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Kinetics of Uptake and Export of Foliar-Applied Radio-Labeled Phosphorus by Leaf and Fruit Rind of Satsuma Mandarin During Fruit Development

Abstract Leaf and fruit rind retention, uptake kinetics, export kinetics, total uptake, uptake speed, and the total export of foliar-applied radio-labeled-32phosphorus (32P) were examined for 11-year-old Satsuma mandarin (Citrus unshiu Marc.) trees on Poncirus trifoliata (L) rootstock. Under the subtropical climate of south china, both leaf and fruit rind took up 32P that was applied on their surface. The linear regression equations for the relationships between retention of 32P and leaf and fruit surface area were, respectively, y = 0.0182x + 0.6237 (r 2 = 0.8589) and y = 0.0003x + 0.6413 (r 2 = 0.8231). Leaf retention was greater than that of fruit rind. An average of 24 h after treatment, the leaf retained 2.20 ± 0.02 µmol, 74.84 ± 0.68% of original amount of 32P applied, but fruit rind only retained 47.45 ± 0.68% of the total 32P applied. Leaf uptake of 32P per g dry weight per hour during the first hour after application was nearly 10 times faster than that of the fruit rind. Due to differences in surface characteristics and structure, the leaf took up 0.1128 µmol/g · h; the rind absorbed 0.0129 µmol/g · h. The quantities of 32P exported during 72 h were identical to those for uptake of 32P. Within 72 h after application, the leaf exported 1.61 µmol/g dry weight tissue, whereas the rind exported 0.143 µmol/g dry weight tissue of the initial amount of 32P applied. The retention of 32P by leaves on shoots with fruit (+F) and leaves of shoots without fruit (−F) did not differ significantly. On the third day after treatment, the content of 32P per single leaf reached maximum value. On average, leaves on +F shoots and −F shoots retained, respectively, 2.80 ± 0.05 µmol and 2.82 ± 0.03 µmol. Some 32P was transported into the adjacent fruit fives days after the application of 32P. Leaves of +F shoots transported 10.89 ± 5.8% of their 32P into adjacent fruit seven days after treatment.