Posts Tagged ‘global warming’

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.

HowPlantsWork © 2008-2011 All Rights Reserved.

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