**4. Acknowledgment**

This study was performed under the collaborative research project, no. 3241, of Japan International Research Centre for Agricultural Sciences (JIRCAS) with the Southern Horticultural Research Institute of Vietnam (SOFRI). Prof. Emeritus Su, H.-J. at the Taiwan University provided us with precious suggestions in the study. Dr. Chau, N. M. supported our studies both officially and scientifically. Dr. Koizumi, M. and Dr. Kano, T. suggested us about works on citrus both in Japan and other countries. Dr. Yonemoto, M. and Mr. Ogata, T. at JIRCAS provided information necessary for citrus cultivation. Dr. Hoa, N. V. and Mr. Dien, L. Q. gave us information of citrus in Vietnam. Miss Nga, V. T. and Miss Oanh, T.T.K. helped our works both in laboratory and field. We would like to express our sincere thanks to them.
