The Kok effect: beyond the artefact, emerging leaf mechanisms (KOALA)

18th New Phytologist Workshop

6–8  July 2016, University of Angers, France




The aim of the workshop will be to gain decisive insight into the mechanisms of the Kok effect, which has intrigued plant scientists for more than half a century, by bringing together the community of world scientists who are committed to examining this effect. Therefore, the output of the meeting will be to provide a specific definition of the Kok effect and a set of clear hypotheses about its origin and possible interpretations, so as to figure out whether this effect is relevant for ecophysiological respiration measurements.


Rational and scope


The Kok effect (discovered by Kok and colleagues in 1948) is defined as the abrupt increase in the slope obtained with a plot representing net leaf CO2 assimilation against light, at low light levels. There is a huge body of literature associated with the Kok effect, since it has been used for years to infer the rate of respiration in illuminated leaves, and has been observed at the ecosystem or mesocosm scale. In most studies that involve measurements of day respiration (non photorespiratory CO2 evolution by illuminated leaves), the Kok effect is exploited since it is much less demanding than alternative techniques, and technically easier to carry out. Nevertheless, the intrinsic mechanisms of the effect are still unknown. Unexpected relationships with O2 fixation (photorespiration) seem to contradict the presumed, purely mitochondrial origin of the effect, and the possible involvement of the oxidative pentose phosphate pathway remains unresolved. In other words, there is still uncertainty on whether the estimates of leaf day respiration obtained so far with the Kok effect are valid or not. Recent investigation using either contrasted environmental conditions (e.g., CO2-enriched atmosphere) or metabolic analyses suggest that the effect can be altered under certain circumstances. Furthermore, new gas exchange techniques (direct O2 measurements in an open system, or coupling to 13C/12C isotopes) have revealed yet poorly documented aspects of the Kok effect (associated respiratory quotient, 13C signature of ‘Kok-evolved’ CO2). 


The implications of changing our view on the Kok effect are to be recognized: this effect has been proposed to be used at the ecosystem scale to estimate ecosystem respiration in the light – rather than assuming a similar ecosystem respiration in the light and in darkness. At the metabolic scale, should the effect not be as unambiguous a measure of mitochondrial respiration in the light as is currently perceived, the scientific community would have to reconsider the basis of the inhibition of leaf respiration by light, take into account the involvement of other pathways (e.g., pentose phosphates or photorespiration).


Organising Committee


Guillaume Tcherkez (Australian National University, Australia & University Paris-Sud France)

Tom Buckley (University of Sydney, Australia)

Paul Gauthier (Princeton University, USA)




Note that participation in this workshop is by invitation only. For more information please contact us by email: