Carotenoids are organic pigments found in plants and other organisms that are responsible for red, orange, and yellow colors. They serve two main functions in plants - absorbing light for photosynthesis and protecting chlorophyll from damage. In higher plants, carotenoids like carotenes and xanthophylls are ubiquitously present in chloroplasts. The xanthophyll cycle involves the interconversion of violaxanthin, antheraxanthin, and zeaxanthin in response to light intensity and protects the photosynthetic system. Non-photochemical quenching dissipates excess light energy as heat through mechanisms like singlet-singlet energy transfer between carotenoids and chlorophyll or carotenoid-mediated alterations to

2. CAROTENOIDS
 Organic pigments; lipid soluble; found in the chloroplast &
chromoplast of plants and other organism ( bacteria & some fungi).
 600 known carotenoids.
 Derivatives of tetraterpene produced from 8 isoprene molecule
(having 40 carbon atom).
 Absorb wavelength ranging from 400-500 nm (violet to green light)
 Responsible for many of the red, orange, and yellow hues of plant
leaves, fruits, and flowers.( for examples – oranges of carrots and citrus
fruits, the reds of peppers and tomatoes)
 Dominant pigments in autumn leaf coloration of tree species ( 15 -
30%)
 Two main function :
1. Absorption light energy for photosynthesis. (Accessory pigment)
2. Protection of chlorophyll from phtodamage. (Photoprotective
pigments)

3. CAROTENOID COMPOSITION OF HIGHER PLANTS
 In higher plants , uniform carotenoid i.e., the carotene , xanthophylls are
ubiquitously present in the photosynthetic (thylakoid) membranes of
chloroplast of higher plants.
 α-carotene, not ubiquitous but can be found in some species .
 Within the thylakoid membranes, carotenoids are bound mostly to specific
chlorophyll carotenoid-binding protein complexes of the two
photosystems (PSI and PSII)
 The distribution of carotenoids between PSI and PSII is highly uneven,
with PSI enriched in β-carotene and PSII enriched in lutein.
 Within PSII, the bulk β-carotene is present in the core complexes closely
surrounding the reaction center. The xanthophylls prevail in the
remaining.
 For algal carotenoids (fucoxanthin, siphonaxanthin, and peridinin) has
role in light collection.
 According to Goedheer carotenes can perform light collection while
xanthophylls cannot
 According to Siefermann-Harms lutein is capable of efficient energy
transfer to chlorophyll.

4. CAROTENOIDS
CAROTENE XANTHOPHYLL

5. CAROTENE
 Unsaturated Hydrocarbon having the formula C40Hx for e.g. β-carotene ;
lycopene.
 No oxygen, fat-soluble molecules.
 Absorb ultraviolet, violet, and blue light and emit orange or red light, and
(in low concentrations) yellow light.
 Carotenes are responsible for the orange colour of the carrot and colour in
dry foliage.
 Carotenes contribute to photosynthesis by transmitting the light energy
they absorb to chlorophyll.
 They also protect plant tissues by helping to absorb the energy from
singlet oxygen, an excited form of the oxygen molecule O2 which is formed
during photosynthesis
Carotene
β
γ
δ
α
ε
ζ

6. XANTHOPHYLL
 Yellow pigment, oxygenated carotenoids are synthesized within the plastids without
light .
 Present in all young leaves as well as in etiolated leaves.
 Highest quantity in the leaves of most green plants. Serve as non photochemical
quenching agent to deal with triplet chlorophyll (an excited form of chlorophyll).
 More polar than the purely hydrocarbon carotenes.
XANTHOPHYLL
Lutein
zeaxanthin
neoxanthin
violaxanthin
flavoxanthin
cryptoxanthin
α
β

7. ACCLIMATION OF THE CAROTENOID COMPOSITION
OF HIGHER PLANTS TO THE GROWTH LIGHT
ENVIRONMENT
 The total number of carotenoid molecules per chlorophyll molecule is typically
greater in sun compared with shade leaves.
 There is an significant change in composition of carotenoids of xanthophyll class
(violoxanthin, antheraxanthin, zeaxanthin ) in bright sun.
 The carotenoid exhibiting a decrease in sun leaves was α-carotene(by -
85%) and Lactucaxanthin,, which is not ubiquitous among higher plants.
 First, the xanthophyll cycle is likely to have a specific photoprotective function
under light stress.
 Decrease in β- carotene and luetin concentration in the shade suggests the role of
these pigment is significant in the collection of light .

8. PATHS FOR CHLOROPHYLL-
CAROTENOID INTERACTION IN HIGHER
PLANTS

9. THE XANTHOPHYLL CYCLE:
 The interconversion of violaxanthin, antheraxanthin and zeaxanthin. A different xanthophyll
cycle, involving diadinoxanthin in some microalgae.
 The violaxanthin content of leaves can be decreased by transfer into a high light intensity
 This is reversed when the leaf is transferred back to a low light or to darkness.
 Intrathylakoid pH is the key factor for the formation of antheraxanthin and zeaxanthin under
excess light. The intrathylakoid pH has a dual role in regulating energy dissipation;
1. Controlling the biochemical conversions in the xanthophyll cycle.
2. In activating energy dissipation directly.
 Two enzyme -Violaxanthin de-epoxidase and the Epoxidase.
 The enzyme Violaxanthin de-epoxidase catalyzing the forward reaction (V - A - Z) ,it
operates at its highest rate when an imbalance between light absorption (causes an
accumulation of protons within the thylakoid lumen).
 The Epoxidase catalyzing the back reaction (Z -* A - V) requires a neutral pH.
 Electron transport supplies the reductant for both above reaction that utilizes oxygen and
NADPH as substrates.
 V. A, and Z are associated with the light-harvesting antennae of PSII and PSI
 distribution of these is uneven .
 greater maximal conversion to A + Z, are found in the inner minor complexes than
the major peripheral complex (LHCII)

