August 2024 - 34

TECHNICALLY SPEAKING
INTENSITY OF LIGHT WAVEBANDS (µmol∙m-2
BLUE
RED
FAR RED
PAR
ePAR
22
154
2
176
178
22
132
22
154
176
∙s-1
)
22
114
45
136
181
INCREASE IN FRESH MASS
+18%
+26%
Figure 1. Lettuce 'Rouxai' grown at 73° F had 18% or 26% more shoot mass when red light was partially substituted by far-red light. This
partial substitution decreased the intensity of photosynthetically active radiation (PAR) while extended PAR (ePAR) was similar among
treatments. Data by Erik Runkle and Nathan Kelly.
For example, an experiment performed indoors with
lettuce showed that plant growth increased as red light
was partially substituted for far-red light (Figure 1). This
occurred even though PAR actually decreased while ePAR
was similar among treatments.
However, more than a moderate amount of far-red
light can have negative effects on plant quality. As the
proportion of far-red in a light spectrum increases,
leaves become larger but thinner and lighter in color. In
the production of ornamentals, plants can also become
undesirably tall.
Therefore, at least in some cases, the maximum
desirable percentage of far-red light in a light spectrum
could be 10% to 20%. More research is needed to fine
tune the delivery of far-red light for a variety of crops and
growing environments.
PAR is measured by quantum sensors that have been
meticulously engineered to specifically measure the
400 to 700 nm waveband. For plant applications, these
sensors are vastly superior to other light meters that
measure in footcandles or lux. Footcandle and lux meters
measure light based on the human eye.
An ePAR quantum sensor is commercially available and
measures the 400 to 750 nm waveband. Like a PAR sensor, an
ePAR sensor measures the number of photons (in micromoles)
per square meter and second (µmol∙m-2
extended range.
∙s-1
) but for the
34
AUGUST 2024 GPNMAG.COM
Many people have defined far-red light as the waveband
of radiation from 700 to 800 nm. However, light in the
750-800 nm waveband has little, if any, effect on plant
growth or development.
Because of this, some plant scientists are now defining
far-red light to cut off at 750 nm. This is consistent with
the ePAR waveband, which also ends at 750 nm.
Does this imply everyone needs to replace PAR sensors
with ePAR sensors? In most cases, probably not.
Far-red light and ePAR are most relevant for supplemental
lighting and especially lighting inside vertical farms. Far-red
photons emitted from lighting fixtures have biological activity
in plants and thus they should be included in metrics, such
as light output (photon flux) and efficacy of a fixture.
Most LED fixtures for plant applications emit little
(e.g., 2%) light in the 700-800 nm waveband. Some
emit a meaningfully higher percentage of far-red light
(7% to 11%), and most of that is within the 700-750 nm
waveband. Although far-red light is not considered in
the photon flux or fixture efficacy metrics, those photons
increase growth. If those far-red photons " counted, " the
output and efficacy of those lighting fixtures would be
higher.
Erik Runkle is professor and floriculture Extension specialist in
the Department of Horticulture at Michigan State University.
He can be contacted at runkleer@msu.edu.

http://www.GPNMAG.COM

August 2024

Table of Contents for the Digital Edition of August 2024