PHOTOSYNTHESIS and PLANT NUTRITION

Certain essential plant elements are required for photosynthesis and related metabolic processes.  Some have a direct role in the structure of the photosynthetic apparatus.  Necrosis and chlorosis (chlorophyll loss), which gives brown or yellow discoloration in the leaf , that accompanies a deficiency of an essential plant nutrient, results in reduced leaf area and plant metabolic activity.  Often by the time symptoms are visible in the plant leaf, the metabolic activity in the chloroplast is severe and plant growth has been reduced to a level of economic loss for the grower.  Essential plant nutrients involved in the translocation of photosynthetic from the site formed (light and dark reactions) to areas of sink formation - fruits, grains and storage organs are another area where deficiencies of a specific plant nutrient can reduce overall plant growth.  The rates of photosynthesis can be reduced significantly if storage sites are not developing properly and acting a sink for the photosynthate formed in photosynthesis. Understanding how essential plant nutrients are involved in the photosynthesis process is necessary for the plant nutritionist to maximize plant health and economic growth of the crop. 

Photosynthesis in General
   
In the most general terms, photosynthesis could be described as the process by which plants use light energy to transform carbon dioxide (CO₂) and water (H₂O) into oxygen (O₂) and carbohydrates.  A general reaction for the photosynthetic process is shown below:

  CO₂ + H₂O + Light Energy  ----> (CH₂O) + O₂

Photosynthesis, however, is not that simple.  It actually consists of several interconnected processes that work together in taking solar radiation and using it to turn carbon dioxide and water into starches and oxygen gas.  Inorganic elements play a vital role in all of the processes of photosynthesis. Nutrients can play both a direct and indirect role in photosynthesis.
   
Direct effects of nutrient deficiencies are usually considered reversible.  Elements can be considered to have direct effects on photosynthesis when deficiencies of a particular element can cause a rapid decline in photosynthetic activity; and upon reintroduction of that element at a proper level, photosynthesis resumes.  Indirect effects are not usually readily reversible. They occur over a longer period of time and involve an element that is not necessarily critical in the photosynthetic process, but instead is crucial in the production of something that is directly involved. An example of an indirect effect would be the depletion of sulfur.  Although sulfur is not directly involved in the reactions of photosynthesis, it is a major component of amino acids, which make up many of the proteins of the chloroplasts.  Without sulfur, chloroplast production would suffer and photosynthesis rates would fall off.  Unlike the elements having direct effects on photosynthesis, the reintroduction of elements that indirectly affect the photosynthetic process may not readily improve rates of photosynthesis.
   
Often, the symptoms of many nutrient deficiencies are simply the visual manifestations of decreased photosynthetic activity by a plant. For example, the chlorosis that occurs when plants are deficient in magnesium (Mg) is due to the fact that magnesium makes up part of the chlorophyll molecule.  Chlorophyll is a pigment in plants responsible for giving them their green color.  Thus, when magnesium is lacking, chlorophyll production is lowered and leaves yellow.  In order to understand the role of the essential elements in photosynthesis we must first have an understanding of where, how and why the various processes of photosynthesis occur.

 Photosynthesis is a series of partial processes:
   
1.  Interception of incident radiation and its conversion into chemical energy as NADPH and ATP.

    The reaction of long-day and short-day plants to time periods and wavelengths of light is shown here.

    2.  Diffusion of CO₂ to the chloroplasts.

Photosynthesis largely occurs in the leaves of green plants.  In particular it occurs within an organelle that is unique to plants called the chloroplast. The chloroplast is one of three general types of plastids.  The other two types are called chromoplasts and leucoplasts.  Chloroplasts are organelles that are bounded by a double membrane system; which encloses an internal membrane system, the thylakoids, and a gel like matrix called the stroma.  Thylakoids are grouped into stack like structures called grana, and several grana can be connected by tube like structures called frets.  It is amongst these structures that photosynthesis occurs. (see figure below)
  
Chloroplasts have also been shown to contain significant concentrations of inorganic nutrient ions.  Studies have shown that more than half of the N, Ca and Mg in a plant, is in the leaves. Concentrations of Ca, Mn, Fe, Cu, Mg, Al and Si have all been detected in the lamellae of the chloroplasts.
  
