Wall stress relaxation refers to the reduction in tensile stress (force per unit area) in cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

s of plants, fungi and bacteria, as a result of movement or rearrangement of the polymeric network that gives the wall its tensile strength

Tensile strength

Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

. Wall stress usually comes from cell hydrostatic pressure or turgor pressure

Turgor pressure

Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...

, which stretches the cell wall. The wall's resistance to stretching simultaneously creates wall stress and its counter force, turgor pressure. In plant cells, the wall-loosening protein named expansin

Expansin

Expansin refers to a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem function, and other developmental processes...

 causes wall stress relaxation by loosening the connections between cellulose microfibrils. The resulting decrease in cell turgor pressure and cell water potential

Water potential

Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

creates the necessary water potential gradient that allows water uptake and expansion of the cell.