Plant Cell, Tissue and Organ Culture 61: 135–142, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 135 Influence of in vitro growth conditions on photosynthetic competence and survival rate of Rehmannia glutinosa plantlets during acclimatization period Jeong-Hoon Seon 1,3 , Yong-Yi Cui 1 , Toyoki Kozai 2 & Kee-Yoeup Paek 1,* 1 Research Center for the Development of Advanced Horticultural Technology, Chungbuk National University, Cheong-ju 361-763, South Korea; 2 Faculty of Horticulture, Chiba University, Matsudo, Chiba 271, Japan; 3 Present address; Dept. of Biological Science, University of Calgary, Calgary, AB, T2N 1N4, Canada ( * requests for offprints; Phone: 82-431-261-2529; Fax: 82-431-252-5369; E-mail: masang@trut.chungbuk.ac.kr) Received 23 December 1999; accepted in revised form 9 June 2000 Key words: carbohydrate, chlorophyll fluorescence, photoautotrophic conditions, photosynthetic rate, survival rate Abstract The photosynthetic responses of Rehmannia glutinosa grown under photoautotrophic or heterotrophic conditions in vitro were investigated after transfer to greenhouse conditions. In addition, the changes in carbohydrate content and survival rates of the plantlets were evaluated. During six days after transplantation, the photosynthetic rate declined and photoinhibitory impairments represented by decrease of Fv/Fm and chlorophyll content were ob- served regardless of environmental conditions in vitro. Excessive transpiration was observed in plantlets grown under heterotrophic conditions during that period. Fructose and glucose content of the plantlets grown under photoautotrophic conditions increased with time and reached almost the same level of field grown plants after day 15. Under heterotrophic conditions, in contrast, the content of these sugars decreased continuously during that period. It is suggested that high survival rate of plantlets grown under photoautotrophic conditions has to be attributed to improvement of photosynthetic competence by imposed high light intensity and CO 2 concentration in vitro. The results strongly suggest that the control of transpiration during early stage after transplantation plays a key role in the acclimatization process, and photoautotrophic conditions could be a solution to solve the problems associated with transplantation stress. Abbreviations: Fm – maximum chlorophyll fluorescence; Fo – minimal fluorescence emission; Fv – variable chlorophyll fluorescence; Fv/Fm – the ratio of variable to maximum chlorophyll fluorescence; MS – Murashige and Skoog medium; PPF – photosynthetic photon flux; PAC – photoautotrophic conditions; PHC – heterotrophic conditions Introduction Rehmannia glutinosa a member of the Scrophulari- aceae family is one of the most important medicinal plants native to China and Korea. The roots are used for the treatment of hematic and irregular menstru- ation. Although the methods for multiplication by plant tissue culture are well known, there are few reports on environmental control in vitro to improve acclimatization rate after transfer to soil. Acclimat- ization of in vitro plantlets to greenhouse or field conditions is a critical step for many plant species, requiring time and expenses that restrict the com- mercial application of the micropropagation process (Fila et al., 1998; Ross-Karstens, 1998). One of the main reasons for a low acclimation rate ex vitro might be low photosynthetic rates in vitro, presumably be- cause of the low photosynthetic photon flux (PPF) with a low CO 2 concentration in the culture vessel. It is known that poorly ventilated or closed vessels maintain a high humidity and the plantlets grown in these vessels are forced to open their stomata in or-