the 2 classes of responses by phytochromes? describe them
phototropism: when plant growth responds to direction or quality of light (ex. stems bending toward light)
photoperiodism: when plant growth responds to length and time of day and night (to the photoperiod)
What is an example of phototropism?
a plant stem bending toward the direction of the light
What is an example of photoperiodism?
leaves changing colour on a deciduous tree when the days begin to shorten
Describe the relationship between R, FR, Pfr and Pr under a canopy and how this relates to shade avoidance
when there is less light, the R:FR ratio decreases (more FR) so Pfr is converted to Pr
this causes:
- stem elongation and etiolation and the plant to grow in the direction of the light (phototropism) - unequal distribution of plant growth regulators (ex. auxin)
- growth of shade leaves which are longer, thinner leaves
What are shade leaves? why do these develop?
leaves that are different in morphology to leaves if appropriate amount of light is received
they are broader, thinner, and have fewer palisade layers, with thinner cuticles
these develop in response to low light conditions which cause more phytochromes to be in Pfr conformation and change the morphology of the plant
less light can be reflected and more can be absorbed if broader, thinner and thinner cuticle
What is etiolation? what causes this?
when stems become really long and thin and weak and lack chlorophyll because they do not receive enough light
a response to high Pfr:Pr in low light conditions
How have shade tolerant trees adapted to low light conditions to overcome the negative morphological changes (etiolation, phototropism, shade leaves)
large seeds with energy reserves to provide enough energy to support a solid root and shoot system before they become autotrophic
How can red or FR light affect seed germination?
the quality of the incident radiation can affect the growth of plants (phototropism)
some seeds require red light to germinate so they can’t be buried too deep in the soil or have dense canopy - especially small seeds
What is heliotropism?
rapid plant response to direction of light - when leaves or flowers move towards or away from the sun
blue light response, not red or FR
T or F: heliotropism involves Pfr-Pr conformational changes
false, it’s a rapid response that’s based on blue light sensitivity
Why is photoperiodism a useful response?
because night/day length is a more reliable cue for environmental change than temperature
What is winter hardiness? how is it an example of photoperiodism?
adaptations to surviving cool winters
the changes in photoperiod after midsummer induces acclimation for winter
as the night length increases, trees may:
- set buds
- retranslocate sugars and proteins from leaves into bark and roots
- develop cold tolerance
- leaf abscission
How is flowering an example of photoperiodism?
phytochromes in leaves sense night length which can trigger the change from vegetative to reproductive growth (flowers)
the amount of Pfr reverting back to Pr overnight will decrease as night length shortens
What are long day plants? When do they flower? give an example
plants that require shorter nights and longer days
usually spring and summer flowering
ex. berries
What are short day plants? When do they flower? give an example
plants that require longer nights
fall flowering
ex. asters
What are neutral day plants? When do they flower? give an example
plants that are unaffected by day length and flower whenever, year round
ex. dandelions
what are some examples of how individual plants are adapted to low light conditions?
higher biomass allocation to leaves than to roots/shoots = plants invest more into light capturing tissues
more resources put into increasing light capture than growth = thin leaves with large SA
higher chlorophyll concentration
lower enzyme concentrations because light is limiting, not CO2
How can canopies in low lights be adapted to the conditions?
the canopy architecture minimizes the amount of leaf overlap
What happens when FR:R is high? What light conditions does this occur in?
when FR:R is high, there’s low light conditions
Pfr is converted to Pr so etiolation will occur because
Pfr is the active form which ACTS to inhibit etiolation, so when there’s more being converted to Pr there’s more etiolation occurring
What happens when FR:R is low? What light conditions does this occur in?
when there’s more R light than FR light, there is more light
this causes Pr to be converted to Pfr which inhibits etiolation
Which wavelength does Pfr absorb at when it is converted back to Pr?
730 nm = FR
Which wavelength does Pr absorb at when it is converted to Pfr?
660 nm = Red
What triggers the conformational change (ie., the activity) of the phytochrome?
the wavelength being absorbed
whether it’s red = inactive = suppressing etiolation more Pr converting to Pfr
or whether it’s FR = inactive = etiolation not inhibited because more Pfr converting to Pr
What are 5 effects of light intensity or duration on animals? give examples
vision - whether animals need light to see (ex. humans have poor eye sight in low light conditions)
rest/sleep cycles - nocturnal, diurnal
photokinesis - speed of movement or activity increases when light intensity increases
phototaxis - animal orientation toward or away from light (positive or negative)
photoperiod - when animal behaviour changes depending on day length (ex. migratory cycles, reproduction and nesting, fur or feather colour changes)
Describe photokinesis and give an example
when animal activity increases with light intensity
ex. insects, crabs
reptiles??
Describe phototaxis and give an example
when animals orientate themselves in relation to light
positive - toward light
negative - away from light
ex. western tent caterpillars toward light; hemlock looper larvae away from light
What are some examples of photoperiod effects on animals?
migratory patterns
reproduction and nesting
changes to fur or feathers
What are the 3 key attributes of light?
intensity
duration
quality
