Decrease, Photosystem II efficiency

The decreased Photosystem II efficiency describes a reduction in the capacity of Photosystem II to convert absorbed light energy into chemical energy, typically quantified as a decline in the maximum or effective quantum yield of PSII (e.g. Fv/Fm or ΦPSII). This impairment reflects disturbances in PSII reaction centers and/or associated electron transport components, resulting in reduced linear electron flow and a consequent limitation in the production of ATP and NADPH required for downstream photosynthetic processes (Maxwell & Johnson, 2000; Baker, 2008).

Decreased Photosystem II (PSII) efficiency can be quantified using a suite of complementary physiological and biochemical methods that assess the functional status of PSII reaction centers and associated electron transport processes. The most widely applied approach is chlorophyll fluorescence analysis, particularly the measurement of the maximum quantum yield of PSII (Fv/Fm), which provides a sensitive, non-invasive indicator of PSII photochemical efficiency and photoinhibitory damage (Maxwell and Johnson, 2000; Xia et al., 2023). Pulse-amplitude-modulated (PAM) fluorometry and related modulated fluorescence techniques allow both dark-adapted and light-adapted measurements, enabling assessment of effective PSII quantum yield and dynamic responses to stressors.

PSII efficiency can also be evaluated through measurements of photosynthetic oxygen evolution rates, typically using polarographic oxygen electrodes with leaf discs, algal cultures, or isolated chloroplasts. These measurements directly reflect the functional integrity of the PSII water-splitting complex and downstream electron transport capacity (DELIEU and WALKER, 1981). Reductions in oxygen evolution are indicative of impaired PSII activity and reduced photochemical performance.

At the molecular and mechanistic level, degradation or modification of the D1 protein of PSII can be assessed to support evidence of PSII damage. The D1 protein is a primary target of photodamage and herbicide interaction, and increased D1 turnover or instability is closely linked to declines in PSII efficiency (Alfonso et al., 1996). Together, chlorophyll fluorescence parameters, oxygen evolution measurements, and D1 protein degradation assays provide robust and mechanistically informative lines of evidence for identifying and quantifying decreases in Photosystem II efficiency.

The key event “decreased Photosystem II efficiency” is broadly applicable across oxygenic photosynthetic organisms that rely on PSII-mediated light reactions, including higher plants, macroalgae, microalgae, and cyanobacteria. The underlying structure and function of PSII, as well as the photochemical principles captured by chlorophyll fluorescence parameters (e.g. Fv/Fm) and oxygen evolution, are highly conserved across these taxa, supporting cross-species relevance of this KE.

This KE is particularly applicable in studies assessing the effects of stressors that directly or indirectly interfere with PSII function, such as photosystem II–inhibiting herbicides, compounds targeting the D1 protein or the QB binding site, excess light, UV radiation, nutrient limitation, and other environmental stressors that induce photoinhibition or disrupt electron transport. It is most reliably measured under controlled laboratory or semi-controlled conditions where light history, acclimation status, and physiological state of the test organism can be standardized.

The domain of applicability is strongest for acute to sub-chronic exposures where changes in PSII efficiency precede downstream effects on carbon fixation, growth, and biomass production. While the KE is highly sensitive and diagnostically informative at the cellular and organelle level, its interpretation at higher levels of biological organization (e.g. whole-plant productivity or ecosystem-level responses) requires integration with additional endpoints to account for compensatory mechanisms such as energy dissipation, PSII repair, and alternative electron transport pathways.

Alfonso, M., Pueyo, J.J., Gaddour, K., Etienne, A.-L., Kirilovsky, D. and Picorel, R. (1996). Induced new mutation of D1 serine-268 in soybean photosynthetic cell cultures produced atrazine resistance, increased stability of S2QB− and S3QB− states, and increased sensitivity to light stress. Plant Physiology, 112(4), 1499–1508.

DELIEU, T. and WALKER, D.A. (1981). Polarographic measurement of photosynthetic oxygen evolution by leaf discs. New Phytologist, 89(2), 165–178.

Maxwell, K. and Johnson, G.N. (2000). Chlorophyll fluorescence—a practical guide. Journal of Experimental Botany, 51(345), 659–668.

Xia, Q., Tang, H., Fu, L., Tan, J., Govindjee and Guo, Y. (2023). Determination of Fv/Fm from chlorophyll a fluorescence without dark adaptation by an LSSVM model. Plant Phenomics, 5, 0034.