Mineral nutrients taken up by the roots are, as a rule, transported in the xylem to the shoot, and photoassimilates transported in the phloem to the roots. According to the Thornley model of photosynthate partitioning, nutrient deficiencies should favour photosynthate partitioning to the roots. Examples are cited to show that this preferential partitioning is dependent on phloem mobility and hence on nutrient cycling from shoot to roots. Thus, root growth is enhanced under nitrogen and phosphorus deficiencies, but not under deficiencies of nutrients of low mobility in the phloem, such as calcium and boron. Enhanced root growth under nutrient deficiency relies on the import of both photosynthates and mineral nutrients. Cycling of mineral nutrients serves a number of other functions. These include the root supply of nutrients assimilated in the shoot (nitrate and sulphate reduction), maintenance of cation-anion balance in the shoot, providing an additional driving force for solute volume flow in the phloem and xylem, and acting as a shoot signal to convey nutrient demand to the root. Cycling of certain mineral nutrients through source leaves has a considerable impact on photosynthate export as demonstrated in impaired export under magnesium, potassium, or zinc deficiencies. Mineral nutrient deficiency can, therefore, affect photosynthate partitioning either directly via phloem loading and transport or indirectly by depressing sink demand.