THE FORM, DISTRIBUTION AND SEASONAL ACCUMULATION OF CALCIUM IN KIWIFRUIT LEAVES

Abstract

Seasonal changes in the content of Ca and Mg in leaf blades collected from mature kiwifruit [Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson var. deliciosa] vines in a high‐producing orchard were measured by a sequential fractionation procedure (14 M acetic acid, 0.25 M HC1 and residual). Total Ca and Mg concentrations decreased within the first 4 weeks of growth, but increased linearly thereafter to be 1322 and 197 μmol g⁻¹ dry weight, respectively, by leaf fall. Total quantities of Ca and Mg accumulated at this time were approximately 2500 and 380 μmol per blade. Calcium oxalate, dissolved in the HC1 extract, was the predominant fraction at every harvest, accounting for up to 79 % of the total Ca content shortly after leaf emergence, and decreasing to 42 % at leaf fall. The quantity of physiologically active Ca (acetic acid‐soluble) ranged from a minimum of 15 % (8 weeks after emergence) to a maximum of 41 % at the end of the season. Forms of Ca remaining in the solid residue after extraction made only a minor contribution (3 to 21 %) to the total content at any time during the season. In contrast, however, 89 to 96% of the total Mg was extracted by acetic acid at every harvest, with the remainder dissolved by HC1. Scanning electron microscopy and X‐ray microanalysis of the lamina of mature leaves indicated the presence of two crystalline, Ca‐rich products which differed in their morphology, location and distribution. Large styloids (120 to 270 μm in length) were concentrated in idioblasts adjacent to vascular tissue, while bundles of four‐sided raphides, containing wedge‐shaped apices, were located predominantly between the major transport vessels of the vascular tissue. X‐ray diffraction analysis of crystalline isolates confirmed that both morphological forms are calcium oxalate monohydrate (whewellite).