Light influences transpiration and calcium accumulation in fruit of kiwifruit plants (Actinidia deliciosa var. deliciosa)

Calcium (Ca) is an essential plant nutrient involved in determining fruit quality of several fruits, including kiwifruit. The objective of the present study was to evaluate the influence of light intensity on transpiration and water flow into fruit of kiwifruit (Actinidia deliciosa var. deliciosa, C.F. Liang et A.R. Ferguson) and the resulting effects on Ca accumulation. At fruit-set two light treatments were imposed on single canes. The exposed treatment was maintained through summer pruning, while the shade treatment was imposed using shade cloth. Fruit transpiration was measured on attached fruit using a portable photosynthesis system. The inflow of water into the fruit via the xylem was estimated from fruit transpiration and its relationship with fruit Ca accumulation determined. The concentration of Ca was measured in xylem sap extracted using a Scholander pressure chamber. Following the first 40–50 days after fruit-set (AFS) the cumulative xylem water inflow into the fruit increased exponentially in both treatments, but by the end of the growing season the total influx was 30% higher in the exposed treatment than in the shaded treatment, reaching 140 g per fruit. The shade treatment influenced the concentration of Ca, causing accumulation in the fruit to be about 50% of that in the exposed treatment. Our results suggest that transpiration is not the only factor controlling Ca transport, and that light also influenced the Ca concentration in xylem sap. Taking into account that auxin is able to stimulate Ca uptake and that light promotes the biosynthesis of auxin protecting phenols (hydroxycinnamic acids), a new working hypothesis is proposed: light, induces the biosynthesis of such phenols, indirectly decreases auxin degradation, and therefore, increases Ca accumulation.