Lecture 6 - Seed Abscission

Question 1

Defining abscission

Answer 1

Abscission refers to the breaking off of any plant part:
* seed abscission
* petal abscission
* fruit abscission
* leaf abscission
* branch abscission

During abscission, the middle lamella that joins plant cell walls is enzimatically digested.

With the matrix of polysaccharides destroyed, a small assist from the wind or an animal is required to severe the connection. That’s abscission.

Question 2

Leaf Abscission

Answer 2

The abscission zone (AZ) is always very narrow, only a few cells wide, and they occur in characteristic places.

Abscission is stimulated by ethylene and inhibited by auxin.

The AZ in leaves is always located at the junction of the petiole.
and the shoot.

Question 3

Fruit abscission

Answer 3

The AZ in fruit abscission develops at the junction of the fruit to the pedicel.

Ethylene levels begin to increase and the pedicel turns brown and becomes even drier;
Soon the AZ is apparent as an indentation and then as a perforation.
At this point samaras are only tenuously attached to the pedicel.
The stage in now set for abscission.

Question 4

Dehiscence

Answer 4

Dehiscence refers to a rupture, a splitting **within a plant part. **
As with AZ, plant hormones trigger development of DZs.

Question 5

Premature fruit dehiscence: dealing with agricultural losses

Answer 5

Premature dehiscence of fruits (ie before we are ready to harvest them) is a major problem in certain crops.

~15% of canola seeds are lost by premature dehiscence before they are fully inside the combine harvester.

A great deal of on-going work (conventional breeding as well as transgenic studies) to develop a variety of canola that not only delays dehiscence but creates the DZ more or less simultaneously among all individuals so that we can harvest all of the crop on a single day before any seed loss occurs.

Question 6

Wind-induced seed abscission

Answer 6

1. The dehiscence of fruits and the abscission of seeds is dependent on low relative humidity.
   2.** Wind provides the motive force for abscising the seed. **

Question 7

Abscission and relative humidity

Answer 7

Humidity refers to the total of water vapor in the air.

Relative humidity refers to the humidity as a percentage of the total amount he air can hold.

%RH = (actual vapour density/saturation vapour density) *100

100% relative humidity means that the air can hold no more water vapor. Any additional water molecules end up as condensation.

RH minimized during the early afternoon when temperature is maximized. But why?
Because warm air can hold more water than cold air. In warm air molecules are moving around more than in cold air.

Relative humidity controls the rate of development of the AZ or DZ .

Question 8

Boreal forest: a familiar example of the RH-mediated readiness for abscission

Answer 8

The scales of ovulate cones move in response to RH. At low RH scales open; at high RH, scales close up. The cells in a mature cone are dead, so the mechanism is passive.

Question 9

Taraxacum officinale (common dandelion)

Answer 9

Mediated by high RH, the flower head (capitulum) closes.

Effect of a sudden thunderstorm:
* the pappi close up.
* The pappi (fibers that function in dispersal) are the modified calyx.
* Drying out after the rain: pappi expansion after 1 h
* Hours later: the fibers of each pappus are fully extended

Question 10

Swietenia humilis

Answer 10

DZ’s on woody pericarp develop much faster with low RH.
Wings remain appressed to each other until dried out.
Gravity pulls the wing down.
As more wing area is exposed, the wind can abscise it.

Question 11

Ceiba aescufolia

Answer 11

• Woody pericarp dehisces much faster in low RH.
• Matted damp fibers do not expand unless in low RH.
• Side of fruit exposed to sun dries the fastest
• Wind can pull at fully extended fibers.

Question 12

We can conlcude that:

Answer 12

• Abscission is maximized in the early afternoon
• Little to no abscission until the sun is shinning on the fruit
• Virtually NO ABSCISSION AT NIGHT

Question 13

Now let’s turn to wind speeds
Seed abscise when:

Answer 13

1. wind speeds are high
2. atmospheric instability is maximized

These two conditions enhance the dispersal potential of the seeds.

Question 14

Seed abscission id episodic

Answer 14

Most seeds abscise during 3 to 5 (?) single events.
The rest of the time-nothing happens.
WHY?
* RH to high
* Right skew distibution of wind speeds