Plant Physiology
Transpiration
Transpiration is defined as water loss in the form of vapour from the internal tissues of aerial parts of the plant body through stomata,lenticels,etc. under the influence of sunlight and other factors .
Antitranspirants
A plant utilised 1% of absorbed water in metabolism, the rest transpired. Moreover, during drought plants show wilting which reduces 50% of plant yield. In horticulture practices some chemicals in the form of antitranspirants are used to reduce the rate of transpiration without hampering other metabolic process like photosynthesis and growth.
Types of Antitranspirants :
(i) Stomatal closing type: These chemicals like phenylmercuric acetate(PMA) and absecic acid (ABA) induce closer of stomata to cut down the rate of transpiration. On application of PMA at very low concentrations, stomata closed partially for 2 weeks.
Drawbacks: Due to closure of stomata the processes like respiration and photosynthesis also become inhibited on application of such chemicals as stomata constitute common pathways for both evaporation and exchange of gases.
(ii) Film forming types: Colourless plastics and low viscocity waxes and silicon oils act as physical barrier on leaf surface on application by forming a thin film. It reduces transpiration but at the same time allow diffusion of oxygen and carbon dioxide.
Mechanism:
(i) Antitranspirants reduce absorption of solar energy by leaves and lower leaf temperature, as a result transpiration reduced.
(ii) Antitranspirants form a thin transparent layer on leaves to hinder the escape of water vapour from the leaves.
Effect on field crops: Fuahring,1973, sprayed antitranspirants on sorghum leaves under limited irrigation conditions which resulted in 5 % to 17% increase in yield.
K(+) Malate Hypothesis of opening and closing of stomata
A: Closed stomata
B: Open stomata
The role of K(+) ion in stomatal opening was first discovered in 1943 by Imamura and suppported and elaborated by Fujino(1967), Levitt (1974), Raschke(1975),Noggle-Firtz(1976) .
Photo courtesy: Wikipedia
Fig. Red drop of photosynthesis yield
Hence, Emerson observed that when both and long wavelengths of red light acted on at the same time the rate of photosynthesis (Z) was higher than the sum of photosynthetic rates obtained by using short and long wavelengths separately (X+Y).
Courtesy: Plant Physiology by P.L. Kochhar and H.N.Krishnamoorty,p- 207
Fig: Action spectrum of photosynthesis
Fig:Hill & Bendal's Z-scheme of light reaction
Transfer of e from water to PS II:
Fig: Calvin Cycle of Photosynthesis
. Fig: Kranz anatomy in Maize leaf
Photograph of Kalanchoe daigremontiana
Mechanism:
In Day time:
1. Levitt (1974) observed that proton (H+) uptake by guard cells takes place with the help of ATP and results into increase of pH. At increased pH ,starch is hydrolysed to form Phosphoenol Pyruvic Acid (PEP).
2. In presence of light PEP of guard cells reacts with carbon dioxide in presence of PEP carboxylase to form C4 substrates OAA.
2. In presence of light PEP of guard cells reacts with carbon dioxide in presence of PEP carboxylase to form C4 substrates OAA.
3.The OAA is then converted to malic acid and NAD(+) by the activity of enzymes malate dehydrogenase.
4. Malic acid is then dissociated into H(+) and Malate(-)ion.
5.In the next step H(+) effluxed from the guard cells due to the activity of H(+) ion pump of guard cell membrane.
6. The malate (-)ion remained in the cell and the cell interior becomes negatively charged and pH of the cell increased avobe 7.0 which helps in more malate synthesis.
7.To balance the charge of the cell interior k(+) ion passively influxed into the guard cells from the adjacent subsidiary cells and epidermal cells. Thus the concentration of K+ ion increases from 50 mM to 300 mM in the guard cells.
8.The K(+) rushes to the cell vacuole where it combines with malate (-) ion to form K-malate( potassium malate).
9.The k-malate induces endosmosis of water from the adjacent cells into the guard cells. As a result turgor pressure (TP) of the guard cell increases which leads to opening of stomata.
