Developmental regulation of sterol biosynthesis inCucurbita maxima L.

Twenty‐two sterols were identified by capillary gas chromatography and capillary gas chromatography/mass spectroscopy inCucurbita maxima grown under green‐house conditions. Both whole plants and individual tissues (leaves, stems, roots, cotyledons, flowers) were analyzed at weekly intervals during the 12‐week development of the plant. In whole plants, sterol accumulation parallels plant growth except for a period in the mid‐life cycle where there is a reduction in the amount of sterol accumulated on a total sterol/plant and mg sterol/g dry wt basis. This reduction in the amount of sterol is coincident with the visual onset of flowering. During development, the percent contribution of each class of sterol (Δ5_, Δ7_, Δ0_‐sterols) remains relatively constant. However, the percent contribution of an individual sterol species varies depending on the tissue examined and the developmental period selected for analysis. While the young plant (<2 weeks) possesses elevated levels of sterols with the Δ25(27)‐double bond, the trend was toward a reduction in the amounts of these sterols with development. Leaves and stems accumulate large quantities of 24ζ‐ethyl‐5α‐cholesta‐7,22‐dien‐3β‐ol (7,22‐stigmastadienol) and 24ζ‐ethyl‐5α‐cholest‐7‐en‐3β‐ol (7‐stigmastenol), while roots accumulate only 7,22‐stigmastadienol as their principal sterol. Male flowers and roots were found to contain elevated levels of Δ5_‐sterols.