  When essential elements are deficient or present in toxic amounts, a number of common changes can occur in the chloroplasts of a plant.  Often, the chloroplasts of affected plants display the following irregularities:
· Chloroplasts are more spherical than ovoid in shape.
· The number of grana, tend to be reduced or appear as indistinguishable plastids.
· Deficiencies in calcium can result in membrane irregularities.
· Deficiencies in sulfur can result in general chloroplast irregularities.
· Deficiencies in magnesium of iron can result in destruction of lamellar systems as well as irregularities in chloroplast membranes.
· Ammonium toxicity can result in a decrease in the number of grana present as well as a general swelling of the chloroplast. This is likely related to the uncoupling effect that ammonia has on photosynthetic phosphorylation.

 See also a typical and cross-section of a plant cell.
   
It must be noted that although we have split photosynthesis into several different processes for the purpose of studying it; all of the steps listed above are invariably linked together and can all occur at the same time in the plant.  Below is a diagram showing the various processes of photosynthesis together as they occur in the leaf.

    3.  Synthesis from triosephosphate of starch in the chloroplasts or sucrose in the cytoplasm.
The Light Reaction

The light reaction of photosynthesis involves the adsorption of light radiation of particular wavelengths and the utilization of this absorbed energy to split water molecules in order to generate ATP and NADPH, which can be used as a source of chemical energy in the dark reaction. The light reaction takes place within two working units called photosystems.  Photosystems consist of chlorophyll and other pigment molecules, such as xanthophylls and carotenoids that absorb light energy from particular wavelengths. The pigments that absorb light energy are called antenna pigments.  These pigments absorb light energy and deliver it to an area called the reaction center.  The reaction center consists of some proteins and chlorophyll molecules that use this gathered energy and convert it into chemical energy.
   
The various pigments in plants help give them the unique ability to absorb light energy and convert it into useable chemical energy. Though all the pigments play important roles in the functioning of green plants, the one whose role is the most obvious is chlorophyll and thus it will be discussed here.
   
Chlorophyll is a large molecule with a heme structure, much like the hemoglobin in our red blood cells.  However, instead of having an iron atom in the center like a nitrogen porphyrin ring hemoglobin does, it has a magnesium atom.   There are different types of chlorophyll: chlorophyll a, b and c.  Chlorophyll a makes up about 75 percent of the chlorophyll in a green plant and is thus most essential to the photosynthetic process.  What makes chlorophyll so special is its ability to absorb light.  Chlorophyll absorbs some wavelengths of light better than others.  It absorbs very little light in the 500nm range of wavelength.  Not surprisingly, this corresponds to green light in the electromagnetic spectrum.  Chlorophyll best absorbs light from the 680nm to 700nm range. This corresponds to red/far red light on the electromagnetic spectrum.   Chlorophyll takes the energy it absorbs from these wavelengths of light and converts it to useable chemical energy within the photosystems of the light reaction of photosynthesis. There are two separate photosystems over which the light reaction takes place.  Appropriately enough, they are named Photosystem I and Photosystem II, after the order in which they were discovered.  One of the main differences between the two photosystems is that antenna pigments of Photosystem I absorb light predominately of a wavelength of 700 nm; while the pigments of Photosystem II absorb light mostly from the 680 nm wavelength.  The two photosystems are joined by an electron transport chain made of a number of proteins, many of which contain inorganic nutrients within them.

In the top figure, the visible spectrum is shown.  Bottom, the porphyrin ring structure
of chlorophyll is shown.  Note the magnesium ion in the center of the ring.

When light energy is absorbed by the antenna pigments of the leaf it will be funneled to the reaction centers of the two photosystems. Once the absorbed light energy reaches the reaction center of Photosystem II a water molecule is split in the Hill reaction. When the water molecule is split O₂ is evolved and electrons are "bumped" to a higher energy level.  Two inorganic nutrients, manganese and chlorine are necessary for the Hill reaction to occur.  Without them water molecules will not be split and photosynthesis will not proceed.  Zinc (Zn) has also been linked to the Hill reaction and deficiencies in zinc have been shown to result in impaired Hill reaction activity.
    The Hill Reaction:  2H₂O ---> 4 e - + 4H⁺ + O₂
                                      (Mn + Cl)

