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The hyponastic response is an upward bending of leaves or other plant parts, resulting from growth of the lower side. This can be observed in many terrestrial plants and is thought to be linked to the plant hormone ethylene.
Submerged plants often show the hyponastic response, where the upward bending of the leaves and the elongation of the petioles might help the plant to restore normal gas exchange with the atmosphere.
Plants produce ethylene, and normally this dissolves in the air quite easily. But when the plant is submerged ethylene is trapped in the plant. Plants that are exposed to elevated ethylene levels in experimental set-ups also show the hyponastic response.