Stable Isotope Analysis of Sunflower Leaf Respiration at Low Light Intensities: the Metabolic Origin of the Kok Effect

Abstract

A deeper knowledge of plant metabolism, particularly respiration, is necessary for understanding the role of plants in the global carbon cycle. Plant respiration is inhibited in the light, though reasons for the inhibition remain unclear. The existence and origin of the Kok effect, characterized by a breaking point in photosynthesis rate followed by an increase in slope, have been under debate since Bessel Kok first described the Kok effect in 1948. Because the Kok effect is used to estimate the respiration in the light, investigating its metabolic origin can point to the mechanisms and cause for respiratory light inhibition. Since respiration is difficult to study at high light because of the photosensitive fluxes and the Kok effect occurs at low light intensities, investigating the changes in respiration at low light can illuminate the metabolic sources of light inhibition. By comparing the decarboxylation rates of position-specific \(^{13}\)C-labeled glucoses, the Kok effect can be linked to a metabolic shift in respiration led by pyruvate dehydrogenase that occurs at low light intensities.