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

Area of Science:

• Plant physiology
• Photosynthesis research
• Plant respiration

Background:

• The Kok effect describes a sharp decrease in the quantum yield (QY) of net CO2 assimilation at low photosynthetic photon flux density (PPFD).
• This effect is commonly attributed to light-induced suppression of respiration in the light (R).
• A recent hypothesis proposed that declining chloroplastic CO2 concentration (cc) at low PPFD might explain the Kok effect.

Purpose of the Study:

• To test the hypothesis that the Kok effect is caused by declining cc.
• To investigate the role of light-dependent respiration (R) in the Kok effect.
• To propose an improved method for estimating R using the Kok effect.

Main Methods:

• Measurements of CO2 exchange in Vicia faba leaves at low PPFD under varying oxygen concentrations (2% and 21%).
• Comparison of developing and mature leaves with differing dark respiration rates.
• Evaluation of predictions derived from the cc hypothesis against experimental data.

Main Results:

• Quantum yield (QY) exceeded theoretical maximums for CO2 uptake, contradicting the cc hypothesis.
• QY was higher under 21% oxygen than 2% oxygen.
• The change in QY at the Kok breakpoint was independent of oxygen levels.
• Results indicated a progressive decline in R with increasing PPFD, comprising both oxygen-sensitive and -insensitive components.

Conclusions:

• The Kok effect is primarily driven by a decline in light-dependent respiration (R) with increasing PPFD, not by declining chloroplastic CO2 (cc).
• The cc hypothesis predictions were contradicted by experimental results.
• An improved Kok method accounting for high cc at low PPFD is suggested for more accurate R estimation.