Predicting the Rooting Depth, Dynamic Root Distribution and the Yield of Sunflower under Different Soil Salinity and Nitrogen Applications

•High salinity induced rapid fine root elongation after sunflower bud initiation.•The effect of nitrogen fertilizer on fine root growth depended on soil salinity levels.•Belowground root development was nonlinearly related with aboveground canopy growth.•A model was proposed to describe the flexible fine root distribution under salt stress.