Linking Fruit Ca Uptake Capacity to Fruit Growth and Pedicel Anatomy, a Cros- Species Study

Abstract

Calcium (Ca) in flesh fruits is important for quality formation and maintenance. Moststudies on fruit Ca focus on one species. This study attempted to understand someuniversal relations to fruit Ca uptake across species. Calcium contents in fruit tissueswere analyzed in different fruits, including three cultivars of litchi, two cultivars each ofgrape and citrus, and one cultivar each of loquat, apple, pear, Indian jujube, and longan.In situCa distribution was revealed with electron probe and xylem functionality visualizedby dye tracing. Fruit Ca uptake rate and activity were calculated and correlated withfruit growth and pedicel anatomy. The results showed that fruit Ca uptake rate wasthe highest in pomes (loquat, apple, and pear), followed by Indian jujube drupe, arillatefruits (litchis and longan) and citrus, while grape berries were the lowest. Fruit Ca uptakerate showed a strong positive correlation to growth rate. However, Ca uptake activity,reflecting Ca uptake rate relative to growth, was the highest in arillate fruits and loquatand lowest in grape berries, and had a poor correlation with fruit growth rate. In all fruits,Ca concentration in the pedicel was higher than in the fruit, and they displayed a goodpositive correlation. In the pedicel, Ca was most abundant in the phloem. Dye tracingshowed that xylem function loss occurred with maturation in all species/varieties. Applehad the poorest xylem functionality with the least development of secondary xylem, butits Ca uptake rate was among the highest. Vessel density, size and area in the pedicelshowed no correlation with fruit Ca uptake rate. It is concluded that: (1) fruit growthmay be a key determinant of Ca uptake; (2) the universal pattern of Ca being higherin the pedicel than in the fruit indicates existence of a pedicel-fruit “bottleneck” effectin Ca transport across species; (3) xylem functionality loss with fruit maturation is alsoa universal event; (4) in the pedicel, Ca is more distributed in the phloem; (5) vesselmorphology in the pedicel is not rate-limiting for fruit Ca uptake; (6) phloem pathwaymight contribute to fruit Ca uptake.