Archive Article: February, 2010
It seems obvious enough that plants don't have teeth; but plants must perform the same processes that animals do to grow – they must get carbon, and lots of it. Like all life on Earth, plants are carbon-based. In fact, carbon constitutes the majority of all dried plant biomass, about 45 per cent. We chew food to extract carbon from its source while plants assimilate carbon from the air.
Carbon is the number one element required to grow plant tissue, and it is ‘inhaled’ by plants from the air in the form of CO2 gas. The entire process of assimilating carbon from CO2 into plant tissue while plants are exposed to light is called photosynthesis.
Think of plants as hungry for CO2; they are constantly hoarding carbon at a level that is over 10,000 times the amount found in the air (350 ppm or 0.0035 per cent). Plants grow by assimilating, reducing or fixing CO2 into carbohydrates, but in all that carbon-hoarding and photosynthetic-processing, where are the teeth located?
Figure 1. The Calvin Cycle - where the teeth of photosynthesis are. RuBisCO is the place where carbon enters the food chain. Note the wasteful process called photorespiration occurring when too much oxygen reaches RuBisCO.
RuBisCO
The enzyme known as Ribulose Bisphosphate Carboxylase, or RuBisCO, may be the closest thing plants have to teeth. This enzyme grabs CO2 gas molecules and attaches them to a five carbon sugar molecule, thereby, assimilating more carbon atoms into the Calvin Cycle (see figure one). It fixes a molecule of CO2 to another five carbon sugar (ribulose) making a new six carbon sugar every time it cycles through its enzymatic function. Considering the super low CO2 concentration of 0.0035 per cent present on Earth today, RuBisCO has essentially chewed most of the carbon out of the atmosphere and fixed it into plants.
By grabbing a single CO2 molecule out of the air every 0.3 seconds, every RuBisCO enzyme is at the heart of the photosynthesis machinery. This is a ‘chewing rate’ of around three molecules of CO2 per second, which is quite slow compared to most other enzymes. Because of its abundance and essential role in life, the RuBisCO enzyme also happens to be the most abundant protein on earth. If you could extract the whole biosphere's protein to make a giant Earth-smoothie-protein-shake, RuBisCO alone would make up over 50 per cent of the total protein.
RuBisCO is a large complex molecule (figure two), which in more advanced plants is built from 16 separate sub-units forming an amazingly complex protein structure. It is located in the chlorophyll-containing organelles (chloroplasts) found inside every green plant and algae cell. RuBisCO is a miracle of nature that has been evolving since life began - a true protein nano-machine that drives all life on Earth.
Figure 2. A model for the structure of RuBisCO in chloroplasts from higher plants. RuBisCO consists of eight large (L) and eight small (S) subunits arranged as four dimers. Small subunits are shown in red (only four of the small subunits are seen); large subunits are shown in blue and green, in order to show the boundaries of the dimers.
The enzyme does not restrict its diet to CO2 alone; it will also 'chew' on oxygen molecules, which slows photosynthetic yields. There are eight locations for CO2 to bind to in each RuBisCO enzyme, and these are also able to bind oxygen molecules. For this reason, RuBisCO is located in an oxygen-reduced environment inside plant cells and tucked away inside chloroplasts. Chloroplasts contain dense accumulations of the large enzyme in patches; in some algae there are super complexes of RuBisCO accumulated into structures called pyrenoids. Discoveries with unicellular algae have shown just how localized the teeth can be; in efforts to concentrate CO2 around RuBisCO when CO2 becomes depleted, some algal plants will make RuBisCO only on one side of the chloroplast - meaning only one half has teeth and can actually grab CO2, while the other half actually has no teeth and acts as a reservoir of Calvin Cycle intermediates, shunting the required substrates to the side with teeth. This is a mechanism to compensate for depleted CO2 when O2 becomes a competing substrate, unique to some algae.
