Crop root system traits cannot be seen as a silver bullet delivering drought resistance

Background: The role of roots in determining the yield of a crop in water-limited environments (drought resistance) is controversial as individual root traits for water uptake do not necessarily guarantee a root system is efficient at capturing soil water. For increased drought resistance do we need deep or shallow root systems?, more or less roots?, high or low root resistance?, prolific root branching or restricted branching?, high sensitivity or low sensitivity to water deficits? Scope: The root system characteristics to benefit yield under drought vary with the amount and distribution of precipitation; texture, depth and water-holding capacity of the soil; as well as root system characteristics. Our analysis shows that other drought-resistance traits such as early vigour, osmotic adjustment, and stay-green, increase drought resistance through increased root growth and water use. Conclusions: The relationship between the root system and drought resistance is complex because the expression of the root system traits that increase drought resistance depends on a number of factors and their interactions. Breeding for root systems that increase drought resistance cannot depend on identifying a silver bullet, single marker or single gene.