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

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 (]). Lack of detailed pH_x measurements at high temporal resolution could be a potential source of error when evaluating ] 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 ] based on T_stem and the corresponding measured CO₂]. No statistically significant difference was found between mean ] 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 ] 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 ] 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 ] will negatively affect pH_x. Our results are the first quantifying the error in ] 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 ] 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).