Carbon dioxide fixation by detached cereal caryopses

Immature detached cereal caryopses from barley (Hordeum vulgare L. var distichum cv Midas) and wheat (Triticum aestivum L. cv Sicco) were shown to be capable of fixing externally supplied (14)CO(2) in the light or dark. Green cross cells and the testa contained the majority of the (14)C-labeled material. Some (14)C-labeled material was also found in the outer, or transparent, layer and in the endosperm/embryo fraction. More (14)C was recovered from caryopses when they were incubated in (14)CO(2) without the transparent layer, thus suggesting that this layer is a barrier to the uptake of CO(2). In all cases, significant amounts of (14)C-labeled material were found in caryopses after dark incubation with (14)CO(2). Interestingly, CO(2) fixation in the chlorophyll-less mutant Albino lemma was significantly greater in the light than in the dark. The results indicate that intact caryopses have the ability to translocate (14)C-labeled assimilate derived from external CO(2) to the endosperm/embryo. Carboxylating activity in the transparent layer appears to be confined to phosphoenolpyruvate carboxylase activity but that in the chloroplast-containing cross-cells may be accounted for by both ribulose-1,5-bisphosphate carboxylase-oxygenase and phosphoenolpyruvate carboxylase activity. Depending on a number of assumptions, the amount of CO(2) fixed is sufficient to account for about 2% of the weight of starch found in the mature caryopsis.