June 2021 - 19

LIGHTING
the red and far-red part of the spectrum, but not
bright enough for us to see. The higher the PPFD,
the more light energy will be converted into heat
and less energy will be used for photosynthesis.
Compare that to a sink that is unable to drain large
amounts of water; at some point the plant cannot
photosynthesize more, no matter how much extra
light it receives. Inevitably, a plant's light use
efficiency decreases with increasing PPFD.
We can measure how much of the provided
light is used to create the electrical currents that
power photosynthesis by measuring changes
in chlorophyll fluorescence. In other words, by
measuring fluorescence we can quantify the plant's
light use efficiency when exposed to different PPFDs.
Understanding how efficiently plants use the light
they receive helps us to develop more cost-effective
lighting strategies.
Because plants use light less efficiently as the
PPFD increases (Figure 1), providing supplemental
lighting when sunlight levels are high is an
inefficient use of energy and money. On the other
hand, supplemental light is most beneficial when
sunlight levels are low, because the plants' light
use efficiency is higher: supplemental light will be
utilized more efficiently to power photosynthesis
and increase plant growth.
But the decrease in light use efficiency with
increasing PPFD also suggests another way to
increase the cost effectiveness of supplemental
1.0
0.8
0.6
0.4
0.2
0.0
200 400 600 800 1000 1200 1400 1600
PPFD (μmol·m-2
·s-1
)
Figure 1. The light use efficiency of lettuce decreases with increasing PPFD. This is true for all crops,
but the rate of this decrease differs among crops. Data collected by Geoff Weaver.
lighting. We hypothesized that by providing
lower PPFDs, over longer photoperiods to
achieve the same DLI, plants would be more
light-use efficient and grow faster. On the
other hand, when plants are provided a high
PPFD over shorter photoperiods, their light
use efficiency will decrease. We tested those
hypotheses in greenhouse and controlled
environment conditions on ornamentals and
leafy greens.
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Light use efficiency

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June 2021

Table of Contents for the Digital Edition of June 2021