Fruit Calcium: Transport and Physiology

Abstract

Calcium has well-documented roles in plant signaling, water relations and cell wall interactions. Significant research into how calcium impacts these individual processes in various tissues has been carried out; however, the influence of calcium on fruit ripening has not been thoroughly explored. Here, we review the current state of knowledge on how calcium may impact the development, physical traits and disease susceptibility of fruit through facilitating developmental and stress response signaling, stabilizing membranes, influencing water relations and modifying cell wall properties through cross-linking of de-esterified pectins. We explore the involvement of calcium in hormone signaling integral to the physiological mechanisms behind common disorders that have been associated with fruit calcium deficiency (e.g., blossom end rot in tomatoes or bitter pit in apples). This review works toward an improved understanding of how the many roles of calcium interact to influence fruit ripening, and proposes future research directions to fill knowledge gaps. Specifically, we focus mostly on grapes and present a model that integrates existing knowledge around these various functions of calcium in fruit, which provides a basis for understanding the physiological impacts of sub-optimal calcium nutrition in grapes. Calcium accumulation and distribution in fruit is shown to be highly dependent on water delivery and cell wall interactions in the apoplasm. Localized calcium deficiencies observed in particular species or varieties can result from differences in xylem morphology, fruit water relations and pectin composition, and can cause leaky membranes, irregular cell wall softening, impaired hormonal signaling and aberrant fruit development. We propose that the role of apoplasmic calcium-pectin crosslinking, particularly in the xylem, is an understudied area that may have a key influence on fruit water relations. Furthermore, we believe that improved knowledge of the calcium-regulated signaling pathways that control ripening would assist in addressing calcium deficiency disorders and improving fruit pathogen resistance.

Ultrasound enhances calcium absorption of jujube fruit by regulating the cellular calcium distribution and metabolism of cell wall polysaccharides

“…Hocking et al . suggested that loss of firmness likely involves modification of pectin side chains, depolymerization of apoplasmic calcium–pectin crosslinks and degradation of cell wall polysaccharides.…”

Control efficacy of Ca‐containing foliar fertilizers on bitter pit in bagged ‘Fuji’ apple and effects on the Ca and N contents of apple fruits and leaves

“…We also found that, compared with the control, the three tested Ca‐containing foliar fertilizers influenced the Ca content and N/Ca ratio of bagged apple fruits much more than the fruit N content, leaf Ca content, leaf N content and N/Ca ratio of leaf (Figs and ). The results showed that the tested Ca fertilizers containing CaCl 2 , Ca(NO 3 ) 2 and especially Ca(HCOO) 2 could help to supply much more Ca nutrient to the bagged apple fruits, confirming there was much higher Ca content in leaves than in fruits, and a lower rate of nutrients supplied to fruits, which indicated that the three tested Ca‐containing foliar fertilizers could potentially improve the quality of bagged apple fruits by improving the distribution of the mineral nutrients in bagged apple fruits and the rest of the apple plant.…”

“…The Ca content is generally much higher in leaves, which have greater transpiration rates than do fruits, because Ca is imported into fleshy fruits only in small amounts . The rate of water supply and nutrients to fruits is lower than to the rest of the plant, and Ca 2+ delivery to organs depends on the concentration of Ca 2+ in the xylem sap, transpiration and growth rate …”

Assessment of calcium and zinc accumulation in cultivated and wild apples

“…In addition, calcium has importance as an intracellular secondary messenger, and it is known to participate in auxin, gibberellic acid GA, and abscisic acid ABA signaling to regulate fruit set and ripening, cell division and expansion, and fruit softening . However, the influence of calcium nutrition, transport and signaling pathways on fruit development and ripening still remains largely unclear …”