But back in the greenhouse, imagine starving a crop of CO2. CO2 in a well sealed greenhouse or grow room will be assimilated starting the moment after dawn when light becomes bright enough. From a typical concentration of 350 ppm CO2 in the air, CO2 can drop to 160 ppm in under two hours. This will stop plant growth by starving their cells' RuBisCO enzymes of their favorite thing to chew on. At this point oxygen can even out-compete the reduced levels of CO2 and cause photorespiration (figure one) - a wasteful use of captured light energy, sending the plant's teeth to chatter and chew on junk food.
Figure 3. Photosynthesis has two parts - the teeth are in the Calvin Cycle.
The Mouth and Teeth Working Together
RuBisCO runs on the energy captured from light in the growing environment. In a discussion about feeding plants CO2, we must remember how photochemistry first makes power for RuBisCO to operate. In addition to needing bright light, photosynthesis requires water, correct temperature, adequate mineral nutrition and proper humidity. If any of these environmental variables change, photochemistry and CO2 assimilation are affected, and likewise, growth rates.
When light shines on green chlorophyll pigments, the energy of PAR photons is captured by plants in a way that is similar to charging a battery; after chlorophyll molecules trap photons of PAR, light energy is stored in high-energy molecules called ATP and NADPH. We need to mention phytochemistry since the teeth of photosynthesis cannot chew at all without being fuelled by ATP and NADPH. The part of photosynthesis involving chlorophyll is called photochemistry, the light-dependant reactions of photosynthesis. But our focus here is on the light-independent part of photosynthesis, also known as the Calvin Cycle, involving RuBisCO - the supposed plant teeth (figure three). The products of photochemistry are a charge of 'reducing power' used up by RuBisCO to drive CO2 fixation. So without a fully charged battery from being in bright light, a plant does not have enough ATP or NADPH to operate RuBisCO and CO2 uptake grinds to a tiny fraction of what is possible.
Figure 4. Anatomy of Stomata - The brown colored parts are the guard cells. Notice the abundant and flaky wax deposits (grey-green).
Imagine clonal lines of plants growing with ample light in an ideal state of fertigation, temperature and humidity; let's say they have moist, fertigated soil, it is noon on a sunny day at 72°F soil and an air temperature with 67 per cent relative humidity. On this perfect day, what is going on that enables plants to uptake CO2 for growth? We've got the image of photosynthetic teeth chewing CO2 out of the air, but we must not forget how there is a 'mouth' that also affects photosynthetic rates in plants - this would be the stomata that are all over green tissues on plants (figure four).
On this perfect, sunny day, at the anatomically minute scale of the plant-cell, imagine wandering through a stomata and drifting deep inside the leaves, where we'd find it is sopping wet, and that plants are always trying to keep themselves inflated with water pressure (plant turgor). Leaves are organs that need to remain open to exchange gasses with the atmosphere, but they must also maintain a saturated, 100 per cent relative humidity inside themselves. This happens using tiny pores called stomata that open and close by action of guard cells (figure four). And, since most of the surface of mature leaves is coated with a wax that seals the leaf from outside air, the main way in and out of a leaf is through the stomata, and plants keep turgid by regulating the stomatal water loss. Water vapor leaving the plant can be conserved by closing the stomata, which happens under drought conditions to prevent wilting - the 'mouths' close and carbon feeding also ceases.
Figure 5. Measuring Stomatal Conductance with an Infrared Gas Analyzer (IRGA) and porometer. When combined into one machine, this kind of device can accurately measure the millimoles of water vapor and CO2 gas leaving and entering a leaf per second. The amount of water vapor detected leaving the leaf is divided by an estimate of stomatal aperture or by leaf area, to give the measurement called stomatal conductance.
In these ideal sunny conditions when the plants are not stressed, the stomata are wide open and CO2 can enter the leaf. This happens by diffusion through the air - while water vapor is pouring out of the plant by transpiring, CO2 is entering through the same stomata. This process is also known as stomatal conductance, measuring how much water vapor, CO2 and O2 can pass through the pores on the leaf surface (figure five). Once CO2 gas has diffused into a leaf through stomata, and into a plant cell, the enzyme RuBisCO immediately fixes it into carbohydrates (CH2Os), and this clears the way for more CO2 to diffuse in.
