Entwicklungs- und lichtabhängige akkumulation von C-glycosylflavonen im haferkeimling (Avena sativa L)

Abstract

Summary In the growing shoot of oat seedlings ( Avena sativa , «Gelbhafer») generally four C-glycosylflavones are synthesized. They probably are closely related biochemically and accumulate to 97–99 % in the primary leaf (fig. 4). The chemically simplest flavone iso-vitexin (F 5 = saponaretin) is a minor component, whereas isovitexin-O-arabinoside (F 2 ), vitexin-O-glucoside (F 3 ) and 7-O-methoxy(?)-vitexin-O-rhamnoglucoside (F 1 ) accumulate as main substances (fig. 5). This «basic» flavone pattern is found in all developmental stages, independent of light. In light-grown shoots additional minor flavones, the «chloroplast flavones» (F 6 –F 9 ), and water soluble esters of p-coumaric-, ferulic- and p-hydroxybenzoic acid can be detected. Roots contain only traces of flavones, but instead three water soluble main compounds of unknown nature, having blue fluorescence. In addition, insoluble, hydro-lyzable esters of p-coumaric- and ferulic acid are found. During the 3rd and 4th day of shoot growth (stage 0–1, fig. 1) flavone synthesis starts light-independent. In the beginning of the greening of the primary leaf (stage 1–2) there is an eruptive increase in the content of the main flavones (F 1 – 3 ), with isovitexin (F 5 ) showing a lag phase (fig. 3, 5). The total amount of the leaf flavones at this stage, until 6th day (stage 3), is 3–4 times higher than the chlorophyll (a + b) content. Afterwards chlorophyll content increases further, absolutely as well as relatively, whereas glycoflavone accumulation comes to a stationary phase earlier (stage 4–5) than chlorophyll. Nevertheless, 10 day old differentiated primary leaves contain 1,4 times more flavone than chlorophyll. In the dark the growth of the primary leaf is retarded as is the rate of glycosylflavone synthesis (fig. 2, 3, 5). Single flavone compounds accumulate up to the 9th–10th day at different rates. The relatively more derivatized methoxy-trioside (F 1 ) accumulates to a greater extent than do the biosides (F 2 , F 3 ), and reaches 80 % as compared to levels in green leaves. In contrast, the monoside (F 5 ) accumulates at the lowest rate and shows together with F 3 the strongest light dependency. Illumination for 26–34 hours of 4, 5 and 6 day old dark grown seedlings (stage 2–4) induces different levels of total flavones in the primary leaves (fig. 7, 8, 10). When 4 day old etiolated leaves are exposed to light for one day, they produce within this day flavones and chlorophylls to an extent, which amounts to about 70–80 % and 100 %, respectively, of the levels found in leaves from plants grown for 5 days in the light. In older leaves this reconstitution is not observed any more. The light-induced accumulation of individual glycosylflavones is different, the compounds F 3 and F 5 are the most inducible, F 1 the least (see above). In 4 day old etiolated leaves flavone synthesis begins 2 h after illumination and before chlorophyll synthesis (tab. 1). In 5 and 6 day old leaves greening begins first. Later on, in all cases a higher amount of flavones than chlorophylls is produced and this ratio remains constant during the period of illumination and is largely independent of the age of the leaf as well as of the duration of etiolation. The above described correlation between flavone accumulation and differentiation of primary leaves and plastids, together with the observation that single flavone compounds, such as isovitexin, were found to be enriched in plastids ( Weissenbock et al., 1972; Weissenbock , 1973; Weissenbock and Schneider , 1974), whereas etioplasts and chloroplasts possess their own specific flavone pattern, point to an important role of plastids in the C-glycosylflavone metabolism in the primary leaves of Avena sativa . For illustration and discussion this hypothesis is put into a working scheme.