Xylem diameter of petioles and peduncles
The xylem lumen diameter of three petioles and three peduncles from each
of three additional plants was examined to test for any association
between organ xylem vulnerability to cavitation and vessel size.
Transverse sections of peduncles and petioles were made close to the
base where they joined the main stem using a sliding microtome (Leica
Microsystems, North Ryde, NSW, Australia) and a BFS‐3MP Freezing Stage
(Physitemp Instruments, Clifton, NJ, USA) at 30 µm thickness. Cross
sections were stained with toluidine blue (5%) and mounted on glass
slides with phenol glycerine jelly. Slides were photographed at 20x
magnification using a Nikon Digital Sight DS-Fi2 (Tokyo, Japan) mounted
on a compound microscope (DM1000; Leica Microsystems, Wetzlar, Germany).
Vessel diameter was measured using ImageJ. Four representative fields of
view (20x magnification) of entire xylem bundles were captured for each
transverse section. Using IMAGEJ, a binary image was produced for each
section, and vessel lumens were selected using the threshold function.
An ellipse was automatically fitted to each xylem vessel lumen (fig. 1).
The lumen area of each vessel was automatically measured using the
‘analyse particles’ function to calculate an average cross-section
diameter for each vessel. Given that the flow rate of water through
vascular tissue is directly proportional to the xylem diameter to the
4th power (Hagen–Poiseuille law), only the largest of
xylem vessels contribute significantly to the water transport capacity
of the vascular system (Petit, Anfodillo, & De Zan, 2009; Melvin T.
Tyree & Ewers, 1991). Thus, for each section, xylem diameters were
ranked and the mean diameter of the largest 5% of vessels measured.
This allowed us to compare the theoretical water supply capacity and
resilience to xylem collapse in vegetative and reproductive support
structures.