Xylem, Phloem and Transpiration Flows in Developing Apple Fruits

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Abstract
Xylem, phloem and transpiration flows were measured in developing fruits of the apples Royal Gala and Cox's Orange Pippin at early, mid and late stages during their development. Fruit volume growth was first computed from measurements of diameter made using a system of sensitive displacement transducers. Xylem, phloem and transpiration flow components (of which fruit volume growth is the integral) were then separated using a scheme of treatments which disabled one or other flow at a time. Changes observed during development in the patterns of the three flows are in general agreement with expectations based on less direct observations (loc. cit. Ferguson and Watkins, 1989). Recognizing the distinctive mineral compositions of xylem and phloem streams, the changes also accommodate, and seem to explain, published observations of mineral accumulation in apple. Significant differences in the pattern of xylem and phloem flow were observed between the varieties examined. These may explain varietal differences in susceptibility to the mineral imbalance disorder bitter-pit. Xylem flows were shown to reverse at times; that is they flowed from fruit to tree. This occurred particularly during periods of high evaporative demand. Xylem reversal is of importance to the overall water economy of a tree but may also have special importance to the mineral composition of the fruit.