Effects of hydroponic solution EC, substrates, PPF and nutrient scheduling on growth and photosynthetic competence during acclimatization of micropropagated Spathiphyllum plantlets Y.H. Dewir 1,2 , D. Chakrabarty 1 , M.B. Ali 1 , E.J. Hahn 1 and K.Y. Paek 1, * 1 Research Center for The Development of Advanced Horticultural Technology; Chungbuk National University, Cheong-ju 361-763, Korea; 2 Department of Horticulture, Faculty of Agriculture, Tanta University, Kafr El-Sheikh 33516, Egypt; *Author for correspondence (e-mail:paekky@cbucc.chungbuk.ac.kr; phone: +82-43-261-3245; fax: +82-43-272-5369) Received 4 January 2005; accepted in revised form 23 June 2005 Key words: Antioxidant enzymes, Electrical conductivity, Fv/Fm, Hydroponics, Photosynthesis Abstract In vitro regenerated shoots of Spathiphyllum from bioreactor were hydroponically cultured for 30 days. The response of plant growth and photosynthesis to different substrates, photosynthetic photon flux (PPF), nutrient scheduling and electrical conductivity (EC) of hydroponic solution were studied. The best plant growth response was observed in perlite based substrates with moderate PFF (70–100lmol m À2 s À1 ). Highest fresh weight, dry weight, shoot length, root length, root number and photosynthetic characteristics (chlorophyll, carotenoids and Fv/Fm) was observed in continuous immersion system. Plant growth responses, photosynthetic rate, stomatal conductance and transpiration rate were also found to be affected by EC levels. The optimum EC of a balanced nutrient solution was recorded as 1.2 dS m À1 . Photosynthetic activity was also characterized in terms of photochemical efficiency using measurements of chlorophyll fluorescence. Fv/Fm (it is a measure of the intrinsic or maximum efficiency of PSII i.e. the quantum efficiency if all PSII centers were open) also decreased significantly in plants grown under higher EC level; a decrease in this parameter indicates down regulation of photosynthesis or photoinhibition. Antioxidant defense enzymes such as catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), glutathione reductase (GR) and monodehydroascorbate reductase (MDHAR) significantly elevated in the leaves and roots of plantlets at higher EC levels. This increase could reflect a defense response to the cellular damage provoked by higher EC levels in the nutrient solution. Introduction For many species the major drawback of micro- propagation is the acclimatization process. Plants produced in nutrient-rich substrate under sterile conditions are fragile. Transfer and acclimatization to the ex vitro environment is the final but frequently most hazardous step in a successful micropropagation system (Preece and Sutter 1991). The specific in vitro environment, with artificial medium usually supplied with sugar(s), the growth of plantlets in small air-tight vessels with high air humidity, low gas exchange and thus a CO 2 -shortage during almost the whole photoperiod, ethylene production and relatively low photosynthetic pho- ton flux density (PPFD), induces disturbances in Plant Growth Regulation (2005) 46:241–251 Ó Springer 2005 DOI 10.1007/s10725-005-0161-1