, Volume 49, Issue 2-3, pp 137-146
Date: 29 Sep 2006

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

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Abstract

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.