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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Light Enables a Very High Efficiency of Carbon Storage in Developing Embryos of Rapeseed¹

Michigan State University, Department of Plant Biology, East Lansing, Michigan 48824

The conversion of photosynthate to seed storage reserves is crucial to plant fitness and agricultural production, yet quantitative information about the efficiency of this process is lacking. To measure metabolic efficiency in developing seeds, rapeseed (Brassica napus) embryos were cultured in media in which all carbon sources were [U-¹⁴C]-labeled and their conversion into CO₂, oil, protein, and other biomass was determined. The conversion efficiency of the supplied carbon into seed storage reserves was very high. When provided with 0, 50, or 150 µmol m^–2 s^–1 light, the proportion of carbon taken up by embryos that was recovered in biomass was 60% to 64%, 77% to 86%, and 85% to 95%, respectively. Light not only improved the efficiency of carbon storage, but also increased the growth rate, the proportion of ¹⁴C recovered in oil relative to protein, and the fixation of external ¹⁴CO₂ into biomass. Embryos grown at 50 µmol m^–2 s^–1 in the presence of 5 µM 1,1-dimethyl-3-(3,4-dichlorophenyl) urea (an inhibitor of photosystem II) were reduced in total biomass and oil synthesis by 3.2-fold and 2.8-fold, respectively, to the levels observed in the dark. To explore if the reduced growth and carbon conversion efficiency in dark were related to oxygen supplied by photosystem II, embryos and siliques were cultured with increased oxygen. The carbon conversion efficiency of embryos remained unchanged when oxygen levels were increased 3-fold. Increasing the O₂ levels surrounding siliques from 21% to 60% did not increase oil synthesis rates either at 1,000 µmol m^–2 s^–1 or in the dark. We conclude that light increases the growth, efficiency of carbon storage, and oil synthesis in developing rapeseed embryos primarily by providing reductant and/or ATP.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.063628.

^* Corresponding author; e-mail goffman{at}msu.edu; fax 517–353–1926.

Received March 31, 2005; returned for revision May 13, 2005; accepted May 16, 2005.

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