Chapter 3 - Gas Exchange
Contents - Class Homepage
Light Reactions Dark Reactions C4 and Photorespiration
Environmental Factors Respiration Growth and Photosynthesis

When you have successfully completed this section you will:

1.  Understand the role of light energy  and basic energy transformation involved in carbon fixation .

2.  Know the products (and their functions) of the light and dark reactions of photosynthesis.

3.  Understand how environmental factors (light, water, CO2, temperature, nutrients  and air pollutants) impact carbon uptake.

4.  Be able to explain the physiological and morphological differences in sun and shade leaves and their ecological importance.

5. Understand the role of and relationship between photosynthesis, respiration, and photorespiration in tree growth.

Life on earth ultimately depends on the energy from the sun.  Photosynthesis is the only process of any significance that can store this energy.  All the carbon in your own body was gathered out of the air and transformed into a carbohydrate by a plant through the process of photosynthesis.  Put simply, we owe our existence to the ability of plants to utilize light energy.  Even the oxygen we breathe is a by-product of the process of photosynthesis.

You may recall the general equation for photosynthesis:

                                      CO2 + H2O --------> C6H12O6 + O2.

What you may not have realized is that combustion (burning) of wood, fossil fuels or any carbohydrate has an equation that is basically the reverse of photosynthesis.  A general combustion equation looks like:

                                      C6H12O6 + O2 --------> H2O + CO2 + heat.

Chlorophyll, the green pigment responsible for capturing light, is found within the chloroplasts arranged in discrete patterns on a membrane system.  Chlorophyll absorbs light in the blue-violet region and orange-red region.  Chlorophyll has minimal absorption in the green and yellow wavelength which is why most leaves are green.  Other pigments also occur in tree leaves but normally are masked by the large amount of green chlorophyll.  These other pigments play no direct role in photosynthesis but are believed to absorb and transfer light energy to chlorophyll and protect the plant against photooxidation.  These other pigments (anthocyanins which are reddish to purple and carotenoids which are yellow to orange) are best known for the spectacular fall coloration they produce.

Photosynthesis occurs in the chloroplasts of the leaf.

The light reactions are composed of Photosystem II and I, which are joined by electron carriers. Light energy is first captured by Photosystem II (hence the name "light reactions"), causing the splitting of water. This can be seen at position one where the P680 chlorophyll molecule loses an electron (becomes oxidized) when it absorbs light. The P680 molecule becomes reduced and ready to donate electrons again by removing electrons from water.

At position 2, electrons are passed along electron carriers towards photosytem I. The electron carriers alternately become reduced (gain electrons) and oxidized (lose electrons). At the first electron carrier (position 2), hydrogen ions are pulled from the stroma into the lumen. These hydrogen ions, along with those produced by the splitting of water are used to produce ATP.

At photosystem I, additional light energy is absorbed by the chlorophyll molecule P700 (position 3), further exciting electrons to a higher energy state. At the end of electron transport (position 4), excited electrons are donated to NADP (oxidized state) producing NADPH (reduced state).

Shown at position 5, is the ATPase complex where the hydrogen ions produced by the splitting of water and those pulled in during electron transport are used to generate ATP. During electron transport, a higher concentration of hydrogen ions develops inside the lumen than outside. As they diffuse through the ATPase complex ATP is produced.

Measuring chlorophyll fluorescence is a way to diagnose the efficiency of the light reactions.  Using a fiber optic cable, bright light is shined on the pine needles.  If the light is not converted to chemical energy the chlorophyll molecules will re-emit (fluoresce) some of the light energy.

This tube of chlorophyll on the left was extracted from a white ash tree. When exposed to a bright light in an otherwise dark room the chlorophyll absorbs
energy. As energy is absorbed, electrons in the chlorophyll molecule are kicked to a higher energy state. As the electrons return to a lower energy state (ground state) they can re-emit light energy of a longer wavelength causing the chlorophyll to appear red (right)

The dark reactions of photosynthesis are so named because they do not require light to take place. They do, however, require the ATP and NADPH produced during the light reactions. Therefore the dark reactions do not actually occur in the dark.

The path of carbon in photosynthesis has become known as “the Calvin cycle”. The Calvin cycle involves a large series of chemical reactions which utilize the ATP and NADPH produced during the light reactions. We will not be going over all the steps in this cycle. The actual step which fixes a CO2 molecule is catalyzed by the enzyme ribulose bis-phosphate (RuBP) carboxylase. RuBP is a 5 carbon sugar which when it is combined with a CO2 molecule produces two new 3 carbon molecules. Notice there has been a net increase of one carbon (2x3=6). These 3 carbon molecules through a large and complicated series of reactions form new hydrocarbons which eventually result in growth of the plant. Species which solely rely on this pathway for carbon fixation are known as C3 plants because the first products formed are 3 carbon molecules.

