Individual fruit of `Delicious' apple (Malus domestica Borkh.) were exposed to low, high, or ambient relative humidity (RH) levels during different stages of fruit development to study the importance of transpiration and the xylem system in supplying Ca to fruit. The Ca content of fruit exposed to low RH was the same or higher than that of fruit exposed to high RH. Treatments imposed early or late in the season usually affected fruit Ca levels similarly. Fruit weight was not consistently affected by RH treatments. The xylem may be a significant source of fruit Ca throughout the season.
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