Potato Research 31 (1988) 297-304 Subcellular localisation of calcium in potato tubers K. J. OPARKA and H. V. DAVIES Department of Physiology and Crop Production, Scottish Crop Research Institute, lnvergowrie, Dundee DD2 5DA, UK Accepted for publication: 31 August 1987 Additional keywords." Antimonate, microscopy, Solanurn tuberosurn L. phloem, xylem Summary Deposits, believed to contain calcium, were located in different cells of the potato tuber after using an antimonate procedure. In the starch-storage parenchyma of the medulla and cortex antimonate deposits were abundant within vacuoles, on the tonoplast, and to a lesser extent in cell walls. Phloem tissues contained a high concentration of deposits, mainly within compan- ion cells and phloem parenchyma, consistent sites of deposition being mitochondria, plastids and tonoplasts. Deposits were abundant within vacuoles o fcells of thexylem parenchyma. Within vessel elements deposits were limited to the surface of secondary wall thickenings. Antimonate deposits associated with the middle lamella of xylem parenchyma were rarely found. The results are discussed in relation to previous findings for potato tubers and for higher plant cells in gener- al. Introduction The cytochemical location of sites of calcium in higher plant cells has been investigated both in relation to its physiological role within cells and tissues (Van Steveninck et al., 1976; Wick & Hepler, 1980; Slocum & Roux, 1982, 1983; Tandler et al., 1973) and its potential role in disease resistance (Stockwell & Hanchey, 1982). Physiological disord- ers related to calcium deficiency are common in many crops although the reasons why calcium should be scarce within a given tissue are not well understood. Clarkson (1984) has suggested that calcium-deficiency symptoms in higher plants seldom arise because of a failure of calcium supply to the roots but rather from the plant's inability to distrib- ute calcium once it has been taken up from the soil. Potato tubers are low in calcium (Wiersum, 1966; Davies & Millard, 1985) and some of the physiological disorders of the tuber, such as internal rust spot (Collier et al., 1978) are known to be associated with calcium deficiencies within the tuber. However, the exact location of calcium in cells of the potato tuber has not been determined, so if the role of Ca in physiological disorders is to be examined further, then it is clearly desirable to know the cellular and subcellular distribution of Ca in the tubers. In a previous study of calcium distribution and fractionation in the potato tuber, Davies & Millard (1985) found that over 90% of the calcium was in a physiologically active form, only a small proportion being present in insoluble forms such as calcium oxalate. Autoradiography of tubers grown in the presence of 45Ca further revealed high activity in the periderm and vascular ring and in regions occupied by internal and external phloem bundles. The experiments reported here were designed to determine the subcellular location Potato Research 31 (1988) 297