We cannot overstate the importance of the enzyme RuBP carboxylase in the fixation of carbon. This enzyme makes up anywhere from 50 to 70 percent of the total protein found in leaves. Without this enzyme catalyzing the first reaction no carbon fixation would occur. RuBP carboxylase can also function as an oxygenase, meaning it can catalyze the reaction between RuBP and oxygen. When the enzyme functions as an oxygenase no carbon is fixed and CO2 is released in a process known as photorespiration. C3 plants actually lose anywhere from 20 to 40 percent of their fixed carbon through photorespiration. In summary, RuBP carboxylase can also function as an oxygenase. In other words carbon dioxide and oxygen compete for the same enzyme. If oxygen in the air is lowered photorespiration decreases. If carbon dioxide in the air increases photorespiration decreases.

Other plants (including many important crop species, weeds, but no temperate tree species) have an additional pathway for carbon fixation. This pathway initially incorporates CO2 into 4 carbon acids using the enzyme phosphoenol pyruvate (PEP) carboxylase. The PEP carboxylase enzyme is more efficient at capturing CO2 than is RuBP carboxylase so it can function at lower concentrations of CO2. Plants that utilize PEP carboxylase as the first step in carbon fixation are known as C4 plants. The PEPcarboxylase is located in the mesophyll cells of C4 plants. Mesophyll cells are located towards the outsides of the leaves so they are in a location to quickly grab up CO2 which enters the leaves through the stomata.

C4 plants contain RuBP carboxylase and perform the Calvin cycle as well. The PEP carboxylase simply functions as a very efficient mechanism to capture CO2. Following the initial capture of CO2 into 4 carbon acids they are transported to specialized cells called bundle sheath cells. The bundle sheath cells surround the vascular bundles (leaf veins) in the leaves. In the bundle sheath cells, the 4 carbon acid is split to a 3 carbon compound and CO2. This CO2 is then fixed by the Calvin cycle. In a sense the PEPcarboxylase functions as a highly efficient CO2 pump which captures carbon and moves it into the bundle sheath cells where it is incorporated into sugars by the Calvin cycle. Because of this unique system, C4 plants have little photorespiration resulting in generally higher photosynthetic rate. Any CO2 that may be released by photorespiration in the bundle sheath cells would quickly be recaptured by PEPcarboxylase in the surrounding mesophyll cells.

As has already been discussed, light is the source of energy for photosynthesis and therefore is a major factor which influences the rate of photosynthesis. As light is initially increased the rate of photosynthesis increases. This continues up to the “light saturation point”. At the light saturation point further increases in light results in little or no further increase in photosynthesis. As light becomes more and more limiting plants will eventually reach what is called the “light compensation point”. At this point the rate of net photosynthesis is zero. Below this point more respiration is occurring than photosynthesis, above this point more photosynthesis occurs than does respiration. If a plant stays near the light compensation point little or no growth can occur and the plant will likely die. When there is no light, only dark respiration occurs.

The relationship between light and photosynthesis can most easily be seen by examining a "light response curve".

A portable light and infrared gas analyzer may be used to measure the light response of photosynthesis in loblolly pine needles.  The infrared gas analyzer measures CO2 depletion in a closed chamber.

The above leaf cuvette (chamber) contains a LED light source capable of producing light saturation levels well maintaining a constant temperature in the chamber.

Individual leaves on a tree differ in their response to light.  Within a tree canopy, leaves in the lower crown are known a shade leaves, since they developed at lower levels of light.  Leaves in the upper canopy are conversely called  sun leaves.   There are a number of morphological and physiological difference in sun and shade foliage. Shade leaves typically are thinner and have more surface area to capture light more efficiently. Sun leaves on the other hand are thicker and have less surface area.

Although broadleaf trees show some of the more conspicuous morphological differences in response to sunlight, development and display of conifer needles will also change in response to the light environment. Balsam fir (Abies balsamea) needles formed in the shade will become thinner, and they will orient themselves in a flatter almost two-dimensional plane with much less overlap. This orientation obviously improves light capture in low light. Needles formed in the sun will be thicker, and they will orient themselves nearly all around the twig (like a bottle brush), with some even pointing almost straight up.

