Transcript The Plant Cell - Forest Watch

Abscission: How does it occur?
Exploring Leaf Longevity
Martha Carlson, PhD candidate
NRESS, UNH, 2012
Twig, 2 Buds, 2 Petioles
The petiole of each leaf cups forming buds that will open next spring.
Notice the soft hairy trichomes that cushion the buds. The petiole and leaf
must break off, leaving the bud and twig. The twig must seal off the break
to close vascular tissue and prevent invasion by fungi or bacteria.
The Abscission Zone
Even very early in the summer, a clear line shows the
abscission zone. Simple ground meristem cells here do
not differentiate as other cells do. Then, in late
summer, these cells on the twig side of the AZ begin to
thicken. Notice how they bulge out of the petiole at
left.
The AZ layer
Bleecker and Patterson, 1997, reported that the small
cytoplasmic cells do not enlarge and do not obtain vacuoles
as other cells do. The cells fracture dividing petiole from
twig when pectin in the cell walls is dissolved. Pectinase is
prompted by ethylene. Cells on the twig side thicken into
“suberized scar tissue.”
Abscission Layer changes
Zooming in to 288x, the thickening cells show waxy filling.
Cells above in the petiole show no such thickening. Notice
that the thickening cells are not one layer thick but 8 to 10
cells deep.
Stimulated by ethylene, the cells produce
suberin, a wax that water-proofs the twig
side of the abscission layer. Notice that cells
at lower left do not show thicker walls.
These are simple cortex cells of the twig.
Examining a petiole saddle
To explore the petiole-twig zone more, we pulled leaves off
the maple twigs. The base of the twig around each bud
looks like “a saddle,” Dr. Rock said. It does look like a
saddle, a seat for a petiole and its leaf.
What about the xylem and
phloem?
Each leaf is supplied water via xylem
cells and ships sugars out of the leaf via
the phloem. We find at least three
vascular bundles in the maple petiole.
Xylem Cells are easy to see.
Proto-xylem
Phloem
parenchyma with
starch molecules
Meta-xylem
Petiole are young. Their xylem cells must be as springy as slinky toys to
allow the petiole to grow. Spirals of proto-xylem and slightly tighter
and more supportive meta-xylem are easy to find in the vascular
bundle.
Open xylem cells mean
Photosynthesis is still occurring
Is this a phloem cell group?
A sieve
tube
element
with
associated
cells.
Phloem cells are harder to see. They are small soft cells that quickly hide
themselves when the petiole is broke, exuding sugars of course. Phloem cells
are actually groups of sieve cells. The major transport cell, the sieve tube
element (in angiosperms) is surrounded by smaller cells:
Companion parenchyma, thin cells connected with pits into the sieve cell,
regulate sieve cell metabolism—including collecting leaking sugar in petioles;
Phloem fibers provide support with thicker walls.
Phloem parenchyma contain starch molecules and/or a single crystal.
Xylem and Phloem are closely
associated
Crystals in phloem
parenchyma cells
Xylem cells provide water osmotically to the sugary phloem
cells. So they must be closely placed.
Soft, thin-walled phloem cells are hard to spot but crystals
in phloem parenchyma identify the cells at top right. Can
you see any sieve elements?
Staining shows the suberin
A photograph by Michael Clayton on the University of
Wisconsin’s Plant Teaching Collection is stained to clearly show
the suberization of the twig side of the abscission layer. Even the
xylem cells appear suberized. But not the phloem—the blue
river of cells that goes through the abscission zone. The phloem
stays open until right before leaf drop.
http://botit.botany.wisc.edu/Resources/Botany/Shoot/Leaf/Abscission%20layer%20IAA/Abscission%20lay
er%20MC.jpg.html
Phloem Closed or Open?
At left, Tree 859B, a tree whose spectral scans show it was
already senescing in late August, shows a waxing covering
on many phloem cells. At right, Tree SS3, not yet senescing,
shows no such suberin wax in the phloem. Which tree will
have more time to dismantle and ship down the petiole leaf
nutrients—N, K, Mg, Mn, Fe, S and sugars?
Phloem Sieve Tube Elements
Esau says transport phloem cells may leak sugar, as these
many starch cells suggest. The cells are long with
plasmodesmata connections to companion cells.
No nucleus, no tonoplast, no
vacuole.
As it develops, the
phloem sieve tube
elements loses
major cell parts “to
increase
protoplasmic
continuity” with
adjoining sieve
tubes (Easau).
Companion cells
manage
metabolism.
Phloem Complex
Crystal forming parenchyma cells identify a phloem complex. These cells
are laid from upper right to lower left, transporting sugars from leaf to twig
and sinks beyond. Notice the many pores connecting cells. Notice how
thick these cell walls are, a characteristic of some sieve tube elements
(Esau).
Phloem Fibers
Thick-walled fibers protect the delicate
phloem and their cargo of sugar from
insects that might penetrate the twig
epidermis and cortex.
Other differences
Why is 859B senescing now, in early September? Is it a
stressed tree? Is SS3 an unstressed tree? The pithy cells of
the twigs give a clue. At left, cells in the twig adjacent to
859B’s petiole appear to heave some starch grains but not
many. Cells in SS3’s twigs are packed with chubby starch
molecules, food for the twig and bud.
Starch molecules extended far into the center of the twig in
Tree SS3.
The sugar will provide food and supplies for anthocyanin,
phenolics and fast spring growth to the bud and twig.
Does longevity matter?
803
822
813
831
Sept. 28th,
2008, which
tree is
healthier?
834
References
Abscission: How does it occur? copyright 2012, Martha Carlson and Barrett Rock,
University of New Hampshire.
Photographs taken with the University of New Hampshire scanning electron
microscope in Martha Carlson PhD research with help from Nancy Cherim, SEM
technical director, and Dr. Barrett N. Rock, plant anatomist.
Bleecker, A.B., and S.E. Patterson. 1997. Last Exit: Senescence, abscission, and
meristem arrest in Arabidopsis. The Plant Cell. 9:1169-1179.
Clayton, Michael et al., The Plant Teaching Collection, Botany Department, University
of Wisconsin-Madison. These materials were developed for use in the courses taught through the botany
department at the University of Wisconsin-Madison. They may be freely used by educators in the context of their
classrooms. This means that they may be incorporated into lecture presentations, printed supplemental materials, and
used in the teaching laboratory as a visual reference. They may not be distributed on the world wide web, or in
published works, without permission. Any request for these uses should be made through Michael Clayton the collection
manager.
Evert, R.F. 2006. Esau’s Plant Anatomy. Wiley-Interscience, Hoboken, NJ.