As nature would have it, light is critical for keeping stomata open, and plants close their stomata in darkness. Anthropomorphizing this image of 'chewing plants' just a little further, stomata in the grow room might be considered billions of tiny mouths that open up immediately, as soon as the lights turn on, allowing carbon to flow into the leaf. Under conditions of high temperature, drought or water stress, stomata will close more and more, gradually starving the plant of carbon. There are evolutionary reasons for these responses by stomata; however, any closure will limit the CO2 that plants can feed off of. Certain plant species have adapted methods of gulping big mouthfuls of CO2 before the heat of the day causes them to close their stomata. These are the C4 and CAM type plants with specialized leaf anatomies and they also have specialized cellular metabolism allowing them to stash CO2, and in some species their teeth are concentrated into specific inner areas of the leaf (see figures six and seven).
Figure 6a (top). Diagram to show how C4 and CAM plants can 'gulp' CO2 and concentrate this for RuBisCO (RuBp Carboxylase' to chew with less interference from oxygen.
Figure 6b (above). Scanning electron microscopy (left panel) and immunological localization of RuBisCO (right panel) in a photosynthetic cell of the single cell C4 species Suaeda aralocaspica (aka sweep weed and seablights) showing the RuBisCO-containing chloroplasts (orange) are restricted to the proximal end of cell (right panel). (Images by Elena Voznesenskaya and Vince Franceschi)
After considering how the stomata and RuBisCO work to get CO2 into a plant to become CH2Os, we can find out what CO2 level is optimal to increase plant growth, but it takes some tweaking of all growing inputs. As mentioned, any condition that closes stomata inhibits growth; so in fact all our growth inputs of high temperatures, excessive CO2 levels, drought and light intensity will all inhibit plant growth just by closing stomata.
Optimal CO2 fertilization requires that a warm meal be delivered with gravy - an increase in mineral fertilization. Like too much food of any kind, excessive CO2 is also a problem to the mouth and teeth of photosynthesis. In plants, a large imbalance in growth occurs from feeding too much CO2 which happens since the molecular shape of the plant hormone called abscissic acid (ABA) and the CO2 molecule's shape are similar. The plant hormone called abscissic acid is the one that controls stomatal closing under changing environmental conditions. Excessive CO2 fools the guard cell into sensing ABA is present and plants close their stomatal openings when CO2 exceeds tolerable amounts. Therefore, fertilizing with too much CO2 can reduce photosynthesis directly by closing the plants' mouths. But there is another response to over-feeding with CO2 that affects the teeth directly- plants make less RuBisCO enzymes as they adapt to long term, excessive CO2 in the grow room. So when growing under too much CO2, plants shut their mouths and actually make fewer photosynthetic teeth to chew and grow with.
Plants cool their tissues by opening their stomata wide, and by allowing more water to evaporate and transpire, which lowers the leaf's temperature. This happens until too much drought occurs, and it becomes a priority to conserve water. However, if raising the temperature to 77°F from 72°F is just enough to cause the plants to keep their stomata open and increase cooling, then this also allows for the most CO2 possible to enter inside the leaf cells where RuBisCO is already assimilating CO2 at its maximum rate. Under these ideal conditions, truly monstrous growth rates can occur - all mouths are agape, hanging wide and the teeth of photosynthesis are churning all available CO2 into carbohydrates at maximum speed, only the atmosphere is the limit!
Figure 7. C4 and Cam plants pictures and diagrams.
References
The Regulation of RuBisCO Activity in Response to Variation in Temperature and Atmospheric CO2 Partial Pressure in Sweet Potato, Plant Physiology 139:979-990 (2005).
The Intracellular Localization of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase in Chlamydomonas reinhardtii, Plant Physiology 116: 1585-1591 (1998).
Handbook of Photosynthesis, By Mohammad Pessarakli. Published by Marcel Dekker, 1996.
