SUMMARY

The continuing rise of CO2 concentration in the atmosphere since the start of industrialisation, and the associated global warming, pose interesting questions about the response of plants to CO2. However, in the course of evolution, plants have been exposed to a much wider range of CO2 concentrations than we have seen in the last 200 years. It is likely that when photosynthesis evolved, around 3.8 billion years ago, the earth's atmosphere was CO2 rich, just as Mars and Venus are today. In order to understand the adaptability of plants to changing CO2, there are several possible experimental approaches. The presence of CO2 springs in several parts of the world provides a natural laboratory for such studies, with the important advantage that plants growing at such sites will have been exposed to elevated CO2 for many generations.

The CO2 concentration of the earth's atmosphere when photosynthesis began, some 3.8 × 109 years ago, is presumed to have been in the range 90–98%, similar to that found currently on the lifeless sister planets Mars and Venus (Emiliani, 1992; Raven, 1995). This high CO2 atmosphere originated from the outgassing of the planet's crust, which continues today albeit at a diminished rate in volcanic regions of the world.

Photosynthesis in those early days of the earth's history is believed to have been achieved by prokaryote filamentous cyanobacteria, forming crusts as seen now at coastal areas in warm parts of the world. The rates of photosynthesis achieved by these organisms may have been quite high, as a result of the large diffusion gradient from the atmosphere to the sites of carboxylation.