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Archive for the ‘Photosynthesis’ Category

garden.jpgYES! Here’s How (And Why):

Although “Global Warming” is to some people a controversial subject, the one thing that’s not controversial is that the level of atmospheric CO2 has significantly increased in the past 100 years and will likely continue to increase – at least until humans stop burning fossil fuels. (We’ve previously visited this subject on a number of occasions, here and here, for example.)

OK, so atmospheric CO2 is at historically very high levels and is going even higher in the decades to come. How will this likely affect plants?

As previously discussed, since green plants use CO2 as the carbon source in photosynthesis, they will probably do more photosynthesis, i.e., produce more biomass.

But as is often the case with things biological, it’s not quite as simple as that.

Fertilize More, Water Less

For your garden plants to take full advantage of this high CO2 world, you will probably need to add more nitrogen fertilizer, but you may have to water less often. Here’s why.

For optimal plant growth, plants need sufficient amounts of carbon and nitrogen and water.

In a high CO2 world, plants will have a sufficient carbon source. But if the availability of nitrogen is limited, then plant growth will be limited. Therefore, to fully take advantage of a high CO2 world, your garden plants will need to have sufficient amounts of nitrogen (N). In most cases, nitrogen is available to plants in the form of nitrate (NO3) in the soil. So to ensure your plants thrive in a high CO2 world, add plenty of compost or nitrogen-containing fertilizer.

Plants in a high CO2 world will also use water more efficiently. This is because the stomates in the leaves need to open less to obtain sufficient amounts of CO2. This is good, because then the plant transpires less water. The result, in general, is that plants will use less water for a given amount of biomass production in a high CO2 world.

corn.jpgDon’t Plant Corn

Not all plants will benefit from a high CO2 world.

So-called C-4 plants already use CO2 very efficiently. Consequently, their photosynthesis will not be significantly improved with increased amounts of atmospheric CO2.

Corn or maize is a classic C-4 plant.

Other C-4 plants include sugarcane, sorghum, and so-called ”warm season” grasses.

Other cereals such as wheat, barley and oats are not C-4 plants — they are so-called C-3 plants — and should benefit from a high CO2 world.

Bottom line: People on this planet show no signs of throttling back their use of fossil fuels. On the contrary, the production of CO2 from the burning of fossil fuels, especially coal, will likely increase in the coming years.

So, it makes sense to prepare for a high CO2 (and probably warmer) world by learning more about how plants will likely respond to such changes in their environment.

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First, What’s a C4 Plant?

I could answer this question here, but instead I’ll refer you to an excellent introduction provided by Dr. Colin Osborne. Or you can read the Wikipedia entry. (I’ll wait until you come back….)

OK….now you know that C4 plants – such as the crops maize, sorghum, and sugarcane – add a preliminary step to regular old C3 photosynthesis in order to increase the effective CO2 concentration for the enzyme RuBisCo. This results in much less photorespiration in C4 plants, which significantly increases their photosynthetic efficiency. Thus, in environments that promote photorespiration (e.g., hot, arid, and/or saline), C4 plants seemingly have a distinct selective advantage over most C3 plants.

Where Did C4 plants Come From?

Although C3 land plants have existed for nearly 500 million years, C4 plants didn’t arise until about 25 to 35 million years ago.

Why?

It’s probably because when, for a number of reasons, Earth’s atmospheric CO2 decreased to the point where photorespiration – which reduces photosynthetic efficiency – became a significant issue for plants, especially in hot, arid environments.

grassland.jpgThis limitation to photosynthetic productivity was reduced through the convergent evolution of C4 photosynthesis in nearly 50 independent flowering plant lineages.

This happened independently so many times likely because many C3 plants may already have had C4-type photosynthesis occurring in their stems (e.g., see Ref. 1 below).

The first C4 plants were probably grasses (monocots), followed several millions of years later by C4 dicots. Today, grasses represent about 2/3 of the roughly 7,500 species of C4 plants extant, with the rest split about evenly between dicots and sedges.

