Spatial and temporal patterns of xylem sap pH derived from stems and twigs of Populus deltoides L.

Xylem sap pH (pHX) is critical in determining the quantity of inorganic carbon dissolved in xylem solution from gaseous [CO2] measurements. Studies of internal carbon transport have generally assumed that pHX derived from stems and twigs is similar and that pHX remains constant through time; however, no empirical studies have investigated these assumptions. If any of these assumptions are violated, potentially large errors can be introduced into calculations of dissolved CO2 in xylem and resulting estimates of internal carbon transport. We tested the validity of assumptions related to pHX in Populus deltoides L. with a series of non-manipulative experiments. The pHX derived from stems and twigs was generally similar and remained relatively constant through a diel period. The only exception was that pHX derived from lower stem sections at night was higher than that derived from twigs. The pHX derived from stems was similar on clear days when solar radiation and vapor pressure deficit (VPD) were similar, but higher on an overcast day when solar radiation and VPD were lower. Similarly, cloudy conditions immediately before an afternoon thunderstorm increased pHX derived from twigs. The pHX derived from twigs remained similar when measured on sunny afternoons between July and October. Our results suggest that common assumptions of pHX used in studies of internal carbon transport appear valid for P. deltoides and further suggest pHX is influenced by environmental factors, such as solar radiation and VPD that affect transpiration rates.

Research highlights
► Our results suggest xylem sap pH derived from twigs is generally a suitable proxy for xylem sap pH derived from stems when calculating liquid phase [CO2] from gaseous phase [CO2] measurements in internal carbon transport studies.
► We also found that xylem sap pH remained relatively constant over the diel period, indicating high frequency xylem sap pH measurements are not necessarily critical for accurate measurement of liquid phase [CO2] over individual days.
► Although xylem sap pH derived from lower stems during the night was higher than xylem sap pH derived elsewhere and showed a diel pattern should be recognized, but should not be a major impediment to internal carbon transport studies focused on dissolved CO2 movement with the transpirational stream.
► We recommend that relationships between tissue components be examined before making this assumption in other systems as other tree species may not exhibit similar relationships.
► Environmental conditions which influence transpiration affect xylem sap pH—presumably by altering the ratio of cations and anions and not by influencing the gaseous [CO2].