L7 - Over excited leaves and photoinhibition Flashcards
Define photoinhibition.
When does this occcur?
Give to factors that put plants at more risk of photoinhibition?
- The light-dependent reduction in the light-dependent reactions of photosynthesis.
- Occurs when light capture occurs faster than use of ATP and NADPH by Calvin Cycle.
1) Plant exposed to higher light intensities than those in which they were grown.
2) In conditions that inhibit carbon metabolism e.g. low temp, water stress
What is the primary cause of photoinhibition?
- Excessive energy loading on LHCs causing over reduction of e- transfer chain. ROS then form.
1) In PSII excited triplet Chl donate energy to O2, producing singlet oxygen, 1O2, bleaching chlorophyll.
2) In PSI, triplet Chl can reduce O2, generating superoxide (O2-). At the acceptor side this can be converted to H2O2.
Are ROS good or bad and why?
Outline the range of photoinhibition.
- All ROS potentially harmful, destroy purines and polyunsaturated fatty acids.
- Dynamic/protective photoinhibition: reversible, lower quantum efficiency (but not Amax) due to PSII inefficiency.
- Chronic photoinhibition: irreversible photodamage, lower quantum efficiency (and Amax) due to less functional PSII.
How does the ability of leaves to use light energy and to dissipate excess light energy vary with different leaves?
Sketch the relevant diagram.
- Shade leaves have low light capacities for photosynthesis and photoprotection.
- Sun leaves have greater range for photosynthesis and photoprotection.
(See diagram on pg 2 L7)
How do we know what happens with absorbed light?
- Kautsky and Hirsch (1931) showed that absorbed light energy can be dissipated via photochemistry, non-photochemical quenching (NPQ) or fluorescence.
- The relative probabilities of these outcomes sum to 1, forming basis for using fluorescence to probe photochemistry and NPQ.
- Fluorescence light can be distinguished from incidence light due to longer wavelength (lower energy).
What factors can influence the relative probabilities of photochemistry, NPQ and fluorescence? How?
- If quinone in Qa position is reduced, RC is closed and unable to stabilise charge separation.
- At a closed RC, energy dissipation only possible via fluorescence or NPQ. P(Fluorescence) drastically increased.
- Photoprotection can be dynamically induced in LHCs, increasing NPQ.
- This decreases yield of photochemistry and fluorescence.
How do plants photoprotect? Give 3 methods.
Note: I don’t think photoprotection includes fluorescence.
1) Energy/thermal dissipation (NPQ)
2) Short-term avoidance of high light
3) Developmental avoidance of high light
( 2 and 3 discussed in later lectures)
Outline how the central regulator of NPQ works?
Outline the specifics of this sensor/regulator and the evidence for its role.
1) Excess light causes acidification of lumen due to build up of H+ ions.
2) Change in pH detected by PSII subunit S (PsbS) in PSII antennae.
3) PsbS affects LHC conformational change to NPQ state - mechanism unknown.
PsbS:
- Member of LHC family
- Doesn’t bind pigment
- Shown that mutagenised Arabidopsis plants lacking PsbS have lower NPQ
- Senses lumen pH via two glutamate residues on lumen-exposed loops, as mutations of both residues reduce NPQ.
Give the 3 methods of thermal dissipation (NPQ)
1) Conformational change in the LHCII.
- Conformation change reduces efficiency of light harvesting and energy transfer.
2) The xanthophyll cycle, in which specific carotenoids are used to promote energy dissipation.
3) Thermal dissipation through transfer of energy from chlorophyll to carotenoids via RET and subsequent heat dissipation.
Outline the xanthophyll cycle.
What is the enzyme used for this?
How is this enzyme use promoted when NPQ is needed?
Where specifically is this cycle common?
- A decrease in lumenal pH (detected by PsbS) induces the cycle.
- Specific carotenoids cycle from an epoxidised state to a de-epoxidised state.
- Violaxanthin to Antheraxanthin to Zeaxanthin.
- De-epoxidased state have longer chain of conjugated bonds, increasing energy dissipation.
- Operates within minutes and is diurnal.
- Violaxanthin de-epoxidase enzyme (npq1).
- At pH < 6.2, ascorbate protonates, providing co-factor for enzyme.
- Dissipates energy in antennae complexes proximal to RCs
Outline the evidence for the importance of the xanthophyll cycle in photoprotection.
- Mutant plants that overexpress beta-carotene hydroxylase (xanthophyll pigment0 cope better in high light.
- Mutant Arabidopsis plants with violaxanthin de-epoxidase (npq1) knockdown grown in fluctuating light are less productive.
- Transgenic Tobacco w/ overexpression of xanthophyll enzymes and PsbS showed faster NPQ response and higher productivity.
How do xanthophyll to chlorophyll ratios vary between sun and shade leaves?
- Xanthophyll to chlorophyll ratios are higher in sun leaves than shade leaves.
- Sun leaves need more NPQ.
- Chlorophyll preferential if possible in shade leaves as generally a better absorber.
