The Kok effect in V icia faba cannot be explained solely by changes in chloroplastic CO 2 concentration

Summary The Kok effect – an abrupt decline in quantum yield ( QY ) of net CO 2 assimilation at low photosynthetic photon flux density ( PPFD ) – is widely used to estimate respiration in the light ( R ), which assumes the effect is caused by light suppression of R . A recent report suggested much of the Kok effect can be explained by declining chloroplastic CO 2 concentration ( c c ) at low PPFD . Several predictions arise from the hypothesis that the Kok effect is caused by declining c c , and we tested these predictions in Vicia faba . We measured CO 2 exchange at low PPFD , in 2% and 21% oxygen, in developing and mature leaves, which differed greatly in R in darkness. Our results contradicted each of the predictions based on the c c effect: QY exceeded the theoretical maximum value for photosynthetic CO 2 uptake; QY was larger in 21% than 2% oxygen; and the change in QY at the Kok effect breakpoint was unaffected by oxygen. Our results strongly suggest the Kok effect arises largely from a progressive decline in R with PPFD that includes both oxygen‐sensitive and ‐insensitive components. We suggest an improved Kok method that accounts for high c c at low PPFD .