Within-leaf variation in embolism resistance is not a rule in compound-leaved angiosperms

Premise Hydraulic segmentation, caused by the difference in embolism
resistance across plant organs, provides a sacrificial layer of cheaper
plant organs, like leaves, to protect more costly organs, such as stems,
during drought. Within-leaf hydraulic segmentation has been observed in
two compound-leaved tree species, with leaflets being more vulnerable than
the rachis or petiole. Many herbaceous species have compound leaves, and
some species have leaflets that are associated with pulvini at the base of
the lamina, which could provide an anatomical means of preventing embolism
from spreading within a leaf due to the high concentration of
vessel-endings in the pulvinus. Methods We assessed embolism resistance
across the leaf in three diverse palmately and pinnately compound-leaved
herbaceous species, one of each with a pulvinus. We used the optical
vulnerability method to simultaneously investigate whether hydraulic
segmentation occurred across the leaves of six compound-leaved herbaceous
species and one compound-leaved deciduous tree species. Results We found
considerable variation in embolism resistance across the six diverse
herbaceous species measured but no evidence of hydraulic segmentation
across both the palmately- and pinnately-compound leaves of the six
herbaceous species and the one deciduous tree species. In two species with
pulvini, we observed major embolism events crossing the pulvinus,
spreading from the rachis or petiole into the lamina, and embolizing both
tissues at the same water potential. Conclusion   We conclude
that there is little evidence for within-leaf hydraulic segmentation in
compound-leaved herbaceous species and that the presence of a pulvinus
does not provide a barrier to embolism spread in herbaceous plants.
Further, that hydraulic segmentation is not a rule in compound-leaved
angiosperms.