Wilting

We just discussed transpiration, and how leaves are constantly losing water vapour to the environment. However, what happens when there is not enough water in the soil to replace the water that was lost? Similarly, what happens when there is too much water? In the next section we discuss wilting, and why plants wilt and get `floppy' in hot weather or after a long drought. We will also look at ways that plants can rid themselves of extra water when there is too much water in the environment and the plant has to cope with high root pressure and a low transpiration rate.

Wilting

Plants need water to maintain turgor pressure. Turgor pressure is what provides the plant with much of its structural support. Have a look at the figure below which shows the effect of osmosis on the turgidity of cells.

Cells in solutions with different concentrations

Wilting refers to the loss of rigidity or structure of non-woody parts of plants (see the figure below). It occurs when turgidity of plant cells is lost. When a cell absorbs water, the cell membrane pushes against the cell wall. The rigid cell wall pushes back on the cell making the cell turgid. If there is not enough water in the plant, the large central vacuole of the cell shrinks and the cytoplasm decreases, resulting in decreased pressure being exerted on the cell membrane, and in turn, on the cell wall. This results in the cell becoming flaccid (floppy). When the cells of a plant are flaccid, the entire plant begins to wilt.

Crops wilt due to a lack of water.

Wilting occurs due to lower availability of water which may be due to:

• Drought conditions: where the soil moisture drops below conditions that allow plants to grow.
• Low temperatures: which prevent the plants vascular transport system from functioning;
• High salinity (salt concentration): which causes water to diffuse from plant cells to the soil, thus inducing shrinking of cells.
• Bacterial or fungal infections: that block the plant's vascular system.