Dissolved [CO2] in xylem sap of C3 annuals, shrubs, vines, and bamboo

Background/Question/Methods

A portion of the carbon dioxide (CO₂) released during respiration remains within the stem and can be transported internally through the xylem stream.  Photosynthetic cells in stems and leaves may fix this CO₂, or it may diffuse from the plant into the atmosphere. High xylem CO₂ concentrations ([CO₂]) have been observed in many tree species, with measurements in the range of <1% to 26.3%. While most investigations of xylem [CO₂] have focused on trees, high xylem [CO₂] has been measured in two shrub species, suggesting that internal CO₂ may play an important role in the carbon dynamics of shrubs and other plant functional groups. The objective of this study was to survey xylem [CO₂] in C₃ annuals (3 species), shrubs (8 species), vines (2 species), and bamboo (1 species). One to seven individuals were selected from plant species growing in the Athens, GA area and CO₂ microelectrodes were used to measure xylem gaseous [CO₂]. Stem diameter and height of microelectrode insertion were recorded to examine any possible effects on xylem [CO₂]. Xylem temperature and xylem sap pH were measured and used to calculate dissolved xylem carbon dioxide concentration ([CO₂*]) using gaseous [CO₂] measurements and Henry’s law.

Results/Conclusions

Mean xylem [CO₂*] ranged among species from 0.231-3.251 mmol L^-1, which is within the range of concentrations observed in trees. The overall mean [CO₂] of xylem sap was 2.69% gas by volume, which is about 70 times higher than current atmospheric [CO₂]. Differences in mean [CO₂*] were observed between functional groups.  Annuals had a mean (±SE) [CO₂*] of 0.956 (±0.557), shrubs 0.814 (±0.462), and vines 0.300 (±0.142) mmol L^-1. Only bamboo (Phyllostachys aurea) was significantly different (P<0.0001), with a mean [CO₂*] of 3.251 (±2.131) mmol L^-1. In a comparison of species, only Phyllostachys aurea (bamboo) and Lonicera japonica (vine) were significantly different, with mean xylem [CO₂*] of 3.251 (±2.131) and 0.231 (±0.059) respectively. Stem diameter, height of microelectrode insertion, xylem temperature, and xylem sap pH did not have a significant effect on xylem [CO₂*] when examined across all species. Stem xylem [CO₂*] may also be affected by factors not measured in this study, such as climate, soil, herbivory, or community composition.  Future studies should incorporate a larger sample size and additional measurements to gain a better understanding of xylem CO₂ in understudied functional groups.