Calcium uptake by developing apple fruits. I. Seasonal changes in calcium content of fruits

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Abstract
Trends in calcium uptake by developing fruits of four apple cultivars were followed during three seasons. Calcium uptake continued from flowering to harvest, though there was a tendency for the uptake rate to decline in the last two months of fruit growth. An attempt was made to predict Ca uptake rates on the basis of the product of sap Ca concentration and the water flow into the fruit. The water flow was estimated as the sum of the net water uptake for fruit growth and that lost in evaporation from the fruit, and the Ca concentration was assumed to be equal to that of xylem sap extracted from shoots with a pressure chamber. Using these values the mass flow hypothesis consistently underestimated Ca uptake rates by developing fruits early in the season and markedly overestimated them in the month or so prior to harvest. Possible reasons for these discrepancies are discussed.