Is the Kok effect a respiratory phenomenon? Metabolic insight using ¹³C labeling in Helianthus annuus leaves

Abstract

The Kok effect is a well-known phenomenon in which the quantum yield of photosynthesis changes abruptly at low light. This effect has often been interpreted as a shift in leaf respiratory metabolism and thus used widely to measure day respiration. However, there is still no formal evidence that the Kok effect has a respiratory origin. Here, both gas exchange and isotopic labeling were carried out on sunflower leaves, using glucose that was 13 C-enriched at specific C-atom positions. Position-specific decarboxylation measurements and NMR analysis of metabolites were used to trace the fate of C-atoms in metabolism. Decarboxylation rates were significant at low light (including above the Kok break point) and increased with decreasing irradiance below 100 µmol photons m −2 s −1. The variation in several metabolite pools such as malate, fumarate or citrate, and flux calculations suggest the involvement of several decarboxylating pathways in the Kok effect, including the malic enzyme. Our results show that day respiratory CO 2 evolution plays an important role in the Kok effect. However, the increase in the apparent quantum yield of photosynthesis below the Kok break point is also probably related to malate metabolism, which participates in maintaining photosynthetic linear electron flow.

Canopy Position Influences the Degree of Light Suppression of Leaf Respiration in Abundant Tree Genera in the Amazon Forest

“…It is known that the Kok method can underestimate rates of light respiration (Gong et al, 2018;Way et al, 2018;Yin et al, 2020), but on the other hand is the most practical method to apply in logistically challenging field conditions (Tcherkez et al, 2017a). The Kok "effect" has been described as due to not only changes in respiratory rates with light, but also other physiological and biochemical process (Gauthier et al, 2020;Yin et al, 2020). At least four non-mutually exclusive phenomenon could explain the Kok "effect" including a decrease in the photochemical efficiency of photosynthesis, refixation of internal CO 2 sources (respiration, photorespiration, etc.)…”

Non‐targeted 13C metabolite analysis demonstrates broad re‐orchestration of leaf metabolism when gas exchange conditions vary

“…It must be noted that in the previous sentence, the term “probably” has been used. In effect, the measurement of R d is still technically challenging and the two common methods (Kok and Laisk methods) used to determine R d have drawbacks (Atkin, Millar, Gardeström, & Day, 2000; Tcherkez et al, 2017): the Kok effect has recently been shown to involve specific metabolism and numerically, cannot simply be explained by a change in CO 2 evolution (Gauthier, Saenz, Griffin, Way, & Tcherkez, 2020), meaning that Kok‐derived estimates of R d are not fully representative; the Laisk method requires changes in CO 2 mole fraction, thus it is not adapted to examine the effect of the gaseous environment. Conversely, at low O 2 (2% O 2 and below), leaves experience hypoxic metabolism leading to alanine accumulation, diverting pyruvate from mitochondrial oxidation thereby impacting on day respiration (Abadie, Blanchet, Carroll, & Tcherkez, 2017).…”

Consistent diurnal pattern of leaf respiration in the light among contrasting species and climates

“…Mechanisms related to the diurnal variation of respiration in the light at constant measuring temperature Many processes can affect respiratory CO 2 effluxes of leaves in the light (Tcherkez et al, 2017b). These include the oxidative pentose phosphate pathway (Buckley & Adams, 2011;Shameer et al, 2019;Xu et al, 2021), photorespiration (Igamberdiev et al, 2001;Tcherkez et al, 2005Tcherkez et al, , 2008Tcherkez et al, , 2012a, the continued utilization of stored organic acids (Gauthier et al, 2010), the activity of the pyruvate dehydrogenase complex (Budde & Randall, 1990;Gemel & Randall, 1992), the activity of the malic enzyme (Gauthier et al, 2020) and NAD(P)H : NAD(P) ratios (Igamberdiev & Gardeström, 2003). Some CO 2 fluxes may even originate from nonleaf sources that are transported through the vascular tissues to leaves and subsequently released (Stutz et al, 2017;Stutz & Hanson, 2019).…”