A Plants dilemma!

When to open stomata and photosynthesize When to close stomata and conserve water!?

Consider that the limiting factor to photosynthesis is the availability of CO2 in the atmosphere which currently is present in very dilute concentrations of 0.039% whereas O2 is relatively concentrated at 20%.

Logically, a well hydrated plant would keep stomata open at daytime to allow ready access to atmospheric CO2 while keeping stomata closed at nighttime in order to conserve the CO2 generated by respiration. The large vacuoles of plant cells could be considered botanical scuba tanks permitting the storage of waste CO2 generated by nighttime respiration. O2 is never in limiting supply when stomata are open; however, those same large vacuoles could overcome the problems of laggard diffusion during gas exchange, by sequestering copious waste O2 generated during photosynthesis. Vacuoles can therefore provide a reserve of CO2 for photosynthesis or a reserve of O2 for respiration as need be. Waste not want not!

Ideally, a well hydrated plants default setting would be stomata open in daytime and closed at nighttime. And often this is indeed the case. However, there are thee unfortunate wrinkles complicating this straightforward story:

1. Desiccation

2. Photorespiration

3. Nutritional (specifically Nitrogen) requirements

By definition, photosynthesis happens in light. However when conditions are most light, conditions are also most hot and dry. Plants are most prone to water loss precisely when they open stomata for gas exchange during the day.

Another unanticipated wrinkle is a relic of plant evolution called Photorespiration. Lets start with what is supposed to happen during Photosynthesis. The start of the Calvin Cycle combines CO2 with the phosphorylated 5-carbon sugar ribulose bisphosphate to produce two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO).

From an enzymatic standpoint, the fixation of CO2 by RuBisCO is very inefficient (a high Km); however, whatever RuBisCO lacks in efficiency, it makes up for in quantity! RuBisCO can fairly claim to be the most abundant protein on earth accounting (in some plants) for up to 50% of soluble leaf protein.

RuBisCOs active site can actually bind either CO2 or O2. When photosynthetic algae first arose, the early Earths atmosphere contained little, if any, oxygen. RuBisCO would have functioned very well under these conditions. It was only later, when the concentration of oxygen in the atmosphere increased considerably, did the competitive binding of O2 to RuBisCOs active site pose a problem.

What would that problem be? When RuBisCO binds O2, (instead of CO2) only one 3C molecule of PGA is produced (instead of two) and a toxic 2C molecule called Phosphoglycolate is produced. The plant must rid itself of the Phosphoglycolate involving a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria. Since this alternate pathway requires light and produces CO2, it is called Photorespiration. However, no useful energy is gained from Photorespiration.

Higher temperatures melt the tertiary structure of RuBisCO, rendering it less able to discriminate between CO2 and O2. Meanwhile, warmer temperatures also decrease the solubility of CO2 which poses a problem if CO2 is already in limiting supply. (Remember soda pop is more likely to fizz if it is warm as opposed to cold). CO2 availability is further limited by the constraints of diffusion during gas exchange. Meanwhile relative O2 concentrations accumulate in flagrant excess as a result of Photosystem II located in the very same chloroplast as the Calvin Cycles RuBisCO. When the concentration of CO2 drops below 0.01 percent, O2 will out-compete CO2 at RuBisCOs active site, and no net photosynthesis occurs.

To summarize: RuBisCO catalyzes two different reactions:

adding CO2 to ribulose bisphosphate the carboxylase activity during photosynthesis

adding O2 to ribulose bisphosphate the oxygenase activity during photorespiration

Which of these two reactions predominates would depend on the relative concentrations of O2 and CO2 where:

high CO2, low O2 favors the carboxylase action during photosynthesis,

high O2, low CO2 favors the oxygenase action during photorespiration

The light reactions of photosynthesis liberate oxygen and deplete carbon dioxide. Meanwhile, the availability of soluble carbon dioxide is significantly decreased at higher solvent temperatures.

Therefore,

high light intensities and

high temperatures (above ~ 30C)

favour the oxygenase reaction of Photorespiration over the carboxylase activity of regular Photosynthesis.

In other words, if a plant is to survive the deleterious effects of photorespiration, it needs to avoid both high light and temperature conditions or find other ways of storing CO2.while isolating O2 from RuBisCO.

Lets start with the simplest scenario. When water is abundant and temperatures are relatively low, a plants life is pretty straight forward. No special adaptations are required. Plants that immediately bind CO2 during photosynthesis are referred to as C3 Plants because the molecule 3-phosphoglycerate (see diagram above) has a backbone comprised of three carbon atoms. C3 Plants open their stomata during the daylight hours and as a result cannot survive intense light or heat.

C4 plants only open their stomata during cooler parts of the day. That means they require a store of CO2 for photosynthesis when stomata are closed. C4 plants get their name by storing CO2 as a stable product four-carbon organic compound, usually malate.

The details of the C4 cycle (as cut and pasted from John W. Kimball's excellent site)

After entering through stomata, CO2 diffuses into a mesophyll cell.

Being close to the leaf surface, these cells are exposed to high levels of O2, but

have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle).

Instead the CO2 is inserted into a 3-carbon compound (C3) called phosphoenolpyruvic acid (PEP) forming

the 4-carbon compound oxaloacetic acid (C4).

Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is

transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells

are deep in the leaf so atmospheric oxygen cannot diffuse easily to them;

often have thylakoids with reduced photosystem II complexes (the one that produces O2).

Both of these features keep oxygen levels low.

Here the 4-carbon compound is broken down into

carbon dioxide, which enters the Calvin cycle to form sugars and starch.

pyruvic acid (C3), which is transported back to a mesophyll cell where it is converted back into PEP.

In other words, the C4 pathway minimizes photorespiration by separating the so-called light and dark reaction in different locations of the leaf, thereby isolating O2 from RuBisCO. There is a cost to this strategy: every CO2 molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO. As a result, the C4 pathway uses more energy than the C3 pathway. The C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. For tropical plants, this added energy debt is more than compensated by avoiding the expenditure of over half of photosynthetic carbon to photorespiration.

The separation of the so-called light and dark reaction in different locations of C4 leaves explains the separation of Palisade mesophyll cells from the radially oriented bundle sheath cells surrounding the veins. This unique feature of C4 leaves is referred to as Krantz (i.e. wreath in German) Anatomy.

Krantz AnatomyCAM Plants

(CAM stands for Crassulacean Acid Metabolism because it was first studied in members of the plant family Crassulaceae.) CAM plants are also C4 plants but instead of segregating the so-called light and dark reactions of photosynthesis in different locations of the leaf, these reactions occur instead at different times. CAM Plants are unique by only opening their stomata at night, when at all!

CAM Details - (as cut and pasted from John W. Kimball's excellent site)

At night,

- CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them).

-The CO2 joins with PEP to form the 4-carbon oxaloacetic acid.

-This is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells.

In the morning,

-the stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen).

-The accumulated malic acid leaves the vacuole and is broken down to release CO2.

-The CO2 is taken up into the Calvin (C3) cycle.

In summary, when conditions are extremely dry, CAM plants simply close their stomata both night and day. O2 produced during photosynthesis is recycled for respiration and CO2 produced during respiration is similarly recycled for photosynthesis. Like our own planet, CAM plants represent a closed system in terms of matter and an open system in terms of energy. Note the plant cannot grow while CAM-idling. There are many variations of the C3/C4/CAM theme. For example, there seem to be three versions of CAM: "obligate CAM plants vs. "inducible CAM plants" and CAM-idlers aka "CAM-cycling". There are also different versions of C4 - lets leave all these details for later study in university.