In our previous study, it was experimentally demonstrated that the maximum allowable salinity limit in irrigation water to keeping normal growth of rice plants might be around 1,800 μS/cm in terms of EC (Electric Conductivity). This is the result under a particular condition that the salinity is unchanged with such an allowable limit during the entire period of irrigation. The present experimental study considers increase in salinity of irrigation water in the growing process of rice plants, shifting natural groundwater (freshwater) to saline water at different three growing stages (i. e., tillering, panicle formation and booting stages) where physiological vigorousness of their roots significantly changes. The testing is carried out in a greenhouse, potting rice plants and creating the evaporation-free or evaporative environment for water surface in the pot. The results show that any shift in irrigation water in the evaporation-free environment, if a rice plant is trimmed into 20 stems after the tillering stage, produces nearly the same results as the shiftless irrigation with freshwater in all terms of the transpiration, the growth of the plants, the starting period of heading and the yield, and does not cause any recognizable outbreak of etiolation. In the evaporative environment, also, etiolation is not caused though EC-value increases by 100 to 200 μS/cm from that of irrigated water. It is thus concluded that the saline water of 1,800 μS/cm can be used as irrigation water at any growing stage of rice plants if the number of stems in booting is controlled into 20.
