Epinasty

Epinasty refers to how leaves and stems turn downward when their tops grow faster than their bottoms.

While many plants move to follow the sun's path each day (phototropism), sometimes plant movements are more random. These are called nastic movements. Epinasty is a nastic movement.

Top-heavy rose ends up growing downward (Bridget Coila) CC BY-SA 2.0

Physical weight

The weight of a heavy flower or fruit is an example of mechanical epinasty. Over time, the upper portion of a stem grows longer and faster as the fruit or flower pulls downward.
Overspray

Herbicide overspray can cause severe epinasty. This crazy growth occurs because many popular herbicides are synthetic auxins (plant hormones) designed to drive a plant to grow itself to death.

​[If your tomato plants are exhibiting downward curling leaves, it may be that the soil needs more time to dry out between waterings.]

Epinasty of tomato leaves due to herbicide overspray. Courtesy of Les Rosenthal.

Boron deficiency

Boron deficiencies can cause palm trees to exhibit epinasty.

Ethylene gas

The most common cause of epinasty is ethylene. Ethylene is a plant hormone that helps fruits ripen. It is also a survival tactic when flooding occurs. Let me explain. ​

Carpentaria palm tree crown bending due to boron deficiency (Jason Thien) CC BY 2.0

When roots experience flooding, they generate an amino acid that I cannot pronounce, but botanists call ACC. ACC is the ethylene precursor. ACC moves up the xylem, where it converts to ethylene gas. This ethylene stimulates roots to create hollow tubes that connect to adventitious roots. These structures draw oxygen into the plant.

Other signs of ethylene exposure include chlorosis, thickening stems, petal loss, and deformed or aborted flowers. Epinasty from ethylene gas is common among plants grown in greenhouses with poorly maintained propane or natural gas heaters.