Light responses Flashcards
whole plant responses to too much/ too little light
Leaves as photosynthetic organs: light response curves
- Light response curves are useful to see how a plant responds to light
- As plants respire constantly they are always using carbohydrates
- Dark respiration rate is below the dotted line (on graph)
^below this line more carbohydrates are being used than
produced
Light increases towards the right of the graph, at the light compensation point amount of CO2 given off by respiration is balanced by CO2 fixed by photosynthesis above this point (above the horizontal dotted line) there is net gain of carbohydrate/ photosynthesis
Rate of photosynthesis increases up until the light saturation point, for everything to the left of the light saturation point is limited by light availability. After this point (to the right) it is limited by something else probs the Calvin Benson cycle on the graph marked as CO2-limited
Not every leaf is subject to the same light environments, so shaded leaves have lower saturation levels
Comparing sun and shade plants
Shade plants have a lower compensation point – low levels are enough for photosynthesis
Sun plants have a relatively higher compensation point – low levels not enough
Compensation level is reached faster in shade plants but they also have lower saturation point
Trade-off – shade plants grow well in low light but cannot take advantage of high light levels if exposed
Light in short supply
e.g. Understorey plants: some have leaves that have anthocyanins in lower epidermal cells which gives the underside of their leaves a red tone. Anthocyanins also act as deterrents to herbivores so young leaves may also be redder.
e.g. Specialist desert plants: decrease their surface area for water loss and use window tissue to focus ambient light over a small surface. For example: Fenestraria rhopalophylla focuses Ambient light: 2000 μmol m-2 s-1 to Internal light: up to 6000 μmol m-2 s-1
(See: www.desert-tropicals.com/Plants)
Excess light : photostable species
e.g. Cobweb Houseleek has trichomes (hairs.)
Mamallaria also has hairs arranged in an umbrella form to shield.
Maurocenia utilise anthocyanin in the vacuole to quench excess light
Regulation of photosynthesis and repair of photodamage
(Features that act to prevent photoinhibition in diagram in notes)
Resulting products of excess light are shown in the green box – species with excess electrons
Electron accepting pigments balance and quench them
SOD – super oxide dismutase converts damaging oxygen species into hydrogen peroxide which is slightly less damaging and acts as a stress signalling chemical
Photoprotective pigments
e.g. Xanthoria elegans (sunburst lichen), is rich in carotenoids which take up excess electrons for radiation protection - essential as it grows in Ladakh in the Himalayan mountain region
(see:https://heatherkellyblog.wordpress.com/2015/01/12/more-weird-and-wonderful-lichens)
e.g. Carotenoid-rich Stonecrop (Sedum sp.) growing in hot, dry conditions on Malta
-> Redness is usually linked to high stress growing conditions
- Photo protection — evolution of anthraquinones in Teloschistaceae
From: Gaya E. et al. (2015) - Adaptive radiation of lichen-forming Teloschistaceae is associated with sunscreen pigments and bark-to-rock substrate shift.
Teloschistaceae fungi act in symbiosis with algae
Special radiation in lichen is observed where the ability to generate carotenoid pigment is established
^ Allowing lichens to expand their habitat range.
- Photoprotection — violaxanthin, antheraxanthin, and zeaxanthin - the xanthophyll cycle
The Xanthophyll cycle involves switching between these 3 states continuously throughout the day cycle.
Shade-dwelling plants have some xanthophyll to respond to changes in the environment
xanthophyll cycle:
- Cycle between three different xanthophylls
- Uses reducing power to reform water
- Ascorbate is vitamin C
- Genetic modification to produce more ascorbate could allow plants to adapt to higher light levels
Photoprotection - chloroplast movement
Plants are able to rearrange the chloroplasts within their cells
In strong blue light surface of chloroplasts is reduced by clumping
Chloroplasts bind and move along actin filaments within the cell
e.g. See: Sun prints on Begonia grandis leaves Botanic stories, 7/10/16, Posted by Catherine Kidner
If an opaque acetate image is placed over a leaf this results in a temporarily darker area this is because under shade chloroplasts spread out to maximise light usage
Photo-protection - dynamic photo-inhibition
Extra energy dissipated as shown by red line in diagram in notes
Under high stress all photons are being dissipated and the respiration process is not being replenished. The result is that the plant stops growing.
Photoprotection - PSll and PSl
Photosystem 1 is more at risk due to super oxide ions (SOD helps to regulate this)
Resulting in reduced NADPH production and impact on Calvin cycle
Defence against UV light (particularly UVB 280-315 nm)
UV light is even more harmful than visible light
Ozone is thinner at higher altitudes so plants are more vulnerable to damage there
Protections are similar to those for visible light
Thick waxy cuticles and epidermis scatter and reflect light
^ e.g. as seen on pine needles
Higher protection levels on pine needles require higher energy input likely this is related to the fact that the tree retains them for multiple years - so it is worth the investment to avoid damage.
Defence a against UV Iight (UV-B) : Scattering and reflection by hairs
e.g. Saussarea sp. growing at 5360 m in Ladakh
Defencea ainst UV Iight (UV-B): Absorption by pigments
Particularly in epidermal cell vacuoles there are flavanoid pigments
Flavanoids screen out the majority of UV-B whilst allowing in usable light
Flavanoids appear blue and pink
e.g. Li et al 1993 observed die back in mutant arabidopsis that cannot produce flavanoids due to increased UVB sensitivity
^ Genetic modification to improve flavanoid production would increase productivity
Defence against UV Iight (UV-B) : Himalayan rhubarb, Rheum nobile - A UV-absorbing greenhouse!
Himalayan Rhubarb forms a closed protective shelter with its outer leaves - it is up to 10 degrees centigrade warmer inside
- This is attractive to pollinating insects
- Additionally, flowers are protected from UV damage
See: Bo song et al. (2013).
https:// heatherkellyblog.wordpress.com/
2015/03/01 /himalayan-rhubarb-the- greenhouse-plant/
