Cell wall bound ferulic acid, the major substance of the blue-green fluorescence emission of plants

Abstract

Summary When excited by UV-A radiation (e.g. N 2 -laser 337 nm), leaves of green plants exhibit, in addition to the red and far-red chlorophyll fluorescence, a genuine blue-green fluorescence emission with a maximum near 440–460 nm and a lower shoulder near 520–530 nm. Members of the Poaceae (monocotyledonous plants), e.g. maize ( Zea mays L.), wheat ( Triticum aestivum L.) or oat ( Auma sativa L.), possessed a much higher blue-green fluorescence emission than leaves of dicotyledonous plants, such as spinach ( Spinacia oleracea L.), tobacco ( Nicotiana tabacum L.), sunflower ( Helianthus annuus L.), foxtail ( Amaranthus caudatus L.) or purslane ( Portulaca oleracea L.). In the measured dicotyledonous plants, the blue-green fluorescence was in a similar range or considerably lower than the red chlorophyll fluorescence. In contrast, in Poaceae and other monocotyledonous plants, the blue-green fluorescence was generally several times higher than the red chlorophyll fluorescence. By alkaline hydrolysis of cell walls it was shown that the members of Poaceae exhibit a several times higher content of the blue-green fluorescent ferulic acid, both on leaf area and on a dry weight basis, than dicotyledonous plants, such as spinach, foxtail and purslane. Ferulic acid, being covalendy bound to the cell wall, appears to be the main emitter of the blue-green fluorescence of leaves which has been documented by several complementary observations. Other cell wall bound cinnamic acids (caffeic acid, p-coumaric acid), which are present in some plants in low amounts, contribute very little to the overall blue-green fluorescence emission of leaves. This also applies to the extractable flavonoids and cinnamic acids of the vacuole. The high fluorescence yield of ferulic acid containing cell walls is also documented by fluorescence excitation spectra of isolated, dried cell walls before and after alkaline hydrolysis of ferulic acid. Plants which do not possess ferulic acid in their cell walls, such as sunflower, pumpkin ( Cucurbita fici-folia L.) or tobacco, exhibit only a very faint blue-green fluorescence emission. Outdoor plants contained much higher levels of extractable flavonoids and cinnamic acids than greenhouse plants, yet the blue-green fluorescence was hardly modified indicating that the soluble flavonoids and cinnamic acids of the vacuole do not or only very little contribute to the blue-green fluorescence emission of plants.