CO2 refixation characteristics of developing canola seeds and silique wall

The potential for developing canola (Brassica napus L.) seeds and the interior silique (pod) wall to refix respired CO2 has been investigated. From ribulose-1,5-bisphosphate carboxylase–oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities, seeds were estimated to have a greater CO2 fixation capacity than silique wall endocarp during oil filling. The major component of seed fixation capacity was embryo Rubisco, which had a total activity of 6.3 nmol min-1 embryo-1 (3.7 µmol min-1 mg chlorophyll-1) at 28 days after anthesis (DAA) with smaller contributions from seed coat and embryo PEPC. Rubisco activities were probably maximal in vivo because of high silique cavity CO2 concentrations (0.8 to 2.5%). Seed chlorophyll content rapidly increased over 10-fold from 20 to 30 DAA and, with 20% of incident light transmitted through the silique wall, embryos demonstrated appreciable photosynthetic electron transport rates and most energy produced appeared to be used for Rubisco-catalysed CO2 fixation. Endocarp refixation capacity was less than seeds because chlorophyll content was not enriched and PEPC activities were relatively small. These data indicate that developing seeds and also endocarp refix respired CO2 and that embryo chlorophyll plays a critical role in this refixation.