What are the cells that make the leaves curl in plants during water stress?

The cells that make the leaves curl in plants during water stress are Bulliform cells.

The epidermal cell modification in plants which prevents water loss is known as Bulliform Cells.

Bulliform cells are bubble shaped cells which are present in grasses. They help in closing the stomata under stressful conditions and this prevents water loss.

When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.