The sequence of events is as follows:
Light ➞ Malic acid synthesis ➞ Dissociation of malic acid into H(+) --> and malate (-)ions --> Influx of k (+) ions and efflux H(+) ions --> Formation of K- malate in vacoules--> Osmosis of water from adjacent cells -- > Increase of TP of guard cells --> Stomata opens
AT NIGHT-
In the absence of light, the sequence of events of the day time is reversed .
1. During night, photosynthesis stops , carbon dioxide accumulated in guard cells.
2. As a result, the pH of the guard cells decrease.
3. At low pH , k- malate dissociates into K+ ion and malate(-) ion .
4. K+ion efflux from guard cells into the subsidiary cells and H+ ion infflux into the guard cells.
5. Then water comes out of the guard cells ,
and TP of the guard cells decreases which results into the closure of stomata.
Photosynthesis
Photosynthesis occurs in two phases---Light phase and Dark phase.
Light Phase: The light dependent reactions of photosynthesis occurs in grana thylakoids to produce NADPH and ATP which are utilised in the Dark phase .
Dark phase : The light independent reactions of photosynthesis takes place in the stroma of chloroplast.
Red drop & Emerson Effect:
(i) Emerson and Lewis(1943) measured quantum yield at different wavelengths of light during photosynthesis.
(ii) The Oxygen molecules released per quantum of light absorbed is called quantum yield.
(iii) Emerson et.al., (1957)noticed that when a plant was shifted from short wavelength (650nm) to wavelength longer than 680 nm (red region) quantum yield dropped suddenly.
(iv) This sudden fall in the photosynthesis yield beyond red region of spectrum is called red drop or long red wavelength drop-off.
(v) They further observed that photosynthetic rate can be restored if simultaneously shorter wavelength is provided.
(vi) Let, photosynthetic rate of simultaneous shorter and longer wavelength is Z, and the photosynthetic rate at shorter wavelength is X, and photosynthetic rate at longer wavelength is Y.
(vii) Emerson et.al. observed that Z > X+ Y , and this photosynthetic enhancement is called Emerson Enhancement Effect or Emerson Effect.
Photosynthesis at 700 nm = 10 unit. (X)
Photosynthesis at 653 nm = 43.5 unit. (Y)
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Total ( beams at seperate) = 53.5 unit (X+Y)
When wavelength used simultaneously:
Photosynthesis at (653+700) nm = 72.5 unit (Z).
Absorption spectrum
The graphical representation of amount of light of different wavelengths absorbed by a pigment is called absorption spectrum.
The absorption spectra of the chlorophyll a and chlorophyll b differ somewhat from each other:-
(i) The maximum absorption occurs in the blue end of the spectrum:-
449 nm in chl.a and 453 nm in chl.b
(ii) Second maximum absorption occurs in the red end of the spectrum:-
660 nm in chl.a and 642 nm in chl.b
(iii) Virtually no absorption occurs in the green- yellow region. Green rays are reflected so, chlorophyll solution and leaves appear green to the eye.
(iv) Bacteriochlorophyll absorps most in the infrared and the blue violet parts of the spectrum.
The light absorbed by chl.b, chl.c and chl.d is passed onto chl.a to use in photosynthesis.
Action spectrum
The graphical representation of the rate of photosynthesis at different wavelengths of light is called action spectrum.
It shows that maximum photosynthesis occurs in blue- violet and red part of the light. However, sufficient photosynthesis occurs in the mid part of the light spectrum where the carotenoids are active.
Light Reactions of Photosynthesis:
The formation of ATP in presence of light is called photophosphorylation and there are two distinct types of such reactions -- one, in which only ATP is formed (Cyclic ) and other in which NADPH is formed alongwith ATP formation and oxygen liberation ( Non-cyclic).
Non-Cyclic photophosphorylation:
It was illustrated by Robin Hill and Fay Bendal in 1960 by "Z-scheme" in which electrons transferred from (i) water to PS II, (ii) from PS II to PS I, and (iii) PS II to NADPH+. It happens when light of shorter wavelengths and longer wavelengths are illuminated at the same time on the green pigmets of plants.
