Photobiology

CHAPTER 4 PHOTOBIOLOGY

Introduction Sunlight satisfies 2 important needs of biological organisms:  (i) energy  (ii) information n Photobiology; studies on importance of light in the life of green plants n Photobiology studies includes photosynthesis, photomorfogenesis and photoperiodism n

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Role of sunlight : Photosynthesis – sunlight as an energy source Photomorphogenesis & photoperiodisme – sunlight provides the necessary information for proper plant development and measurement of daylength

4.1 Photoreceptors n

defined as pigment molecules

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Pigments molecules absorbs light for use in a physiological process

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process the energy and informational content of light into a form that can be used by the plant

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Photoreseptors roles in various physiological process

(i) Chlorophyll u Consist of 2 parts;  (i) porphyrin; head  (ii)long hydrocarbon @ phytol; tail u Made up of 4-nitrogen containing pyrrole ring u Mg2+ in the center of the ring u Four species of chlorophyll : a, b, c and d 

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Chlorophyll a : u higher plants, algae, cyanobacteria

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Chlorophyll b :

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-CHO (formyl group) substitutes – CH3 on ring II u Found in all higher plants and green algae u

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Chlorophyll c : u lack phytol tail u Found in diatoms, dinoflagellates & brown algae t

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Chlorophyll d : u -O-CHO substitutes –CH=CH on 2  ring I u Found only in red algae

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Chlorophyll does not absorb strongly green of the visible light spectrum (490-550nm)

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Maximum absorption: blue light (425490nm) and red (640-700nm)

Structure of Chlorophyll a (textbook pg 53) CHO O

CH2 CH3

CH

Chlorophyll d

I

CHO Chlorophyll b

II N

N Mg

N

IV

N

III

CH2 CH2 C=O CH2 CH

Chlorophyll a

pheophytin Chlorophyll c

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Chlorophyll b,c and d similar to chlorophyll a, expect :

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Chlorophyll b : that a formyl group replaces the methyl group on ring II

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Chlorophyll c : that it lacks the long hydrocarbon tail

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Cholorphyll d : that a –O-CHO group is substituted on ring I

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Absorption

Absorption spectra of chlorophyll a and chlorophyll b

Chlorophyll a

400

Chlorophyll b

500 600 Wavelength (nm)

700

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(ii) Phycobilins

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Properties similar to the bilin pigments in mamalian

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Pigments of algal origin

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4 phytobilin are known; 3 are phycoerythrin, phycocyanin and allophycocyanin;involved in photosynthes 1 is phytochrome (phytochromobilin); important in photomorphogenesis

n Found

in cyanobacteria and red algae

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n Absorb

light energy in green region

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n Function:

Exist in two forms P660 and P735

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Absorption spectra of phycocyanin and phycoerythrin Adsorption

Phycocyanin

phycoerythrin

400

500 Wavelength (nm)

600

700

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(iii) Carotenoids

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Orange and yellow pigments n present in photosynthetic organisms n Lipid soluble: found in chloroplast membrane or chromoplasts n

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Carotenoids family pigments: (fig. 3.11) (i) carotenes (ii) xanthophylls

Carotenes:

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orange @ red-orange pigments u major carotene is -carotene; found in algae and higher plants u minor form is -carotene u others forms are -carotene; found in green photosynthetic bacteria and Lycopene; pigment in tomato fruit u

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-carotene : - absorbs blue region of visible spectrum & protects chlorophyll from u

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Xanthophylls:

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n Yellow

carotenoid n oxygenated carotenes, eg :  - Lutein (-carotene)  - zeaxanthin (-carotene)

(iv) Cryptochrome n Plant responses to blue and UV-A radiation n Responses :  - to prevalent in lower plant (ferns,  mosses and fungi)  - phototropism and hypocotyl elongation  (Similar action in higher plants) n Include carotenoids and flavins u Riboflavin, u Flavin Mononucleotide (FMN) u Flavin Adenin Dinucleotide (FAD) 

(v) Flavonoids u 3 major groups of flavanoids;  - flavones, flavonols and  anthocyanidins 

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Most strongly colored anthocyanidins and anthocyanins (anthocyanidins + glycoside)

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Various colors of scarlett, blue, pink,purple due to presence of anthocyanin

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Water soluble pigment, found in vacuolar sap

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Composition of some common anthocyanins Anthocyanidin + Glycoside

= Anthocyanin Source

Pelargonidin

3,5-diglucoside

Pelargonin

Cyanidin

3,5-diglucoside

Cyanin

Geranium (pelargonium) petals Red rose petals

Cyanidin

3-galactose

Idaein

Cranbeerry fruits

Delphinidin

3-rhamnoglucoside

Violanin

Violet (Viola) flowers

Malvidin

3-glucoside

Oenin

Blue grapes

(vi) Betacyanins

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Red pigments of beet root and Bougainvillea flowers

