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Photosynthetic photon efficacy: does it really equal growth efficacy?

Grow light efficacy has been a hot topic over the last few years. Although efficacy metrics are essential when choosing a grow light, we want to emphasize the importance of understanding what may lay behind the photosynthetic photon efficacy (PPE) number. Grow light efficacy may not always mean optimum growth efficacy when it comes to your plants, writes the team with Heliospectra in their newest blog. 

Plants grow healthily and most efficiently under a balanced light spectrum. When it comes to LED lighting, this type of spectrum is often referred to as full-spectrum LED, which is in contrast to the more conventional narrow-spectrum LED alternative. It is especially crucial to provide well-balanced lighting conditions to plants grown under sole-source LED lighting.

LED grow lights with a spectrum optimized for plant growth may not excel in PPE due to the PPE variations among LED elements in different colors. PPE does not include the photons outside of photosynthetically active radiation (PAR). PAR consists of the blue, green, and red spectral range (400-700 nm). Generally, a grow fixture with a high content of red light has high PPE due to the high efficiency of red LEDs. It is, therefore, not uncommon for fixtures with the highest PPE to have a spectrum with high red content (90-95%) and some blue content (5-10%).

However, far-red (FR) light (700-800 nm), which lies outside of PAR and is thus not included in PPE calculations, has a positive effect on plant development, architecture, and flowering, as shown in numerous research papers. Therefore, we wanted to compare LED grow lights' overall growth efficacy (g/kWh) with a spectrum optimized for plant growth versus LED grow lights with a spectrum targeted for higher PPE.

Comparing growth Efficacy
Two adjustable-spectrum LED grow lights were tuned to have an equal power consumption of 275 Watts but with different spectral outputs. One light was tuned to have a spectral output only within the PAR range, referred to as “PAR”. The second light had the same spectral ratios within PAR as that in “PAR”, but with some of the output replaced by FR, referred to as “PAR+FR”. The photosynthetic photon flux density (PPFD) output was 6.8 % higher in “PAR” than in “PAR+FR”. The PPE efficacy was 6.1 % higher in “PAR”.

We studied basil in the two different light environments, beginning seven days after seeding for a span of 25 days. After the 25 days, pots of basil containing ten plants each were measured. We collected data for the average plant height and fresh biomass weight. This experiment was repeated three times, and averages were calculated.

Results
The height difference was clearly visible. The basil grown under “PAR+FR” was taller than the basil grown under “PAR”. The results showed that plants grown under the “PAR+FR” spectrum were larger in size and had higher biomass than the ones grown under the “PAR” spectrum, despite the lower PPF output from the “PAR+FR” spectrum. The growth efficacy (gram/kWh) was 10% higher in the “PAR+FR” treatment compared with the higher PPE “PAR” light treatment.

Even though the “PAR” spectrum showed a 6.1% higher PPE efficacy and 6.8 % higher PPFD, the basil grown under “PAR+FR” had a 10 % increase in biomass, 28% increase in height, and 10% increase in growth efficacy.

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