Leaf Water Evaporative Enrichment

Few studies have compared ¿>180 of leaf water with that of organic material formed at the same time. Wang et al. (1998) presented leaf water and cellulose oxygen isotope compositions from a wide range of plant forms grown in a common garden. Re-plotting their data reveals that cellulose was, on average, 19%o more enriched than leaf water, with a slope for the fitted regression line of 0.48. That is, the range in leaf water <5180 was reduced by about half in cellulose. Their data also show a wide variation about the fitted line (Fig. 2.2). A much closer fit was found by Helliker and Ehleringer (2002a) for leaf water and cellulose of a range of grass species grown under constant conditions and a range of relative humidities. They found that 97% of variation in cellulose <5lsO was explained by variation in leaf water <5180. Less of the leaf water signal was dampened in the well-controlled environment, with a slope between the two of 0.72. Dampening of the leaf water signal in cellulose will be explored in more detail later.

Enrichment of leaf water relative to soil water was first demonstrated by Gonfiantini et al. (1965). A model of evaporative enrichment developed by Craig and Gordon (1965) for enrichment of a free water surface is commonly applied to leaf water, with modifications. This model relates enrichment of leaf water above source water (A18Oe) to the kinetic fractionation during diffusion through the stomata and leaf boundary layer (£k)>

42 40 38

d 36

o 34

8 32

4 6 8 10 12 14 16 18 20 22 <S180 leaf water (%<>)

Figure 2.2 The relationship between measured ¡5lsO of leaf water and <5lsO of cellulose for a range of plants species grown in a common garden. The fitted line has a slope of 0.48, and when <5^80 of leaf water is zero, cellulose ,5lsO is fitted to be 26.1%o(r = 0.405;P = 0.003). Data from Wang et al. (1998).

• Evergreen trees o Shrubs ▼ Conifers v Deciduous trees ■ Other

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c

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® vt ° o v v t v

^^v VT ^ v o vv

^ ■ Wo v

o

c

4 6 8 10 12 14 16 18 20 22 <S180 leaf water (%<>)

i-1-1-1-1-1-r i-1-1-1-1-1-r the proportional depression of water vapor pressure by the heavier H2lsO molecule (e*), the oxygen isotope composition of water vapor relative to source water (A18Ov) and scaled by the ratio of ambient to intercellular water vapor pressure (ea/ei) (Craig and Gordon, 1965; Dongmann et al, 1974; Farquhar and Lloyd, 1993) by:

In well mixed conditions, A18Ov is often close to -£*, so that A18Oe, and therefore A18Op (to some extent) are proportional to 1 - ea/ei. As a result, A18Op should be negatively related to relative humidity (RH). Equation 2.2 also predicts that at constant ea, increasing stomatal conductance will result in less enrichment at the sites of evaporation within leaves. This is because as transpiration rate increases with higher stomatal conductance, both leaf temperature and e, decrease.

The negative relationship predicted between RH and A18Op (or between RH and S18Op, if source water 5lsO is constant) has been observed in a number of studies (e.g., Edwards and Fritz, 1986; Saurer etal, 1997; Roden and Ehleringer, 1999; Barbour and Farquhar, 2000). However, other studies report no evidence of a humidity signal in <518Op (DeNiro and Cooper, 1989).

While Eq. 2.2 predicts general trends in leaf water enrichment quite well, in some cases measured leaf water <5180 was found to be less enriched than that predicted (e.g., Yakir etal, 1989; Flanagan etal, 1994; Wang etal, 1998), while in others leaf water was more enriched than predicted (Bariac et al, 1994a; Wang and Yakir, 1995; Helliker and Ehleringer, 2000). A number of approaches have been taken to address these discrepancies, including: (1) pools of water within a leaf (e.g., Yakir et al, 1990); (2) unenriched water within veins lowering the bulk leaf water enrichment (e.g., Roden and Ehleringer, 1999); (3) a string of interconnected pools of water within the leaf (e.g., Helliker and Ehleringer, 2000); (4) diurnal changes in the evaporative environment and water content of the leaf (e.g., Cernusak et al., 2002); and (5) the ratio of convection of unenriched water towards the sites of evaporation to back diffusion of enrichment from those sites (e.g., Farquhar and Lloyd, 1993). These different approaches are outside the scope of this chapter, so here we consider only the broad implications of the different treatments on A18Op. Effects on A18Op additional to those outlined by Eq. 2.2 are seen in the final two treatments.

Diurnal variation in leaf water enrichment is expected to occur largely as a result of diurnal variation in temperature. Ambient vapor pressure usually remains nearly constant over a diurnal cycle, such that variation in the term ea/ei will be driven mostly by temporal changes in the saturation vapor pressure within the leaf, which will vary exponentially as a function of leaf temperature. Such diurnal variation in leaf water enrichment is commonly observed, with the extent of variation depending on environmental conditions (Dongman et al, 1974; Zundel et al, 1978; Yakir et al, 1990; Walker and Lance, 1991; Bariac et al, 1994b; Cernusak et al, 2002), and is likely to affect A18Op. In expanding leaves, it will cause diurnal variation in the isotopic composition of the water in which new leaf material is forming. In mature leaves acting as carbohydrate sources, it will cause diurnal variation in the AlsO of the carbohydrates exported from the leaf. The A18Op in either case should therefore reflect a photosynthesis-weighted average of the diurnal variation in leaf water enrichment. Because both tissue synthesis and carbohydrate export can continue at night, the net effect is likely to be a reduction in A18Op relative to that which would be predicted if only the midday evaporative conditions were taken into account. Further research is necessary to accurately quantify the effects of diurnal variation in leaf water enrichment on

