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Elevated CO2 concentrations - too much of a good thing?
The atmospheric partial pressure of CO2 has been predicted to double by the end of the 21st century. It is widely assumed that increased CO2 concentrations lead to increased rates of photosynthesis and plant growth. However, in reality, the situation is much more complex. This is well illustrated in the paper by Pierce and colleagues from Sheffield, Bangor and Durham, UK (pp. 613-622). They have been working with the grass Poa annua var. vivipara. This is actually a pseudoviviparous variety in which reproductive spikelets revert to the vegetative state, producing photosynthetic plantlets that are shed from the parent and become independently established. This mode of vegetative reproduction is usually regarded as a means of maintaining genotype in populations that are adapted to non-ideal conditions, such as low nutrient availability. The authors have investigated several facets of growth and physiology of this pseudoviviparous grass growing at low nutrient availability typical of its habitat and exposed to ambient and to twice ambient CO2 partial pressures. Acclimation to elevated CO2 reduces photosynthetic capacity and thus the plants do not utilize the increased carbon available. This has been observed in several other species, and for some of these it has been established that the increased CO2 concentrations lead to a depression in the level of expression of genes encoding photosynthetic enzymes. Seed set tends to alleviate this depression because the seeds act as a major sink. However, in Poa annua var. vivipara seeds are not set, and further, the plantlets that are produced suffer the same decline in photosynthetic capacity as the parent plant. Parent plants also tend to senesce earlier, thus shortening the period of connection between the plantlets and the parent. Overall, reproductive efficiency is therefore decreased at the higher CO2 partial pressure: an excess of CO2 combined with low soil nutrient availability is clearly a disadvantageous combination for this pseudoviviparous grass.
j.a.bryant{at}exeter.ac.uk
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