Changes in the chloroplastic CO₂ concentration explain much of the observed Kok effect: a model

Abstract

Mitochondrial respiration often appears to be inhibited in the light when compared with measurements in the dark. This inhibition is inferred from the response of the net CO assimilation rate (A) to absorbed irradiance (I), changing slope around the light compensation point (I ). We suggest a model that provides a plausible mechanistic explanation of this 'Kok effect'. The model uses the mathematical description of photosynthesis developed by Farquhar, von Caemmerer and Berry; it involves no inhibition of respiration rate in the light. We also describe a fitting technique for quantifying the Kok effect at low I. Changes in the chloroplastic CO partial pressure (C ) can explain curvature of A vs I, its diminution in C plants and at low oxygen concentrations or high carbon dioxide concentrations in C plants, and effects of dark respiration rate and of temperature. It also explains the apparent inhibition of respiration in the light as inferred by the Laisk approach. While there are probably other sources of curvature in A vs I, variation in C can largely explain the curvature at low irradiance, and suggests that interpretation of day respiration compared with dark respiration of leaves on the basis of the Kok effect needs reassessment.

The rapid A/Ci response: a guide to best practices

“…This makes assigning ‘truth’ more difficult since the method used can affect the outcome. This is especially true for values like g m , R d , and Γ * such that it is not widely accepted that these are optimally derived from a single standard A/C i (see methods Pons et al ., ; Walker & Cousins, ; Walker & Ort, ; Farquhar & Busch, ). The most widely accepted use for the A/C i is to estimate V cmax and J (with much less certainty about J ) and those values are in ‘reasonably close agreement’ between the standard and RACiR approaches in Taylor & Long () and in Stinziano et al .…”

“…Based on potential procedural or biological concerns mentioned earlier, at high ramp rates RACiR may not generate compensation points similar to a standard A/C i . However, we also point out that standard A/C i methods used to generate estimates of other parameters presented in Taylor & Long (), including g m (mesophyll conductance), R d (dark respiration), and Γ * (photorespiratory compensation point) are potentially problematic (Pons et al ., ; Walker & Cousins, ; Walker & Ort, ; Farquhar & Busch, ) and we would not recommend using them.…”

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“…For each curve, the derived was then fixed in and used to derive the remaining parameters. As the initial slope of the response curve was fitted to data above a potential Kok kink, the derived R d could also be considered to be equal to R light , but note the current debate over this issue (Buckley et al ., ; Farquhar & Busch, ; Tcherkez et al ., ). Our estimates of R d were not corrected for changes in C i (Kirschbaum & Farquhar, ).…”

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“…Both these issues lead to a lower estimate of G 1 at the ecosystem level compared to the leaf level. Estimates of canopy‐scale R L from our simulations amount to 3.4% of GPP on average across sites, whereas overestimation of GPP due to a possible light inhibition of R D would be in the order of 15% (Janssens et al., ; Wehr et al., ; Wohlfahrt et al., ), indicating that the latter is the more relevant source of disagreement between across‐scale estimates of G 1 , but clearly a better process understanding is needed (Farquhar & Busch, ).…”

“…The two have to be interpreted differently for two main reasons: (i) GPP estimated from flux partitioning algorithms integrates carboxylation ( V c ) minus photorespiration ( R p ) and is thus not equivalent to (and to some extent larger than) net photosynthesis ( A n = V c − R p − R L ) measured from leaf‐level gas exchange. (ii) Leaf respiration in the dark ( R D ) has been found to exceed that in daylight ( R L ) (Atkin, Westbeek, Cambridge, Lambers, & Pons, ; Brooks & Farquhar, ), though a recent study suggests that this “Kok effect” could also be explained by changes in the chloroplastic CO 2 concentration ( C c ) caused by a reduced mesophyll conductance at low light (Farquhar & Busch, ), in which case R D would not necessarily be down‐regulated in the light. The consequence of a larger R D compared to R L would be that an extrapolation of nighttime respiration to daytime overestimates GPP (Wohlfahrt & Gu, ).…”