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

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 another post in this blog, I briefly explore the complex subject of how increasing atmospheric CO2 may affect plant photosynthesis.

There is some recent 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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