Structure, Carbon Dioxide Fixation and Metabolism of Stomata

Abstract

Studies were made of the structure and rates of CO₂ fixation of epidermis and of changes in organic metabolites in Commelina cyanea during transition to light and dark in both normal and CO₂-free air.

Guard cells of C. cyanea and Vicia faba contain numerous highly developed mitochondria and starch-forming chloroplasts (mitochondria: chloroplast ratios of 3 : 1) in comparison to other epidermal cells with few mitochondria and rudimentary plastids without starch. Their rates of photosynthesis per chloroplast appeared to be at least as high as those of the mesophyll, but circumstantial evidence suggested that about half of current photosynthate was respired. The rate of CO₂ fixation in the dark was about 0.2–0.4% of that in the light.

Illumination caused an increase, and darkening a decrease, of aperture, malate, and organic acid 1% within the epidermis of C. cyanea. Darkening in CO₂-free air was accompanied by only slight decreases in aperture and malate. There were close positive correlations between aperture and concentration of malate and between aperture and organic acid ¹⁴C. During opening, the rise in organic acid ¹⁴C was associated with a decline in amino acid ¹⁴C. It is suggested that organic acids may be formed through aspartate and possibly also from sugars and other amino acids entering the tricarboxylic acid cycle. Concentrations of sugars were not related to aperture although they increased on illumination and declined about 2 h after darkening. Polysaccharide concentrations in the epidermis of darkened leaves were similar to those in illuminated leaves.