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

Area of Science:

• Plant Physiology
• Biochemistry
• Photosynthesis Research

Background:

• Guard cells are known to assimilate carbon dioxide (CO2).
• The specific biochemical pathways utilized by guard cells for CO2 fixation have been debated.
• Previous studies may have included contaminating mesophyll cells, affecting results.

Purpose of the Study:

• To investigate the CO2 fixation products and metabolic pathways in isolated plant epidermis, specifically guard cells.
• To determine if guard cells possess the reductive pentosephosphate pathway.
• To clarify the role of guard cells in carbon assimilation within the plant epidermis.

Main Methods:

• Isolated epidermis of Tulipa gesneriana and Commelina communis were exposed to (14)CO2 under varying light and stomatal conditions.
• Radioactive labeling patterns of CO2 fixation products were analyzed.
• Enzymatic activity assays were performed on epidermal homogenates to assess specific pathway enzymes.

Main Results:

• The primary products of CO2 fixation in guard cells were malate and aspartate, regardless of stomatal status or light.
• Radioactivity was minimal in 3-phosphoglyceric acid (3-PGA) and sugar phosphates, especially in mesophyll-free epidermal samples.
• Guard cells demonstrated limited ability to convert ribulose diphosphate to 3-PGA and lacked the capacity to convert ribulose-5-phosphate to ribulose diphosphate, indicating absence of the reductive pentosephosphate pathway.

Conclusions:

• Guard cells actively assimilate CO2, primarily forming malate and aspartate.
• Guard cells of Commelina communis and likely Tulipa gesneriana lack the complete reductive pentosephosphate pathway.
• Guard cells depend on imported carbon compounds to meet their metabolic demands, and prior findings of epidermal CO2 reduction may stem from mesophyll contamination.