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. 2000 Jun;123(2):671-80.
doi: 10.1104/pp.123.2.671.

Variation in the oxygen isotope ratio of phloem sap sucrose from castor bean. Evidence in support of the Péclet effect

M M Barbour  1 , U Schurr, B K Henry, S C Wong, G D Farquhar
Affiliations

Variation in the oxygen isotope ratio of phloem sap sucrose from castor bean. Evidence in support of the Péclet effect

M M Barbour et al. Plant Physiol. 2000 Jun.

Abstract

Theory suggests that the level of enrichment of (18)O above source water in plant organic material (Delta) may provide an integrative indicator of control of water loss. However, there are still gaps in our understanding of the processes affecting Delta. One such gap is the observed discrepancy between modeled enrichment of water at the sites of evaporation within the leaf and measured enrichment of the leaf water as a whole (Delta(L)). Farquhar and Lloyd (1993) suggested that this may be caused by a Péclet effect. It is also unclear whether organic material formed in the leaf reflects enrichment of water at the sites of evaporation within the leaf or Delta(L). To investigate this question castor bean (Ricinus communis L.) leaves, still attached to the plant, were sealed into a controlled-environment gas exchange chamber and subjected to a step change in leaf-to-air vapor pressure difference. Sucrose was collected from a cut on the petiole of the leaf in the chamber under equilibrium conditions and every hour for 6 h after the change in leaf-to-air vapor pressure difference. Oxygen isotope composition of sucrose in the phloem sap (Delta(suc)) reflected modeled Delta(L). A model is presented describing Delta(suc) at isotopic steady state, and accounts for 96% of variation in measured Delta(suc). The data strongly support the Péclet effect theory.

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Figures

Figure 1
Figure 1
The measured change in Δsuc over time with a step change in VPd from 16 to 8 mbar.
Figure 2
Figure 2
Change in gas exchange parameters and Δsuc with time for experiment 1. A, VPd; B, ea/ei; C, E; D, gs; E, assimilation rate (A); F, measured Δsuc.
Figure 3
Figure 3
Change in gas exchange parameters and Δsuc with time for experiment 2. A, VPd; B, ea/ei; C, E; D, gs; E, assimilation rate (A); F, measured Δsuc.
Figure 4
Figure 4
Change in gas exchange parameters and Δsuc with time for experiment 3. A, VPd; B, ea/ei; C, E; D, gs; E, assimilation rate (A); F, measured Δsuc.
Figure 5
Figure 5
Change in gas exchange parameters and Δsuc with time for experiment 4. A, VPd; B, ea/ei; C, E; D, gs; E, assimilation rate (A); F, measured Δsuc.
Figure 6
Figure 6
The relationship between Δsuc and ea/ei. Error bars represent se of each mean (n = 2–4). The solid line represents a least squares regression; Δsuc ‰ = 52.90–17.04 ea/ei, r = −0.98, P = 0.0005. The dashed line represents the predicted Δsuc from the Craig-Gordon model (Eq. 1) where Δsuc = Δe + ɛwc, and ɛwc = 27‰; Δsuc = 65.45–29.57 ea/ei. Note that the lines intersect when ea/ei is 1 and Δsuc ≈ 36‰.
Figure 7
Figure 7
The relationship between E and the fractional difference between modeled oxygen isotope composition of leaf water at the sites of evaporation (Δe) and estimated isotope composition of the water with which Suc exchanged (Δsuc − ɛwc). Error bars represent se of each mean (n = 2–4). Values from the same experiment are joined and labeled for reference. The predicted relationships at different L are plotted as dashed lines.
Figure 8
Figure 8
The relationship between mean measured and modeled Δsuc under each equilibrium condition when L = 13.5 mm and ɛwc = 27‰. Error bars represent se of each mean (n = 2–4). The line fitted to the data (—) represents a least-squares regression using the error bars as a weight, and is not significantly different from 1:1. Measured Δsuc ‰ = 0.35 + (0.99 × Modeled Δsuc), r = 0.98.
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References

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