Quantifying the impact of daily and seasonal variation in sap p H on xylem dissolved inorganic carbon estimates in plum trees

In studies on internal CO 2 transport, average xylem sap p H (p H x ) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap ([CO 2*]). Lack of detailed p H x measurements at high temporal resolution could be a potential source of error when evaluating [CO 2*] dynamics. In this experiment, we performed continuous measurements of CO 2 concentration ([ CO 2 ]) and stem temperature ( T stem ), complemented with p H x measurements at 30‐min intervals during the day at various stages of the growing season ( D ay of the Y ear ( DOY ): 86 (late winter), 128 (mid‐spring) and 155 (early summer)) on a plum tree ( P runus domestica L . cv. R eine C laude d' O ullins). We used the recorded p H x to calculate [CO 2*] based on T stem and the corresponding measured [ CO 2 ]. No statistically significant difference was found between mean [CO 2*] calculated with instantaneous p H x and daily average p H x . However, using an average p H x value from a different part of the growing season than the measurements of [ CO 2 ] and T stem to estimate [CO 2*] 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 [CO 2*] data. Measured p H x did not show a significant daily variation, unlike [ CO 2 ], which increased during the day and declined at night. As the growing season progressed, daily average [ CO 2 ] (3.4%, 5.3%, 7.4%) increased and average p H x (5.43, 5.29, 5.20) decreased. Increase in [ CO 2 ] will increase its solubility in xylem sap according to Henry's law, and the dissociation of [CO 2*] will negatively affect p H x . Our results are the first quantifying the error in [CO 2*] due to the interaction between [ CO 2 ] and p H x on a seasonal time scale. We found significant changes in p H x across the growing season, but overall the effect on the calculation of [CO 2*] remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if p H x is in the more sensitive range (p H x > 6.5).