All animals including human beings depend on plants for their food. Have you ever wondered from where plants get their food? Green plants, in fact, have to make or rather synthesise the food they need and all other organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds. Ultimately, all living forms on earth depend on sunlight for energy. The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth. Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the atmosphere by green plants. Have you ever thought what would happen if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform light energy into chemical energy.
WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis. Some simple experiments you may have done in the earlier classes have shown that chlorophyll (green pigment of the leaf), light and CO2
are required for photosynthesis to occur.
You may have carried out the experiment to look for starch formation in two leaves – a variegated leaf or a leaf that was partially covered with black paper, and one that was exposed to light. On testing these leaves
for starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light. Another experiment you may have carried out is the half-leaf experiment, where a part of a leaf is
enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. The setup is then placed in light for some time. On testing for starch later in the two halves of the leaf, you must have found that the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative. This showed that CO2.was required for photosynthesis. Can you explain how this conclusion could be drawn?
EARLY EXPERIMENT :
It is interesting to learn about those simple
experiments that led to a gradual development in our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
performed a series of experiments that revealed the essential role of air in the growth of green plants. Priestley, you may recall, discovered oxygen in 1774. Priestley observed that a candle burning in a closed space – a bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or an animal that breathe the air, both somehow, damage the air. But when he placed a mint plant in the
same bell jar, he found that the mouse stayed alive and the candle continued to burn. Priestley hypothesised as follows: Plants restore to the air whatever breathing animals and burning candles remove. Can you imagine how Priestley would have conducted the experiment using a candle and a plant? Remember, he would need to rekindle the
candle to test whether it burns after a few days. How many different ways can you think of to light the candle without disturbing the set-up?
Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799) showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not. Later he identified these bubbles to be of oxygen. Hence he showed that it is only the green part of the plants that could release oxygen.
It was not until about 1854 that Julius von Sachs provided evidence for production of glucose when plants grow. Glucose is usually stored as starch. His later studies showed that the green substance in plants (chlorophyll as we know it now) is located in special bodies (later called chloroplasts) within plant cells. He found that the green parts in plants is where glucose is made, and that the glucose is usually stored as starch. Now consider the interesting experiments done by T.W Engelmann
(1843 – 1909). Using a prism he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension
of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described.
WHERE DOES PHOTOSYNTHESIS TAKE PLACE?
You would of course answer: in ‘the green leaf’ or you may add, ‘in the chloroplasts’ You are
definitely right. Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. Can you name some other parts where you think photosynthesis may occur? You would recollect from previous unit that the mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light. When do you think the chloroplasts will be aligned with their flat surfaces parallel to the walls? When would they be perpendicular to the incident light? Within the chloroplast there is the membranous system consisting of grana, the stroma lamellae, and the fluid stroma.There is a clear division of labour within the chloroplast. The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.
In stroma, enzymatic reactions incorporate CO2
into the plant leading to the synthesis of sugar, which in turn forms starch. The former set of
reactions, since they are directly light driven are called light reactions. The latter are not directly light driven but are dependent on the products
of light reactions (ATP and NADPH). Hence, to distinguish the latter they are called, by convention, as dark reactions. However, this should not be construed to mean that they occur in darkness or that they are not light-
HOW MANY PIGMENTS ARE INVOLVED
Looking at plants have you ever wondered why
and how there are so many shades of green in
their leaves – even in the same plant? We can
look for an answer to this question by trying to
separate the leaf pigments of any green plant
through paper chromatography. A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is not due to a single pigment but due to four pigments: Chlorophyll a (bright or blue green in the chromatogram), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Let us now see what roles various pigments play in photosynthesis. Pigments are substances that have an ability to absorb light, at specific wavelengths. Can you guess which is the most abundant plant pigment in the world? Let us study the graph showing the ability of chlorophyll a pigment to absorb lights of different wavelengths . Of course, you are familiar with the wavelength of the visible spectrum of light as well as the VIBGYOR.
an you determine the wavelength (colour of light) at which chlorophyll a shows the maximum absorption? Does it show another absorption peak at any other wavelengths too? If yes, which one? Now look at showing the
wavelengths at which maximum photosynthesis occurs in a plant. Can you see that the wavelengths at which there is maximum absorption by chlorophyll a, i.e., in the blue and the red regions, also shows higher rate of photosynthesis. Hence, we can conclude that chlorophyll a is the chief pigment associated with photosynthesis. But by looking at can you say that there is a complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis?
