Effects of three levels of photosynthetic photon flux (PPF: 60, 160 and 300 lmol m -2 s -1 ) were investigated in onemonth-old Phalaenopsis plantlets acclimatised ex vitro. Optimal growth, chlorophyll and carotenoid concentations, and a high carotenoid:chlorophyll a ratio were obtained at 160 lmol m -2 s -1 , while net CO 2 assimilation (A), stomatal conductance (g), transpiration rate (E) and leaf temperature peaked at 300 lmol m -2 s -1 , indicating the ability of the plants to grow ex vitro. Adverse effects of the highest PPF were reflected in loss of chlorophyll, biomass, non-protein thiol and cysteine, but increased proline. After acclimatisation, glucose-6-phosphate dehydrogenase, shikimate dehydrogenase, phenylalanine ammonia-lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD) increased, as did lignin. Peroxidases (POD), which play an important role in lignin synthesis, were induced in acclimatised plants. Polyphenol oxidase (PPO) and b-glucosidase (b-GS) activities increased to a maximum in acclimatised plants at 300 lmol m -2 s -1 . A positive correlation between PAL, CAD activity and lignin concentration was observed, especially at 160 and 300 lmol m -2 s -1 . The study concludes that enhancement of lignin biosynthesis probably not only adds rigidity to plant cell walls but also induces defence against radiation stress. A PPF of 160 lmol m -2 s -1 was suitable for acclimatisation when plants were transferred from in vitro conditions.