A Péclet effect, where the convection of unenriched water to the evaporating sites is opposed by backward diffusion of H2 lsO, will have important and testable effects on A18Op. The Péclet effect (Farquhar and Lloyd, 1993) predicts a somewhat reduced response of A18Op to the changes in the external evaporative environment (i.e., changes in RH), but also a somewhat enhanced response to leaf-driven changes in evaporation (i.e., changes in stomatal conductance). In this model the organic molecules exchange with leaf water somewhat less enriched than predicted by Eq. 2.2, and the extent of the difference between the Craig and Gordon-predicted and Péclet-predicted leaf water enrichments increases with increasing transpiration. Under constant humidity, plants control transpiration rate by stomatal aperture. The Craig and Gordon model predicts that AlsO of leaf water (and so A18Op) should decrease slightly with increasing stomatal conductance. Inclusion of a Péclet effect significantly enhances the dependence of AlsO of leaf water on stomatal conductance, as shown in Fig. 2.3. Strong relationships between stomatal conductance and A18Op have been found for cotton leaves grown in a humidity-controlled glasshouse (Barbour and Farquhar, 2000), and for field-grown wheat (Barbour et al, 2000a).

Some indirect evidence in support of the relevance of a Péclet effect to A18Op is presented by Barbour et al (2000b), who found that AlsO of sucrose was negatively related to ea/ei (as predicted by both the simple Craig and Gordon model and the Péclet effect extension), but that the Craig and Gordon prediction of AlsO of sucrose was significantly more sensitive to changes in ea/ei than was observed (as would be the case with a Péclet effect). In this analysis the Craig and Gordon predicted AlsO of sucrose was calculated from Eq. 2.2, plus the fractionation factor between carbonyl oxygen and water (taken to be 27%o).

Figure 2.3 The predicted dependence of AlsO of leaf water on stomatal conductance (gs) using the Craig-Gordon and Peclet models of leaf water enrichment.

Figure 2.3 The predicted dependence of AlsO of leaf water on stomatal conductance (gs) using the Craig-Gordon and Peclet models of leaf water enrichment.

The leaf water oxygen isotope signal is dampened in organic material formed from exported sucrose in other plant parts. This dilution is mainly a result of isotopic exchange of organic molecules with water in the sink cells forming new organic material. This 'sink cell water' may be isotop-ically rather different to source leaf water. For example, Adar et al. (1995) found that water in the new xylem cells of Tamarix jordanis tree stems was about —2.4%o, reflecting soil water, while leaf water of the same plant was +25.2%o. Barbour and Farquhar (2000) suggested that while tree stem xylem cell water may reflect source water <5lsO, other tissue (particularly those closer to the source leaves) could be more like leaf water AlsO, due to unloading of phloem water (at AlsO of leaf water) with phloem sugar. This idea is supported by calculations (Bret-Harte and Silk, 1994) suggesting phloem water could potentially supply 80% of the water required for cell expansion in corn root tips. Bret-Harte and Silk's (1994) calculations mean that the proportion of water in developing cells sourced from the xylem (px) could be as low as 0.2. Barbour and Farquhar (2000) also included in the parameter px the possibilities that water in phloem and xylem may exchange during sucrose transport, and that water in the developing cells may become enriched by transpiration from these cells.

Evidence of a mixture of phloem and xylem water in leaf sink cells was recently presented by Helliker and Ehleringer (2002b), who show that water in the intercalary meristems of Loliurn multiflorum leaves (—13 to —9.6%o) was intermediate between source water ( —16.3%o) and leaf water (—7.3 to 16.3%o). These authors calculated a value for px of 0.62 for

Lolium multiflorum, and a range in px of between 0.50 and 0.62 for ten grass species in a previous study (Helliker and Ehleringer, 2002a).

Further, Cernusak et al (2002) show that Lupinus angustifolius pod and seed water (S180 = 5%o) was somewhat more enriched than stem xylem water (¿lsO = —3%o), but less enriched than leaf water (5lsO of between 0 and 23%o). Water bled from the phloem of pods was between pod and leaf water (<5lsO between 7 and 15%o) and tended to follow diurnal patterns of leaf water enrichment. Recalculating data from Cernusak et al (2002) gives px values of 0.31 for phloem water and 0.69 for pod and seed water at midday. The observation that phloem water is less enriched than leaf water supports the suggestion (Barbour and Farquhar, 2000) that some exchange between phloem and xylem water has occurred.

As described above, organic molecules reflect the water in which they formed due to isotopic exchange between carbonyl oxygen and water (Sternberg et al., 1986). Oxygen atoms in other functional groups, such as hydroxyl, carboxyl, and phosphate groups, are not exchangeable at normal cellular temperature and pH. The exchange of oxygen atoms between water and carbonyl groups is possible due to the formation of a short-lived gem-diol intermediate (Samuel and Silver, 1965), as shown in Fig. 2.4.

At equilibrium oxygen atoms in carbonyl groups are between 25 and 30%o more enriched than the water in which they formed (Sternberg and DeNiro, 1983). Many intermediates in the biochemical pathways leading to synthesis of structural and non-structural carbohydrates contain carbonyl oxygen groups, so the exchange reaction becomes important in determining the <5lsO of plant tissue as a whole. Acetone, with a single exchangeable oxygen, was found to be 28%o more enriched than the water with which it exchanged (Sternberg and DeNiro, 1983). If a substance contains more than one oxygen atom that has gone through a carbonyl group, an average fractionation factor (ewc) is applicable, even though slight differences in fractionation may occur for different oxygen atoms, depending on the proximity of other atoms (Schmidt et al, 2001).

The rate of exchange of carbonyl oxygen varies considerably between molecules, with larger molecules being much slower to reach equilibrium

Figure 2.4 The exchange of oxygen atoms between carbonyl groups and water via a gem-diol intermediate.

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