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 "'CO2, 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.

M. 1997. Water and sucrose regulate canola embryo development. -Physiol. Plant. 101: 361-366.The effect of water and sucrose on the growth and development of zygotic, 30-day-old canola {Brassica napus L. cv. Bounty) embryos was examined in vitro by manipulating the levels of sucrose and/or sorbitol present in the culture medium. In some experiments, the medium water potential was allowed to vary with sucrose concentration, while in other experiments, the medium water potential was held constant by adding sorbitol to varying amounts of sucrose. Our results showed that embryos cultured on sorbitol alone exhibited two developmental patterns: embryos germinated precociously on media containing up to 0.70 M sorbitol, whereas embryos became yellow and quiescent on media with higher concentrations of sorbitol. For embryos cultured on media containing sucrose alone, three distinct developmental patterns were noted: at low sucrose concentrations, embryos germinated precociously; at intermediate concentrations, embryos continued to grow in an embryonic mode; and, at high concentrations, embryos became yellow and quiescent. Continued embryonic growth was never observed in embryos cultured on media containing sorbitol alone. Embryos never germinated precociously when cultured on media maintained at a constant water potential of -1.4 MPa, rather dry weight increased in these embryos with an increase in sucrose concentration. We envision the effect of sucrose on embryo growth and development to be nested within the effect of water availability. When water availability is restricted, embryos become quiescent. When water is available, embryos have the potential to grow, but the developmental growth pattern depends on the availability of sucrose. In the absence of sucrose, embryos germinate and initiate the transition to autotrophy. If sufficient sucrose is available, embryos remain photoheterotrophic and continue to grow in an embryonic mode.

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.

M. 1997. Water and sucrose regulate canola embryo development. -Physiol. Plant. 101: 361-366.The effect of water and sucrose on the growth and development of zygotic, 30-day-old canola {Brassica napus L. cv. Bounty) embryos was examined in vitro by manipulating the levels of sucrose and/or sorbitol present in the culture medium. In some experiments, the medium water potential was allowed to vary with sucrose concentration, while in other experiments, the medium water potential was held constant by adding sorbitol to varying amounts of sucrose. Our results showed that embryos cultured on sorbitol alone exhibited two developmental patterns: embryos germinated precociously on media containing up to 0.70 M sorbitol, whereas embryos became yellow and quiescent on media with higher concentrations of sorbitol. For embryos cultured on media containing sucrose alone, three distinct developmental patterns were noted: at low sucrose concentrations, embryos germinated precociously; at intermediate concentrations, embryos continued to grow in an embryonic mode; and, at high concentrations, embryos became yellow and quiescent. Continued embryonic growth was never observed in embryos cultured on media containing sorbitol alone. Embryos never germinated precociously when cultured on media maintained at a constant water potential of -1.4 MPa, rather dry weight increased in these embryos with an increase in sucrose concentration. We envision the effect of sucrose on embryo growth and development to be nested within the effect of water availability. When water availability is restricted, embryos become quiescent. When water is available, embryos have the potential to grow, but the developmental growth pattern depends on the availability of sucrose. In the absence of sucrose, embryos germinate and initiate the transition to autotrophy. If sufficient sucrose is available, embryos remain photoheterotrophic and continue to grow in an embryonic mode.