Epinasty

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


While we all know that many plants move to follow the path of the sun each day (phototropism), sometimes plant movements are more random. These are called nastic movements. Epinasty is a nastic movement. Hyponasty is the opposite of epinasty.


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 weight of the fruit or flower pulls downward.

Overspray

Herbicide overspray can also 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.

Severe epinasty due to herbicide overspray (UTIA)

Boron deficiency in palms (T.K. Broschat)

Boron deficiency

Boron deficiencies can cause palm trees to exhibit epinasty.

Ethylene gas

The most common cause of epinasty is ethylene. Ethylene is a gaseous plant hormone that helps fruits ripen. It is also a major survival tactic in cases of flooding, either temporary or permanent. Let me explain. When roots experience flooding, they generate an amino acid that I cannot pronounce, but botanists call it ACC. ACC is the precursor to ethylene. ACC moves up the xylem where it is turned into ethylene gas. This ethylene stimulates roots to create hollow tubes that connect to adventitious roots. These tiny roots and tubes are used to 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.

Healthy tomato (left) with ethylene-induced epinasty (right) from OSU

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