I believe it is K or rather potassium
Vacuole is the answer.
Wilting is the loss of rigidity of non woody parts of plants and occurs when turgor pressure falls.
The vacuole controls turgor pressure. Turgor pressure dictates the rigidity of the cell and is associated with the difference between the osmotic pressure inside and outside the cell.
When a plant receives adequate amounts of water, the central vacuoles of its cells swell as the liquid collects within them creating a high level of turgor pressure which helps maintain the structural integrity of the plant along with the support of the cell wall.
In the absence of enough water , central vacuoles shrink and turgor pressure is reduced compromising the plant's rigidity so that wilting takes place.
