Trees are primarily constructed of secondary xylem, a multi-function complex tissue. One of the key functions is to transport water to transpiring leaves. This is critical because trees trade water for CO2 in photosynthesis, but transpiration has to be carefully controlled because damage to the water transport system can occur if tension on the water within the xylem vessels increases enough to cause cavitation by air seeding. These tensions develop to damaging levels if water loss outpaces supply. One of the other key functions of xylem is water storage (capacitance), which helps to keep water tension in the xylem in a non-damaging range.
We hypothesized that capacitance would vary depending on position within the tree. We predicted that xylem capacitance would be higher in tissues that were more susceptible to water-stress induced cavitation (P50). This was examined by measuring moisture release curves, hydraulic vulnerability curves, and density for xylem samples harvested from the current year growth of narrow, intermediate, and wide diameter roots and shoots of juvenile poplar trees (Populus trichocarpa). Intra-organismal water management is fundamental to tree physiology, and evaluation of changes in capacitance with position within the plant body has been only rarely studied.
Roots and shoots had different xylem tissue characteristics. Capacitance was higher in root xylem tissue than it was in shoot xylem tissue (F1,24 = 11.61, P = 0.02). Roots were also more vulnerable to water-stress induced hydraulic transport failure (i.e., they had less negative P50) compared to shoots (F1,54 = 15.77, P = 0.01) and had lower xylem density (F1,54 = 212.37, P < 0.01). Capacitance was correlated to other xylem tissue traits: capacitance and P50 were correlated (R = 0.86, P = 0.03) and capacitance and xylem density were negatively correlated (R = -0.81, P = 0.05).
Roots xylem is less dense and this is associated with increased water storage, lower construction costs, and increase transport efficiency. A larger supply of stored water may serve to buffer xylem tensions in more vulnerable tissues such as root xylem. Shoot xylem was denser, with reduced capacitance, but had a greater resistance to cavitation. Rather than using stored water to avoid negative pressures, shoot xylem may have to rely on increased tolerance because of biomechanical requirements that differ from roots and that may limit storage potential.