title: Quantifying the impact of daily and seasonal variation in sap pH on xylem dissolved inorganic carbon estimates in plum trees.
abstract: In studies on internal CO2 transport, average xylem sap pH (pH(x)) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap ([CO2 *]). Lack of detailed pH(x) measurements at high temporal resolution could be a potential source of error when evaluating [CO2*] dynamics. In this experiment, we performed continuous measurements of CO2 concentration ([CO2]) and stem temperature (T(stem)), complemented with pH(x) measurements at 30-min intervals during the day at various stages of the growing season (Day of the Year (DOY): 86 (late winter), 128 (mid-spring) and 155 (early summer)) on a plum tree (Prunus domestica L. cv. Reine Claude d'Oullins). We used the recorded pH(x) to calculate [CO2*] based on T(stem) and the corresponding measured [CO2]. No statistically significant difference was found between mean [CO2*] calculated with instantaneous pH(x) and daily average pH(x). However, using an average pH(x) value from a different part of the growing season than the measurements of [CO2] and T(stem) to estimate [CO2*] led to a statistically significant error. The error varied between 3.25 ± 0.01% under-estimation and 3.97 ± 0.01% over-estimation, relative to the true [CO2*] data. Measured pH(x) did not show a significant daily variation, unlike [CO2], which increased during the day and declined at night. As the growing season progressed, daily average [CO2] (3.4%, 5.3%, 7.4%) increased and average pH(x) (5.43, 5.29, 5.20) decreased. Increase in [CO2] will increase its solubility in xylem sap according to Henry's law, and the dissociation of [CO2*] will negatively affect pH(x). Our results are the first quantifying the error in [CO2*] due to the interaction between [CO2] and pH(x) on a seasonal time scale. We found significant changes in pH(x) across the growing season, but overall the effect on the calculation of [CO2*] remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if pH(x) is in the more sensitive range (pH(x) > 6.5).
authors.affiliation: Laboratory of Plant Ecology, Ghent University, Gent, Belgium.
authors.forename: F G
authors.initials: FG
authors.lastname: Erda
authors.affiliation: Laboratory of Plant Ecology, Ghent University, Gent, Belgium.
authors.forename: J
authors.initials: J
authors.lastname: Bloemen
authors.affiliation: Laboratory of Plant Ecology, Ghent University, Gent, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: Steppe
mesh_terms: D002244:Carbon / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007287:Inorganic Chemicals / Q000378:metabolism*; D027861:Prunus / Q000378:metabolism*; D012621:Seasons; D052584:Xylem / Q000378:metabolism*
chemical_list: D007287:Inorganic Chemicals; D002244:Carbon
keywords: CO
  2; [CO2 *]; dissolved carbon dioxide; internal CO2 concentration; internal carbon transport; respiration