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Allagochrome II. Effects of Light and Substrate on Allagochrome and Chlorogenic Acid Levels of Incubated Sunflower Leaf Discs ¹

Department of Biological Sciences, Goucher College, Towson, Maryland 21204

Sunflower leaf discs incubated in the light on carbohydrate substrates exhibit several-fold increases in amounts of extractable allagochrome and chlorogenic acid. These changes are linear with time, and oxygen is required. The light effect saturates at approximately 600 µW/cm² "white" light, roughly the compensation point for photosynthesis. Red light is as effective as white light. Incubation in the dark, or in far red light, produces negligible changes in allagochrome and chlorogenic acid content.

Sucrose (0.2 M) has been used as the standard substrate. At this concentration, glucose and fructose are slightly more effective. The optimum temperature range for incubation is 20 to 30°. Allagochrome and chlorogenic acid values of both light and dark incubated samples decrease between 30 and 50°, approaching zero at 50°. Net light effects decrease to zero between 40 and 50°.

Of the inhibitors tested, 2,4-dinitrophenol, hydroxylamine and salicylaldoxime have no effect on light enhanced allagochrome and chlorogenic acid values except at high concentrations (which are generally deleterious to leaf tissues and cause decreased values for both dark and light incubated samples). Net light effects are completely inhibited, without changes in values of dark incubated discs, by azide (0.1 to 1 mM) and dichlorophenyldimethylurea (1 to 10 µM).

Leaf tissues from the xantha mutant of Helianthus annuus do not exhibit a light effect. The absence of light effects in nonphotosynthetic leaf tissue and the inhibiting effects of photosynthetic poisons suggest that the photosynthetic apparatus is somehow involved.

An hypothesis for light regulated metabolism via phenolic synthesis is discussed.