Photolysis of water :
(i) Splitting of water occurs on the inner side of thylakoid membrane to produce (H)+ ions /protons and (OH)- ions/ hydroxyl ions in presence of Mn++, Cl- and liberating oxygen as a biproduct at light wavelength of less than 680 nm. Reactions of photolysis of water are given below:-
(ii) Simultaneously, PS II also absorbes photons and excited to release electrons from P680 and get ready to receive electrons from OH- of ionised water.
(iii) The exited electrons are transferred to PQ ( plastoquinone).
Transfer of e from PS II to PS I :
(iv) Next electrons are transferred to PS I via Cyt b6 , Cyt f and PC ( plastocyanin) as PS I also liberated electrons after being exited by absorbing photon from light wavelength above 680 nm.
(v) When electrons transferred from Cyt. b6 to Cyt.f , ATP synthesized by phosphorylation of ADP.
Transfer of e from PS I to NADPH+:
(vi) The electron liberated from P700 of PS I is accepted by FD ( Ferredoxin) .
(vii) At last, extruded electron is received by NADP+ with the help of NADPH reductase and reduced to NADPH through H+ ion which was released during photolysis of water.
The electrons released from water never came back to water or to PS II but used to reduce NADP+, hence called Non-cyclic.It involves both PS II and PS I , both short wavelengths and long wavelengths , production of NADPH , oxygen and ATP.
Cyclic Photophosphorylation:
(i) When there is only longer light wavelength and photosynthetic cells are exposed to red light, PS I is activated but not the PS II.
(ii) In absence of shorter wavelength process of photolysis of water and consequently the evolution of oxygen as well as formation of H+ ions and reduction of NADPH+ are stopped.
(iii) The supply of electrons to PS I are not available hence, the electronbreleased from excited PS I is first primary acceptor , passed to Ferredoxin (FD) , then travelsv down to Cyt.b6.
(iv) Then electron moves down to Cyt.f , PC and finally returns to P700 of PS I, thus the cycle completes.
(v) During the journey of electron from FD to Cut.b6 and between Cut b6 and Cyt. f ATP synthesized. According to another view, the electron is transferred from FD to PQ then subsequently to PS I via cytochrome complex and PC.
(vi) The electron which was released from PS I earlier is again returns to PS I , producing ATP so, it is termed Cyclic Photophosphorylation.
If, after a time gap short light wavelength is provided , PS II is activated, the non cyclic photophosphorylation is resumed, ATP, NADPH and oxygen are produced by splitting water.
Chemical Reaction or Dark Reaction of Photosynthesis:
The reactions involved in reductive fixation of carbon dioxide into photosynthetic products through a series of enzymatic steps using ATP and NADPH produced by light reactions are termed as Dark Reactions. In green plants there are three pathways for CO2 fixation, these are:-
(i) Calvin Cycle ( ii) Hatch - Slack cycle and (iii) CAM pathways
Calvin Cycle or C3 Cycle:
Malvin Calvin, Bensen and Bassham and their associates found that 3C compound 3- PGA (Phosphoglyceric acid ) is the first stable product of the dark phases while reduction of carbon dioxide takes place in the stroma of the chloroplasts with the help of the assimilatory power of the cell ( NADPH2 and ATP), produced in the earlier light reaction.
Since , PGA is a 3C compound and the first stable product of the Calvin cycle it also termed as C3 cycle.
Phases of Calvin cycle: Calvin cycle is divided into three phases: carboxylation, glycolytic reversal ,and regeneration of RuBP.
1. Carboxylation: Benson concluded that the dark phases of photosynthesis began with the combination of CO2 to a 5C sugar Ribulose 1-5, bisphosphate ( RuBP), resulting in highly unstable 6C compound 2-Carboxy 3-keto 1,5- biphosphoribotol which was immediately hydrolysed to 2 molecules of PGA.
2. Glycolytic Reversal:
The reactions of this phases are reversal of glycolysis process.
(i) PGA is first phosphorylated to 1,3-diPGA by ATP ( the cofactors produced in the earlier light reaction) with the help of enzyme triose phosphate kinase.