These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis
does take place at the other wavelengths of the visible spectrum. Let us see how this happens. Though chlorophyll is the major pigment
responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. Indeed, they
not only enable a wider range of wavelength of incoming light to be utilised for photosyntesis but also protect chlorophyll a from photo-oxidation.
WHAT IS LIGHT REACTION?
Light reactions or the ‘Photochemical’ phase
include light absorption, water splitting, oxygen
release, and the formation of high-energy
chemical intermediates, ATP and NADPH.
Several complexes are involved in the process.
The pigments are organised into two discrete
photochemical light harvesting complexes
(LHC) within the Photosystem I (PS I) and
Photosystem II (PS II). These are named in
the sequence of their discovery, and not in the
sequence in which they function during the light
reaction. The LHC are made up of hundreds of
pigment molecules bound to proteins. Each
photosystem has all the pigments (except one
molecule of chlorophyll a) forming a light
harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms
the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption
peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.
THE ELECTRON TRANSPORT
In photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes .This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale. The electrons are not used up as they pass through the electron transport chain, but are passed on to the pigments of photosystem PS I. Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700nm and are transferred to another accepter molecule that has a greater redox potential.These electrons then are moved downhill again, this time to a molecule of energy-rich NADP+. The addition of these electrons reduces NADP+ to NADPH + H+. This whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally down hill to NADP+ causing it to be reduced to NADPH + H+ is called the Z scheme, due to its characterstic shape . This shape is formed when all the carriers are placed in a sequence on a redox potential scale.
Splitting of Water
You would then ask, How does PS II supply electrons continuously? The electrons that were moved from photosystem II must be replaced. This is achieved by electrons available due to splitting of water. The splitting of water is associated with the PS II; water is split into H+, [O] and electrons. This creates oxygen, one of the net products of photosynthesis. The electrons needed to replace those removed from photosystem I are provided by photosystem II.
We need to emphasise here that the water splitting complex is associated with the PS II, which itself is physically located on the inner side of the membrane of the thylakoid. Then, where are the protons and O2 formed
likely to be released – in the lumen? or on the outer side of the membrane?
Cyclic and Non-cyclic Photo-phosphorylation :
Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy. Special substances like ATP, carry this energy in their chemical bonds. The process through which ATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation. Photo-phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. When the two photosystems work in a series, first PS II and then the PS I, a process called
non-cyclic photo-phosphorylation occurs. The
two photosystems are connected through an
electron transport chain, as seen earlier – in the
Z scheme. Both ATP and NADPH + H+ are
synthesised by this kind of electron flow
When only PS I is functional, the electron is
circulated within the photosystem and the
phosphorylation occurs due to cyclic flow of
electrons (Figure 13.6). A possible location
where this could be happening is in the stroma
lamellae. While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP+ but is
cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP,
but not of NADPH + H+. Cyclic photophosphorylation also occurs when
only light of wavelengths beyond 680 nm are available for excitation.
Chemiosmotic Hypothesis :
Let us now try and understand how actually ATP is synthesised in the chloroplast. The chemiosmotic hypothesis has been put forward to explain the mechanism. Like in respiration, in photosynthesis too, ATP synthesis is
linked to development of a proton gradient across a membrane. This time these are membranes of the thylakoid. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS .
Let us understand what causes the proton gradient across the membrane. We need to consider again the processes that take place during the activation of electrons and their transport to determine the steps that
cause a proton gradient to develop .
(a) Since splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by
the splitting of water accumulate within the lumen of the thylakoids.
(b) As electrons move through the photosystems, protons are transported
across the membrane. This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier.
Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron
to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.
(c) The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of
electrons of PS I, protons are necessary for the reduction of NADP+ to NADPH+ H+. These protons are also removed from the stroma.
Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates
a proton gradient across the thylakoid membrane as well as a measurable
decrease in pH in the lumen. Why are we so interested in the proton gradient? This gradient is important because it is the breakdown of this gradient that leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0
of the ATPase. The ATPase enzyme consists of two parts: one called the F0 is embedded in the membrane and forms a transmembrane
channel that carries out facilitated diffusion of protons across the membrane. The other portion is called F1 and protrudes on the outer
surface of the thylakoid membrane on the side that faces the stroma. The break down of the gradient provides enough energy to cause a
conformational change in the F1 particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.
Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is used to pump protons across a membrane,
to create a gradient or a high concentration of protons within the thylakoid lumen. ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyses the formation of ATP. Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in
the stroma, responsible for fixing CO2, and synthesis of sugars.
