750 AJCS 4(9):750-756 (2010) ISSN:1835-2707 Effects of temperature and relative humidity during in vitro acclimatization, on physiological changes and growth characters of Phalaenopsis adapted to in vivo Suriyan Cha-um 1* Bolortuya Ulziibat 2 and Chalermpol Kirdmanee 1 1 National Center for Genetic Engineering and Biotechnology, National Science and Technology Development Agency, 113 Thailand Science Park, Paholyothin Rd, Klong 1, Klong Luang, Pathumthani 12120, Thailand 2 Institute of Biology, Mongolian Academy of Sciences, Jukov avenue-77, Ulaanbaatar-51, Mongolia * Corresponding author and present address: Suriyan Cha-um, National Center for Genetic Engineering and Biotechnology, 113 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand *Corresponding author: suriyanc@biotec.or.th Abstract Phalaenopsis plantlets, acclimatized under different air temperatures (15±2, 25±2 and 35±2°C) and relative humidity (RH) (60±5, 80±5 and 95±5%RH), were transferred directly to in vivo environments for 14 days. The experiment was done at the Plant Physiology and Biochemistry Lab, National Center for Genetic Engineering and Biotechnology (BIOTEC) in year 2007. Chlorophyll a (Chl a ), chlorophyll b (Chl b ), total chlorophyll (TC) and total carotenoid (C x+c ) contents in plantlets acclimatized under conditions of low air temperature and low RH were maintained to a higher degree than in those acclimatized under high temperature and high relative humidity by 4.45, 5.79, 4.68 and 4.95 times, respectively. Chl a and TC contents in acclimatized plantlets were positively related to maximum quantum yield of PSII (F v /F m ) (r 2 = 0.61) and photon yield of PSII (Φ PSII ) (r 2 = 0.82), respectively. F v /F m , Φ PSII , photochemical quenching (qP) and transpiration rate (E) in plantlets acclimatized under low temperature and low RH were enriched and were greater than those under high temperature and high RH treatment, while stomatal conductance (g s ) was lower, leading to enhanced net photosynthetic rate (P n ) and growth performances. Low temperature and low RH conditions of in vitro acclimatization should be implemented to produce healthy Phalaenopsis plantlets, defined by pigment stabilization, chlorophyll a fluorescence regulation, P n and growth characteristics, to enable their rapid adaptation to in vivo environments. Keywords: chlorophyll content, chlorophyll a fluorescence, growth, net photosynthetic rate, orchid. Abbreviations: Chl a _chlorophyll a, Chl b _chlorophyll b, TC_total chlorophyll, C x+c _total carotenoids, E_transpiration rate, F v /F m _maximum quantum yield of PSII, g s _stomatal conductance, MS_Murashige and Skoog medium, NPQ_non photochemical quenching, P n _net photosynthetic rate, Φ PSII _photon yield of PSII, PPF_photosynthetic photon flux, qP_photochemical quenching, RH_relative humidity. Introduction Phalaenopsis, or the moth orchid, is one of the most important genera of ornamental plants in the world. 75% of the market share of orchids produced in the year 2000 was a potted Phalaenopsis orchid, representing seventy-five million US dollars. Large scale production of Phalaenopsis is carried out in The Netherlands, Germany, China, Taiwan, The United States and Japan (Griesbach 2002). Phalaenopsis originates in temperate regions, which have low temperatures (≤25°C). It has been reported as being sensitive to high temperatures and this is especially the case with hybrid types (Kano 2001). In the floral transition stage, low temperatures are necessary for endogenous cytokinin and gibberellin accumulation, as well as photosynthetic enhancement, leading to sucrose gathering for flower bud initiation and stalk elongation (Chou et al. 2000; Su et al. 2001a; Kataoka et al. 2004; Blanchard and Runkle 2006; Lee et al. 2007; Chen et al. 2008; Penfield 2008). High temperature environments strongly affect oxidative stress in Phalaenopsis orchids, resulting in inhibition of flower development (Su et al. 2001a; Ali et al. 2005). In the present study, air temperature is mentioned as a key factor in controlling Phalaenopsis plantlet growth and development in vitro, prior to in vivo transplantation. On a commercial scale, Phalaenopsis plantlets have been produced by micropro- pagation through plant tissue culture, which is successfully implemented in many countries, Japan, Taiwan and China (Griesbach 2002). There are many reports into developing an effective protocol of Phalaenopsis micropropagation via protocorm-like bodies (Islam et al. 1998; Chen et al. 2000; Park et al. 2000; Tokuhara and Mii 2001; Park et al. 2002; Tokuhara and Mii 2003; Liu et al. 2006; Shrestha et al. 2007). Normally, the environments in in vivo are quite different when compared to in vitro conditions, in terms of relative humidity (RH), constant temperature, air ventilation, nutrient levels, etc (Kozai et al. 1997; Chen 2004; Hazarika 2006). In vitro acclimatization, or hardening, is one of the main processes in the production of healthy plantlets before their transplantation to in vivo (Pospíšilová et al. 1999a; Hazarika 2003).