(biology), the incomplete saturation with water of plant cells, resulting from an intensive water loss by the plant that is not replenished by absorption of water from the soil. A water deficit is usually observed during the hottest hours of the day. The leaves of many plants, such as thorn-apple and pumpkin, lose their turgor at this time and droop. The external appearance of plants with a large amount of mechanical tissue (immortelles) does not change even when they die of water deficit. With the arrival of dry weather one can observe residual water deficit, when the plant cannot make up its lack of water during the night. Water deficit, especially residual water deficit, causes many physiological and biochemical changes in plants. Some plants (certain groups of xerophytes, the so-called euxerophytes) can with-stand a very large water deficit (up to 50-60 percent), while other plants (mesophytes) are harmed by a relatively small water deficit. Since there is no reliable method for determining water deficit, one must sometimes resort to determining the wilting coefficient. For plants that are raised without watering, one determines the amount of water in the soil at the moment when their leaves begin to wilt.