(ii) Further diphosphoglyceric acid is reduced by NADPH ( the cofactor produced in the earlier light reaction) to Glyceraldehyde 3-P with the help of enzyme triose phosphate dehydrogenase.
This is the primary energy-storing chemical reaction in the biosphere.
(iii) A part of the Glyceraldehyde 3-P so formed is converted to its isomer Dihydroxy acetone-3-P with the enzyme phospho triose isomerase.
(iv) Next, one molecule of each isomers react each other to condense Fructose1,6-diP with the help of enzyme aldolase.
(v) Fructose1, 6-diP loses one P-group, forms Fructose 6-Phosphate with the help of enzyme phosphatase.
(vi) Subsequently, Fructose 6-Phosphate is changed to its isomer Glucose 6-Phosphate .
(vii) At last, Glucose 6-Phosphate produces a hexose sugar Glucose.
As glucose is a 6C compound, six turns of Calvin cycle are required to synthesise it's one molecule.
3.Regeneration of RuBP
The reactions and enzymes involved in the regeneration of RuBP are given below:
(i) 6C compound F-6-P and 3C compound 3-PGAld interact and transketolase catalysed it , and resulted into 4C sugar E-4-P and 5C sugar Xylulose -5-P.
(ii)4C sugar E-4-P condensed by aldolase with triose PGAld (3C) to form (7C) Sedoheptulose 1,7- diP.
(iii)Next 7C compound Sedoheptulose 1,7-diP loses a molecule of P and converted to 7C compound S 7-P .
(iv) S 7- P (7C) reacts with Glyceraldehyde 3-P (3C,triosephosphate) to produce 5C sugars Xylulose 5-P and Ribose 5-P .
(v)Next Xylulose 5-P is converted to Ribulose 5-P by epimerase.
(vi) Ribose 5-P also converted to Ribulose 5-P by an isomerase.
(vii) At last Ribulose 5-P picks up a P from ATP to become RuBP .
This cycle is called Calvin Cycle after Malvin Calvin who worked out its details. In this cyclic reduction of 6 molecules of carbon dioxide, 18 molecules of ATP and 12 molecules of NADPH are consumed.
C4 Cycle or Hatch Slack Pathway
Discovery: In 1965 , Kortschak, Hartt and Burr observed 80% of radioactive 14CO2 is found in 4-carbon dicarboxilic acids( malic acid and aspartic acid) and only 10% of it in Phospho Glyceric Acid (PGA) during an experiment on photosynthesis.
In 1967, M.D.Hatch and C.R. Slack explained that radioactive malic acid appeared in photosynthetic cells much earlier than PGA in plants like Maize, Sugarcane, Sorghum, Amaranthus , Panicum,etc.
These plants are termed as C4 plants and have Kranz anatomy where vascular tissue is surrounded by large bundle sheath (BS)cells contain agranal large chloroplasts with RUBP carboxylase ,and undifferentiated mesophyll cells with normal chloroplast with grana and PEP carboxylase enzyme. Plasmodesmata connects a BS cell to a mesophyll cell.
Steps:
Fig: C4 Cycle in Sugarcane leaf cells
Inside the mesophyll cells:
(i) In mesophyll cells , CO2 reacts with H2O and rapidly converts into HCO3- and protons . The reaction is catalysed by the enzyme carbonic anhydrase .
(ii) Subsequently HCO3- fixed onto PEP , a 3C compound, directly yielding OAA (Oxalo acetic acid) ,a 4C compound , catalysed by the enzyme PEPcarboxylase.
CO2 +H2O —Carbonic anhydrase—> HCO3- +H+
PEP + HCO3- ___PEPcarboxylase____>OAA
(iii) The unstable oxaloacetate (OAA) is soon reduced to 4-C malic acid by enzyme malic dehydrogenase in presence of the coenzyme NADPH + H+. In some plants OAA converts to aspartic acid by reductive amination.
OAA + NADPH2—Malic dehydrogenase → malate + NADP+
OAA + Alanine___Transaminase___>aspartate + pyruvate
Malic acid or aspartic acid is translocated to BS cells through plasmodesmata.
