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 and here for recent examples)
Some of the effects of heat on photosynthesis were 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.
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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