7. Light Flashcards
What does strong light do to leaves?
How can leaves be protected against ROS?
It can damage as there is high PPFD (photosynthetic photon flux density)= Produces free radicals (reactive oxygen species, ROS)
Leaves must dissipate excess light energy to avoid damage to the photosynthetic apparatus
Increasing light= Increasing photosynthesis until CO2 or another factor becomes the limiting factor
Protection: Leaf movements, chloroplast position and chloroplast movements and a range of molecular mechanisms
Example: xanthophyll cycle: Dissipates excess light energy to avoid damaging the photosynthetic apparatus = Prevents formation of ROS
What is photoinhibition?
Damage due to strong light which is reversible by repair mechanisms
Dynamic photoinhibition: When moderate levels of excess light decreases quantum efficiency (reduced slope of the curve) WITHOUT reducing maximum photosynthetic rate
Dynamic photoinhibition temporarily diverts excess light absorption to heat but maintains maximal photosynthetic rate
Examples of photoprotection: Removal of ROS, thermal energy dissipation and decreased absorption
While chronic photo inhibition: High levels of excess light decreases quantum efficiency AND maximum photosynthetic rate= Chloroplast damage
What is photo-oxidation?
Irreversible damage due to strong light
Shade leaves have only very limited capacity for damage repair and are easily photo-oxidised
What can temperature sensitivity curves allow you to determine?
Leaf absorption of light energy generates a heat load that must be dissipated
Curves=
1) Can determine a temperature range where enzymatic events are stimulated
2) Range for optimal photosynthesis
3) Range where destructive events occur
What happens to leaf temperatures when CO2 concentrations increase?
Increased CO2 levels= Stomatal aperture is smaller= Lower transpirational cooling= leaf temperatures increases
Leaves must also dissipate very large amounts of heat
What does UV radiation do to plant leaves?
Increasing altitude and lower latitude= UV radiation generally increases
UV radiation= Penetrate cells= Causes acute injuries due to high quantum energy
Longer wave UV-A= Chiefly photo oxidative
UV-B causes photolesions
UV damage is able to break the disulphide bridges in protein molecules
Can also prevent xanthophylls cycle
What is a counteracting process of UV radiation being penetrated?
Enzymatic repair mechanisms like reverse reactions through DNA photolyase during blue light
Elimination of damaged parts via endonuclease
Can also protect: Effective absorption of UV radiation by epicuticular waxes, carotenoids and flavonoids dissolved in the cell sap in cells of higher plants
