Print PageThe concentration of hydrogen ions ([H+]), commonly referred to as pH, is fundamental to numerous plant physiological processes. The pH of xylem sap (pHX) can influence leaf elongation, xylem hydraulics, and changes in pHX have been associated with root to shoot stress signaling. The pHX also affects the solubility of ions and molecules and is important in determining the quantity of dissolved CO2 that can be transported internally through the transpiration stream of trees—a pathway for CO2 movement that is gaining appreciation as an integral component of forest carbon dynamics. Specifically, 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 designed to compare pHX derived from different tissues at different temporal scales.
Results/Conclusions:
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, demonstrating that 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. 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 that dramatically affect transpiration rates. Studies designed to understand how transpiration, gaseous [CO2], and pHX interact will improve our ability to measure and predict internal carbon transport.
