Abstract
Canola embryos are green during development, but the role of embryo chloroplasts is not known. The objective of this study was to characterize the structure and function of embryo chloroplasts. Observations obtained by SDS‐PAGE and transmission electron microscopy showed that thylakoids from embryo chloroplasts contained the same chlorophyll‐protein complexes, but exhibited a greater proportion of granal stacking, compared with thylakoids from leaf chloroplasts. When assayed using an oxygen electrode, photosynthetic electron transport and respiration were enhanced in canola embryos bathed in concentrations of sucrose below 0.47 M. Photosynthesis, measured as the rate of incorporation of 14CO2, was much lower in embryos than leaves even though significant electron transport was detected. These results indicate that the primary role of chloroplasts in embryos is not to photoassimilate CO2. Instead, canola embryos are photoheterotrophic and may use the light reactions to generate the ATP and NADPH required to fuel the conversion of maternally supplied sucrose to the fatty acids used in oil synthesis and storage. These results led us to propose that the current model of plastid differentiation be modified to include the development of photoheterotrophic chloroplasts in sink tissues. This pattern of plastid differentiation can then be used to explain patterns of embryo development.
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