It is from this reaction which oxygen is evolved in photosynthesis.  It was once believed that the oxygen released from photosynthesis came from the breakdown of carbon dioxide, but that has been shown not to be the case. After being sent to a higher energy state by the absorption of light energy in Photosystem II, the electrons from the water molecule are trapped by a protein complex.  This protein complex strongly resembles an electron transport chain.  It includes cytochromes, ferrodoxin, iron-sulfur proteins and plastocyanin. Again, the importance of plant nutrients is evident by the presence of iron (Fe) in ferrodoxin, copper (Cu) in plastocyanin, and sulfur (S) in the iron-sulfur proteins.

Once the electrons have reached this protein complex, they are moved along the chain from one protein to another down an energy gradient.  As the electrons move down this transport chain, the energy that they lose is used to add an extra phosphate to ADP in order to make ATP.  This process is termed photophosphorylation.

  The figure above shows Photosystems I and II and their relation to each other in the
light reaction of photosynthesis. Notice that the oxygen that is evolved from photosynthesis
comes from a water molecule, and not carbon dioxide as was once thought.

As the electrons make their way down the protein transport chain they will eventually come to Photosystem I.  Once the electrons have made it to Photosystem I, a process occurs that very closely mirrors what happened at Photosystem II.  Various antenna pigments absorb light energy of a wavelength of 700 nm and that energy is funneled to the reaction center.  Once the energy reaches the reaction center it is used to again elevate the two electrons to a higher energy level.  After reaching a higher energy state the electrons again move down an electron transport system much like they did after Photosystem II.  The big difference here is that the electrons are not used to manufacture ATP, but instead they are used to reduce NADP to NADPH₂.  The NADPH₂ is later used in the Calvin cycle in the manufacturing of carbohydrates from carbon dioxide. Noncyclic photosynthesis.

As stated earlier, photophosphorylation is the process by which ATP is created in the light reaction.  This is a very important process, for it is the only time that ATP is created during photosynthesis.  And just like nearly every other event that occurs in photosynthesis, photophosphorylation is greatly affected by inorganic nutrients.  Photophosphorylation takes place in the protein chain that is located between Photosystem II and Photosystem I; more specifically, it occurs at the thylakoid membrane.
   
Initially, in Photosystem II, a water molecule is split up (the Hill reaction) into an oxygen molecule, two electrons and four protons (H⁺).  As was discussed earlier, the two electrons are sent to a state of elevated energy due to the absorbance of light energy by chlorophyll molecules as well as some other pigments.  Then these electrons proceed to move to Photosystem I via a protein chain, in the process losing much of the energy that they had gained.  This energy is not just "lost" though.  It is used to pump the protons that were released from the splitting of water across the thylakoid membrane from the stroma region of the chloroplast into the thylakoid.  Pumping all of these protons across the thylakoid membrane sets up an electrochemical gradient, which drives the synthesis of ATP.

The process of photophosphorylation as it occurs across the thylakoid membrane.
This is the only reaction in photosynthesis in which ATP is formed.
NADPH (not shown) is formed in this process as well.

Protons naturally will try to flow down the electrochemical gradient, from high potential to low potential.  In this case the protons will try to move back into the stroma from the thylakoid.  The protons move back across the thylakoid membrane into the stroma through an enzyme called ATP-synthase.  When they move through this enzyme, into the stroma they are moving to an area of lower potential. The protons then must lose some energy along the way, right?  Well they do, and this energy is used to put an inorganic phosphate ion, denoted Pi, to ADP, thus creating ATP.
   
Inorganic nutrients are very crucial to the functioning of this process.  Iron, sulfur, and copper are all parts of proteins that are critical in the movement of electrons from Photosystem II to Photosystem I.  Calcium is also very important in this process because it maintains membrane integrity.  Obviously, this is very important when considering the flow of protons and electrons across the thylakoid membrane.  Phosphorous too, plays an important role.  Not only is phosphorous added to ADP to form ATP, it, like calcium, is important in maintaining membrane integrity.
   
