Flickering LEDs Boost Photosynthesis When Pulses Stay Under a Millisecond

For nearly a century, plant scientists have toyed with a deceptively simple idea: because photons are only needed at the very first step of photosynthesis, perhaps plants do not need light all the time. If the later, light-independent stages of the process could keep running in the dark, then delivering energy in rapid flashes rather than a steady stream might squeeze more photosynthesis out of every photon. A new study from the University of Helsinki and the Estonian University of Life Sciences has now mapped, with unprecedented precision, exactly which flash patterns deliver on that promise—and which ones quietly sabotage plant growth. The findings, published in Discover Plants, arrive at a moment when LED technology has made flickering light essentially free, and they could reshape how vertical farms, greenhouses and space habitats illuminate their crops.

The research team, led by Krõõt Aasamaa together with Ivar Sibul, measured photosynthetic rates in six temperate species spanning dramatically different ecological strategies: the light-demanding herbs Plantago major and Taraxacum officinale, the shade-midtolerant Alchemilla vulgaris, the shade-tolerant herb Aegopodium podagraria, the light-demanding tree Betula pendula, and the shade-tolerant shrub Corylus avellana. Each leaf was clamped into the chamber of a gas analyser and exposed, in rapid alternation, to continuous light and to pulsed light carrying exactly the same integrated photon flux—the same total number of photons delivered over time. This careful matching is critical, because simply inserting dark gaps into a light regime reduces the total light a plant receives, confounding any comparison. By boosting the intensity during each light phase in proportion to the duty ratio, the researchers isolated the pure effect of intermittency itself.

The technical achievement behind the study lies in the sheer breadth of the pulsing regimes tested. Using a custom-built microcontroller apparatus based on a field-programmable gate array clocked at up to one gigahertz, the team generated cycle periods ranging from 100 milliseconds down to a mere 4 microseconds, with duty ratios—the fraction of each cycle spent illuminated—spanning 43 to 88 percent. That is more than a hundred-fold range of flash frequencies, far beyond what any previous study had systematically explored. The result is the first coherent picture of how the temporal texture of light shapes photosynthesis across an entire ecological spectrum of plants.

The headline finding is strikingly clean. Pulsed light with cycle periods between 100 and 13 milliseconds actually reduced photosynthetic rates compared with continuous light of equal total flux. Flashes with cycle periods between 12 and 1 milliseconds were neutral—neither helping nor harming. But once cycle periods dropped below 0.9 milliseconds, into the sub-millisecond and microsecond territory, photosynthesis accelerated above continuous-light levels in every species and every leaf type tested. Within this efficient range, the exact cycle period barely mattered: efficiencies remained comparable even as the cycle length varied more than 200-fold. What mattered instead was the duty ratio. Across all species, efficiency rose linearly as the duty ratio fell, meaning short, intense flashes separated by generous darkness outperformed long, dimmer pulses—provided the peak intensity stayed within the bounds of natural sunlight.

The physiological explanation the authors propose hinges on a race between two processes that begin the instant the light goes off. During each dark phase, energy absorbed in the preceding flash continues its journey downstream through the photosynthetic machinery, allowing the reaction centers of photosystems I and II to reset to an open state, ready to catch new photons. Simultaneously, however, photosynthetic enzymes begin to slip from their active, light-activated working state toward a resting state—a shift that begins within milliseconds of darkness. Short dark phases, up to roughly half a millisecond, appear beneficial because they reopen enough reaction centers while leaving enzyme activity essentially untouched. Between about half a millisecond and seven milliseconds of darkness, the two effects cancel out. Beyond seven milliseconds, enzyme deactivation wins: the machinery goes half-silent, and the next flash cannot restore photosynthesis quickly enough to use the available light efficiently. This framework elegantly explains why flashes that are too slow actively suppress photosynthesis.

