Abscission: How does it occur?

Exploring Leaf LongevityMartha 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.

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.

Phloem parenchyma with starch molecules

Proto-xylem

Meta-xylem

Open xylem cells mean Photosynthesis is still occurring

Is this a phloem cell group?

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.

A sieve tube element with associated cells.

Xylem and Phloem are closely associated

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?

Crystals in phloem parenchyma cells

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%20layer%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?

Sept. 28th, 2008, which

tree is healthier?

803

813

822

831

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.