Variations in cell wall microstructure and components and activities of their degradation enzymes in grapes with soft or hard textures

Abstract:【Objective】Grape berries have thin skin and juicy flesh, and is prone to drop and rot after harvest, which seriously affects transportation, storage, and shelf life, especially for grape with soft flesh. Cell wall components, arrangement and structural integrity are important factors affecting firmness of grape berry. To investigate the relationship between berry firmness and cell wall components, microstructure, as well as activities of their degradation enzymes.【Methods】Vitis vinifera‘Dawn Seedless’with hard flesh and Pinot Gris with soft flesh were used as materials in this study. Cell microstructure, cell wall components, including propectin, water- soluble pectin (WSP), hemicellulose and cellulose, and their degradation enzymes including PME (pectin methylesterase), PG (polygalacturonase), PL (pectin lyase), and cellulase activities were analyzed at 35, 49, 63, 77 and 91 days after anthesis (DAA).【Results】The results showed that the firmness of Dawn Seedless and Pinot Gris first increased and then decreased during berry development, and the berry firmness of Dawn Seedless and Pinot Gris peaked at 35 and 49 DAA, respectively. The firmness of Dawn Seedless was significantly higher than that of Pinot Gris except 49 DAA, and firmness of Dawn Seedless was 2.8 times higher than that of Pi-not Gris at maturity. During the berry development and ripening, propectin content of the two varieties showed a general decreasing trend, and propectin content of Pinot Gris was significantly higher than that of Dawn Seedless. In Dawn Seedless, propectin content peaked at 35 DAA, and then decreased rapidly and remained relatively constant from 49 to 91 DAA. WSP content in Dawn Seedless and Pinot Gris was relatively stable from 35 to 77 DAA but significantly increased in Dawn Seedless at 91 DAA. Hemicellulose content of the two varieties generally decreased during berry development with a rapid decrease occurring from 35 to 49 DAA by 73% and 35%, respectively, and there was no significant difference on hemicellulose content between the two varieties from 63 to 91 DAA. Cellulose content of Pinot Gris showed a general downward trend, and there was no clear change pattern in cellulose content in Dawn Seedless. PG activity of Dawn Seedless berry did not change significantly from 35 to 77 DAA, but increased rapidly and reached the highest at maturity. PG activity in Pinot Gris fruit increased first and then decreased, and the peak appeared at 77 DAA. PME activity of the two varieties decreased first and then increased, and was the lowest at 49 DAA. PME activity in Dawn Seedless was significantly higher than that in Pinot Gris from 35 to 63 DAA, and there was no significant difference in PME activity between the two varieties from 77 to 91 DAA. PL activity also showed different trends in the two cultivars. In Dawn Seedless, PL activity first decreased and then increased, and reached the lowest at 77 DAA, while it remained relatively stable in Pinot Gris. The cellulase enzyme activity of both varieties increased gradually and was significantly higher in Dawn Seedless than in Pinot Gris during 35-63 DAA, and there was no significant difference between the two varieties from 77 to 91 DAA. The berry firmness had a significant positive correlation with propectin content, and was negatively correlated with cellulase activity. During the development and ripening of Dawn Seedless, berry firmness was positively correlated to the contents of propectin and hemicellulose, and significantly negatively correlated with cellulase activity. WSP content was significantly positively correlated with PG activity, and propectin and hemicellulose contents were significantly negatively correlated with cellulase activity. The berry firmness of Pinot Gris was positively correlated to cellulose content, but significantly negatively to cellulase activity. During berry development and ripening, changes were observed in size and arrangement of the flesh cells in both varieties. With the berry development, flesh cells of the two varieties became larger, and the cell size was not uniform and the arrangement became loose. Cell layers began to separate between skin and flesh in the fruit of Pinot Gris at 49 DAA. At the beginning of fruit softening and veraison (63 DAA), the arrangement and size of cells of the two varieties were irregular, and the cell wall boundary of Pinot Gris began to degrade. In contrast, the cell wall of Dawn Seedless remained basically intact without cell wall degradation.【Conclusion】In the ripening and softening process of grape fruit, degradation of propectin and hemicellulose is rapid, leading to the disruption of the cell wall, which is the key factor of grape fruit softening. The degradation enzymes, especially cellulase, play comprehensive roles in the cell wall component changes.