Studies on the translocation characteristics of C-photoassimilates to fruit during the fruit development stage in 'Fuji' apple.

In order to define translocation characteristics of C-photoassimilates to fruit during the fruit development stage in 'Fuji' apple, the C labeled tracer method was used in whole five-year-old 'Fuji'3/M26/Malus hupehensis (Pamp.) Rehder apple trees at different days after flowering (DAF). The changes in C translocation to the fruit, source strength of the leaves, and sink strength of the fruits were assessed. The results indicated that the δC value and C distribution rate of the fruit increased first and then decreased with the increase in the fruit development period, being higher from 120 to 135 DAF. The leaves appeared to moderately senesce in an attempt to maintain high photosynthesis during 120-135 DAF, which promoted the outward transport of photoassimilates. The single fruit weight and longitudinal and transverse diameter of the fruit increased rapidly during 120-150 DAF, which increased the sink zone for the unloading of photoassimilates in the fruit. The activity of sorbitol dehydrogenase (SDH) and amylase (AM), the content of indole-3-acetic acid (IAA), the gibberellin (GA) and abscisic acid (ABA) in the fruit flesh, and the gene expression levels of MdSOT1, MdSOT2, MdSOT3, MdSUT1, and MdSUT4 in the fruit stalk tissue were higher during 120-135 DAF. At this point, the difference in the sorbitol content between the fruit stalk and fruit flesh was also at the highest level of the entire year. These factors together increased the sink activity of the fruit, thus improving the photoassimilate transport efficiency to the fruit.