Evaluation of Definitive Water Status Using the ‘Water Stress Indicator Sheet’ in Deciduous Fruit Trees

We evaluated the water status of peach, grape, Japanese pear, and satsuma mandarin trees using a ‘water-stress indicator sheet’. This sheet indicates the water status of a sample by changing color (from blue to light pink); this is caused by a reaction between the sheet and water evaporating from the leaf onto which it is pressed. Decreases in the evaporation rates of grapes, peaches, and Japanese pear under water-stress conditions can be estimated using the time required for the indicator sheet to change color. This color change occurs in 100–110 s when in contact with the leaves of non-water-stressed plants, but when the plant is under sufficient water stress to reduce the evaporation rate by half, the change in color takes 200 s. In satsuma mandarin, it takes 130 and 230 s, respectively. Thus, the time elapsing between initial contact and the change in sheet color can function as a measure of the water status. However, when taking measurements for practical use, it is necessary to maintain conditions such as solar irradiance at a constant level.