347 Mapping the Distribution of Calcium on Apple Tissue with Proton- Induced X-Ray Emission – after Application of Additional Pre-Harvest Foliar or Soil Calcium R.E. Wilsdorf and E. Lötze J. Mesjasz-Przybylowicz and Department of Horticultural Science W.J. Przybylowicz a Stellenbosch University Materials Research Department Stellenbosch iThemba LABS, National Research South Africa Foundation Somerset West South Africa Keywords: elemental mapping, PIXE, X-ray microanalysis, Ca deficiency, Malus domestica Abstract The development of calcium (Ca) deficiencies in the apple fruit are ascribed to a lack of accumulation of Ca in specific areas of the fruit. Localized deficiencies therefore commonly occur in spite of sufficient levels of total fruit Ca. A trial was conducted during two consecutive seasons (2008/9 and 2009/10) to evaluate the contribution of either foliar or soil-applied Ca to the localization of Ca in “Braeburn” apple fruit. Ca (Ca(NO 3 ) 2 ) was applied either in the form of soil pellets (Tropicote TM ) at fruit set or after harvest, or as a series of weekly foliar applications (Calflo TM ) between 21 and 70 days after full bloom (dafb). Additional treatments consisting of combinations of these were also applied. Elemental Ca mapping across the radius of the fruit was achieved via the use of micro-PIXE (particle induced X-ray emission). In all treatments, Ca was concentrated in the skin and core of the fruit, with the lowest values occurring in the outer cortex. At 80 dafb, Ca was highly associated with vascular bundles in an otherwise homogenous cortex. Fruit set Ca soil applications consistently resulted in fruit with less Ca at 80 dafb compared to the other treatments. The prevalence of vascular bundles mostly determined the average Ca concentrations in the core, inner and outer cortex regions and emphasizes the importance of the functionality of vascular bundles throughout the season. Foliar Ca altered the distribution of Ca in the fruit and resulted in a significant increase in Ca in the outer cortex relative to the core. INTRODUCTION Calcium (Ca) differs from other nutritional elements as most of it is incorporated in the foliage and permanent structures of the tree, with only small amounts being translocated to the fruit, resulting in Ca-related deficiencies in apple orchards worldwide (Saure, 2002; 2005). Although the total Ca content in the fruit at harvest is usually sufficient for good quality, the distribution of Ca within the fruit tissues often leads to localised deficiencies in the fruit (Saure, 2005). Ca is typically concentrated within the skin and core of the fruit (Ferguson and Watkins, 1983; Himelrick and McDuffie, 1983), decreasing gradually across the radius of the fruit, with the lowest values occurring in the outer cortex (Wilkinson and Perring, 1961). This dilution of Ca concentration in the cortex is due to the higher growth rate in the outer regions of the fruit as it expands during the season (Saure, 2005). A decreasing number of vascular bundles towards the outer areas of the fruit may also contribute towards this distribution (Hanekom, 1973). Soil applied Ca is absorbed through the root surface and translocated to the fruit and leaves via the vascular system (White, 2001). The xylem vessels in fruit are very a On leave from AGH University of Science and Technology, Faculty of Physics & Applied Computer Science, 30-059 Krakow, Poland. Proc. VII th IS on Mineral Nutrition of Fruit Crops Eds.: S. Poovarodom and S. Yingjajaval Acta Hort. 984, ISHS 2013