Effects of light intensities on antioxidant enzymes and malondialdehyde content during short-term acclimatization on micropropagated plantlet

We investigated the effects of light stress on the activities of antioxidant enzymes, such as superoxide dismutase (SOD), dehydro ascorbate reductase (DHAR), monodehydro ascorbate reductase (MDHAR), ascorbate oxidase (AO), glutathione reductase (GR), guaiacol peroxidase (G-POD), catalase (CAT), glutathione S transferase (GST), lipoxygenase (LOX), lipid peroxidation (LP), leaf protein content and photosynthetic efficiency (Fv/Fm) in order to evaluate their role during acclimatization in Phalaenopsis orchids. Six months old in vitro grown plantlets were exposed to low light (LL-60μmolm−2s−1), intermediate light (IL-160μmolm−2s−1) and high light (HL-300μmolm−2s−1) photosynthetic photon flux density (PPFD), respectively under controlled condition. Plantlets exposed to HL intensity had lower level of Fv/Fm ratio than the LL grown plantlets during acclimatization. Regarding antioxidants enzymes, SOD activity increased in leaves with increasing light intensity but light stress had no significant effect in roots. DHAR and MDHAR activities increased in LL and IL but decreased at HL. The CAT activity increased in both leaves and roots with increasing light intensity. While G-POD activity increased in roots, POD activity was not detected in leaves. No significant change in GR activity has been found at IL and HL, though it decreased significantly at LL compared to in vitro grown plantlets. There was an increase in AO activity in leaves of about 50% at HL compared to in vitro grown plantlets, whereas no changes in roots were observed. GST activity showed pronounced stimulation in both leaves and roots of the plantlets exposed to HL compared to in vitro grown ones. Total leaf protein content increased in light stressed plantlets compared to in vitro grown plantlets. LP and LOX increased during light stress compared to in vitro grown plantlets suggesting that LOX mediated lipid peroxidation contributed to the oxidative damage occurring in the present study. These results suggest that increase in enzyme activities were an adaptive response of the plantlets to higher amounts of reactive oxygen species (ROS) generated during acclimatization under light stress.