Some Biochemical Characteristics of Guard Cell and Mesophyll Cell Protoplasts from Commelina communis L.

Guard cell and mesophyll cell protoplasts of Commelina communisL., were isolated and used to investigate their various biochemicalcharacteristics. Contamination of the samples by other celltypes was very low and viability of the protoplasts, assessedby the use of neutral red, Evans blue and fluorescein diacetate,was high (89–98%). Mesophyll cell protoplasts containedmore chlorophyll (x 47), more soluble protein (x 10), more totalN (x 36) and more DNA (x 9) than guard cell protoplasts. Theabsorption spectra of protoplast extracts were similar for bothcell types except that below 400 nm there was a large increasein absorption by the guard cell protoplast extract. In guardcell protoplast extracts, high levels of activity of phosphoenolpyruvatecarboxylase (E.C. 4.1.1.31 [EC] ), NAD malate dehydrogenase (E.C.1.1,1.37), NADP malic enzyme (E.C. 1.1.1.40 [EC] ) and carbonic anhydrase(E.C. 4.2.1.1 [EC] ) were detected while only low levels of pyruvate-orthophosphatedikinase (E.C. 2.7.9.1 [EC] ) activity were detected. Glycollate oxidase(E.C. 1.1.3.1 [EC] ), ribulose-l,5-bisphosphate carboxylase (E.C 4.1.1.39 [EC] ),NADP malate dehydrogenase (E.C. 1.1.1.82 [EC] ) and NAD malic enzyme(E.C. 1.1.1.39 [EC] ) were not detected in guard cell protoplast extracts.High levels of ribulose-1, 5-bisphosphate carboxylase, glycollateoxidase, NAD malate dehydrogenase and carbonic anhydrase weredetected in mesophyll cell protoplast extracts which is typicalof C₃ plants. A pathway of carbon flow during stomatal openingand closing is proposed. Key words: Carbon metabolism, Commelina communis, guard cell protoplasts, mesophyll cell protoplasts, stomata     

Carbon Dioxide Fixation by Guard Cell Protoplasts of Commelina communis

Biochemical studies of epidermal tissue may not reflect metabolismof the guard cells which represent less than 5% of the tissuevolume. Pure samples of guard cell protoplasts of Commelinacommunis were therefore used to investigate CO₂ fixation ratesand ¹⁴C-labelling patterns of metabolites in the light and thedark. Qualitatively, results were similar in most respects tothose obtained in a previous study (Schnabl, 1980) for guardcell protoplasts of Vicia faba. CO₂ fixation rates by guardcell protoplasts of C. communis were the same in the light andthe dark but about 50 times lower than the values Schnabl obtainedfor V.faba. The ¹⁴C-labelling pattern of metabolites in C. communiswas also similar in the light and the dark: over 60% of thetotal fixed was in malate with only 1% in sugar phosphates.Label was also detected in starch, aspartate, glutamate andcitrate but not in glycollate as previously recorded in V. fabaguard cell protoplasts. The results confirm the view that the reductive pentose phosphatepathway does not occur in guard cells of C. communis. Key words: CO₂ fixation, Guard cell protoplasts, Stomata