Isotope fractionation processes

Carbon isotope ratio

In C₃-plants, the carbon isotope composition of organic material, d¹³C_plant, is approximately related to the ratio of partial pressure of CO₂ inside the leaf, p_i, and the partial pressure of CO₂ in the atmosphere, p_a (Farquhar, O'Leary & Berry 1982):

d¹³C_plant = d¹³Catm - a - (b - a)p_i/p_a
(1)

where d¹³C_atm is the d¹³C of atmospheric CO₂, a is the fractionation occurring due to the diffusion in air (= 4.4‰) and b is the fractionation caused by carboxylation (27‰). This equation has been validated convincingly with on-line techniques (Evans et al. 1986), and has proven useful in numerous studies. Strictly speaking, equation (1) is valid only for the first product of photosynthesis and does not include fractionations caused by biochemical processes later on. For instance, lipids are known to be isotopically lighter than the bulk material, whereas cellulose is heavier. Therefore, the difference between d¹³C in cellulose and in bulk plant material, D_c-p, has to be considered for the inference of p_i/p_a ratios from d¹³C values of cellulose, d¹³C_cell (Marshall & Monserud, 1996):

d¹³C_cell = d¹³C_plant + D_c-p = d¹³C_atm - a - (b - a)p_i/p_a + D_c-p
(1')

D_c-p is approximately 1 to 2‰. This value may dependent on species and may also vary between different cellulose extraction techniques.

Oxygen isotope ratio

In summary, the oxygen isotope composition of organic material is determined by (i) the isotopic composition of the source or soil water, d¹⁸O_s (ii) the enrichment taking place in the leaf water due to the transpiration, resulting in an increased d¹⁸O_l of leaf water compared to d¹⁸O_s, and (iii) biochemical fractionations. The degree of leaf water enrichment is dependent on the ratio of the atmospheric and intercellular vapour pressures (ea and ei, respectively), the isotopic composition of water vapour in the air, d¹⁸O_v, the fractionation due to the change of phase from liquid to vapour, e_e (9.8‰ at 20°C, Majoube 1971), and the kinetic fractionation due to the diffusion of vapour into undersaturated air, e_k. A formula integrating these processes has been developed by Dongmann et al. (1974):

d¹⁸O_l = d¹⁸O_s + e_k + e_e + (d¹⁸O_v - d¹⁸O_s - e_k) e_a/e_i
(2)

e_k (=26.5‰ according to Farquhar et al. 1989) is calculated based on the degree of turbulence in the leaf-boundary and on the ratio of the resistance for diffusion through the stomata to the resistance for diffusion through the leaf-boundary. This value is usually assumed as constant but is actually sensitive to the nature of the boundary layer which is controlled by leaf size and morphology (Buhay, Edwards & Aravena 1996). When water vapour is in isotopic equilibrium with soil water, then equation (2) can be simplified because d¹⁸O_v - d¹⁸O_s = -e_e. This is often the case in European summer climate conditions (Förstel & Hützen, 1983). Equation (2) is treating the leaf water as one well-mixed pool which is a crude approximation only. Water in the chloroplasts can be expected to be less enriched than water directly subject to transpiration (Yakir, DeNiro & Gat 1990b). For high transpiration rates especially, such differences should be important. Accordingly, a dependence of d¹⁸O_l on the transpiration rate has been found, but this effect can not easily be quantified at present and is therefore neglected in equation (2).

The oxygen isotopic composition of organic matter is determined by the oxygen of the water at the site of synthesis. Cellulose is enriched by 27‰ compared to water (DeNiro & Epstein 1981). This enrichment may be termed biochemical fractionation (e_c) and probably is caused by isotopic exchange of carbonyl oxygen atoms of intermediate products of photosynthesis with water (Sternberg, DeNiro & Savidge 1986). This exchange does not only occur in the leaf, but also in the stem during cellulose production (Hill et al. 1995). Therefore, the d¹⁸O signal of leaf water imprinted on the sucrose may be partly lost when sucrose transported to the stem exchanges oxygen with less enriched stem water. It is not known at present what percentage of oxygen atoms undergoes this exchange. Sternberg et al. (1986) estimated a value of 45% based on experiments with cellulose-producing bacteria.

To calculate the d¹⁸O of cellulose in the stem we assume a factor f (where f is in the range from 0 to 1) which summarises the dampening effect of (i) leaf water inhomogeneity and (ii) exchange of oxygen atoms of sucrose with stem water. Using the expression for the leaf water enrichment (eq. 2) we propose the following equation:

d¹⁸O_cell = d¹⁸O_s + f [e_k + e_e + (d¹⁸O_v - d¹⁸O_s - e_k) e_a/e_i] + e_c
(3)

References

Buhay W.M., Edwards T.W.D. & Aravena R.(1996) Evaluating kinetic fractionation factors used for ecologic and paleoclimatic reconstructions from oxygen and hydrogen isotope ratios in plant water and cellulose. Geochimica et Cosmochimica Acta 60, No. 12, 2209-2218.
DeNiro M.J. & Epstein S. (1981) Isotopic composition of cellulose from aquatic organisms. Geochimica et Cosmochimica Acta 45, 1885-1894.
Dongmann G., Nürnberg H.W., Förstel H. & Wagener K. (1974) On the enrichment of H218O in the leaves of transpiring plants. Radiation and Environmental Biophysics 11, 41-52.
Evans J.R., Sharkey T.D., Berry J.A. & Farquhar G.D. (1986) Carbon isotope discrimination measured concurrently with gas exchange to investigate CO2 diffusion in leaves of higher plants. Australian Journal of Plant Physiology 13, 281-292.
Farquhar G.D., O'Leary M.H. & Berry J.A.(1982) On the relationship between carbon isotope discrimination and the intercellular carbon dioxide concentration in leaves. Australian Journal of Plant Physiology 9, 121-137.
Farquhar G.D., Ehleringer J.R. & Hubick K.T. (1989) Carbon isotope discrimination and photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 40, 503-537
Förstel H. & Hützen H. (1983) 18O/16O ratio of water in a local ecosystem as a basis of climate record. In Paleoclimates and Paleowaters: A collection of environmental isotope studies, IAEA, Vienna, 67-81.
Hill S.A., Waterhouse J.S., Field E.M., Switsur V.R. & Ap Rees T. (1995). Rapid recycling of triose phosphates in oak stem tissue. Plant, Cell and Environment 18, 931-936
Majoube M. (1971) Fractionnement en oxygen et en deuterium entre l'eau et sa vapeur. Journal de Chimie et de Physique 68, 1423.
Marshall J.D. & Monserud R.A. (1996) Homeostatic gas-exchange parameters inferred from 13C/12C in tree rings of conifers. Oecologia 105, 13-21.
Saurer M., Aellen K. and Siegwolf R. (1997). Correlating d¹³C and d¹⁸O  in cellulose of trees. Plant, Cell and Environment, 20, 1543-1550 Abstract .
Sternberg L.S.L., DeNiro M.J. & Savidge R.A. (1986) Oxygen isotope exchange between metabolites and water during biochemical reactions leading to cellulose synthesis. Plant Physiology 82, 423-427.
Yakir D., DeNiro M.J. & Gat J.R. (1990b) Natural deuterium and oxygen-18 enrichment in leaf water of cotton plants grown under wet and dry conditions: evidence for water compartmentalization and its dynamics. Plant, Cell and Environment 13, 49-56.
 
 

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