Despite the fact that C4 plants make up only about 3% of plant species, they account for nearly 25% of terrestrial photosynthesis (see Refs. 2 & 3 below). “C4 grasses and sedges dominate nearly all grasslands in the tropics, subtropics and warm temperate zones, and are major representatives of arid landscapes from the temperate zones to the tropics.” (from: Ref. 2)

The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed.” (from: Ref. 3)

Considering the Increasing Levels of Atmospheric CO2, Whither C4 Plants?

smokestack.jpg Fast forward to today, with the increasing levels of atmospheric carbon dioxide, thanks mainly to the burning of fossil fuels.

Are C4 plants losing their CO2 advantage over C3 plants? And with atmospheric CO2 levels projected to double or even triple within the next 100 years, could C4 plants eventually disappear from the landscape? And what about C4 crop plants such as maize that constitute a major source of food and fuel?

Though C3 crop plants may benefit from increased atmospheric CO2, C4 crop plants will likely not benefit much, if at all. (e.g., see Ref. 4)

And what about native plant communities?

Although there is some evidence that increased CO2 may promote the displacement of some C4 grasses by C3 dicots on some rangelands, a great deal of uncertainty currently exists regarding the effects of CO2 alone. This is because of the multiple environmental effects (such as increased heat and drought) that accompany increasing levels of atmospheric CO2. (e.g., see Ref. 5)

Bottom line: Though C4 plants likely arose as a result of decreased levels of atmospheric CO2, their fate is very uncertain in the face of the increasing levels of CO2 that will likely occur in the centuries to come.

References

1. Hibberd, J.M. and Quick, W.P. (2002) “Characteristics of C4 photosynthesis in stems and petioles of C3 flowering plants.” Nature vol. 415, pp.451-454. (Abstract)

2. Sage, R.F. (2004) “The evolution of C4 photosynthesis.” New Phytologist vol. 161, pp. 341–370. (Abstract)

3. Edwards, E.J., Osborne, C.P., Strömberg, C.A.E., Smith, S.A., and C4 Grasses Consortium. (2010) “The origins of C4 grasslands: Integrating evolutionary and ecosystem science.” Science vol. 328, pp. 587-591. (Abstract)

4. Leakey, A.D.B. (2009) “Rising atmospheric carbon dioxide concentration and the future of C4 crops for food and fuel.” Proceedings of the Royal Society B vol. 276, pp. 2333-2343. (Abstract)

5. Bradley, B.A., Blumenthal, D.A., Wilcove, D.S., and Ziska, L.H. (2010) “Predicting plant invasions in an era of global change.” Trends in Ecology & Evolution vol. 25, pp. 310-318. (Abstract)

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3926153221_3bdc08a53a_m.jpgThe world’s most abundant and most important enzyme is RuBisCo.

It’s the most abundant because it’s present in relatively large quantities in every photosynthetic organism on the planet – from microscopic cyanobacteria and phytoplankton in the oceans to the leaves of giant-sized trees in the tropics.

It’s also the most important enzyme on Earth because it catalyzes the first step in the photosynthetic conversion of CO2 into sugars (a.k.a., the Calvin cycle). Indeed, all the organic carbon in the
biosphere is ultimately derived from the CO2 that RuBisCo captures from the atmosphere.

Because of its primary role in photosynthesis, the enzymatic efficiency of RuBisCo has a major impact on plant productivity. It turns out, however, that RuBisCo is a relatively inefficient enzyme and typically is the chief rate-limiting factor in agricultural productivity.

One reason RuBisCo is so inefficient is that it can react with O2 instead of CO2 much of the time. Another reason is that the current levels of atmospheric CO2 are roughly half the concentration required for RuBisCo to run at top speed. (Scientists hypothesize that the reason for these problems is that RuBisCo first evolved in cyanobacteria 3 billion years ago, when there was little atmospheric O2 and much higher levels of CO2.)

rubisco1.jpgThis is why scientists have tried to use the genetic engineering of RuBisCo to improve photosynthetic efficiency.

