CARBOHYDRATES, ORGANIC ACIDS AND MINERAL ELEMENTS IN XYLEM SAP BLEEDING FROM KIWIFRUIT CANES

Comparison of xylem sap mineral concentrations between kiwifruit shoot types using spittlebugs for non-destructive sampling of sap

Excreta of the meadow spittlebug [Philaenus spumarius L. (Homoptera: Cercopidae)] feeding on leaves and pedicels of kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa 'Hayward'] were collected from insects from two different positions in the vine: from long, non-terminating axillary shoots producing fruit that are high in Ca²⁺ and low in K⁺ and from short-terminating axillary shoots producing fruit that are low in Ca²⁺ and high in K⁺. The Ca²⁺, Mg²⁺, K⁺ and P concentrations in the excreta were determined, and found to be similar to those in the xylem sap. Daily and seasonal changes in xylem sap composition were compared in excreta collected from the two different shoot types. On average, Ca²⁺ and Mg²⁺ concentrations were higher and K⁺ and P concentrations were lower in xylem sap collected from pedicels on long, non-terminating axillary shoots than in sap collected from pedicels on short-terminating shoots. Differences in the mineral concentration between these two shoot types may therefore be due to differences in the xylem sap mineral concentration reaching the fruit. There was no measurable gradient in xylem sap composition within the parent shoots that could explain the differences between sap composition of the two axillary shoot types. Long, non-terminating shoots had higher leaf area, were more exposed, had higher stomatal conductance and rates of transpiration, and more negative leaf water potentials than short-terminating shoots. The higher xylem sap Ca²⁺ and Mg²⁺ concentrations of long shoots were therefore associated with higher rates of water transport to the long shoots. Xylem sap concentration differences between these two shoots types may have been because of differential loading or unloading of minerals between shoot types, associated with differences in transpiration rate or shoot growth rates. The higher transpiration rate of long shoots may cause phloem immobile minerals such as Ca²⁺ to accumulate to higher levels at cation exchange sites in the shoot apoplast, resulting in increased xylem sap concentrations arriving at the fruit.

Modelling malic acid accumulation in fruits: relationships with organic acids, potassium, and temperature

Malic acid production, degradation, and storage during fruit development have been modelled. The model assumes that malic acid content is determined essentially by the conditions of its storage in the mesocarp cells, and provides a simplified representation of the mechanisms involved in the accumulation of malate in the vacuole and their regulation by thermodynamic constraints. Solving the corresponding system of equations made it possible to predict the malic acid content of the fruit as a function of organic acids, potassium concentration, and temperature. The model was applied to peach fruit, and parameters were estimated from the data of fruit development monitored over 2 years. The predictions were in good agreement with experimental data. Simulations were performed to analyse the behaviour of the model in response to variations in composition and temperature.