Jeffrey A. Pierce, Frank W. Ewers, Jameel Al-Haddad, and Frank W. Telewski


Abstract

The mechanical properties of lignin have long been studied. However, there has been little research into how lignification of the cell wall affects hydraulic properties in water conducting cells. Most experiments that have looked into this have focused on comparing hydraulic properties across species and wood density could be used as a proxy for lignin content. Observed differences in these experiments could be attributed to differences in vascular anatomy and not solely differences in lignin composition. The ability to genetically manipulate the ratios of lignin monomers present in tissues allows us to examine the role of lignin content and lignin monomers in hydraulic movement of vascular tissues without modifying the overall anatomy of the vascular system. In this experiment, we used a line of poplar (Populus tremula x P. alba) clones genetically modified to over-express the Ferulate 5-Hydroxylase (F5H) enzyme. Over-expression of the F5H enzyme resulted in an increased syringyl:guaiacyl ratio within the plants' cell walls. The F5H clones and wild type poplars were subjected to water stress over a 4 week period, and hydraulic conductivity and vulnerability to embolism were measured. Gas exchange was monitored throughout the experiment to assess stress levels in the plants. Initial measurements have shown an increased initial hydraulic conductivity in the F5H clones over the wild type before water stress was induced. However, as water stress increased the F5H clones showed an increase in vulnerability to embolism and a decreased hydraulic conductivity compared to the clones. This may suggest an increased brittleness and vulnerability to cavitation in the modified cell walls. No differences in gas exchange values have been observed between the F5H and wild type clones.