Effects of Drought Stress on the Growth, Water Relations and Vascular Flows of Young 'Summerkiwi' Fruit

The early stage of kiwifruit development is characterized by high water in/outflows via xylem and epidermis transpiration. Therefore, early drought stress may heavily impact on the biophysical mechanism of fruit growth. This study tests how reducing irrigation at this time of the season affects kiwifruit growth rate, vine-to-fruit water relations and vascular flows. At 6 weeks after full bloom, 50% irrigation was applied to four vines of the cultivar 'Summerkiwi', while four fully irrigated vines were used as control. The daily patterns of fruit growth, vascular and transpiration flows were determined on one fruit per vine from four days after treatment imposition by continuous monitoring of fruit diameter by automatic, custom-built fruit gauges. The daily courses of leaf, stem and fruit water potential were measured on stressed and control vines, using a Scholander pressure chamber. Leaf, stem and fruit water potentials were negatively and heavily affected by drought at all times of measurement. As stem and fruit water potentials were similarly reduced in stressed vines, the resulting stem-to-fruit water potential gradients were not affected during most of the day. However, in late afternoon they were higher in control vines, and determined higher xylem flows and relative growth rates in control fruit. For the rest of the day, fruit growth rates, vascular and transpiration flows were not affected by water depletion. In spite of this, stressed fruit imported a lower amount of phloem sap at the end of the day, probably due to a decrease in canopy carbon assimilation. In conclusion, while both fruit and vine water status are deeply affected by drought stress, differences in the relative gradients are minimal, thus reducing, in the short term, the stress impact on water imports and fruit growth.