WHERE ARE THE ATP AND NADPH USED?
We learnt that the products of light reaction are ATP, NADPH and O2. Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis. This process does not directly depend on the presence of light but is dependent on the
products of the light reaction, i.e., ATP and NADPH, besides CO2 and H2O. You may wonder how this could be verified; it is simple: immediately after light becomes unavailable, the biosynthetic process continues for
some time, and then stops. If then, light is made available, the synthesis starts again.
Can we, hence, say that calling the biosynthetic phase as the dark reaction is a misnomer? Discuss this amongst yourselves. Let us now see how the ATP and NADPH are used in the biosynthetic phase. We saw earlier that CO2
is combined with H2O to produce (CH2O)n
or sugars. It was of interest to scientists to find out how this reaction proceeded, or rather what was the first product formed when CO2
is taken into a reaction or fixed. Just after world war II, among the several efforts to put radioisotopes to beneficial use, the work of Melvin Calvin is exemplary. The use of radioactive 14C by him in algal photosynthesis
studies led to the discovery that the first CO2
fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called Calvin cycle after him. The first product identified was 3-phosphoglyceric acid or in short PGA. How many carbon atoms does it have? Scientists also tried to know whether all plants have PGA as the first product of CO2 fixation, or whether any other product was formed in other plants. Experiments conducted over a wide range of plants led to the discovery of another group of plants, where the first stable product of CO2 fixation was again an organic acid, but one which had 4 carbon
atoms in it. This acid was identified to be oxaloacetic acid or OAA. Since then CO2
assimilation during photosynthesis was said to be of two main types: those plants in which the first product of CO2 fixation is a C3 acid (PGA), i.e., the C3 pathway, and those in which the first product was a C4 acid (OAA), i.e., the C4
pathway. These two groups of plants showed
other associated characteristics that we will discuss later.
The Primary Acceptor of CO2 :
Let us now ask ourselves a question that was asked by the scientists who were struggling to understand the ‘dark reaction’. How many carbon atoms would a molecule have which after accepting (fixing) CO2, would have 3
carbons (of PGA)? The studies very unexpectedly showed that the acceptor molecule was a 5-carbon ketose sugar – ribulose bisphosphate (RuBP). Did any
of you think of this possibility? Do not worry; the scientists also took a long time and conducted many experiments to reach this conclusion. They also believed that since the first product was a C3 acid, the primary
acceptor would be a 2-carbon compound; they spent many years trying to identify a 2-carbon compound before they discovered the 5-carbon
The Calvin Cycle
Calvin and his co-workers then worked out the whole pathway and showed
that the pathway operated in a cyclic manner; the RuBP was regenerated.
Let us now see how the Calvin pathway operates and where the sugar is synthesised. Let us at the outset understand very clearly that the Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3
or C4 (or any other) pathways . For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.
1. Carboxylation – Carboxylation is the fixation of CO2 into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is
catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.
2. Reduction – These are a series of reactions that lead to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for
phosphorylation and two of NADPH for reduction per CO2 molecule fixed. The fixation of six molecules of CO2 and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.
3. Regeneration – Regeneration of the CO2
acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration
steps require one ATP for phosphorylation to form RuBP.
Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.To make one molecule of glucose 6 turns of the cycle are required. Work out how many ATP and NADPH molecules will be required to make
one molecule of glucose through the Calvin pathway.It might help you to understand all of this if we look at what goes in and what comes out of the Calvin cycle.
THE C4 PATHWAY :
Plants that are adapted to dry tropical regions have the C4 pathway mentioned earlier. Though these plants have the C4 oxaloacetic acid as
the first CO2 fixation product they use the C3
pathway or the Calvin cycle as the main biosynthetic pathway. Then, in what way are they different from C3 plants? This is a question that you may reasonably ask.C4
plants are special: They have a special type of leaf anatomy, they tolerate higher temperatures, they show a response to high light intensities,
they lack a process called photorespiration and have greater productivity of biomass. Let us understand these one by one. Study vertical sections of leaves, one of a C3 plant and the other of a C4 plant. Do you notice the differences? Do both have the same types of
mesophylls? Do they have similar cells around the vascular bundle sheath?The particularly large cells around the vascular bundles of the C4 pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means
‘wreath’ and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. You may like to cut a section of the leaves of C4 plants – maize or sorghum – to
observe the Kranz anatomy and the distribution of mesophyll cells.It would be interesting for you to collect leaves of diverse species of
plants around you and cut vertical sections of the leaves. Observe under the microscope – look for the bundle sheath around the vascular
bundles. The presence of the bundle sheath would help you identify the C4 plants.