The species comparisons added a second layer of insight. Herbaceous plants responded to the most efficient pulsed lights significantly more strongly than woody plants—so much so that even the least responsive herb, the shade-tolerant ground elder, outperformed the most responsive woody species, silver birch. Within each growth form, the most light-demanding species showed the strongest responses. And in a first for pulsed-light research, the team compared sun-grown and shade-grown leaves of the same species: light-adapted leaves consistently responded more strongly, by roughly a factor of two, and more than threefold in the sun-loving herbs. Because these sun leaves had never previously experienced natural pulsed light such as wind-driven sunflecks, the authors conclude that the ability to exploit rapid flickering is an inherent property of a high-capacity photosynthetic apparatus, not a trained skill acquired during a leaf’s lifetime.

One species broke the pattern in a revealing way. Alchemilla vulgaris, a herb known for its extraordinary ecological flexibility, produced unusually strong responses in its shade-adapted leaves—comparable to those of sun leaves. The authors suggest this reflects the genus’s strategy of building a powerful metabolic engine even in unfavorable environments, ready to exploit resources whenever they sporadically appear. The broader lesson is that both genetics and growth environment shape a plant’s sensitivity to flickering light, with photosynthetic capacity emerging as the common thread linking strong responses across all comparisons.

Perhaps most importantly, the efficient ranges identified here appear to be universal rather than species-specific. When the authors re-examined decades of earlier pulsed-light literature, a confusing patchwork of contradictory results snapped into focus. Studies reporting that pulsed light failed or even harmed plants had almost invariably used cycle periods that were too long—tens of milliseconds to seconds—or duty ratios far below the efficient band. Studies that found benefits had, often unknowingly, operated within the newly defined efficient window. The sub-millisecond regime and duty ratios around or below 50 percent emerge as the sweet spot, valid across herbs, trees, sun leaves and shade leaves alike, and consistent with findings from unrelated crop species such as lettuce and potato.

The practical implications are immediate. Because pulsed light in the efficient range drives higher photosynthetic rates than continuous light while consuming the same total energy, the authors strongly recommend its adoption in plant cultivation. Their specific guidance is refreshingly simple: use cycle periods of up to 0.9 milliseconds—there is no need for expensive generators producing ultra-short cycles, since all efficient periods perform similarly; keep duty ratios at or below 50 percent, lowering them further only as long as flash intensity stays within natural maxima; and favor light-demanding herbaceous species, the plants with the strongest responses, for pulsed-light cultivation. For an indoor farming industry wrestling with the energy costs of artificial lighting, the prospect of meaningful efficiency gains from nothing more than a change in the timing of light is a compelling one.

The study also closes a historical loop. Experiments on intermittent light date back to the 1930s, when researchers first separated the light-dependent and light-independent reactions of photosynthesis using flashing illumination. For decades afterward, generating pulses was costly and technically awkward, and the field languished. The LED revolution of the 2000s changed the economics entirely, since LEDs can switch rapidly without additional starting current and their lifespan is unaffected by flickering. What remained missing was a systematic map of the parameter space—and a test across plants with genuinely different light ecologies. By supplying both, this work transforms pulsed lighting from a curiosity with inconsistent results into an engineering discipline with clear design rules, and it suggests that the future of efficient plant lighting may be measured in microseconds rather than hours.

Subject of Research: Effects of pulsed LED light cycle period and duty ratio on photosynthesis in herbaceous and woody plants

Article Title: Efficient pulsed lights for herbaceous and woody plants with different light demands

Article References: Aasamaa, K., & Sibul, I. (2026). Efficient pulsed lights for herbaceous and woody plants with different light demands. Discover Plants, 3(1), Article 388. https://doi.org/10.1007/s44372-026-00864-x

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00864-x

Keywords: pulsed light, photosynthesis, LED lighting, plant cultivation, duty ratio, cycle period, herbaceous plants, woody plants, light-demanding species, shade tolerance, vertical farming, light-use efficiency