Understanding the role of calcium in kiwifruit: ion transport, signaling, and fruit quality

Calcium (Ca2+) plays a fundamental role in metabolic processes, and it is involved in several structural functions at the cell level, such as vacuole osmotic regulation, cell wall strengthening, and plasma membrane stability, as well as acting as a secondary messenger for several different signals. The role of Ca2+ in signal transduction and cell wall organization is crucial for stress responses, cell activity, and plant tissue development. In addition, Ca2+ is essential in modulating enzymatic activities, hormonal control, water, and ion transport across the plasma membrane. Although calcium’s role in fruit trees is well studied, many of its specific functions in kiwifruit remain unclear, including the optimal amount of Ca2+ in fruit and its distribution in fruit cells for the best pre- and post-harvest fruit quality. Calcium transport to the fruit is mainly regulated by the xylem sap flow; however, the contribution of fruit transpiration and the requirements of fruit cells are not clear. Understanding the kinetics of Ca2+ accumulation in fruit under different environmental conditions can help establish correct nutrient management. This review addresses the current knowledge on Ca2+ involvement in plant physiology, metabolic processes, structural functions, and fruit growth, quality, and storage, with particular emphasis on Actinidia chinensis. In addition, the different analytical techniques used for the quantification and definition of Ca2+ in different plant organs, including stain technology, X-rays, and advanced imaging methods, are here explored.