Relative Water Content of Shrubs

Definition

• Relative water content is the amount of water a plant contains when it is incapable of taking in more water. This state is known as full saturation. A plant does not need to be in this state in order to survive but, knowing the percentage of water a plant is capable of holding is one way to determine if a plant is stressed.

Formula

• To determine the relative water content of a particular shrub or plant, remove leaf samples and determine their fresh weight (FW), dry weight (DW) and turgid weight (TW). You will then plug the weights into a formula. Determine relative water content (RCW) by subtracting fresh weight from dry weight and multiplying this number by the difference between turgid weight and dry weight. This is also expressed mathematically by the formula: RCW = (FW minus DW) times (TW minus DW).

Testing

• To derive the fresh, dry and turgid weights, leaf samples must be prepared. Wrap fresh leaf samples -- about three or four shrub leaves are sufficient -- in clear plastic. The plastic prevents water from evaporating. Weigh and record the leaf weight. This will be the fresh weight. Let the leaves sit in water overnight.and weigh them in the morning. This weight is the turgid weight. Allow the leaves to dry overnight and then weigh them in the morning. This measurement will be the dry weight.

Considerations

• There are many variations in relative water content, both between species and within the plant itself. Leaves higher in the canopy may be capable of holding more water than those lower in the canopy. Overall, however, the relative water content of a shrub's leaves will give you an idea about the amount of water the shrub is capable of holding. Some shrubs, like barberry, juniper and mugo pine, are capable of withstanding drought. These shrubs are able to survive with a lower relative water content than other shrubs. New growth, on any woody plant, holds more water and requires more water than mature wood. Developing plants need more water than established plantings.