Now study the pathway .This pathway that has
been named the Hatch and Slack Pathway, is again a cyclic process. Let us study the pathway by listing the steps. The primary CO2
acceptor is a 3-carbon molecule phosphoenol
pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase. It is important
to register that the mesophyll cells lack RuBisCO enzyme. The C4 acid OAA is formed in the mesophyll cells. It then forms other 4-carbon compounds like malic acid or aspartic
acid in the mesophyll cells itself, which are transported to the bundle sheath cells. In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule.
The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle. The CO2
released in the bundle sheath cells enters the C3 or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase. Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4 plants.
Did you note that the Calvin pathway occurs in all the mesophyll cells of the C3
plants? In the C4 plants it does not take place in the mesophyll cells but does so only in the bundle sheath cells.
Let us try and understand one more process that creates an important difference between C3 and C4 plants – Photorespiration. To understand photorespiration we have to know a little bit more about the first step of the Calvin pathway – the first CO2 fixation step. This is the reaction where RuBP combines with CO2
to form 2 molecules of 3PGA, that is catalysed by RuBisCO.
RuBisCO that is the most abundant enzyme in the world (Do you wonder why?) is characterised by the fact that its active site can bind to both CO2 and O2– hence the name. Can you think how this could be possible? RuBisCO has a much greater affinity for CO2 than for O2. Imagine what would happen if this were not so! This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased. Here the RuBP instead of being converted to 2 molecules of PGA binds with O2
to form one molecule of phosphoglycerate and
phosphoglycolate in a pathway called photorespiration. In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP. In
the photorespiratory pathway there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process.
In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle sheath cells to release CO2– this results in increasing the intracellular concentration of CO2. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.now that you know that the C4 plants lack photorespiration, you
probably can understand why productivity and yields are better in these plants. In addition these plants show tolerance to higher temperatures.
FACTORS AFFECTING PHOTOSYNTHESIS :
An understanding of the factors that affect photosynthesis is necessary. The rate of photosynthesis is very important in determining the yield of plants including crop plants. Photosynthesis is under the influence of
several factors, both internal (plant) and external. The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2
concentration and the amount of chlorophyll.
The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.The external factors would include the availability of sunlight, temperature, CO2
concentration and water. As a plant photosynthesises, all these factors will simultaneously affect its rate. Hence, though several factors interact and simultaneously affect photosynthesis or CO2 fixation,
usually one factor is the major cause or is the one that limits the rate. Hence, at any point the rate will be determined by the factor available at sub-optimal levels.When several factors affect any [bio] chemical process, Blackman’s
(1905) Law of Limiting Factors comes into effect. This states the following: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.
For example, despite the presence of a green
leaf and optimal light and CO2 conditions, the
plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.
We need to distinguish between light quality, light intensity and the duration of exposure to light, while discussing light as a factor that affects photosynthesis. There is a linear relationship between incident light and CO2
fixation rates at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting . What is interesting to note is that light saturation occurs at 10 per cent of the full sunlight. Hence,except for plants in shade or in dense forests, light is rarely a limiting factor in nature. Increase in incident light beyond a point causes the breakdown of chlorophyll and a
decrease in photosynthesis.
Carbon dioxide Concentration :
Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2
is very low in the atmosphere (between 0.03 and 0.04 per cent). Increase in concentration upto 0.05 per cent can cause an increase in CO2 fixation rates; beyond this the levels can become damaging over longer periods.
The C3 and C4 plants respond differently to CO2 concentrations. At low light conditions neither group responds to high CO2 conditions. At high light intensities, both C3 and C4 plants show increase in the rates of photosynthesis. What is important to note is that the C4
plants show saturation at about 360 µlL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 µlL-1
. Thus, current availability of CO2 levels is limiting to the C3 plants.The fact that C3 plants respond to higher CO2 concentration by
showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.
The dark reactions being enzymatic are temperature controlled. Though the light reactions are also temperature sensitive they are affected to a much lesser extent. The C4
plants respond to higher temperatures and
show higher rate of photosynthesis while C3
plants have a much lower temperature optimum. The temperature optimum for photosynthesis of different plants also
depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate
Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing
the CO2 availability. Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.