10.  Diurnal changes in xanthophyll content as a function of irradiance in
sunflower (Helianthus annuus)(As the amount of light incident to a leaf
increases, a greater proportion of violaxanthin is converted to
antheraxanthin and zeaxanthin, thereby dissipating excess excitation
energy and protecting the photosynthetic apparatus)

11. Non-Photochemical Quenching
 When leaves or tissue are exposed to excess light, a process of non-
photochemical thermal dissipation of the excess absorbed photons
occurs.
 This permits the changes in light intensity, allowing protection of the
photosynthetic membrane against damage- non-photochemical
quenching of Chl fluorescence, qN
Three important features of qN:
1. Acidification of thylakoid lumen viz associated with the formation of
the proton motive force(qE)
2. Energy is dissipated in the light-harvesting system of photosystem II.
3. Formation of zeaxanthin as a result of de-epoxidation of violaxanthin
via the xanthophyll cycle.

12. POSSIBLE MECHANISMS OF FLUORESCENCE QUENCHING
 Two main models have been suggested:
i. Singlet-singlet energy t transfer
ii. Carotenoid mediated alteration to LHC organisation.
SINGLET-SINGLET ENERGY TRANSFER:
 The photochemical and spectroscopic properties of carotenoids are derived from
their low-lying energy states.
 The low-lying singlet states of carotenoids are denoted as (S2) and the (Sl)
states.
 The energies and lifetimes of these singlet states are important in their roles in
photosynthetic systems.
 An electronic transition from the ground state ( SO) to the (S2) state gives rise to
the familiar visible absorption spectra of carotenoids.
 The energy of the S2 state has been shown to be dependent upon the extent of π-
electron conjugation of the carotenoid.
 Dual emission(S2 -S1-S0) can be observed for compounds with eight and nine
conjugated double bonds but, as the length of the conjugated system increases
further, emission from the S2-S0 electronic transition dominates.

13.  Energy level Chl a is lower than that S1 state of violaxanthin but higher than
that of zeaxanthin Thus, it is energetically possible for the S1 state of zeaxanthin
to quench Chl fluorescence via deactivation of the Chl excited singlet state.
 In contrast, the higher S1 value obtained for violaxanthin would lead it to act
preferentially as a light-harvesting pigment, transferring its excitation energy on to
Chl a. It is possible that zeaxanthin may also function as alight-harvesting pigment
using energy transfer from its S2 state to Chl.
 This mechanism is called ‘Molecular Gear Shift’.
 AT high intensity when the dissipation of excess excitation energy is required,
zeaxanthin is formed which serves to deactivate the excited singlet state of Chl a
(resulting in a reduction in Chl fluorescence) and dissipate excitation energy
harmlessly as heat.
 No direct evidence to show that singlet-singlet energy transfer from Chl to
carotenoid occurs in vivo. The data only suggest that such a direct quenching
process may indeed be a possible route of deactivation.The interconversion of
zeaxanthin - violaxanthin would not act as an on off' switch for light-

14. Carotenoid-mediated alteration to LHC organisation
 According to zeaxanthin formation may serve simply to amplify fluorescence
quenching in the LHC rather than acting as the sole driving force.
 The zeaxanthin-associated quenching seen in vivo shares similar spectroscopic
features with quenching brought about by LHCII aggregation.
 This suggests that qE occurs via a common mechanism. qE requires a ΔpH, and
number of studies have demonstrated 'light-activation' of such pH-dependent
quenching so that quenching could be achieved at a lower ΔpH in thylakoids and
chloroplasts in the presence of zeaxanthin than in its absence.
 Such 'light-activation' suggests an indirect role for the xanthophyll-cycle carotenoids in
controlling qE
 This has led to the development of an allosteric LHCII model for zeaxanthin-
mediated regulation of qE
 LHC aggregation has been shown to be associated with distinct changes in the
properties of the bound Chl and carotenoid
 The absorption changes associated with qE formation in isolated LHCII (in both the
blue and red regions of the spectrum) are similar to the changes seen when both
chlorophylls and xanthophylls are aggregated in vitro.
 This suggests that xanthophylL -Ch1 associations may be formed in situ within the
LHC, giving rise to the quenched state of the complex.
 In this model, the carotenoids are proposed to control quenching by a mixture of
quenching and anti-quenching effects

15. LHCII model for carotenoid-mediated regulation of non-photochemical quenching (qE).
 In this model the rate of energy dissipation is controlled by structural changes to LHCII
brought about allosterically by de-epoxidation of violaxanthin into zeaxanthin and by
protonation
 (i) unquenched, unprotonated, binds violaxanthin;(ii) slightly quenched
(zeaxanthin chl), unprotonated, binds zeaxanthin; (iii) quenched (ChYChl),
protonated, violaxanthin displaced from its binding site; (iv) highly quenched
(zeaxanthin/Chl), protonated, binds zeaxanthin)

16. Carotenoids as anti-quenchers
 The addition of violaxanthin and zeaxanthin affected both the aggregation
state of the complex and fluorescence quenching.
 The addition of violaxanthin inhibited both aggregation of LHC and
quenching; this carotenoid could in fact be considered to be acting as an
“anti-quencher”.
 In contrast, zeaxanthin acted to stimulate both LHC aggregation and
quenching.