Dear Haitao

you might refer in a first instance to the back diffusion of 18O enriched water from the sites of evaporation to the mesophyll itself, which is described by the Péclet effect (radial and longitudinal as described by Farquhar and Gan 2003). If I understand you correctly your point is that this back diffusion could or should also occur from one leaf to another. What we do know is that the water in the phloem sap is 18O enriched compared to the xylem (e.g. Gessler, A., Peuke, A. D., Keitel, C., & Farquhar, G. D. (2007). Oxygen isotope enrichment of organic matter in Ricinus communis during the diel course and as affected by assimilate transport. The New Phytologist, 174(3), 600–613. doi:10.1111/j.1469-8137.2007.02007.x - Fig 4 c; and also some papers of Lukas Cernusak). We also know that newly developing leaves (which do not transpire, yet) are supplied with C via the phloem and thus via the mass flow in the sieve tubes (slightly) enriched water might be transferred to such leaves. This would however be symplastic (phloem) transport and not apoplastic transport and only from transpiring to non-transpiring leaves. Apoplastic diffusion through the petiole and the twig would need to occur against the advective flow in acropetal direction over a real long distance (compared to the back diffusion in the leaf lamina). So I would guess that it is not very likely for mesophyll water of the upper canopy to have a measurable impact on leaves of the lower canopy. What do you think?

Cheers

Arthur

Thanks Arthur! I agree with your answer, so if we think canopy as a big leaf, there is no necessary to consider the péclet effect or give péclet number a very small value.

Perhaps big leaf model is not adequate to your task. I guess some applcation to O18 can be made by the reasons given by De Pury and Fraquhar in PCE 1997. Even so moving from a canopy to a crown at a frictional distance from soil and with leaf clumps makes the O18 enrichment quite experimental.

I meant De Pury and Farquhar PCE 97. The problem is that direct measurents over a canopy found evidences of Peclet effect even if the relationship was blurred by other micrometeorological effects like in Welp et al. PCE 2008. It is therefore suggested to use a canopy model that allows some heterogeneity in temperature and light and one of the less complex is the PCE 97. The idea of evaporative discrimination as source of xylem/phloem water would introduce the possibility of leaves acting as feeders, but it requires movements against suction or osmotic gradients. O18 stem values reflect soil water because enriched water does not goes against flow in the time course of the measure. Further any surface condensation of water on outher surfaces would enrich air humidity of O18.

Thanks a lot, I would like to discuss another question. if I have divided canopy into upper level and lower level, and I know the transpiration rate for upper and lower level, respectively, when we treat upper and lower canopy as two big leaves, can we consider peclet effect into leaf water enrichment model on canopy scale. Cause when the transpirtion rate increase or decrease, the effective paht length in the upper level or lower level should increase or decrease towards one direction.

The lower level is affected by sunflecks, diffuse radiation and LAI. Whereas in De Pury actually they use a sun shade partition, their aim is to measure net photosynthesis and compare measurements with the model. Would then try to redraw a similar scheme for transpiration and then follow soybean experiment of Welp et al. Arthur Gessler is one of the few with direct experience. A end it is probable that crop top is affected more than shaded portions at high LAI and that water enriched can exchange O with carbon cycle, but does not travel down as it is. If there parameters for crop modelling I would be interested.