CO2 REFIXATION CHARACTERISTICS OF DEVELOPING CANOLA SEEDS AND SILIQUEWALL

The potential for developing canola (Brassica napus L.) seeds and the interior silique (pod) wall to refix respired CO2 has been investigate d. From ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities, seeds were estimate d 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 mu mol min(-1) mg chlorophyll(-1)) at 28 days after anthesis (DAA) with smaller contributions from seed coat and embryo PEPC. Rubis co activities were probably maximal in vivo because of high silique ca vity CO2 concentrations (0.8 to 2.5%). Seed chlorophyll content rapidl y increased over 10-fold from 20 to 30 DAA and, with 20% of incident l ight transmitted through the silique wall, embryos demonstrated apprec iable photosynthetic electron transport rates and most energy produced appeared to be used for Rubisco-catalysed CO2 fixation. Endocarp refi xation capacity was less than seeds because chlorophyll content was no t enriched and PEPC activities were relatively small. These data indic ate that developing seeds and also endocarp refix respired CO2 and tha t embryo chlorophyll plays a critical role in this refixation.