Physiologically, shade leaves light saturate at lower light levels and have lower maximum photosynthesis rates.  Shade leaves also have lower light compensation points which allows them to maintain positive rates of photosynthesis at lower light levels.  Shade leaves may also have a greater quantum efficiency.  Quantum efficiency is the initial rate of increase in photosynthesis with increasing light.

Taking all these factors into account means that shade leaves will have positive photosynthesis at lower light levels, will increase their rates rapidly with an increase in light, but will saturate at low levels.  They additionally have more surface area per unit weight (meaning they spread out).  All these changes allow shade foliage to function more efficiently in low light.

Tree species differ in their response to light.  Some tree are known to be more shade tolerant than others. That is they are better suited for growing at lower light levels.  Shade trees will often show photosynthetic characteristics similar to shade foliage.  Shade intolerant trees grow poorly and often die quickly when grown in the shade.

Since carbon dioxide is directly involved in photosynthesis as a substrate it is not to suprising that photosynthesis shows a strong response to carbon dioxide concentration.  All tree species increase their photosynthetic rate when carbon dioxide is increased in the air.  In fact, photosynthesis in trees which are well watered, exposed to sufficient light and nutrients  is largely limited by the low concentration of CO2 in the air.  Our research at Virginia Tech for a wide range of tree species typically has found  a 2-fold or greater increase in photosynthesis with a doubling of CO2. Currently the concentration of carbon dioxide in the air is about 380 parts per million or 0.035 percent by volume.  This concentration, however, is rising and is expected by scientists to reach 600 or more ppm by the middle of the next century.

This increasing trend in CO2 concentration has stimulated a large amount of research on the effects of CO2 on tree growth and photosynthesis.  The majority of studies find increased photosynthesis and growth when plants are grown in elevated CO2.  The greatest effects are found when adequate water and nutrients are supplied but even when these factors are limited, some enhancement in growth is found.  Although long-term effects on forest growth are less certain, more and more  scientists are concluding that forest productivity will be increased in a future, elevated CO2 environment.  The chambers to the left were used to study the effects of increased CO2 on pine and sweetgum growth.

Shown here is atmospheric carbon dioxide data collected from the Mauna Loa observatory in Hawaii. Note the steady rise in carbon dioxide with time largely due to the burning of fosil fuels. Also interesting is the consistent rise and fall within each year. The decrease each year is coincident with the growing season in the northern hemisphere. This occurs because the northern hemisphere has more landmass than the southern hemisphere and the vegetation draws down the carbon dioxide level.

Here is a chronosequence of the earth "breathing" that was developed from satellite imagery. Notice the northern hemisphere greening up and as it does the carbon dioxide level (red graph) falling. Carbon dioxide again begins to increase as the northern hemisphere vegetaion loses its foliage in the winter months.

This increasing trend in CO2 concentration has stimulated a large amount of research on the effects of CO2 on tree growth and photosynthesis. The majority of studies find increased photosynthesis and growth when plants are grown in elevated CO2. The greatest effects are found when adequate water and nutrients are supplied but even when these factors are limited, some enhancement in growth is found. Although long-term effects on forest growth are less certain, more and more scientists are concluding that forest productivity will be increased in a future, elevated CO2 environment. The chambers to the left were used to study the effects of increased CO2 on pine and sweetgum growth.

Another large question about increasing global CO2 levels involves the effect on interspecific competition. These large open-top chambers helped to answer this question. Mixed "stands" of loblolly pine and sweetgum were raised in large boxes in these chambers. Coincidentally, results indicate that competitive interaction between loblolly pine and sweetgum probably will not shift under high CO2 levels.

Here, entire stands of loblolly pine are being exposed to elevated CO2 at the Duke Forest in North Carolina. In this experiment, CO2 is emitted out of rings of standing pipes directly into the stand of trees. Researchers call this "Free Air Carbon dioxide Enrichment" (FACE). This represents the most natural of experimental conditions and entire ecosystem responses can be evaluated. However, very high costs discourage large sample sizes.

Water availability has a strong influence on photosynthesis in tree species. Both excesses (flooding) and deficits (drought) in water decrease rates of photosynthesis. For most of our temperate forests, a lack of water is the major cause of decreased productivity. Forest trees have the dilemma of avoiding water loss out of their leaves but at the same time requiring CO2 for photosynthesis. Leaves are equipped with many sophisticated mechanisms to avoid water loss, but at the same time these mechanisms prevent the uptake of CO2 into the leaves. Therefore plants open their stomata which allows CO2 to enter, but at the same time allows water to gush out of the leaves at a rapid rate. As a result trees often experience water deficits leading to reductions in photosynthesis and growth.