But a major obstacle toward this goal has been scientists’ inability to reconstitute functioning RuBisCo in vitro, that is, in a test tube.

This is mainly because the active form of RuBisCo consists of 16 proteins. (e.g., see image on right)

But a recent report in the scientific journal Nature may represent a major step toward rebuilding RuBisCo.

Reassembling RuBisCo

According to the leader of this research group at the Max Planck Institute of Biochemistry in Martinsried, Germany, Dr. Manajit Hayer-Hartl, the keys to the in vitro assembly of RuBisCo are chaperone proteins.

Chaperone proteins facilitate the correct 3D folding of newly synthesized proteins, which is critical for the optimal enzyme activity. According to Dr. Hayer-Hartl, “With 16 subunits like those of Rubisco, the risk is very high that wrong parts of the protein clump together and form useless aggregates” (1).

The next goal for this research group is to genetically modify the genes coding for the RuBisCo proteins so as to minimize binding to O2 and maximize the reaction with CO2.

“Because the modified Rubisco is predicted to absorb carbon dioxide from the atmosphere more effectively,” says Manajit Hayer-Hartl, “it would enhance crop yields and could also be interesting for climate protection.” (1)

Reference

(1) For more information, see the Press Release from the Max Planck Institute of Biochemistry (PDF)

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881400078_5dd598fbbf.jpgArticles about photosynthesis in the popular press or online often make me cringe.

Why?

Because sometimes they lead people to think that the oxygen (O2) produced by photosynthesis is derived from carbon dioxide (CO2).

Some even further compound their mistake by stating that plants actually convert CO2 into O2 at night!

Ack!

This is simply NOT true!!

Please allow me to explain…

The Oxygen You Breathe Comes from Water

Yes, that’s correct, water… H2O

chloroplastsfigure1.jpgHere are how, where, and when this works in green plants:

How: Photosynthesis is basically a two-step process, and the first step is where water is converted into oxygen.

The first step directly requires light energy, which is captured chiefly by the photosynthetic pigments called chlorophyll. The chlorophyll converts light energy (photons) into chemical energy, in the form of high-energy electrons.

This chemical energy is used in the photosynthetic reaction centers to split 2 water molecules, producing 4 electrons, 4 protons, and 2 oxygen atoms, which combine to form oxygen gas (O2).

2H20 –> 4 e + 4 H+ + O2

Where: In green plants, photosynthesis occurs in chloroplasts, about two to four dozen of which float around in the cytoplasm of some plant cells.

2927613566_5dd518d2d7.jpg

The first step, described above, takes place in the thylakoid membranes (see Figure 1 above).

When: Since the splitting of water to form oxygen requires light energy, this only occurs naturally during the daytime.

Where Does the CO2 Come In?

The chemical energy captured in step one above is used in step two of photosynthesis, that is, to convert CO2 into carbohydrates (sugars). This is called carbon fixation, a.k.a., the Calvin cycle, which takes place in the chloroplast stroma. (see Figure 1 above)

What is the scientific evidence that O2 isn’t derived from CO2 in photosynthesis?

Well, one way to test this is to use water or CO2 containing the radionuclide, a.k.a., radioactive isotope, of oxygen (e.g., oxygen-18 = O18) in photosynthesis and see which one, H2O18 or CO218, produces radioactive O218. Turns out, it’s the water.

Cyanobacteria, Green Algae and Plants All Do This

All of the photosynthetic organisms – plants, green algae (e.g., phytoplankton in the oceans), and cyanobacteria – that use water as an electron source do this.

So, where does the oxygen you enjoy breathing mostly come from?

For a probable answer, see here.

Bottom line: Green plants DO NOT convert carbon dioxide (CO2) into oxygen (O2). The oxygen comes from water. Green plants DO, however, convert atmospheric CO2 into sugars. So, the oxygen atoms in the CO2 wind up in the sugars (e.g., glucose = C6H12O6).

