Mineral nutrients affect fruit quality both directly and indirectly, as in the case of nitrogen, that if applied in excess overstimulates shoot growth and causes poor colour development of the shaded fruits. Direct effects of nutrients on fruit quality depend upon nutrient content and nutrient balance in the fruit. It is generally accepted that some aspects of fruit quality such as soluble solid concentration are positively related to fruit potassium content, while during storage, fruit quality is favoured by low nitrogen and high calcium fruit levels. In order to manipulate mineral content and balance in fruits it is of upmost importance to know the dynamics of nutrient accumulation in this organ. Total amounts of nutrients in the fruit account for both elements taken up by the roots in the current season and for those recycled from previous years. Within one season, for most nutrients (e.g. nitrogen, potassium, phosphorus) translocation to fruits occurs both via the xylem and phloem, while calcium is provided only via the xylem. In apple, both phloem and xylem fluxes to fruits contribute to growth in early and mid stages of development, while phloem sap mainly contributes to enlargement close to harvest. High rates of xylem sap enter the fruit of peach even close to harvest, providing the water needed for transpiration. Mature leaves export via phloem organic nitrogen, magnesium and potassium to the developing fruits. Potassium requirements of fruits progressively increase as fruit maturity approaches. Published evidence indicates that at least in apple and peach, xylem sap reaches the fruit also at night when stomata are closed and root pressure drives the flux of water and solutes.