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Betacyanins and betaxanthins differ from anthocyanins by containing nitrogen molecules

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4.2 Photosynthesis in leaves

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Absorption of light by leaves depend on leaves structure (fig.4.1 page 64)

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laminar surface serves to maximize interception of light

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To collect light on the surface and diffuse light on the lower surface

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internal cellular arrangement play importance role

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Anatomy of dicot leaves :

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leaf sheathed with upper & lower epidermis u surface epidermal cells were coated by cuticle u mesophyll tissue : - are located between the 2 epidermal layers - consists of 2, i.e. palisade & spongy cell u palisade mesophyll cell : 1 @ 2 layers cell; located upper layers; large number of chloroplast u Spongy cell : air space between the cells u

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Leaves help to redistribute incoming light and maximize interception by chlorophyll

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A.

photon strikes a chloroplast and absorbed by chlorophyll

B.

Sieve effect : photon passes through the 1st layer of mesophyll cells without being absorbed

C.

Planoconvex nature of epidermal cells creates a lens effect, redirecting incoming light to chloroplasts along the lateral walls of palisade cells.

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Light-guide effect : light reflected at the cell-air interfaces may be channeled through the palisade layers to the spongy cell; because the refractive index of cells is greater than that of air

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4.3 Leaf and gas exchange Through stomatal pores u Stomata :(fig. 5.1 page 90) u

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Leaves epidermis contains pores @ opening called stomata

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Stoma provide for gases exchange between the internal air spaces and ambient environment

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Boarded by a pair of guard cells

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Guard cells urrounded by epidermis cells called subsidiary cells

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stoma + guard cells + subsidiary cells = stomata complexes @ stomatal apparatus

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stomata control 2 important processes : t Uptake of CO for photosynthesis 2 t Water loss through transpiration 

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2 types of stomata :

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(i) graminaseus – dumbbell shaped, thin-walled (ii) elliptic – kidney beans, thickening wall 

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Types of stomata Stoma Guard cells

Elliptic/kidney shaped

Subsidiary cells

dumbbell

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Monocots – contains stomata on both adaxial (upper) and abaxial (lower)

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Woody dicots and trees – stomata only on lower leaf surface

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Herbaceous dicots: frequency lower on the upper surface

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Floating leaves: stomata only on upper surface

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CO2 Diffusion

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CO2 diffusion into the leaf through the stoma

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Size of fully open stoma : 5-15  m wide x 20  m long

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Amounts open stoma : 0.5-2% of total area of the leaf

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Rate of CO2 uptake approach 70% of the rate over an absorbing surface leaf High diffusion efficiency related to special geometry of small pores (size of stomata)

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As the pore size decreases, CO2 diffusion through perimeter of stoma will increase

Diffusion of CO2 through a stomatal pore Guard cell

Guard cell

Outside

Inside

CO2 flow

epidermis Spillover CO2

Fig : Pattern of diffusion flow as the gases enter and exit the stomatal pore n

Fick’s Law of diffusion: u CO molecules can flow only straight 2 through and diffusion is proportional to the cross-sectional area of the throat of the stoma 

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When the gas molecules pass through the aperture into the substomatal cavity, they can ‘spill over’ the edge @ perimeter of the pore

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How is the high concentration gradient for CO2 established and maintained? @ to ensure a constant diffusion of CO2 from the air into the leaf :

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i. CO2 concentration within the stomatal cavity and leaf air space must be less than the CO2concentration in the air above the leaf

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ii. Light, chloroplast continuously fix CO2.  chloroplast within the leaf mesophyll cell  continuously convert gaseous CO2 into a stable compound

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iii. Biochemical cycle constantly removes CO2from intercellular air spaces of the leaf. Thereby, ensuring that the internal leaf CO2 concentration is less than the ambient CO2 concentration in the leaf

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iv. In the dark, photosynthesis stops but respiration generates CO2 . Internal CO2 concentrations are greater than the ambient CO2 concentration . Thus, CO2 diffuses out of the leaf in the dark.

