The vessel wall thickness-vessel diameter relationship across woody angiosperms

Premise Comparative anatomy is necessary to identify the extremes of combinations of functionally relevant structural traits, to ensure that physiological data cover xylem anatomical diversity adequately, and thus achieve a global understanding of xylem structure-function relations. A key trait relationship is that between xylem vessel diameter and wall thickness of both the single vessel and the double vessel+adjacent imperforate tracheary element (ITE). Methods We compiled a comparative data set with 1093 samples, 858 species, 350 genera, 86 families, and 33 orders. We used broken linear regression and an algorithm to explore changes in parameter values from linear regressions using subsets of the data set to identify a threshold, at 90-mu m vessel diameter, in the wall thickness-diameter relationship. Results Below 90 mu m diameter for vessels, virtually any wall thickness could be associated with virtually any diameter. Below this threshold, selection is free to favor a very wide array of combinations, such as very thick walls and narrow vessels in ITE-free herbs, or very thin-walled, wide vessels in evergreen dryland pioneers. Above 90 mu m, there was a moderate positive relationship. Conclusions Our analysis shows that the space of vessel wall thickness-diameter combinations is very wide, with selection apparently eliminating individuals with vessel walls too thin for their diameter. Most importantly, our survey revealed poorly studied plant hydraulic syndromes (functionally significant trait combinations). These data suggest that the full span of trait combinations, and thus the minimal set of hydraulic syndromes requiring study to span woody plant functional diversity adequately, remains to be documented.

Automated summary

Comparative anatomy is crucial for understanding the relationship between xylem structure and function. This study focuses on the relationship between vessel diameter and wall thickness. The findings show that below a threshold of 90 μm, virtually any wall thickness can be associated with any diameter, while above this threshold, there is a moderate positive relationship. Additionally, the study reveals poorly studied plant hydraulic syndromes.