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192742837_e99d0c1a40.jpgAs previously mentioned….if most climatologists are correct, then parts of the Earth’s surface may experience increasing episodes of heat and drought as a result of global “weirding”. (see here and here for recent examples)

Some of the effects of heat on photosynthesis was considered in part 1 of this post.

But what about drought (a.k.a., long term water stress)?

How does the lack of water affect plant photosynthesis?

When plants lose more water than they can take up from the soil, they become water stressed.

Short-term or diurnal water stress can often be manifested in plants on hot, dry afternoons by drooping or flaccid leaves.

Long-term water stress may occur over days or weeks or longer. Such drought conditions certainly inhibit the growth of plants. But such conditions may even inhibit the most critical process in plants, namely, photosynthesis.

The Stomata Are The Keystoma1.jpg

What also may be happening to water-stressed leaves can not be observed without a microscope. That is, the small pores on the leaves called stomata that allow for leaf/air gas exchange may be closed.

This stomatal closure in response to water stress is often triggered by the plant hormone abscisic acid (ABA). In many plants ABA is produced in the leaves in response to water stress.

When plants close their stomata to conserve water, then they effectively cut off the main supply of CO2 for photosynthesis.

Interestingly, as atmospheric CO2 increases from the continued burning of fossil fuels, this may partially compensate for the inhibitory effects of water deficits on plant photosynthesis. (see here for more on this)

Drought + Sunlight May Also Damage A Key Photosynthetic Enzyme

Research on the effects of water stress on photosynthesis has revealed that decreased CO2 availability in bright light leads to formation of reactive oxygen species. These damage the chloroplast ATP synthase, decreasing ATP content and disrupting the photosynthetic Calvin cycle.

Under these circumstances, photosynthesis becomes insensitive to elevated CO2.

Bottom line: Water deficits inhibit photosynthesis by causing stomatal closure and metabolic damage.

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20070621_drought.jpgIf most climatologists are correct, then parts of the Earth’s surface may experience increasing episodes of heat and drought as a result of global “weirding”. (see here for a current example)

In a previous post in this blog, I briefly introduced the complex subject of how increasing atmospheric CO2 may affect plant photosynthesis.

There is some evidence that suggests that, on a global scale, plant photosynthesis may increase due to the elevated levels of the carbon source for this biochemical process, namely, CO2.

But will any increases in global photosynthesis provided by higher levels of CO2 be lost due to heat and drought resulting from global weirding?

Some recent research suggests that the answer probably is yes. These investigators showed that a 4 degree C (about 8.5o F) increase in temperature above background led to decreased carbon absorption by a simulated grassland.

chloroplastsfigure1.jpgHow Heat Affects Photosynthesis

Among the many biochemical processes in plants, photosynthesis is one of the most sensitive to inhibition by elevated temperatures.

Is it the proteins (enzymes) that catalyze the chemical reactions that comprise photosynthesis that are so heat sensitive or the chloroplasts themselves? Since the chloroplasts consist of intricate lipid bilayer structures, is it likely that even moderately high temperatures “melt”, and thus disrupt, the whole process? It appears that the chief suspect is an enzyme.

The world’s most abundant and most important enzyme is RuBisCo, since it catalyzes the first step in carbon fixation (a.k.a., the Calvin cycle), that is, the conversion of CO2 into sugars in the stroma of chloroplasts. (see Figure 1 above)

2322699374_ee8f2b7711.jpgBut it’s not RuBisCo that is the heat-sensitive culprit, but apparently an associate enzyme called RuBisCo activase. Rubisco activase’s chief role is to serve as an activator and regulator of RuBisCo. Specifically, RuBisCo activase helps convert RuBisCo from its inactive to active state.

Much scientific evidence (see here and here, for example) supports the hypothesis that RuBisCo activase may be the key to the heat sensitivity of plant photosynthesis. Despite this, there is still controversy over the limiting processes controlling photosynthesis at elevated temperature.

Bottom line: Photosynthesis in land plants may both benefit (higher CO2) and suffer (higher temps) as a result of global weirding.

Next time: Part 2, how drought (long-term water stress) effects photosynthesis.

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