Can Your Garden Benefit From Increasing Atmospheric CO2?

YES! 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.

Don’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.

HowPlantsWork © 2008-2011 All Rights Reserved.

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Less Nutritious Plants as a Result of Higher Atmospheric CO2?

No Controversy Here

Despite the denials of global warming caused by increased atmospheric CO₂ from the scientifically ignorant or from the oil/coal corporations (or from politicians bought or rented by these corporations), there is one thing they can not deny.

The level of atmospheric CO₂ on Earth has been steadily increasing for the past century and will continue to increase. Indeed, the rate of increase may actually be accelerating.

Because plants rely on CO₂ as their carbon source for photosynthesis, how does/will this increased CO₂ affect green plants?

Less Nutritious Plants in a High-CO₂ World?

In a previous post, I briefly discussed the possible effects of higher CO₂ on plants.

A recent report indicates that under high CO₂ conditions some crop plants may produce higher levels of toxins and lower levels of protein, rendering them less nutritious.

The lower levels of protein as a component of plant biomass may be due to a decreased production of the protein RuBisCo in response to higher levels of CO₂. RuBisCo may account for up to 40% of the protein in leaf tissue, for example.

Despite early predictions that higher CO₂ would lead to increased crop yield more recent information suggests otherwise.

Bottom line: Increasing levels of atmospheric CO₂ will be a significant factor affecting plants now and in the future. We need more research into the effects of increased levels of CO₂ on crop plants in order to better prepare for a high-CO₂ world.

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Plants Don’t Convert CO2 into O2

Articles 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

Here 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.

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).

HowPlantsWork © 2008-2011 All Rights Reserved.

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Photosynthesis and Global “Weirding” – How Heat and Drought Affect Photosynthesis (part 2)

As 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 Key

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.

HowPlantsWork © 2008-2011 All Rights Reserved.

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Photosynthesis and Global “Weirding” – How Heat and Drought Affect Photosynthesis (part 1)

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 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 8.5^o F) increase in temperature above background led to decreased carbon absorption by a simulated grassland.

How 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)

But 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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