Annual Variation in the Effect of Red Light on Sterol Biosynthesis in Digitalis purpurea L.

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Abstract
The effect of varying sequences of red and far-red light on sterol biosynthesis in etiolated seedlings of D. purpurea L. was examined. Red light caused a marked increase in the amounts of free and glycosidic sterols and a small decrease in esterified sterols during the first 4 h after illumination. Far-red light elicited the same response but to a lesser degree. Exposure to red followed by far-red light or the reverse caused little or no increase in the amounts of free and glycosidic sterols. The magnitude of the increase in the amounts of sterols varied, depending upon the season in which the experiments were performed. The largest increments were obtained during the summer and fall, whereas the smallest were observed during the winter and spring. Correlation of these data with previous observations of an annual cycle in the sterol content of Digitalis seedlings showed that the maximum stimulation in sterol biosynthesis occurs when the endogenous level of sterols is minimal. Sterol monoglycosides, acylmonoglycosides and an unidentified sterol conjugate from the lipid extracts were quantitated. Changes in conjugated sterol content were related to the particular light conditions of each experiment. The results are discussed in terms of physiological cycles and the possible influence of hormones upon the control of sterol biosynthesis in Digitalis.