Two experiments were carried-out to study the effect of irrigation frequency and timing on root developments, tomato yield and soil water content at JIRCAS Okinawa Subtropical Station, Ishigaki, Japan. First experiment was conducted in root containers (31 x 15 x 60 cm) with one transparent side for visual viewing of the root development. Sandy clay loam soil mixed with CaCO3 and P2O5 fertilizers was packed in the containers to 50 cm height with bulk density of 1.5 g/cm3. Three irrigation frequencies, 1, 3 and 5 days were investigated. Under each irrigation interval two vegetables, tomato (var. first power) and chile pepper (Takano tsu me) were studied. The soil water content in the containers was kept at field capacity by compensating the loss in weight by adding water. The second experiment was conducted in a greenhouse grown with tomato (var. first power) with two irrigation frequencies, 1 and 3 days and three irrigation timings, early morning (8:00h), afternoon (14:00h) and night (20:00h). Soil water content at 0-60 cm and soil temperature at 15 cm depth were measured at 15 cm distance far from dripper by installing TDR sensors and thermocouples, respectively. The supplied amount of water was the same for all irrigation frequencies and gradually increased to cover the crop water requirements during each growing stage.
The root containers results indicated that increasing water supply increased the root development and root biomass. The 1-day irrigation frequency produced the highest root biomass while the least root biomass was obtained from the 5-days irrigation frequency, indicating that water stress promoted the development of root system in the deeper layer where available soil moisture content was higher than the top layers. The 3-days irrigation frequency in chile pepper and 5-days irrigation frequency in tomato saved 12 % and 18 % of irrigation water respectively at early growth stage compared to the other frequencies.
The results of greenhouse experiment showed that the best irrigation frequency was 3-day. The average yield in 3-day frequency was 70 ton/ha while 63 ton/ha in 1-day frequency. The effect of irrigation timing varied with irrigation frequency. For 3-day frequency, irrigation at early morning was better than afternoon and night irrigations. The average yield for irrigation at early morning was increased by 15% and 14% than irrigation at afternoon and night, respectively. For 1-day frequency irrigation at night increased the yield by 11% and 3% than irrigation at early morning and afternoon correspondingly. The lowest soil water content and soil temperature were displayed by the treatment, which produced the highest yield. With the same amount of water, the early morning irrigation after every 3-days increased tomato yield by 11- 20 % compared to night and early morning irrigation of 1-day frequency. A similar increase in water use efficiency in the early morning irrigation every 3-days was also recorded. Therefore, a selection of the proper irrigation frequency and timing led to a higher yield and high water use efficiency.