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Stomatal opening and closure mechanism n

Mechanical forces are involved in guard cells movement

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Driving force for stomatal opening is known to the osmotic uptake of water by guard cells

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Consequently, increase in hydrostatic pressure

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Deformation of opposing cells that increase the pore size (opening size)

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Thickened walls of elliptic guard cell become concave

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Dumbbell-shaped cells, handles separate but remain parallel

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Stomatal closure ⇨ follows a loss of water ⇨ consequently, decrease in hydrostatic pressure & guard cell walls relaxed

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Deformation of elliptic guard cell due to unique structural arrangement of guard cell wall

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Microfibrils in elliptic guard cell wall are arranged longitudinally within the ventral wall thickenings ⇨ oriented in radial fanning out from ventral wall

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Crosslinking with radial band will restrict expansion along ventral wall

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When guard cells take up water, expansion follow least resistance path to push the thin dorsal wall outward into neighboring epidermis cell ⇨ this causes the cell to arch along the ventral surface and form the stomatal opening

Role of microfibrils in guard cell movement Microfibrils orientation

Direction of expansion

Cells buckle and increase opening

What controls stomatal opening and closure? n

K+ levels are very high in open guard cell and very low in closed guard cells

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K+ content of closure guard cells is low compared with that the K+ move from the subsidiary and epidermal cell into the guard cells

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Upon opening ⇨ large amounts K+ move from subsidiary and epidermal cells into guard cells

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Consequently, accumulation of K+in guard cells occur resulting in stomatal opening

Guard cell metabolisme and stomatal movement:

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Stomatal opening:

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Accumulation of ion (K+) is driven by ATPase proton pump located on plasma membrane

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By removing +ve ion charged, the cell, proton extrusion would tend to lower the electrical potential inside the cell compare to the outside ⇨establish a pH gradient

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Open K+channel in the membrane allows the passive uptake of K+ by charge gradient

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To maintain electrical neutrality in cell, excess K+ must be balanced with negative charge ions

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Charge balance ⇨ Partly by balancing K+ uptake against proton extrusion, partly by influx of Cl- and partly by production within the cell of organic anion such as malate

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Accumulation of K+, Cl- and malate in vacuoles of guard cells would lower osmotic potential & water potential of guard cells ⇨ uptake of water increase the turgor ⇨ Stomata open

Stomatal closure:

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Signals for stomatal closure stimulate the uptake of Ca2+ into cytosol. n Ca2+ uptake would depolarize the membrance n Opening anion channels allows the release of Cl- and malate n Loss of anions would further depolarize the membrance ⇨ opening K+ channels ⇨ allows the passive diffusion of K+ into the adjacent subsidiary & epidermal cells n

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Fig: Ion flow in guard cells during stomatal opening

ATPase-proton pump H+

ADP + Pi

H+ ATP

K+

K+

ClStimulate osmotic uptake of water and increase turgor

2H+ CO2 PEP

ClMalate starch

Accumulation of ions in vacuoles lower the water potential

Control of stomatal movements n

Stomata allow entry of CO2 into the leaf for photosynthesis; and at the same time preventing excessive water loss

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Stomatal movement is regulated by variety of environmental and internal factors, such as : t Light t CO levels 2 t Water status of plant t Temperature

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For example: In light ⇨ stomata will openin order to admit CO2 for photosynthesis ; or partially stomatal will close when CO2 level are high in order to conserve water ⇨ photosynthesis to continue

(i) Light and CO2 n

Light and CO2 are important in regulating stomatal movement

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Stomatal opening: u Low CO concentrations and light stimulate 2 stomata opening u High CO concentrations cause rapid stomata 2 closure even in the light u Response of stomata is to the intracellular concentration of CO2 in guard cells u Once induce to close by high CO , stomata not 2 easily open by CO2-free air u Consequently, CO content of substomatal 2 chamber rather than the ambient atmosphere ⇨ stomatal opening at dawn

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Stomatal closure:

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Closed guard cell remain equilibrium with high CO2 concentration trapped in substomatal chamber u Stomata closed by exposure to high CO 2 can induced to open, if placed in the light u Indirect effect of light attributed to a reduction in intercellular CO2 levels due to photosynthesis in mesophyll cells u In the dark, guard cells closure and accumulation of respiratory CO2 inside the leaf u

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Light :

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Blue and red light stimulate stomatal opening u Stoma more sensitive to blue light compare to red u 15 mol m-2 s-1 , of blue light stimulate stomatal opening u Stomata respond towards red light is indirect mediated by guard cells chloroplast and involves photosynthetic ATP production u

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iii. Water n n n

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Stomatal opening referred to the water status of guard cells Guard cells exposed directly to atmosphere & can lose water by evaporation Rate of water loss from guard cell might exceed rate of movement into guard cell from the surrounding epidermal cells Cells become flaccid and stomata close and called hydropassive closure Plants sense water deficit and initiates specific mechanism to induce closure called hydroactive closure

iv. Temperature n n n n n n n

Temperature stimulate stomatal opening Increase temperature rise ⇨ metabolic activity in guard cells and leaves increase High temperature ⇨ stimulate respiration and inhibit photosynthesis High CO2 ⇨ stomatal closure Stomatal of some species become “Midday” closure ⇨ result in reduction in photosynthesis Closure attributed to water stress by high temperature and excessive transpiration Reopening occurs by midafternoon when water deficit has been satisfied by redistribution of water within the plant