The Growth of the Kiwifruit in its Final Stages

The growth of nine fruit of Actinidia deliciosa ‘Hayward’ (kiwifruit) was monitored over several days in September 2005, during the last stage of fruit growth, using highly sensitive, custom-built fruit diameter gauges, which allow determination of minute variations in fruit size. These gauges were placed on fruit which, after their growth had been monitored for a few days, were subjected to girdling of the cane above and below the fruit, so to sever the phloem connection; subsequently, the fruit was detached and left in the same position in the canopy. The fruit gauges were left on the fruit throughout. Data collected allowed determination of: total fruit growth; the component of that growth due to phloem and to xylem contributions, and water loss by transpiration. In addition, an automated custom-built open system was used for direct measurement of fruit transpiration over time. Fruit were examined at the specific time of the season, which is indicated in the literature as the one when loss of xylem and fruit trichome functionality occurs. The goal of this preliminary study was to verify experimentally whether xylem and fruit transpiration were reduced to zero at this time. Fruit grew slowly over the entire period of study (about 10 days in September), and the changes due to xylem inflow-outflow from the fruit were virtually zero, as was the shrinkage of fruit due to transpiration losses. These results confirm previous observations that, at the beginning of the third stage of fruit growth, there are no changes in fruit volume due to xylem flows, and to transpiration losses via the skin. These observations, coupled to the increase in dry matter content reported for the same time of the season, indicate that at stage 3 in kiwifruit an active model of phloem unloading must be adopted. In fact, without xylem and transpiration contributions to the fruit daily shrinking and enlarging, it would be difficult for the fruit to adjust its osmotic properties and turgor pressure in order to attract more solutes from the phloem by bulk flow (as appears to be the case in peach at stage III, for example). These data support either a diffusion based process of phloem unloading, or an active apoplastic mechanism. A careful examination of the velocity of fruit weight increase might shed some light on the type of process involved in phloem unloading.