IS LATE STAGE REDUCTION OF KIWIFRUIT SAP FLOW DUE TO LOSS OF XYLEM FUNCTIONALITY?

The late stage of fruit growth in Actinidia deliciosa is characterized by slow volume increases, low transpiration and xylem flows. While fruit transpiration is reduced due to anatomical modifications of the fruit epidermis, it is not clear yet whether decreases in xylem flow may be due either to a diminished hydrostatic pressure gradient in the stem-to-fruit path, or to a loss of functionality of the xylem vessels. The daily patterns of xylem and transpiration flows were determined on two consecutive years at 14 and 13 weeks after full bloom, respectively, on 4 control and 4 fruit whose water loss was enhanced by removing half of their epidermis. Measurements were carried out using highly sensitive, custom-built fruit diameter gauges, which allow determination of minute variations in fruit size. These gauges were placed on fruit which were subjected to girdling of the cane above and below the fruit, so to severe the phloem connection. After one day fruit were detached and left in the same position in the canopy. The fruit gauges were left on the fruit throughout the study. The xylem flow was calculated for each fruit as the difference in its growth rate between girdled and detached conditions. In addition, fruit and stem water potentials were determined in control and treated fruit, using a Scholander pressure chamber. Peeling off the kiwifruit epidermis resulted in a three-fold rise of fruit daily water loss that maintained higher rates during the whole day. Despite such great differences in water loss, control and treated fruit did not show differences either in the daily amount and in the pattern of xylem flow, which remained low, albeit present, during the whole day. In both years, stem-to-fruit water potential gradients were not related to daily changes in xylem flow. According to these results, the reduction of kiwifruit xylem flow during the last part of the season must be attributed to a decrease in the xylem hydraulic conductance, which may be caused by disruptions or occlusions of the vessels, and is not due to decreases in the hydrostatic pressure gradients along the stem to fruit pathway.