[(14)C]Carbon-dioxide fixation by isolated leaf epidermes with stomata closed or open

Isolated epidermes of Tulipa gesneriana L. and Commelian communis L. were exposed to (14)CO2 in the light and in darkness, when stomata were either closed or open. The labelling patterns did not differ: the main products of CO2 fixation were malate and aspartate. Small amounts of radioactivity appeared also in acids of the tricarboxylic-acid cycle and their transamination products. Since the capacity of epidermis to assimilate CO2 is known to reside in the guard cells, we can state that guard cells continuously take up CO2 if present, and are thus able to recognize the presence of CO2 in their environment at all times. Epidermal samples exposed to (14)CO2 in the light contained only small amounts of radioactive 3-phosphoglyceric acid (3-PGA) and sugar phosphates, or none at all. Epidermal samples from Commelina communis did not contain labelled 3-PGA if all adhering mesophyll cells had been removed before exposure to (14)CO2. Homogenates of clean epidermal strips of Commelina communis were able to convert exogenous ribulose diphosphate to 3-PGA at a low rate, but could not catalyze the conversion of exogenous ribulose-5-phosphate to ribulose diphosphate. Guard cells of Commelina communis, and probably also those of Tulipa gesneriana, appear not to possess the reductive pentosephosphate pathway, despite the presence of chloroplasts. In such species, the guard cells will have to rely on import in order to maintain their carbon balance. Earlier findings of photosynthetic reduction of CO2 by epidermal tissues were probably obtained with samples that were contaminated with mesophyll cells.