Effect of Light on Sterol Changes in Medicago sativa.

Most vascular plants contain Delta(5)-sterols as the predominant type; however, a few species such as Medicago sativa, have mainly Delta(7)-sterols. The Delta(7)-sterols of alfalfa are isomers of the common Delta(5)-sterols and are generally assumed to be their immediate precursors. Light had a significant influence on the sterol status of M. sativa. High light intensity and a long day favored the accumulation of dihydrospinasterol; a short day and low light intensity, particularly darkness, favored spinasterol accumulation. These data for Delta(7)-sterol plants agree with those reported for Delta(5)-sterol plants; light favors the accumulation of the monounsaturated 29 carbon sterols and darkness favors the accumulation of the diunsaturated sterols. Proposed is a mechanism to explain the effect of light on the accumulation of Delta(7)- and Delta(5)-sterols.