Quantifying plasticity in vessel grouping – added value from the image analysis tool ROXAS

Abstract

The functional role of the connectivity of the xylem network, especially the arrangement of solitary and grouped vessels in a cross section, has often been discussed in the literature. Vessel grouping may improve hydraulic integration and increase resilience to cavitation through redundancy of hydraulic pathways. Alternatively, a high degree of hydraulic integration may facilitate the spread of cavitations among neighboring vessels. Here we show how automated image analysis tools such as ROXAS (see www.wsl.ch/roxas) may greatly enhance the capacity for studying vessel grouping while avoiding some methodological limitations of previous approaches. We tested the new analysis techniques by comparing the xylem network of two populations of the herbaceous species Verbascum thapsus collected at a dry and moist site on Big Island (Hawaii, USA). ROXAS accurately, objectively and reproducibly detected grouped and solitary vessels in high-resolution images of entire root cross sections, and calculated different and partly novel vessel grouping parameters, e.g. the percentage of grouped (vs. solitary) vessels among different vessel size classes. Individuals at the dry site showed a higher degree of vessel grouping, less solitary vessels, greater maximum vessel sizes and an increase of the percentage of grouped vessels with increasing vessel size. The potential, but also some limitations of automated image analysis and the proposed novel parameters are discussed.