Toxicities, not just deficiencies can have a detrimental effect on photophosphorylation.  One, well documented toxicity, that can have adverse effects on the production of ATP is ammonia toxicity. When ammonia reaches toxic levels in the plant, one of the many problems that can occur is the uncoupling of photosynthetic phosphorylation.  The production of ATP becomes "uncoupled" due to the detrimental effects of ammonia on the thylakoid membranes.  As was mentioned earlier, the functioning of the thylakoid membrane is integral in the production of ATP. When the membrane becomes distorted in any way, ATP production will inevitably fall, and the plant will suffer.  Thus it is important to not only know if you are deficient in an inorganic nutrient but to know when nutrients may be at toxic levels as well.  This is why tissue testing should be done on a regular basis.
   
Indeed, it is evident that inorganic nutrients have a major impact on the light reactions of photosynthesis.  Nitrogen is in every amino acid in a plant; thus, it must also be part of every single protein in a plant as well as being a major component of the chlorophyll molecule.  Thus, nitrogen is involved in nearly every aspect of the light reactions as well as photosynthesis as a whole.  Phosphorous also plays a big role in the light reactions of photosynthesis.  It is phosphorous that is added to the ADP to form ATP which will be used elsewhere in the plant for energy. Phosphorous is also part of NADP, which is reduced to the NADPH₂ that goes on to the Calvin cycle.  Magnesium is the central component of the chlorophyll molecule and therefore is vital to the functioning of the light reactions of photosynthesis.  Research has shown that up to ten percent of the magnesium in the plant is held in chlorophyll.  Manganese, chlorine and possibly zinc are essential for the Hill reaction to function.  Iron, sulfur and copper are all parts of proteins that help move electrons between the two photosystems.

    4.  Transport of photosynthates to phloem and then to other regions of the plant.
The Dark Reaction/The Calvin Cycle

The Calvin cycle is often referred to as the dark reaction of photosynthesis.  Referring to the Calvin cycle as the dark reaction can be misleading.  Though called the dark reaction, the Calvin cycle can occur both during day and night.  However, light is not required for this reaction to proceed; hence the name.  In the most general terms, the dark reaction of photosynthesis involves the evolution of carbon dioxide from the atmosphere into the plant, where it is used to manufacture carbohydrates.  The actual cycle is much more in-depth than that however, and will be discussed below.  Once again, inorganic nutrients play an important role in the many facets of the dark reaction of photosynthesis.  As in the light reaction, inorganic nutrients have both direct and indirect effects on the dark reaction, and these will be discussed below, as well.
   
The dark reaction of photosynthesis begins with the diffusion of carbon dioxide into the leaf via the stomata while oxygen, created in the light reaction diffuses out.  One plant nutrient that plays an essential role in the movement of carbon dioxide into the leaf is potassium (K).  As stated earlier, carbon dioxide moves into the leaf through the stomata.  The stomata, which are located mostly on the underside of the leaf can be opened and closed by a plant as needed.  The opening and closing of the stomata is regulated by guard cells that are located on either side of the stomata.  The movement of potassium into the guard cells will determine whether they allow the stomata to remain open or closed.  With potassium deficiencies, there will be problems in the movement of carbon dioxide into the leaves, as well as movement of oxygen out.  Without much needed carbon from carbon dioxide, the plant may have to resort to "mining" carbon from inside the plant itself in order to manufacture much needed carbohydrates that will be utilized in respiration.  In addition, oxygen has an inhibitory effect on photosynthesis, and when levels of oxygen inside a plant get too high, photosynthesis rates may drop.
   
Once carbon dioxide has diffused into the intercellular spaces of the leaves, it then moves into plant cells where it is transported into the chloroplast so that it can be integrated into a carbon skeleton via the Calvin cycle to form the precursors of starches, sugars, proteins and fatty acids.  The Calvin cycle begins with one molecule of carbon dioxide entering the cycle and combining with ribulose 1,5 bisphosphate, a five-carbon sugar to form a series of three-carbon compounds.  It should be noted that this reaction is catalyzed by the enzyme Rubisco.  Rubisco is important because it is thought to be the single most common protein on the planet Earth. In addition, magnesium and perhaps manganese are essential for the functioning of this enzyme.  Without the presence of at least one of these elements this enzyme will not function, carbon dioxide will not be fixed by ribulose 1,5 bisphosphate and the Calvin cycle will not proceed.

