In plants, gas exchange occurs through small pores in the leaf surface called st
ID: 10806 • Letter: I
Question
In plants, gas exchange occurs through small pores in the leaf surface called stomata (an illustration can be found on Book 5 page 113). Stomata can open to allow the exchange of gases and close to reduce water loss from the plant. In a regenerative chamber, the transpiration of water via the stomata represents a major source of clean water. The fossil record suggests that the density of stomata is influenced by the concentration of atmospheric CO2. Stomatal density was found to be high in fossilised ginkgo leaves during the late Permian and again in the Pleistocene, periods when the geological record suggests the levels of atmospheric CO2 were low. Conversely, stomatal densities were low during the Cretaceous; a time when CO2 levels were high and the climate was considerably warmer than it is today.(i) Suggest two possible reasons why stomatal density is linked to the concentration of atmospheric CO2.
(ii) Some scientists claim that plants have the capacity to ameliorate the excesses of climate change by removing more carbon from the atmosphere; based on what the fossil record tells us about stomatal density is this a justifiable view?
Explanation / Answer
1.
Reason 1: stomata are also used for the intake of CO2 by plants, so when [CO2] in the atmosphere is low, plants need to upregulate stomata numbers to take in sufficient CO2 for photosynthesis.
Reason 2: during the Cretaceous period when [CO2] was high but the climate was warmer, plants reduced the number of stomata and kept them closed in order to conserve water, avoid excess evaporation from surfaces.
2. No, this is not justifiable. Although some plants seen here have the ability to regulate response to rise/fall in [CO2], when CO2 rose and warming occurred during the Cretaceous, plants downregulated their stomata numbers. This suggests that plants won't respond to global warming by increasing activity, but will decrease activity to avoid conducting excess photosynthesis.