Enhancement of phenylpropanoid enzymes and lignin in Phalaenopsis orchid and their influence on plant acclimatisation at different levels of photosynthetic photon flux

Effects of three levels of photosynthetic photon flux (PPF: 60, 160 and 300 μmol m-²s-¹) were investigated in one-month-old Phalaenopsis plantlets acclimatised ex vitro. Optimal growth, chlorophyll and carotenoid concentations, and a high carotenoid:chlorophyll a ratio were obtained at 160 μmol m-²s-¹, while net CO₂ assimilation (A), stomatal conductance (g), transpiration rate (E) and leaf temperature peaked at 300 μmol m-²s-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 β-glucosidase (β-GS) activities increased to a maximum in acclimatised plants at 300 μmol m-²s-¹. A positive correlation between PAL, CAD activity and lignin concentration was observed, especially at 160 and 300 μmol m-²s-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 μmol m-²s-¹was suitable for acclimatisation when plants were transferred from in vitro conditions.