Abstract

Summary The Kok effect refers to the abrupt decrease around the light compensation point in the slope of net photosynthetic rate vs irradiance. Arguably, this switch arises from light inhibition of respiration, allowing the Kok method to estimate day respiration (R d). Recent analysis suggests that increasing proportions of photorespiration (quantified as Γ*/C c, the ratio of CO2 compensation point Γ* to chloroplast CO2 concentration, C c) with irradiance explain much of the Kok effect. Also, the Kok method has been modified to account for the decrease in PSII photochemical efficiency (Φ2) with irradiance.Using a model that illustrates how varying R d, Γ*/C c, Φ2 and proportions of alternative electron transport could engender the Kok effect, we quantified the contribution of these parameters to the Kok effect measured in sunflower across various O2 and CO2 concentrations and various temperatures.Overall, the decreasing Φ2 with irradiance explained c. 12%, and the varying Γ*/C c explained c. 25%, of the Kok effect. Maximum real light inhibition of R d was much lower than the inhibition derived from the Kok method, but still increased with photorespiration.Photorespiration had a dual contribution to the Kok effect, one via the varying Γ*/C c and the other via its participation in light inhibition of R d.

Highlights

  • The Kok effect refers to the abrupt change in the slope of the linear relationship between net photosynthetic rate and irradiance that occurs at very low irradiances, as observed initially in unicellular algae (Kok, 1948, 1949; Healey & Myers, 1971)

  • This switch has been interpreted as a consequence of light inhibition of respiration, allowing the so-called Kok method to estimate respiration in the light, or day respiration (Rd), and quantum yield of CO2 assimilation (ΦCO2), using the part of the relationship with the lower slope

  • The absolute value of the estimated Rd is lower than the respiration in the dark (Rdk) (Fig. 1)

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Summary

Introduction

The Kok effect refers to the abrupt change in the slope of the linear relationship between net photosynthetic rate and irradiance that occurs at very low irradiances, as observed initially in unicellular algae (Kok, 1948, 1949; Healey & Myers, 1971). Light inhibition of respiratory activities occurs at a stand scale (Gong et al, 2017), suggesting that it is a general phenomenon, and would have a significant impact on projecting the net ecosystem carbon fluxes in biomes across the globe (Heskel et al, 2013). For this reason, understanding the Kok effect and its related light inhibition of respiration has continuously received attention (Tcherkez et al, 2017a,b)

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