Abscission

Discarded limbs, scattered petals, seeds and fruits thrown to the ground - abscission!

Abscission is the intentional shedding of body parts. Some lizards do it with their tails, we do it with our teeth, and mushrooms do it to their spores. All plants use abscission as a normal part of their lifecycle, but for different reasons and in different ways.

Anatomy of abscission

The area where abscission occurs is called the separation zone, or the abscission zone. When abscission occurs, enzymes are released that breakdown the structural cellulose and the adhesive pectin within cells in the abscission zone. The timing and methods of abscission depend on the reasons behind the action.

Autumn leaves (Pixabay)

Forms of abscission

There are four basic forms of abscission:

• seasonal - deciduous trees shed their leaves each fall to avoid snow load in winter; fruit drop occurs to focus resources on fruit yet to come; dandelions and other flowers and fruits shed seeds to continue the species
• conservation - damaged leaves or twigs are dropped to reduce water loss in drought, or in response to shade tree decline
• defense - plants may drop heavily infested leaves or blossoms to rid itself of some pests
• senescence - natural aging

Tomato blossom drop

Drought-stressed tree

The abscission process

When a plant decides to get rid of a body part [How weird would that be for us? I guess it’s like having a tooth pulled. Yikes!] Anyway, the plant goes through three steps to get rid of unwanted parts: remobilization, protection, and then detachment. During remobilization, resources are pulled from the unwanted part to be used elsewhere. Then, because gaping holes aren’t good for anyone, plants use cork to seal off soon-to-be exposed areas. Detachment can occur in several ways:

• cannibalism - at the abscission zone, the plant may digest an area called the 'middle lamella’ causing the leaf or other part to fall away
• fracking -  plants inject a lot of water into the area, to the point of bursting, forcing the unwanted appendage to fall
• sabotaging -  the plant may alter the structure of each petiole’s base (a leaf’s stem) by making a top layer with weak cells and a bottom layer with swollen cells, causing the leaf to fall away, leaving behind a leaf scar

Detachment triggers

Leaf, fruit, and blossom drop are not uncommon, and there are several causes that trigger detachment:

• chemical - plants generate abscission enzymes in response to stressors, such as pests, disease, salinity, excessive sunlight, cooler temperatures, and shorter daylight hours
• chlorophyll loss - as days shorten, reduced chlorophyll levels cause leaves to turn yellow; since aphids are attracted to yellow, many leaves add a red pigment as protection
• hormonal - reduced auxin levels and increased sensitivity to ethylene gas trigger abscission in many plants

If your plants are losing body parts, see if you can use this information to determine the cause!