Definition
The Emerson effect, also known as the Emerson enhancement effect, is a photosynthetic phenomenon in which the rate of oxygen evolution (or overall photosynthetic productivity) under simultaneous illumination with red (≈ 660 nm) and far‑red (≈ 730 nm) light exceeds the sum of the rates observed when each wavelength is applied separately.
Overview
The effect was first reported in 1957 by American plant physiologist Robert H. Emerson, who was investigating the action spectrum of photosynthesis using isolated chloroplasts and algal suspensions. Emerson observed that illumination with far‑red light alone produced only a modest photosynthetic response, but when far‑red light was added to red light the total rate of oxygen evolution increased disproportionately. This non‑additive response indicated that photosynthesis involves at least two light‑driven components that cooperate when excited by different spectral regions. The Emerson effect provided key experimental support for the later formulation of the two‑photosystem (Photosystem I and Photosystem II) model of the light reactions of photosynthesis.
Etymology / Origin
The term is derived from the surname of Robert Hammond Emerson (1912–2004), whose experiments on the spectral dependence of photosynthesis led to its identification. “Effect” in this context follows the scientific convention of naming a reproducible, measurable phenomenon after its discoverer.
Characteristics
| Aspect | Description |
|---|---|
| Spectral regions involved | Red light (≈ 660 nm) primarily excites Photosystem II; far‑red light (≈ 730 nm) preferentially excites Photosystem I. |
| Experimental setup | Typically measured with isolated chloroplasts, algal cultures, or intact leaves placed in a sealed oxygen‑evolution chamber; oxygen production is monitored under monochromatic and combined illumination. |
| Quantitative observation | The combined‑light rate can be 1.5‑ to 2‑fold higher than the arithmetic sum of the individual monochromatic rates, depending on intensity and organism. |
| Physiological explanation | The effect reflects the sequential use of two photochemical reaction centres (PSII → PSI) linked by an electron transport chain. Excitation of both centres permits more efficient charge separation and electron flow than excitation of either centre alone. |
| Implications | Provided early evidence for the existence of two distinct photosystems; influenced the development of the Z‑scheme of photosynthetic electron transport; informs modern studies of light‑energy harvesting and the design of artificial photosynthetic systems. |
| Limitations | The magnitude of the effect diminishes at very high light intensities where one photosystem becomes saturated; it is not observed in organisms lacking a functional PSI/PSII division (e.g., certain anoxygenic photosynthetic bacteria). |
Related Topics
- Photosynthesis – the overall process of converting light energy into chemical energy in plants, algae, and cyanobacteria.
- Photosystem I (PSI) and Photosystem II (PSII) – the two major pigment‑protein complexes that drive the light reactions.
- Action spectrum – the wavelength dependence of a photobiological response, of which the Emerson effect is a notable deviation.
- Chlorophyll fluorescence – a diagnostic tool often used alongside Emerson‑effect experiments to assess photosystem activity.
- Quantum yield of photosynthesis – the efficiency with which absorbed photons are converted into chemical products; the Emerson effect can raise the effective quantum yield under mixed‑light conditions.
- Z‑scheme – the representation of electron flow between PSI and PSII, incorporating insights gained from the Emerson effect.
The Emerson effect remains a foundational observation in plant physiology and bioenergetics, illustrating the cooperative interaction of multiple photochemical systems within the thylakoid membrane.