Studies on the Morphology Development and Autolytic Process of Peach Endocarp Cell During Pit Hardening Process

Deterioration of peach fruit quality because of split-pit formation had bad influence on market. Though many studies have mentioned the mechanism of split-pit formation, further studies are still needed to uncover the development mechanism of endocarp cell during pit hardening process, especially the Programmed cell death (PCD) process of peach endocarp cell. Here, the development of the peach endocarp cell from the two Chinese peach varieties "Jingyu" and "Okubo" was detailedly studied to understand the PCD of endocarp cell during the lignin formation process. According to our morphology observation using transmission electron microscopy, the PCD process of the endocarp cell, characterized by vacuolar enlargement, might have been initiated as early as eight days after full athesis (maybe even more earlier), which is earlier than that of the split-pit formation process. In our studies, almost all the endocarp cell organelles were found to degenerate at forty days after full athesis, when thickening of the endocarp cell wall was also observed. Furthermore, the involvement of hydrolytic enzymes, in particular proteases and nucleases, in the autolytic process of PCD of peach endocarp cells was quantitatively analyzed during pit hardening process. The peach endocarp protein was found to be rapidly degraded by newly expressed proteases since 35 days after full athesis. Elevation of nuclease activities was found to be also coupled with PCD of peach endocarp cells. Furthermore, DNA degradation was found to happen earlier than RNA degradation and hydrolyzing of endocarp soluble protein during the peach pit hardening process.