Inside the BS cells:
(iv) In BS cells malic acid is decarboxylated or aspartic acid is deaminated to form pyruvic acid and CO2.
Malate + NADP+ — Malic enzyme —-> Pyruvate + CO2 + NADPH
Aspartate + pyruvate—-Transaminase—> Alanine +OAA
(v) The CO2 is taken up by RuBP (the final acceptor of CO2) and enters the Calvin cycle, producing PGA.
Pyruvic acid is sent back to mesophyll cells.
Inside the mesophyll cells:
(vi) Pyruvic acid reacts with ATP under the influence of phosphopyruvic diakinase to regenerate PEP ( primary CO2 acceptor) and thus completing the cycle.
Pyruvate +ATP + H3PO4 –phosphopyruvate diakinase -> PEP + AMP + PPi
Significance:
(i) C4 plants are more efficient than C3 plants because PEP carboxylase is more active in CO2 fixation than RuBP carboxylase .
(ii) C4 plants perform photosynthesis even when stomata are closed ( from CO2 produced in respiration).
(iii) Even at very low concentration of CO2 a C4 plant can continue photosynthesis ( 10 - 50 ppm).
(iv) Hatch- Slack plants show greater increase in dry weight than the Calvin plants.
(v) Hatch - Slack plants can utilize higher light intensities than Calvin plants and the rate of photosynthesis at optimum light intensities may be twice that of Calvin plants.
Crassulacean Acid Metabolism
CAM is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions ( e.g., cacti) or in tropical epiphytes (e.g., orchids & bromeliads) .
CAM plants close their stomata in day time to prevent transpiration, and open their stomata in night when the air temperature is low to collect atmospheric CO2, converting it into Oxaloacetate (OAA) and subsequently storing it as malic acid in vacuoles.The following day ,this malic acid is decarboxylated to release CO2 internally to run Calvin cycle in chloroplast for carbohydrate synthesis in stroma . CAM is named after Crassulacrae, the family of succulent plants, in which this type of metabolism was first discovered.
Mechanism:
Processes behind open stomata in night:
(1)CAM plants take up atmospheric CO2 predominantly at night. CO2 enters the mesophyll leaf tissue through open stomata at night.
(2)Then CO2 reacts with PEP to form C4 substrates OAA. This carboxylation of PEP is catalysed by the enzyme Phosphoenol Pyruvate Carboxylase ( PEP carboxylase ) .The precursor (PEP) is being generated via glycolysis or soluble sugars formed during the previous day .
(3) Subsequently unstable OAA is reduced to Malic Acid by the enzyme malic dehydrogenase in the presence of the coenzyme NADPH2 and stored in the central cell sap vacuole increasing the acidity.
Malate (-) ion moves passively into the vacuole down an electrochemical gradient through a specific channel, following the active transport of protons via a vacuolar H(+) - ATPase pump.
Processes behind closed stomata at day time:
(4) The following day in presence of light , malic acid is remobilized from the vacuole and transported to chloroplast for decarboxylation to form CO2 and Pyruvic acid and subsequently resulting in the decrease of acidity. With respect to the carboxylation process CAM plants are of two types:
(i) NAD(P)-ME- type species: where the decarboxylation is brought about by NAD- dependent -mitochondrial/ cytoplasmic Malic enzyme (ME).
(ii) PEPCK-type species:where cytosolic ME serves decarboxylation.
(5) Regenerated CO2 is refixed via the C3 photosynthesis( Calvin - Benson Cycle ) by RubisCo as CO2 concentration turns 60- fold(upto 10,000 ppm) as compared to the atmospheric CO2 level and fixed to RuBP then carbohydrates are reformed.This process also suppresses photorespiration .
(6) The pyruvic acid is again converted to PEP, then PEP is converted to triose by gluconeogenesis ,hexose phosphate and subsequently into starch.
(7) In the following night, starch is converted to PEP to fix CO2 behind the closed stomata to regenerate the cycle again.
CAM is found in 33 families and 328 genera comprising monocots and dicots ( Smith and Winter,1996). e.g., Kalanchoe daigremontiana.
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