13.3 Factor interaction and CO2 enrichment

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Controlled environment and field studies clearly demonstrate that biomass production and yield are increased in both C3 and C4 species in response to CO2 enrichment. A survey of experiments carried out on a large number of species (Figure 13.2) established that doubling atmospheric CO2 concentration from 350 to 700 µmol CO2 mol–1 stimulates vegetative growth by an average of 37%. Although this is a substantial response, it remains lower than is expected, based on short-term CO2 response curves of leaf photosynthesis. While photosynthetic downregulation (Section 13.2.5) can account for much of this, additional field factors such as water, nutrient or light availability may also account for some of this difference between expected and observed growth responses. Furthermore, these factors interact with CO2 concentration and modify the impact of CO2 enrichment. Section 13.3 addresses these issues.

Potential growth rate is set by genetic factors. However, field growth is generally limited by one or more environmental variables such as sunlight, temperature, availability of mineral nutrients (especially nitrogen and phosphorus) and water.

In a linear sequence of events, limitation of an outcome by one component is common. In complex biological systems such as vascular plants, single-factor limitation is unusual. This is partly because plants balance resource acquisition and allocation to optimise several factors which then become colimiting for growth. For example, photosynthesis can be colimited by several factors despite large variations in the rate of assimilation that occur throughout a day as photon irradiance and temperature vary. Consequently, a growth response to relief of one limiting factor, such as atmospheric CO2, will depend not only on genetically determined potential but also on the relative availability of other colimiting environmental factors. Moreover, increased atmospheric CO2 can also alter plant responses to other environmental variables. Clearly, a study of plant growth responses to high CO2 calls for some discussion of genotype × environment interactions.