The Kok effect in Vicia faba cannot be explained solely by changes in chloroplastic CO2 concentration.

Thomas N Buckley,Heather Vice,Mark A Adams

doi:10.1111/nph.14775

Thomas N Buckley, Heather Vice + Show 1 more

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https://doi.org/10.1111/nph.14775

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Abstract

The Kok effect - an abrupt decline in quantum yield (QY) of net CO2 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 CO2 concentration (cc ) at low PPFD. Several predictions arise from the hypothesis that the Kok effect is caused by declining cc , and we tested these predictions in Vicia faba. We measured CO2 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 cc effect: QY exceeded the theoretical maximum value for photosynthetic CO2 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 cc at low PPFD.

Highlights

Leaf respiration rate is suppressed in the light, by up to 85% of the rate in the dark (Rdark; Hoefnagel et al, 1998)
Light suppression of R has long been suspected as the mechanism of the ‘Kok effect’ – an abrupt change in quantum yield (QY) of net CO2 assimilation rate (A) that occurs at very low photosynthetic photon flux density (PPFD or i), often near the photosynthetic light compensation point (Fig. 1; Kok, 1948, 1949)
Our data contradict each of the three predictions (Table 1) from the hypothesis that changes in cc at low PPFD explain the Kok effect, which strongly indicates that the Kok effect in V. faba is at least partly attributable to suppression of R by light

Summary

Introduction

Leaf respiration rate (nonphotorespiratory CO2 release, R) is suppressed in the light, by up to 85% of the rate in the dark (Rdark; Hoefnagel et al, 1998). There is no consensus as to what causes this suppression or why it varies so widely (Kr€omer, 1995; Hoefnagel et al, 1998; Buckley & Adams, 2011; Tcherkez et al, 2017a,b) This uncertainty confounds reliable prediction of CO2 exchange, as well as interpretation of processes related to CO2 exchange, including photosynthesis and photorespiration, carbohydrate metabolism, anabolism and stable isotope discrimination (Kr€omer, 1995; Hoefnagel et al, 1998; Noctor & Foyer, 1998; Ghashghaie et al, 2003; Tcherkez & Hodges, 2008). Evidence from other methods supports the hypothesis that R is suppressed by light (Atkin et al, 1997, 2000; Peisker & Apel, 2001; Tcherkez et al, 2005, 2008), relatively few experiments have estimated changes in R across the narrow range of very low

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