Maximum Yield Cannabis USA August / September 2017 | Page 24

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Photosynthetic photon flux is the most commonly used
method of measuring PAR. It gives equal value to all photons
generated by a light source between the wavelengths of 400
and 700 nm and is measured as micromoles per second. This
happens to be the same range at which humans use light to
see. PPF Efficacy, in either micromoles per second per watt or
micromoles per joule, is the energy efficiency term. It is akin to
LPW, but in plant talk.
While PPF or PPF Efficacy are good overall metrics for
what plants use to see light, plants do more than just
see. They feel light. That spectrum falls across a broader
range. It’s called photomorphogenesis. 
Figure 2
Absorption spectra for 2
forms of phytochrome.
Adapted from Biochemistry
and Molecular Biology of
Plants by the American
Society of Plant Biologists.
Photomorphogenesis “The
control exerted by light
over growth, development,
and differentiation of
plants that is independent
of photosynthesis.”
Different receptors,
such as phytochromes,
cryptochromes, and
phototropins in plants react differently to those parts of the spectrum
that are missing from PPF. For example, UVA and UVB (<400nm),
as well as far reds (>700nm) have a significant effect on plant growth.
So they shouldn’t be ignored.
How is Lighting for Plants Measured
and Compared?
As previously stated, PPF is based upon the wavelengths
emitted between 400 and 700 nm. That region is where
humans see, though human sight is weighted heavily around
the 560 nm range. While most humans do not see below 400
nm or above 700 nm levels, they do “feel” light radiation at
those levels; one in the form of tanning by means of UVA
(350-400 nm) and some UVB, the other in the form of infrared or
heat-warmth beginning at 700 nm and extending higher.
Humans need to feel good. Plants do too. The shortfall of using
PPF as a metric for plant growth is that it normalizes the spec-
trum of light, treating each nanometer the same.
But if humans need a weighting curve to help characterize their
ability to see, shouldn’t plants also have a weighting curve to
help them see and feel the light to grow? Yes, they should.
In numerous casual but controlled experiments of different
plants, a higher PPF did not always produce the better results.
That’s because the spectrum to produce higher PPF does not
necessarily contain the best components of light that allow all
plants to flourish, especially cannabis. In other words, not all
micromoles are created equal.
In other highly controlled third-party evaluations of different
spectra on cannabis plant growth, there were some interesting
outcomes. Over a six-week veg period using different spectra
but the same PPF, cannabis sativa clones exhibited measurably
different results. At the end of that period, experienced growers
reviewed what they deemed important for best plant yields:
plant height, plant width, the number of leaves, leaf size, the
number of nodes, and the average spacing between nodes. They
also observed each plant qualitatively.
In the final outcome, there were definitely some “best spec-
trum” winners among the test specimens, even though the PPF
was the same. Spectrum mattered.
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