Effects of a PAL inhibitor on phenolic accumulation and UV-B tolerance in Spirodela intermedia (Koch.)

Abstract

Duckweed (Spirodela intermedia) was grown axenically on 1/2 strength Hutner's nutrient solution plus 1% sucrose, with the L-phenylalanine ammonia-lyase (PAL) inhibitor 2-aminoindan-2-phosphonic acid (AIP) at 0.0, 0.05, or 10 μM, at constant 25°C and a light intensity of 300 μmol m -2 s-1 photosynthetically active radiation from CW fluorescent lamps. Growth with 10 μM AIP led to decreased frond area and fresh weight, but dry weight was unchanged. Microscopic examination of fronds revealed increased frond thickness and a lack of reticulate aerenchyma. Ultraviolet epifluorescence microscopy and UV-Vis spectroscopy of methanolic extracts confirmed the dose-dependent inhibition of secondary phenolic synthesis with the near total elimination of secondary phenolic accumulation at the 10 μM level. AIP-treated plants showed increased sensitivity to UV-B as shown by a reduced Fv/Fm. The results provided direct evidence of the working hypothesis that phenols function to screen UV radiation from reaching photosynthetic tissues or damaging other sensitive tissues. A novel histochemical method employing zirconyl chloride to visualize phenols is discussed.