Transport of Photosynthate
   
After the products of photosynthesis: sugars, starches, fatty acids and proteins are formed they must be moved from their point of origin or source, to a location where they are needed, a sink.  Photosynthates are moved around the plant via the phloem tissue.  Two elements in particular are crucial for the movement of photosynthate from source to sink.  Potassium, in addition to its many other roles in the plant serves to transport the products of photosynthesis around the plant.  When organs such as flowers are forming, there is a great need for potassium in a plant.  When deficiencies in potassium occur, there is often decreased flower set and/or a decrease in flower size and "quality."
   
The importance of potassium during times of high metabolic demand cannot be understated.
   
Calcium is also important in the movement of photosynthate around a plant.  Calcium has been found to be important in the "loading" of the phloem with the products of photosynthesis.  If calcium were deficient, there would be problems with getting photosynthate into the phloem tissue and thus it would not be available to sink areas on the plant.

Inorganic nutrients have both direct and indirect effects on photosynthesis:

Direct effects -
 - usually reversible, such as rapid recovery of rate of net photosynthesis when a deficient element is reintroduced at the proper level (Manganese in chloroplasts, for example).

Indirect effects -
 - ions involved in synthesis of enzymes and pigments, those involved in transport, etc. Potassium has an indirect effect via control of stomatal opening and closing.

The roles of inorganic nutrients in photosynthesis: an overview

Nitrogen
Nitrogen is important in nearly every aspect of a plant's life cycle.  This is why it is usually found in higher concentrations than any other nutrient in plant tissue.  Regarding photosynthesis, nitrogen deficiencies can result in a loss of chlorophyll content; after all, nitrogen is an important part of the chlorophyll molecule.  Nitrogen is also a part of every amino and nucleic acid.  Thus nitrogen deficiencies can result in loss of proteins as well as genetic deformities that could negatively impact photosynthesis.  Low nitrogen levels can result in generalized chloroplast irregularities.  Decreased carbon dioxide assimilation is associated with nitrogen deficiencies, as well as resistance to carbon dioxide transfer within the mesophyll of leaves.  Lastly, nitrogen availability affects the amount of substrate (glycolate) that is available for respiration.  This will increase the inhibitory affect of oxygen on photosynthesis. Nitrogen metabolism in a leaf cell and root cell.  Nitrogen fixation in a root nodule.

Phosphorous
Phosphorous is also considered important in photosynthesis.  Obviously phosphorous is required for ATP synthesis, though there has been evidence showing that only at severe deficiencies are ATP levels affected substantially.  It is clear that low levels of phosphorous will lead to lowered rates of photosynthesis within a plant, though the exact mechanism(s) by which this happens have not completely been elucidated. Phosphorous metabolism in a leaf cell and root cell.

Potassium
Potassium has a number of roles in photosynthesis.  Two roles in particular stand out though.  One, potassium is required in maintaining stomatal aperture and the subsequent movement of carbon dioxide into the leaves.  Second, potassium is vital for the transport of photosynthate to sink areas around the plant.  Other problems associated with potassium deficiencies include a reduced number of grana in the chloroplast and a general increase in respiration.  This increase in respiration is likely associated with the decreased exchange of oxygen for carbon dioxide at the stomata.  High oxygen levels within the leaves will lead to increased rates of respiration as well as a generalized lowering of photosynthesis. Potassium metabolism in a leaf cell.

Calcium
Calcium plays a role in ATP synthesis through maintaining membrane integrity in the thylakoids.  Calcium is also important in the loading of photosynthate into the phloem tissue of the plant for transport to sink areas such as flowers and fruit.  Calcium deficiency can also cause leaf distortion; this will lead to a generalized lowering of photosynthesis, either due to decreased surface for light absorption or a disfigurement of the contents of the leaf. Metabolism of calcium in a leaf cell.

Magnesium
Magnesium is an important co-enzyme in many of the reactions of photosynthesis.  In addition, a large amount of magnesium, up to ten percent of total plant magnesium, is located in chlorophyll molecules.  Obviously, if chlorophyll concentrations drop, photosynthesis rates will likely do the same.

