SPUR LEAVES INCREASE CALCIUM IN YOUNG APPLES BY PROMOTING XYLEM INFLOW AND OUTFLOW

The effects of spur leaf removal on xylem sap flows and calcium accumu lation in fruit of apple (Malus xdomestica Borkh. 'Royal Gala') were d etermined 56 to 61 days after full bloom. Fruit calcium concentrations were reduced but fruit size was not influenced by partial spur defoli ation at bloom. Apples exchanged xylem sap with the tree in daily cycl es of flow reversal. The presence of local spur leaves promoted this e xchange by accentuating the xylem sap drawn out of the fruit during th e day, requiring more to flow back into the fruit at night to replace it. Calcium concentrations were lower in the xylem sap leaving the fru it than in that entering it. The reduced calcium accumulation in the f ruit borne on defoliated spurs can therefore be attributed to the redu ced volume of xylem sap exchanged between tree and fruit.