Saline irrigation water is a stress factor that can reduce crop productivity; yet, limited information regarding water salinity in sugarcane production areas and its effects on sugarcane is available in Japan. Therefore, we first analyzed water samples collected from 54 locations on six islands and found that Cl- and Na+ were the primary ions in irrigation water. The ion concentrations varied depending on the sampling locations and the types of water source: most reservoirs showed total ion concentrations below 500 mg L-1, while the highest exceeding 2500 mg L-1 was obtained in a natural swamp on Minamidaitoujima, where brine was considered to merge underground. Secondary, sugarcane was cultivated with water containing up to 3000 mg NaCl L-1. The saline waters tended to decrease sugarcane height and leaf number; however, no harmful effects were observed at harvest when the NaCl levels were up to 500 mg L-1. Meanwhile, the saline waters with 1000 mg NaCl L-1 or higher resulted in significant reductions in stem and leaf dry weights as well as sugars in juice. A regression analysis using data sets of total ion concentration and electrical conductivity (EC) from the survey as independent and dependent variables, respectively, revealed the coefficient of determination being 0.995, suggesting the possibility to accurately grasp water salinities through easy EC measurements. Taken together, it was concluded that sugarcane could be damaged by salt stress in fields where EC of irrigation water is over 250 mS m-1.
