Stomatal Metabolism: Primary Carboxylation and Enzyme Activities

Malate and aspartate are the major labelled products when 14CO2 is offered to epidermal strips of Commelina cyanea R.Br. Thirty seconds after initial exposure to 14CO2 of strips in light with open stomata, 75 % of 14C in aspartate was a [4-14C]aspartate. As the exposure time increased, the ratio [4-14C]aspartate to [U-14C]aspartate declined. This evidence indicates that 14C enters aspartate after a ß-carboxylation reaction and that the newly formed aspartate is further metabolized by the tissue. Ribulosebisphosphate carboxylase fractionated similarly on sucrose density gradients whether from intact leaf or epidermal strips and, from either tissue, the activity was inhibited by antisera to purified fraction 1 protein from wheat. Phosphoenolpyruvate carboxylase from mesophyll and epidermis also coincided on sucrose density gradients. A survey of enzymic activities in extracts of leaves and epidermes showed that the latter were specialized towards the metabolism of C*4 acids. They have a high ratio of phosphoenolpyruvate to ribulosebisphosphate carboxylase activities, and enhanced amounts of aspartate aminotransferase, malate dehydrogenase (NADP+), and NAD : malic and NADP : malic enzymes. Pyruvate, orthophosphate dikinase or phosphoenolpyruvate carboxykinase, enzymes possibly involved with C*4 acid metabolism, were not detected.