Far-red light may change how plants protect a vulnerable photosynthetic system

Laboratory lighting can reveal how photosynthesis breaks down—but its spectrum can also shape the answer. A new preprint review argues that far-red light, often missing from experimental illumination, changes how plants protect photosystem I from damage during shifts between dim and bright conditions.

Photosystem I helps turn absorbed light into the reducing power plants need for carbon assimilation and other metabolic processes. Once damaged, it is slow to repair or replace. Yet understanding its vulnerability requires more than measuring how bright the light becomes: temperature, the timing of illumination and the wavelengths reaching a leaf all matter.

In a bioRxiv preprint, Ko Takeuchi and colleagues bring together research on chilling-induced damage and damage caused by fluctuating light. Their review, which has not been peer reviewed, also analyzes 107 experiments from 32 papers. Its central argument is that laboratory light regimes can favor different protective mechanisms—and may not accurately represent conditions beneath a plant canopy.

How cold can turn moderate light damaging

The review traces a key discovery to cucumber experiments reported in 1994: below about 10°C, moderate illumination could damage photosystem I while initially leaving photosystem II relatively unaffected.

The authors describe a progression based mainly on cucumber leaves exposed to 4°C under relatively weak illumination. Cold first slows metabolic reactions that consume electrons downstream of photosystem I. When electron supply exceeds consumption, electron carriers become excessively reduced, increasing opportunities for reactive oxygen species to form.

Over roughly 30 minutes to two hours, additional protective systems weaken. The catalytic portion of chloroplast ATP synthase becomes more loosely associated with the membrane. Proton leakage then undermines the acidity difference across that membrane, weakening controls that normally slow electron delivery to photosystem I. Meanwhile, a complex joining photosystem I to the electron-recycling machinery called NDH destabilizes, and NDH activity declines.

Marked losses of photosystem I function follow over approximately two to five hours in the sequence described. These timings are not universal: they depend on cultivar, leaf age and treatment conditions. Some early links also remain hypothetical. Although cucumber membrane lipids change physical state near the temperature threshold for injury, no study has directly tested that transition and photosystem I damage under the same conditions.

A protective pathway depends on the light regime

Plants have two major routes for cyclic electron flow around photosystem I: the PGR5/PGRL1-dependent pathway and the NDH-dependent pathway. Both recycle electrons and support the proton gradient used for ATP production and regulation of electron transport.

Experiments using sustained alternations between high and low red light often identify PGR5-dependent protection as especially important. In one Arabidopsis experiment reviewed, the maximum photooxidizable P700 signal—a measure used to assess photosystem I—fell to about 20% of its starting value in a PGR5-deficient mutant, compared with 70% in wild-type plants.

A different experiment, using 800-millisecond bright periods separated by 10 seconds of low light, showed why spectrum matters. Without supplemental far-red light, the signal fell to roughly 70% across the tested genotypes. When far-red was supplied during both phases, wild-type plants retained 97% and the PGR5-deficient mutant retained 92%. Two NDH-deficient mutants retained about 80%.

These results support a role for NDH in protection under that far-red-supplemented regime. They do not establish that NDH always replaces PGR5: pulse duration, background illumination and species all influence the outcome.

An explanation still awaiting direct tests

Far-red light preferentially excites photosystem I, changing the balance of excitation between the two photosystems. The authors propose that this reduces the need for the rapid, large proton-gradient response associated with PGR5-dependent protection, while leaving NDH to support lower-rate electron recycling.

Their more specific hypothesis is that electron-carrier redox states and the proton gradient together determine whether NDH can operate effectively. But the relevant net driving force has never been determined inside a leaf under the contrasting light regimes. Regulation by ATP could also contribute, and the molecular role of PGR5 remains unsettled.

The cross-study analysis adds context rather than resolving those mechanisms. Greater P700 oxidation was associated with less damage per photon exposure in the selected fluctuating-light experiments. That association alone does not prove protection by P700 oxidation itself. Studies differed in species, leaf characteristics, illumination and measurement methods, with sparse data for some extreme-pulse treatments.

Bringing the question back to leaves outdoors

Measurement itself introduces uncertainty. Near-infrared signals used to track P700 can contain contributions from other electron carriers, and saturating measurement pulses can alter the state being assessed. Different depths within a leaf also receive different amounts of light.

The ecological gap is equally important. Evidence that NDH supports growth under canopy-like intensity fluctuations does not, by itself, show that it prevents photosystem I damage: that damage was not measured in the cited canopy-mimicking work.

The authors therefore call for experiments that reproduce natural spectra and measure photosystem I damage alongside growth. Far-red protection is experimentally supported under particular conditions; whether it makes NDH the dominant protective route in real canopies remains a testable proposition, not an established field result.