In Japan, the highest salt concentration in irrigation water for sugarcane cultivation has been reported to be above 2500 mg L⁻¹, which may cause harmful effects to the crops; however, little information is available on the relationship between the salinity of irrigation water and sugarcane. To investigate its effects on agronomic and physiological traits, a Japanese cultivar, Saccharum spp cv. NiF8, was grown with 0, 200, 500, 1000, 2000, and 3000 mg NaCl L⁻¹ under pot conditions. The treatments significantly lowered leaf area; however, NaCl levels up to 500 mg L⁻¹ did not greatly reduce culm weight and juice sugar concentration. These traits were impaired when the tested cultivar was grown with 1000 mg NaCl L⁻¹ or higher, indicating that salt concentration is desired to be lower than 1000 mg L⁻¹. CO₂ assimilation rate was inhibited mainly due to stomatal closure caused by salt stress. The treatments significantly altered Na⁺, Cl⁻, and K⁺ concentrations in juice but not those in leaf, suggesting that juice analysis is an effective method to estimate its salinization status. Culm weight and juice sugar concentration were severely affected as juice conductivity exceeded 900 mS m⁻¹; thereby, sugarcane plants of NiF8 possessing conductivity above this level could be considered salt-stressed where water salinity is a concern.

It is important for the effective use of soil resource and sustainable sugarcane production to study the root architecture and plasticity because soil resources are distributed unevenly. However, information on sugarcane root architecture and plasticity is limited because of its larger plant size and longer growth period. The mechanism of root formation is divided into internal (main stem and tillers) and inter-individual (sole and mixture) interactions. Our study attempted to reveal the effect of tiller regulation (internal interaction) and mixed cultivars (inter-individual interaction) on root formation in sugarcane. Tiller regulation decreased the total root biomass but distributed the roots in deeper soil, indicating that high-tillering characteristics may not necessarily contribute to a deeper root system and drought tolerance, at least during the early growth stages of tillers. Our results also revealed that the total shoot biomass, including the main stem and tillers, was not influenced by tiller regulation, suggesting a plasticity of shoot growth under tiller regulation. Roots under mixed cultivars grew well in the middle soil layer (20–50 cm depth) and were thoroughly distributed in each soil layer. These facts suggested that root habitat segregation of each cultivar may have occurred. Such root densification did not increase shoot biomass in the present study; however, it has the potential for improving lodging resistance and resource use efficiency under some abiotic stress conditions.