Ben Fletcher
Stomata control how the atmosphere affects plants
Stomata are pores in the protective cuticle around
the outside of the aerial parts of land plants. Stomata are the
main link between plants and the atmosphere. They allow carbon
dioxide in for photosynthesis, and let oxygen out. They also
allow water vapour to escape, leading to transpiration.
Stomata are able to respond to a shortage of water in the plant by closing. They also begin to close if carbon dioxide concentrations in the atmosphere are increased, because this means that the plant can take up all the carbon dioxide it needs and lose less water.
Eventually, the plant responds to increases in carbon dioxide by producing fewer stomata on its new leaves. This can be used to predict the carbon dioxide concentration of past atmospheres, by examining the number of stomata on fossil leaves.
High carbon dioxide enhances photosynthesis. In the long-term, it allows plants to adapt and make better use of other resources, such as water and nutrients. In the early colonisation of the land, when water and nutrients may have been hard to obtain, the high carbon dioxide levels could have been crucial in allowing plants to colonise the land. The simple, leafless form of early land plants may also have helped them cope. I will be testing theories like these by growing simple plants, which have not been examined in high carbon dioxide conditions, to see how well they adapt.
Oxygen in the atmosphere interferes with photosynthesis. This is because Rubisco, the enzyme that catalyses the carbon dioxide fixing reaction, can react with either carbon dioxide or oxygen. So, the greater the ratio of oxygen to carbon dioxide in the atmosphere, the less efficient photosynthesis becomes.
This can be detected in fossils, as a change in the discrimination of Rubisco between the two stable forms or isotopes of carbon, the common 12C and the rare 13C. Normally, Rubisco reacts preferably with 12CO2. This leads to a lower proportion of 13C in plant tissue than in the air. However, factors that cause stomata to close reduce this effect, because as the 12CO2 inside the leaf begins to run out, more and more 13CO2 is used in photosynthesis. Therefore, dry conditions or high carbon dioxide:oxygen ratios will be detectible as higher than expected 13C : 12C ratios in plant fossils.
In the carboniferous period, when plants grew in the swamps that eventually formed coal, oxygen levels were high, and this would have affected how plants grew then. By this time, fossils show us that plants were much more diverse and complicated, but few of these plants remain today. To get a better idea of how these plants were affected by these conditions, I will also be growing representatives of these plants in high oxygen levels.
Hence, plant fossils are
a source of information about past changes in the composition
of the atmosphere and how this affected life on land. However,
to get the most information out of the earliest fossils of plants,
it is important to compare them with the plants that most closely
resemble them, because most work so far is on the more complex
and recently evolved plant groups, like flowering plants.
My project home
Stomata and the early evolution of land plants
