The exchange of isotopes in water on the plant scale

We have discussed that isotopic signature of CO2, H2O, and plant and soil material contain information about plant physiological processes. This information is used to help understand climate and ecosystem scale processes that affect the exchange of gases between ecosystems and the atmosphere. While it's impossible to list all of the numerous applications of stable isotopes to ecological and global change research, we will highlight some of the most common applications with an emphasis on studies of the interaction between global environmental change and the biosphere.


 It is well known that scientists study tree rings to gain information about the past. Tree rings occur because certain types of trees produce different types of xylem at the beginning and the end of the growing season. The large xylem vessels produced at the beginning of the season result in one visible ring every year. Therefore, tree rings provide a way to obtain plant material (wood) from a certain, known period of time in the past. The variations in stable isotope composition in wood contain information about environmental conditions in the year that the wood was formed.

We have shown that the isotopic composition of the water used in photosynthesis is affected by the water source of the plant and the atmospheric vapor pressure (Equation 2.9).  The oxygen and hydrogen ratios in wood can be accurately predicted by using the model of leaf water isotope composition given above, and correcting for fractionation during the synthesis of cellulose, the complex carbohydrate that forms plant cell walls.  We will discuss the factors that control the isotopic composition of plant source water in the next section.  Atmospheric vapor pressure is a function of air temperature and humidity; therefore researchers look for correlations between D/H or 18O/16O and climate in the tree ring record.

We have also discussed that fractionation of carbon isotopes during photosynthesis is correlated with the ratio of CO2 concentration inside and outside the leaf (ci/ca).  This theoretical relationship has been often demonstrated experimentally.  To further understand the carbon isotope composition of plant material, the following relationship becomes useful:

photosynthesis   =  ca - ci    
   transpiration       1.6v

where v is the water vapor concentration difference outside and inside the leaf, and 1.6 is the ratio of diffusion coefficients, or rates of diffusivity, of water vapor and CO2 molecules in air.

 The ratio of photosynthesis to transpiration is often termed the water use efficiency. Transpiration is a "wasteful" process - only a tiny fraction, on the order of 1%, of the total water absorbed by the root system of the plant is used in photosynthesis.  The rest evaporates into the atmosphere as a necessary consequence of keeping stomates open to absorb CO2. Stomatal conductance must vary to adjust the transpiration rate according to the moisture available in the soil and in the air, or a plant can suffer injury related to water stress during times of drought. Stomatal conductance decreases when humidity is low, the temperature is high, or the soil has dried due to lack of rain. To the extent that these climatic factors influence stomatal conductance and therefore the rate of transpiration, they are recorded in the carbon isotope composition of wood and other plant material. Therefore, the carbon isotope composition of tree rings also contains a record of past changes in climate.

It is interesting to note that 13C/12C in the atmosphere has not remained constant during the past century, a fact that has often been observed in tree rings. d13C of atmosphere is currently about -8‰. Before the beginning of the industrial revolution, the value was about -6.4‰.  Since we began to release CO2 from the combustion of oil and coal as sources of fuel, we have progressively diluted the 13CO2 concentration in the atmosphere. "Fossil fuels" originate from fossilized plant and animal material - they are depleted in 13C because they contain negative carbon isotope ratios.  For instance, fossil fuels that originated from C3 plants contain a d value of about -26‰.


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