A typical response to water availability shows a region where photosynthesis remains high followed by a rapid reduction in photosynthesis as water becomes scarce. Eventually photosynthesis reaches zero or even becomes negative.

Why does photosynthesis decrease when trees experience water deficits? Often the first answer given to such a question is that since water is required for photosynthesis a shortage of water will result in less photosynthesis. Actually the amounts of water required for photosynthesis is so small that this direct effect is not a contributing factor to the decrease. There are two reason why photosynthesis decreases during a water deficit. These two reason are often lumped under the terms stomatal and non-stomatal effects.

Stomatal effects refer to the fact that during a drought trees often respond by closing their stomata. This of course results in a reduction in water loss which is good. However, closure of the stomates results in less CO2 uptake into the plant reducing photosynthesis. The second reason, non-stomatal effects, refer to general internal problems which occur during water stress which limit photosynthesis. Even if carbon dioxide uptake into the leaf is not limited during a drought typical plants still experience a decrease in photosynthesis. These non-stomatal effects can be due to several factors such as chloroplast disruption, decreased enzymatic activity, and decreased chlorophyll content.

To simplify lets draw an analogy between carbon fixation (photosynthesis) and the production of automobiles. Stomatal limitations to photosynthesis would be equivalent to disruption in the flow of steel into the automobile plant. Non-stomatal limitations would be the equivalent of machines breaking down inside the factory resulting in fewer cars being produced. It is easy to image the machines breaking down with drought. The cell on the left is hydrated and functioning efficiently. The cell on the right is very dry.

Air pollutants can have a dramatic impact on plant growth. In large quantities, such as directly downwind of smelters, entire forest communities have been killed. In the past, this type of large dose, “point source pollution” was common in certain parts of the country. With new technologies and environmental legislation, this type of destruction no longer takes place. More recently, attention has turned to wide spread, “non-point source pollution” effects on tree growth.

Air Pollutants can travel long distances from their source.  During this transport, secondary pollutants such as acid rain and ozone are produced.  Tree foliage may act as a filter, concentrating the pollution.

One of the most phytotoxic and wide spread air pollutants is ozone (O3). Ozone is formed by a series of reactions involving oxygen, hydrocarbons, nitrous oxides and sunlight. Hydrocarbons and nitrous oxides are emitted from various sources with automobiles and electric generating facilities as primary contributors. Ozone and its precursor can be transported far from their original sources so that even vegetation in remote areas can be impacted. Ozone is a very reactive chemical and can be particularly disruptive to plant membranes.

On white pine ozone damage is usually expressed as tip burn.

Many studies have shown that ozone has the potential to reduce photosynthesis and growth in plants. Decreased photosynthesis and growth is particularly large in annual crop plants but less dramatic in tree species. A very large study involving many scientists recently evaluated the impact of ozone on southern pines. They concluded that at present levels of ozone little effect was occurring to growth and photosynthesis; however, at artificially increased ozone levels, both photosynthesis and growth was decreased. A particular concern at high levels of ozone was increased rates of leaf abscission which occurred in most pine species. In the long term this could result in large losses in forest productivity. It is important to remember that since trees are so long lived that small negative effects could accumulate to large effects over time.

Ozone damage on tulip- poplar appears as dark stipples.

Tree seedlings can be exposed to controlled levels of air pollutants through the use of Continuously Stirred Tank Reactors (CSTR's).  Here Fraser fir seedlings are being exposed to ozone.

Many other pollutants, including sulfur dioxide, fluoride and heavy metals have been documented to suppress growth and photosynthesis in tree species. The effect of acidic precipitation which received significant attention in the 1980’s is less certain. Although the popular media concluded that acidic precipitation was wrecking havoc on the forests of the U.S., scientists were much less certain. Few studies were able to document a negative impact, with many finding positive responses from the rain. Acidic mist and clouds, which are often more acidic than rain, have been shown to reduce the ability of red spruce to survive winter temperatures. This could in fact be related to the decline in red spruce forest in the northeastern U.S. which has occurred in recent years.

Deficiencies in nutrients often cause reductions in photosynthetic rates. There are documented examples of deficiencies for most every “macro” and micronutirent. However, two of the most common deficiencies found in forests are due to nitrogen and phosphorus.

Forests are more often limited by nitrogen than any other nutrient. Low nitrogen levels will result in reduced chlorophyll formation and chlorotic (yellow) leaves. However, photosynthesis can be reduced even before visible symptoms occur. Leaf area production is particularly sensitive to nitrogen. Some studies have found leaf area to increase 50 % following application of nitrogen. The increased leaf area results in increased photosynthate production and increased growth. Forest managers take advantage of this effect and, when needed, will often fertilize plantations with nitrogen.

