Stomatal Metabolism: CO₂ Fixation and Respiration

Abstract

The rate of fixation of CO2 by epidermis attached to the leaf of Commelina cyanea was linear with photon flux density up to 1.08 mE m-² s-¹ and with CO2 concentration from 0 to 355 ppm. Detached epidermis, on the other hand, showed no response to photon flux density and had rates in the light which were only twice those in the dark. There was substantial leakage of all labelled methanol-soluble substances from isolated epidermis. Its apparent reduced functioning may well be associated with this leakage rather than reflecting transport from the mesophyll. The early fixation products in light and dark were similar in attached and detached epidermis. These were mainly aspartate and malate in contrast to the Calvin cycle intermediates formed in mesophyll in the light. It seems likely that phosphoenolpyruvate carboxylase is closely implicated in the initial fixation of CO2 by stomata and is responsible for the much higher rates of fixation per unit of chlorophyll by epidermis than by mesophyll. Although aspartate and malate are the major products detectable after 2 min feeding with 14CO2 and remain as a high proportion of labelled products during a subsequent 12CO2 chase, amino acids, sugars and sugar phosphates together with polysaccharides and other methanol-insoluble products are eventually labelled. Attempts to measure respiration rates of attached epidermis were unsuccessful due to fixation of mesophyll-respired CO2; the rate in detached epidermis was high, suggesting a high rate of turnover of carbon products by stomata, but we were unable to relate this to rates in attached epidermis. Rates were higher in CO2-free than in CO2-containing environments, indicating a possible explanation of the CO2 effect on stomata.