Xylem Functioning and Water Relations of the Elastic Living Tissue of the Bark: New Insights about Their Coordination

The interesting species-specific relationship between maximum daily shrinkage (MDS) and stem water potential (stem) was investigated in order to understand the mechanisms involved. Vulnerability curves of Vitis, Citrus, Olea and Prunus were analysed and the air entry stem in the xylem (Pe) was calculated. The water relations parameters of the living tissue of the bark were also studied by building P-V curves. A strong linear correlation was obtained between the stem at which MDS occurs (MDS) and Pe in the studied species. Furthermore, P-V curves showed that, the species with the highest resistance to cavitation, i.e., Prunus, had the lowest osmotic pressure and the largest bulk elastic modulus. The conclusions of this study reinforce the idea of a tight correlation among plant capacitance, transpiration and maintenance of water status of the plant. These results illustrate the importance of studying the integration of long-distance transport systems in plants. INTRODUCTION Monitoring of trunk diameter variations (TDV) on the bark of woody plants is becoming a useful tool for managing irrigation in commercial orchards. In addition to being a valuable feature for agronomists, these sensors supply important information about the physiological mechanisms that allow the plant to transpire under a wide range of environmental conditions. Currently, it is not possible to mechanistically explain the daily and seasonal dynamics of transpiration without considering the role played by water storage in trunks, branches and even leaves (Steppe et al., 2006). Trunk diameter fluctuations have been reported to reflect the addition of four components: irreversible radial growth, reversible living-cell dehydration/rehydration, thermal expansion and contraction, and expansion of dead conducting elements due to the increase and relaxation of internal tensions. Therefore, the interaction between both the living parenchyma in the cortex and the xylem is important to understand the dynamics of TDV and its consequences on the water relations observed in the plant. Maximum daily shrinkage (MDS) is one of the most used indicators of water stress in fruit trees (Fernandez and Cuevas, 2010). When plotted against stem water potential (stem) MDS shows a parabolic response, i.e., as stem decreases MDS increases up to a maximum value after which MDS decreases following further decreases in stem. Furthermore, the stem value at maximum MDS has been reported to be species-specific (Ortuño et al., 2010). The aim of this work was to investigate potential relationships between the pattern of MDS described above and the vulnerability to embolism of the xylem of several fruit tree species of high agronomical interest. MATERIALS AND METHODS Four species of high agronomical interest were selected to search for shared Proc. 9 International Workshop on Sap Flow