Developmental changes in xylem functionality in kiwifruit fruit: implications for fruit calcium accumulation

In kiwifruit, as in most fruits, storage quality is related to calcium concentration and many disorders are associated with low fruit calcium status. Previous studies show that after an early rise, fruit calcium concentration decreases because calcium influx ceases by the mid-growth stage whereas volume growth continues till harvest. Calcium transport to the fruit is exclusively via the xylem - calcium is not phloem mobile. We postulate that declining fruit xylem functionality is responsible for this pattern of calcium accumulation. Xylem functionality was measured from fruit setting to harvest in 'Zespri Gold(TM)' kiwifruit. The applied measurement technique consists in "transpiring" an apoplast-mobile dye into detached fruit through the stalk and recording the percentage of dyed (functional) bundles in sections cut along the length of the fruit. For the middle part of the fruit analysed, the dye method revealed high bundle functionality till about d10 (after bloom). A drastic reduction in the number of functional bundles occurred around d20 and, again, around d55 and d90. Some recovery was observed between these time steps. An almost complete permanent dysfunction was evident nearly at d120 up to harvest time. Fluctuations in bundle functionality are interpreted as being due to vessel breakage (resulting from stretching caused by fruit expansion) and temporary functional recovery resulting from early season differentiation of new vessels. The length growth rate is very high in the first 8 weeks from full bloom with a maximum value of 1.3 mm d(-1) at the 4(th) week. In this period, the fruit achieves 70% of the total length at harvest time. We conclude that patterns of xylem dysfunction in Zespri Gold(TM) provide a satisfactory explanation of the well-known patterns of fruit calcium accumulation.