Which Matters More for Healthy Growth and Bloom--Spectrum or Efficacy?
When evaluating any grow light fixture or horticultural lighting technology, there’s really only one benchmark that matters: the Sun.
Light is the first order of life and pretty much everything on our planet is dependent on it.
Plants, of course, have also evolved to exploit incident solar radiation—the sunlight that reaches the surface of our planet.
There are approaching 400,000 different plant species currently known to science and each one is the result of adaptation to a particular environment over millions of years.
We don’t have to go back that far to remember a time when indoor growers encountered far fewer options when choosing a grow light.
There was seldom much discussion on spectra—save “Metal Halide for growth” and “HPS for bloom”.
Any talk of “photon efficacy” was largely confined to academia.
Photon Efficacy isn’t the Whole Story.
Today, there’s a mind boggling array of options in the horticultural lighting sector, especially within the relatively new subcategory of LED grow lights.
No wonder, then, that so many growers have embraced photosynthetic photon flux (PPF) and photosynthetic photon efficacy (PPE) as handy “universal metrics” when evaluating and comparing different grow light fixtures.
PPF gives growers a simple, comprehensible number to represent the total output of a grow light during one second (e.g. 2100 μmol/sec) and PPE provides valuable information on the efficiency of a fixture by dividing PPF by the amount of electrical energy (Joules) required to generate it (e.g. 2.1 μmol/J).
Indoor growers, especially those interested in growing light-loving plant species like tomatoes, are drawn to the higher PPF and PPFD numbers—a fact not overlooked by many grow light fixture manufacturers and marketers.
Take our two new iLogic™8 LED lighting fixtures for example. The “full-spectrum” model boasts a PPE of 2.7 μmol/J but the alternate version with full-spectrum supplemented ultraviolet and far-red has a PPE of just 2.0 μmol/J.
If you were choosing a new LED lighting fixture based on PPE numbers, you might overlook the latter model—but you’d be missing out!
UV and far-red photons are not counted in PPF, PPFD or PPE statistics because they lie outside of PAR.
Are PPF / PPFD / PPE Metrics Outdated?
PPF and PPE are based on an outdated definition of photosynthetically active radiation as PAR only counts those photons within a wavelength range of 400 and 700 nanometers.
Recently, scientists have demonstrated that plants actually use and respond to a wider range of radiation than PAR, perhaps reflecting the fact that incident solar radiation is only made up of around 42% PAR.
At the more energetic side of the spectrum, there’s ultraviolet (UV-A) radiation (320 – 400 nm). This makes up around 3% of incident solar radiation. And just beyond the 700 nanometer mark is far-red (FR) radiation (700 – 800 nm).
UV-A (and the even more energetic UV-B) can considerably enhance the production of flavonoids and terpenes in a broad range of plant species.
UV also promotes increased resistance to stress and disease, stimulating the production of 15 different defence proteins.
Far-red photons regulate shade avoidance responses and can help to increase branching to create more flower sites.
Far-red can also help to speed up the transition from vegetative growth to flower and contribute to overall flower density.
ePAR, ePPF, and ePPFD Explained
ePAR stands for “Extended Photosynthetically Active Radiation”. It defines a broader range of photon wavelengths compared to the old standard of PAR.
Whereas PAR is defined as 400 nm to 700 nm, the range defined by ePAR is nearly 27% wider: 380 nm to 760 nm. Of course, changing the “goalposts” of PAR in this way has significant knock-on effects for PPF and PPFD as these measurements are derived from PAR.
That’s why two new metrics, ePPF and ePPFD have been created to reflect this fundamental change in the way that we count photosynthetically active photons.
ePPF stands for “Extended Photosynthetic Photon Flux”. It’s a measure of the total output of photons from a light fixture—but now it’s counting from a broader range.
Just as PPF refers to only PAR photons, ePPF is a count of ePAR photons.
Similarly, ePPFD stands for “Extended Photosynthetic Photon Flux Density”. If PPFD is a count of PAR photons arriving on a square meter during a single second, then ePPFD is a count of ePAR photons.
ePPF and ePPFD are two important updated metrics of incident light intensity for plants, based on the broader ePAR range.
Be Careful When Comparing Photosynthetic Photon Efficacy (PPE) Between Lighting Fixtures.
Photosynthetic photon efficacy (PPE) numbers are based on the old PAR standard—so, while the upcoming generation of LED fixtures with UV and far-red may appear to have lower PPE numbers, remember that PAR doesn’t count photons below 400 nm or above 700 nm, so PPE—by definition—does not see the whole picture.
Therefore there’s a strong argument to update our notion of PPE too (ePPE) to reflect the wider ePAR range.
Exciting New Discoveries in Photobiology.
In a recent study by Shuyang Zhen and Bruce Bugbee at Utah State University, far-red photons in the range of 700 – 750 nm were shown to contribute to photosynthesis when supplied in addition to regular PAR light.
This research effectively changed the goalposts for measuring PAR—so where does this leave us when choosing a grow light?