Sulfur
Sulfur is essential in two amino acids, methionine and cysteine.  Thus it is part of nearly all proteins.  In addition, disulfide bonds that occur within proteins help form their overall structure and, thus, their functioning.  Chloroplasts will be reduced if sulfur levels are low because they are rich in sulfur containing proteins.  Sulfur deficiency can also increase stomatal resistance to carbon dioxide diffusion.  Sulfur deficiency can mimic nitrogen deficiency in plants, due to the fact that they have a number of similar roles.  However, sulfur deficiencies will appear in new leaves first as it is immobile in plants; whereas, nitrogen is a mobile element, and thus deficiencies of which will show up in older leaves first. Sulfur metabolism in a leaf cell.

--Minor Nutrients--

Copper
Copper is an important part of the protein, plastocyanin.  This protein is part of the transport chain that moves electrons between Photosystem II and I.  When copper levels are low so are plastocyanin levels and ATP synthesis as well as photosynthesis are negatively affected.  Copper deficiency can also result in lowered Hill reaction activity in plastids.  In toxic concentrations, copper can result in decreased oxygen evolution in photosynthesis.

Iron
Iron is present in relatively high concentrations in the chloroplast.  It is part of several proteins associated with photosynthesis.  These include ferrodoxin and some cytochromes. Iron is also necessary in forming chlorophyll.  Iron deficient plants will have deformed chloroplasts in general.

Manganese
Manganese functions primarily as a co-enzyme in the Hill reaction, and thus low levels of manganese in a plant will adversely affect the splitting of water and the subsequent evolution of electrons and oxygen.  Manganese deficient plants also have severe malformations of their chloroplasts and associated structures.

Chlorine/Zinc
Chlorine, like manganese, functions in the Hill reaction, specifically the transport of electrons from a hydroxyl group to chlorophyll.  Zinc deficiencies have also been linked to decreased activity of the Hill reaction.

Boron/Molybdenum
Though the exact role of these two elements in photosynthesis is unclear, it is known that deficiencies in either of them will hinder photosynthesis to a degree. Metabolism of boron in a leaf cell.

Chloroplasts
Chloroplasts are a major site of inorganic nutrient ion accumulation in leaves.  Studies of tobacco showed that over half of the total N, Ca and Mg is located in the leaves (but less than half of the K and Na which lurks in the phloem and xylem).  Mn content is divided between chloroplasts and cytoplasm, and Ca, Mn, Fe, Cu, Mg, Al and Si are contained within the lamellae of the chloroplasts.

By the time visible symptoms of deficiency or toxicity have occurred (such as necrosis or chlorosis), the chloroplasts structure has been altered or destroyed.

Changes in chloroplast structure in nutrient-deficient plants:

• Chloroplast conformation often more circular than oval.
• Grana number reduced or indistinguishable in plastids.
• Mg and Fe deficiencies can result in destruction of lamellar systems and disruption of chloroplast membranes.
• Ammonium toxicity can also change the structure of chloroplasts - the loss of grana, vesicle formation, membrane distortion and uncoupling of photosynthetic phosphorylation from electron transport. Chloroplasts normally shrink during electron transport, but this disruption causes them to swell - this is probably linked to ATP formation.

Alteration of Photochemistry and Carbon Metabolism by Nutrient Deficiency

Nitrogen
Nitrogen deficiency decreases net photosynthesis.  N and chlorophyll contents of leaves are closely correlated, and N deficiency results in a sharp drop in chlorophyll content.

 N deficiency also results in :

• decline in CO₂ assimilation
• increased resistance to CO₂  transfer through stomata and within mesophyll.
• enhanced inhibitory effect of O₂ on net photosynthesis - N availability regulates the amount of substrate (glycolate) available for photorespiration.

The form of N provided also effects photosynthesis.  NH₄-N accumulates in leaves if used for an extended period; resulting in chlorophyll loss, degraded chloroplast structure and decreased net photosynthesis.  Ammonium accumulation also inhibits transport of glucose produced in photosynthesis.

Phosphorous
The connection between phosphorous and photosynthesis is not completely clear.  However, the following observations have been made:

• The leaf area reduction caused by prolonged P deficiency does lead to lower net photosynthesis by the entire plant.
• It takes a severe P deficiency to result in changes in chloroplast structure and reduction in chlorophyll.  Since such a small amount (1.0-2.0%) of total leaf P is in ATP, only the most severe P deficiency would affect ATP levels.

