Comparative observation on the anatomical structure of Longan fruit during ripening and sugar-dropping on tree

This paper studied the changes of pericarp and aril structure of longan fruit during ripening and senescence（ sugar-dropping） on tree，and the two longan cultivars cv. Shixia and Chuliang were used as the materials，and the TSS content was also been determined. The results showed that duration of ripening of Shixia and Chuliang fruit was 14 d and 21 d，then the sugar-drpping began，and their TSS content dropping rate was 0.60% and 0.31% every day，respectively. When the fruit got ripening，the pericarp structure was still intact，the exocarp was composed of severral layers of parallel arranged metamorphic cells with some discontinuous cuticle and wax blocks attached to the outer epidermis，the mesocarp composed of large lamellar parenchyma cells with lumpish suberins， stone cells and vascular bundles among them，and the endocarp composed of 1～2 layers of tightly arranged parenchyma cells with thick cuticle in cell wall. Meanwhile，the aril surface was smooth，and the parenchyma cell was complete，clear and arranged neatly. When the TSS content dropping，there was less waxy layer and epidermal hairs on epidermis，while much larger chinks or layered gaps within exocarp and mesocarp，and some crackes， holes and some prejections like a nail on the endocarp surface. At the same time，the aril structure collapsed with many creases，and the cell wall might turn thicken with sags and fractures. It’s found that the difference in pericarp between cultivars when ripening was mainly due to the mesocarp，where the cells in Shixia were arranged more closely with smaller and fewer gaps than Chuliang，as well as the significant differences in aril. The aril cell wall of Shixia was firmer in a oblong shape than that of Chuliang，yet the latter’s cell wall at the incision crooked in a narrow shape with weaker rigidity than the former. Forthermore，when sugar-dropping，the shrinkage and collapse of Chuliang’s aril were more serious than those of Shixia，but in which the thickening of cell wall was more obvious.