Abstract: The infected and normal Ya pear were inspected microscopically,and the activities of tyrosinase,peroxidase, cytochrome oxidase and the content of free proline were determined through the chemical analysis.The result indicated that the black kernel disease of Ya pears was not produced by bacteral infection but belonged to a physiologlcal disease. The results of biochemical analysis also indicated that the respiratory pathway of the infected fruits was partly changed from EMP-TCA pathway to HMP, and the brown cotor appeared under the action of polyphenol com pound enzyme seemed to result directly in black kernel. Free oxygen accelerated this process.

Key words: Respiratory chain, Terminal oxidase, Black kernel disease, Ya pear, Mechanism of disease