The white pine twig on the right was grown on a nitrogen deficient site.

Nitrogen fertilization typically occurs when the crowns of trees start to touch each other (crown closure). Since nitrogen quickly moves through the soil, forest managers apply it when the trees are larger to be sure it is quickly captured.

Phosphorus levels can also be highly correlated with photosynthetic rates and growth. In certain parts of the country, particularly the Coastal Plain of the southeastern U.S., phosphorus is often limiting to pine growth. On some of these sites growth increases can be very dramatic.

Growth and maintenance of living organisms cannot occur without the process of respiration.  Respiration is the oxidation of food (carbohydrates) which results in the release of energy in the form of ATP.  Respiration occurs in the cytoplasm and mitochondria in a controlled step-by-step fashion.  It produces not only ATP but also a range of intermediate carbon compounds which can be used as building blocks for growth.  A summary equation for respiration can be written as:

                             C6H12O6 + 6O2 ------->  6CO2 + 6H2O + ATP

The two major steps of respiration are glycolysis which occurs in the cytoplasm and the Krebs cycle which occurs in the mitochondria. Glycolysis in simple terms is the breaking down of large carbohydrates into smaller carbon units. From glycolysis, these smaller carbon units enter the Krebs cycle where they are broken down into CO2 and in the process reducing power in the form of NADH and FADH2 are released. This reducing power then enters an electron transport chain where ATP is formed and water is generated. This electron transport chain is analogous to the electron transport chain of photosynthesis. In photosynthesis water is split and reducing power is generated. In respiration, reducing power is used and water is formed. The two electron transport chains basically run in opposite directions.

Total respiration can be separated into growth and maintenance respiration. Growth respiration is that which is used for the synthesis of new plant material. Maintenance respiration is that which is used to keep existing tissue alive, functioning and healthy.

Growth respiration is highly correlated to the total growth of the plant. Rapid growth results in high levels of growth respiration (actually high growth respiration is responsible for the high growth rate). Maintenance respiration is influenced by environmental stress on the tree. If stress levels are high and cells are being damaged, maintenance respiration rates will increase. The proportion of respiration in growth and maintenance is highly variable.

This CO2 analyzer measures the rate of CO2 production by the bole (the respiration rate).

Bole respiration rates can be measured by placing small chambers on trees (right). Precise measurements of radial growth are obtained by reading the digital dendrometer (above).

Generally when trees are young and growing rapidly, seasonal growth respiration is higher than maintenance respiration. As a tree ages, maintenance respiration increases due to the ever increasing mass of living tissue. At the same time growth is slowing. As a result the majority of respiration is being spent on maintenance. Maintenance respiration can also be quite high in tree seedlings being held in cold storage. Seedling weight during cold storage can actually be reduced as a result of this maintenance respiration. If storage carbohydrates become too depleted, survival of these plants can be decreased.

For many years, scientists have measured photosynthesis - with machines like this cuvette to the left - with the hope of predicting future growth. However, these efforts were never very successful. It is true that when photosynthesis is greatly impaired, growth declines. However, there appears to be a wide range of photosynthetic rates which do not appear to be related to growth. In fact there are situations where growth appears to control photosynthesis. It is known that apple trees with fruit present have higher photosynthetic rates than when the fruit is removed. Similarly, when root growth is stimulated photosynthetic rates increase. In other studies investigating differences in growth among seed sources, the smallest seed sources had the highest photosynthetic rates. How can this be if photosynthetic rates control growth?

Although photosynthesis is responsible for all the carbohydrate production in a tree, respiration rates also need to be known before growth can be estimated. If maintenance respiration rates are very high, all the carbohydrates produced by photosynthesis can be burned up with no growth occurring. In fact if respiration rates are high and exceeding photosynthesis the tree will eventually die.

What is important is the overall carbon balance of the tree. Photosynthesis is responsible for carbon inputs or deposits. Photorespiration, growth respiration and maintenance respiration are responsible for carbon outputs or withdraws. We can actually track where carbon molecules go after they enter a tree. Here radioactive carbon is being applied to a leaf.

In simple terms, when inputs exceed outputs the tree grows. Forest managers manipulate stands by thinning, fertilizing and other cultural manipulations to maximize carbon gains. Pollutants, insect attacks and drought all reduce carbon gains. To visualize the utilization of carbon in a tree, try these simple carbon budget exercises.