THE PHOTOSYNTHETIC POTENTIAL OF CANOLA EMBRYOS

Canola embryos are green during development, but the role of embryo ch loroplasts is not known. The objective of this study was to characteri ze the structure and function of embryo chloroplasts. Observations obt ained by SDS-PAGE and transmission electron microscopy showed that thy lakoids from embryo chloroplasts contained the same chlorophyll-protei n complexes, but exhibited a greater proportion of granal stacking, co mpared with thylakoids from leaf chloroplasts. When assayed using an o xygen electrode, photosynthetic electron transport and respiration wer e enhanced in canola embryos bathed in concentrations of sucrose below 0.47 M. Photosynthesis, measured as the rate of incorporation of (CO2 )-C-14, was much lower in embryos than leaves even though significant electron transport was detected. These results indicate that the prima ry role of chloroplasts in embryos is not to photoassimilate CO2. Inst ead, canola embryos are photoheterotrophic and may use the light react ions to generate the ATP and NADPH required to fuel the conversion of maternally supplied sucrose to the fatty acids used in oil synthesis a nd storage. These results led us to propose that the current model of plastid differentiation be modified to include the development of phot oheterotrophic chloroplasts in sink tissues. This pattern of plastid d ifferentiation can then be used to explain patterns of embryo developm ent.