Tree vascular architecture: Why be sectored?
Zanne, Amy*,1, Sharma, Monisha1, Sweeney, Kate1, Orians, Colin1, 1 Tufts University, Medford, MA
ABSTRACT- Plant vascular architecture should be under strong selection as it is the major pathway of water and nutrient delivery. Vascular integration is a character that varies across species with some species having relatively restricted movement of resources along particular xylem pathways (sectored) and other species having relatively unrestricted movement of resources across many xylem pathways (integrated). We explored how sectoriality in trees relates to a) xylem anatomy and b) the evolutionary relationships among species. We randomly sampled 18 species broadly across the angiosperm phylogeny and measured direct and lateral conductance of 20 mMol KCl through 5 cm branches. We found that ring-porous species had higher direct conductance through their xylem, as well as, greater sectoriality than diffuse-porous species. But, variation occurred between degree of sectoriality and xylem type. Some of this variation may be explained by differences in stem anatomy. More sectored ring-porous species had fewer and more isolated vessels and these vessels became more frequent and randomly dispersed in the more integrated ring-porous species. These results suggest that the degree of sectoriality depends upon frequency with which xylem vessels come into contact with larger more isolated vessels coming into contact less frequently than smaller more randomly scattered vessels. Furthermore, both xylem types and degree of sectoriality were widely scattered across the angiosperm phylogeny. Thus at the level of the major clades, these characters appear highly labile. But, at the family and genera levels, both xylem type and sectoriality are fairly conserved. These results suggest that selective pressures influencing sectoriality and xylem type have been more recent, when families and genera were separating. A knowledge of which environments (e.g., seasonally dry) have more sectored trees should shed light on the selective pressures leading to sectoriality.
Key words: sectoriality , xylem anatomy, angiosperm phylogeny, vascular architecture
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