Potassium

• Potassium deficiency decreases photosynthesis and increases respiration, resulting in reduced net photosynthesis.
• K deficiency decreases stomatal aperture and affects CO2 transfer into leaves.
• Chloroplasts have reduced number of grana.
• Leaf area may be reduced by necrosis as a result of formation of toxic N compounds.
• Net photosynthesis diminishes first in older, lower leaves.
• Transport of photosynthesis is inhibited and in cases of severe K deficiency, conversion of photosynthetic intermediates into sucrose and other end products is disturbed.

Calcium
Calcium deficiency does not alter the rate of photosynthesis until leaf distortion occurs.

• Carbon translocation may be effected according to plant species.
• Ca deficiency may also interfere with energy conversion in chloroplasts.
• Ca has a role in ATP synthesis.

Magnesium
Magnesium deficiency lowers the net rate of photosynthesis.  Mg is a constituent of the chlorophyll molecule which holds about 10% of the total Mg found in leaves.

• Mg deficiency results in interveinal chlorosis of leaves.
• Mg is the catalyst for many enzymatic reactions involving phosphate transfer and ATP metabolism.

Sulfur
Chloroplasts contain proteins rich in sulfur, so a deficiency reduces chlorophyll content and net photosynthesis.

• S deficiency resembles N deficiency because of the importance of each in the formation of protein.
• Severe S deficiency increases stomatal resistance to CO₂ diffusion.

Minor Nutrients:

Copper
Copper deficiency results in decreased Hill reaction activity in plastids.

• Cu is used in the synthesis of plastocynin - a Cu-containing protein that participates in electron transport between photosystems II and I.
• Cu, in toxic concentrations, will decrease photosynthetic O₂ evolution.

Iron
A major portion of iron is localized in chloroplasts and Fe has several roles in photosynthesis.  Fe-deficient plants are chlorotic in their young leaves.

• Fe is necessary for forming a precursor of chlorophyll.
• Fe-deficient chloroplasts are severely disrupted and have reduced size and number of grana.
• Cytochromes have Fe in the heme form and function in electron transport in photosynthesis in cyclic and noncyclic photophosphorylation.

Manganese
Manganese-deficient chloroplast structures of grana and stroma are greatly disrupted.

• Hill reaction activity of Mn-deficient plastids is depressed.
• Mn participates in evolution of O₂ in photosystem II.

Chlorine
Chlorine is also utilized in photosystem II in electron transport from OH to chlorophyll.

Zinc
In the case of severe Zn deficiency, Hill reaction activity in the chloroplasts may be impaired.

Boron and Molybdenum
The role of these elements in photosynthesis is unclear.  When deficient, they may both lower net photosynthesis, alter chloroplast structure, and affect Hill reaction activity.

Sodium
Sodium has been shown to be essential for certain plants that have the C4 pathway for CO₂ fixation.

In certain species, the presence of NaCl can alter the balance between the C3 and C4 pathways, increasing the significance of the C4 pathway.
 
 

    Summary of the Function of Nutrient Elements in Photosynthesis*

*From Barker, Allen V.  1979.  Nutritional factors in photosynthesis of higher plants.  Journal of Plant Nutrition.  1(3):309-342.

    Photosynthesis is the life giving process of a plant.  When photosynthesis is not functioning properly, the plant will undoubtedly suffer and, more often than not it will die.  However, a plant does not have to die in order for a photosynthetic problem to destroy a crop.  Visual appearance is extremely important in the ornamental industry, and if a product is not of the highest visual quality, it is worthless.  After all, would you buy a rose with several dead petals on it?  This can happen if the photosynthetic process is impeded in any way.  This could be due to lack of light, water or proper nutrition.  The effects of a nutritional deficiency may not kill a plant, but they will likely cause a drop in quality of the plant by depressing one or more of the metabolic processes in a plant; this includes photosynthesis.  One of the worst parts about a nutritional deficiency is that by the time one sees visual symptoms, it is probably too late to fix the problem.  Thus, it is imperative that one not only has a detailed nutritional program for each crop he or she is growing, but they must keep continual measurements of nutrient levels in plant tissue.  If plant nutrition is not closely monitored, most growers will be left with acres of plants that do not do much more than take up space.

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