Vulnerability of water conduction in compression and normal wood
Résumé
The water flow is interrupted by the cavitation phenomena due to the generation of air bubbles in vessels or tracheids; unless it is solved only water located above the gap will flow upwards to the crown. Some species have a special tissue structure that prevents that vulnerability to cavitation. In the case of conifers, the membrane in the bordered pits connecting the lumen of adjacent cells functions as a valve.On the other hand, to allow the control of stem orientation, conifer species often produce the so-called “compression wood”, consisting of tracheids with thicker cell walls and smaller lumens (cavities) than other wood tissues. Such features can be expected to be disadvantageous for water conduction. In this study to compare the vulnerability of compression wood and normal wood, we observed the cavitation in the compression and normal part of the branch of Pinus halepensis using two methods. One method is a calculated air volume in lumen from double weight measurement data. Another is an observation of air area in lumens by using X-ray tomography. We found a direct proportional relationship between both air fractions calculated from weight data and tomography, and confirmed that the weighting method is valid. Although the cavitation ratio from weight data of inner part was larger than that of outer part, no significant difference between compression and normal wood was observed in either species. This result was also confirmed by tomography, suggesting that the cavitation does not occur that easily in compression wood. Moreover the images by X-ray tomography gave additional information on the localization of cavitation. The cavitation was not spread homogeneously in the wood tissue. It was, in particular, observed in the latewood of all samples.