Abstract
Because of the humid climate in Japan, it is sometimes difficult to demonstrate the effect of irrigation on crops under natural rainfall conditions. To evaluate the effect of irrigation on Japanese persimmon, a field observation was conducted for two years at a farmland in Sado island, which is one of the famous production areas of the fruit. We revealed that irrigation could do the following: 1) minimize the probability of producing small fruits, which cannot be supplied to the market and 2) increase the proportion of larger fruits (fruits larger than L size would increase by approximately 10%) in the period when the Japanese persimmon trees experienced water stress due to low amount of precipitation in the early cultivation period or due to drought conditions lasting more than 1 month in summer. Since Japanese persimmon trees have wide root zones, irrigation water supplied from a drip tube located 1.2 m away from the trunk would be used effectively by the plant. In addition, we can expect adequate size of fruits in the drought years if irrigation water does not have a matric potential head less than > pF3.0. This value is consistent with the lowest matric potential head (or highest pF value) for normal growth, which is generally used in field irrigation planning in Japan.
