Calcium is one of the most poorly reutilized nutrients. Its deficiencies cause various physiological disturbances and, consequently, reduce the quantity and quality of yields. Reduced content of Ca2+ ions in cells leads to development of, e.g., bitter pit in apples. Efficient and instantaneous mitigation of Ca2+ deficiencies is provided by foliar feeding. There are no detailed data on the effect of foliar feeding with various calcium forms on the cell structure or on the microanalysis and mapping of this element in apple fruit cells. Therefore, we carried out comparative studies of the ultrastructure of epidermis and hypodermis cells, to assess the content and distribution of calcium in the cell wall, cytoplasmic membrane, cytoplasm, and precipitates of Malus domestica Borkh. 'Szampion' fruit exposed to four Ca treatments, including the control with no additional Ca supplementation (I) and foliar applications of Ca(NO3)2 (II), CaCl2 (III), and Ca chelated with EDTA (IV). Light and transmission electron microscopy and an X-ray microanalyzer were used and showed a beneficial effect of calcium preparations on the ultrastructure of fruit epidermis and hypodermis cells, manifested in the presence of a normally developed cell wall with a regular middle lamella, preserved continuity of cytoplasmic membranes, and stabilized cell structure. In the selected elements of apical epidermis cells, the highest level of Ca2+ ions was detected in the middle lamella, cell wall, plasmalemma, and cytoplasm. The highest increase in the Ca2+ content in these cell constituents was recorded in treatment IV, whereas the lowest value of the parameters was noted in variant III.
Keywords: Ca mapping; apple tree; cell wall; cytoplasmic membrane; epidermis; hypodermis.