Summary In a previous paper was demonstrated that cotyledons of Fagopyrum esculentum are not able to synthesize flavonoids by own photosynthetic products, but that carbohydrates from endosperm are necessary for their synthesis (Krause, 1978). This paper describes the role of cotyledons in the biosynthesis of rutin and neochlorogenic acid, the main phenolic compounds in primary leaves of buckwheat. Kinetics of phenylpropanoid accumulation show that before unfolding (8 days after sowing) primary leaves contain a rather large amount of rutin, but very little of neochlorogenic acid. If cotyledons are removed at that time rutin accumulation shows a sigmoid course with a much smaller increase than plants with attachedcotyledons. The maximal level also remains significantly below that of intact plants. On theother hand neochlorogenic acid synthesis is also smaller immediately after decotylization but starts very intensively after unfolding and reaches the same level as is observed in leaves of intact plants about 18 days after sowing. Increase of fresh weight runs parallel to neochlorogenic acid synthesis. It is evident that there are two different mechanisms controlling rutin and neochlorogenic acid synthesis. The manner of participation of cotyledons in the synthesis of these phenylpropanoids was tested in two ways. First was investigated if photosynthetic products of cotyledons are responsible for rutin and neochlorogenic acid synthesis in primary leaves. Therefore intact and decotyledonized plants were gased with 14 CO 2 shortly before unfolding of primary leaves. The highest total radioactivity was measured in primary leaves of intact seedlings. Percentage of incorporation of 14 CO 2 into rutin was about three times higher in intact plants than in decotyledonized. On the other hand percentage of incorporation into neochlorogenic acid was unaffected by decotyledonizing. To elucidate this effect PAL-activity of primary leaves of intact and decotyledonized plants was measured. After decotylization PAL-activity decreased rapidly during 24 hrs while it increased in intact seedlings. The results indicate that - like endosperm does with cotyledons (Krause, 1978) - substances from cotyledons induce PALactivity in primary leaves. But the decrease of PAL-activity cannot be the only reason for the strong delay of rutin accumulation immediately after decotylization. Also subsequent enzymes in rutin synthesis are injured by the manipulation as can be seen in percentage of 14 CO 2 incorporation into rutin. But much more significant is the missing of precursors from cotyledons. During the subsequent growth photosynthetic products of primary leaves cannot enhance rutin synthesis sufficiently to reach the accumulation of intact plants. because the enzymes involved in rutin synthesis cannot be reactivated. Synthesis of neochlorogenic acid is not affected in the same way by the loss of cotyledons as rutin. The comparison of increase in fresh weight and accumulation of neochlorogenic acid indicates a direct dependence of photosynthetic activity during development of primary leaves.