the dli you get from natural sunlight depends on …
TRANSCRIPT
LIGHT MATTERS
In the series “Light Matters”, Theo Tekstra discusses different aspects to lighting,
such as quantity, quality, efficacy, special applications, new developments, and
the science behind it. In this first episode we focus on quantity. How much
light do you give your plants? And how does that matter?
PART
1
THE DLI YOU GET FROM NATURAL SUNLIGHT DEPENDS ON YOUR GEOGRAPHICAL POSITION
BY THEO TEKSTRA – MARKETING MANAGER GAVITA HOLLAND BV
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LIGHT MATTERS I GARDEN CULTURE
reaching a surface of a square meter every second.
This is called Photosynthetic Photon Flux Density, or
PPFD.
Unfortunately, photons are so numerous that that
would easily lead to a 20 digit number, which is a bit
hard to read and value. There is, however, a standard
unit of measurements which defines a large number
of particles such as atoms, molecules, electrons, and
photons. It is the mole. By all means, if you want to
learn more about moles, take a look at Wikipedia,
but for now, it is enough to know that 1 mole of light
is 6.22 x 1023 (the Avogadro number) photons.
The notation for mole is mol, just like ‘s’ is for second,
and ‘m’ is for meter. As we already saw light intensity
is Photosynthetic Photon Flux Density, which is moles
of light per square meter per second. The scientific
notation of “per square meter per second” is “m-2
s-1” - so for space’s sake, and to make it look real
scientific, we are going to use mol m-2 s-1 from now.
Full Sunlight at midday is about 0.0025 mol m-2 s-1, or
2.5 millimol m-2 s-1, or 2,500 micromol (µmol) m-2 s-1.
I think you will agree with me that the µmol m-2 s-1 is
the easiest to use here. Which is fortunate, because
this is the way we measure the photosynthetic
photon flux density.
To recap:
• Photons are so numerous that we count them in
moles of photons.
• Photosynthetic Active Radiation (PAR) is defined
in the range between 400 nm light (blue) and 700
nm (red).
• Light intensity is defined as the number of PAR
photons per square meter per seconds, so mol
m-2 s-1. In practice, we use µmol m-2 s-1.
Plants are Photon CountersPlants use photon strikes for the synthesis of chemical
energy, such as sugars. I say strikes, and not light energy,
because it is the number of photons that is primarily
responsible for the process, and not the individual varying
energy of those photons. Blue photons for example,
contain a much higher amount of energy. That extra
energy, however, is mostly dissipated into heat. To bind
a CO2 molecule, you need about 8-12 photons. So, you
see it is a numbers game! We need to know how many
photons hit our plants to get an idea of the total potential
photosynthesis.
Plants are photon counters. Look at photons as rain
drops: the lighter the rain, the less water reaches the
surface. It’s the same for light: the fewer the photons, the
less light plants get for photosynthesis.
Counting LightTo quantify grow light, we first need to establish which
photons to count, and how to express that in numbers.
It has been established that photons with a wavelength
ranging from 400 nm (blue) to 700 nm (red) contribute
most to the photosynthetic process. That is why we call
photons in this range Photosynthetic Active Radiation, or
PAR for short.
In order to quantify a stream of particles, we need to
count how many reach the surface, at a given time, on
a standard size surface. The international standards for
time and surface are second and square meter. Taking
this back to raindrops again: the rate of the raindrops is
defined by the number of raindrops that fall on a square
meter of surface in one second. It gives you the density
of the rain.
The same applies to light: the intensity of the
(photosynthetic) light is defined by the PAR photons
THERE IS AN OPTIMAL AND MAXIMUM AMOUNT OF LIGHT PER DAY, AND ALSO A MAXIMUM INTENSITY
LIGHT MATTERS
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Q: So basically for a higher yield, I should just give more light?
A: Yes, but there is an optimal and maximum amount of light
per day, and also a maximum intensity you can give your
plant. A shade plant, for example, can only take a limited
intensity, and short day plants do have a maximum intensity
and DLI. It is also a function of what we call the limiting
factors for photosynthesis:
- Light
- Carbon Dioxide
- Temperature
Here is a graph representing the three limiting factors:
These three have to be in a balance. When there are one or
two too low, it will cause the plant to perform sub-optimally,
and there are absolute maximum and optimal levels as well.
So more light might require a higher temperature, and/
or more CO2. It is the grower’s mission to find the right
balance for his crop, and this is just one of the balances.
Other factors are the climate (as in humidity, for example),
available water, and nutrients.
Q: What is the optimal PPFD to give my crop in an indoor
environment?
A: For that you need to know the photosynthetic response
curve of your plant, and you need to make a choice -
whether you want to harvest as much crop per invested
energy (grams per Watt), or crop per square meter (grams
per square meter). It requires an experienced grower to
do the last, as you will be growing up to your plant’s limits.
Let me explain this with
a diagram, showing
photosynthesis (Pn)
against irradiation
(I) of a specific crop
(for other crops this
may be different). A
second variable in this
graph is temperature:
Amount of Light Per DayA light rainfall that continues for 20 hours can result in
much more water than a short heavy shower. There is a
relationship in the intensity of the rain, the length of the
shower, and the amount of water that reaches the ground.
The same goes for light: the total amount of photons
reaching your crop is based on the intensity of the light,
and the light period. The intensity, or PPFD, is defined as
mol m-2 s-1, so by multiplying this by the number of seconds
to get this intensity per day, you get the number of photons
per day, expressed in mol m-2 d-1 (moles per day). This is the
DLI - ‘daily light integral’.
Let’s work on an example.
- PPDF is 1000 µmol m-2 s-1
- Light period daily is 12 hours in a 24 hour cycle
To convert PPFD to DLI, multiply by the number of seconds
you are lighting your crop:
1000 (µmol m-2 s-1) x 12 (hours) x 3600 (seconds per hour)
= 43,200,000 µmol m-2 d-1, or 43.2 mol m-2 d-1.
And there you have it. The relationship between the light
intensity, and the amount of light per day.
Questions and AnswersArmed with this information, let’s try to answer the
following questions:
Q: If I give half the intensity of light, and double the time the
plants get it, does that have the same effect on photosynthesis?
A: Yes, it does. This is how we light tomatoes and roses in
greenhouses. They are long day plants (which flower and
fruit when there are long days of light), and they get up to
20 hours of light per day on dark days. However, if you are
flowering short day plants (which flower when the nights are
long), there is a limited period of about 12 hours in which
you can give that to your plants. So, in that case, you will
use a higher PPFD to get the same DLI in a shorter period.
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MORE LIGHT MIGHT REQUIRE A HIGHER
TEMPERATURE, AND/OR MORE CO2
LIGHT MATTERS I GARDEN CULTURE
At low intensity, you see a more linear increase of
photosynthesis when the light intensity increases.
However, with increased light levels, at some point the
photosynthesis tapers off, and at a certain level may even
cause photoinhibition. So doubling the amount of light does
not automatically mean that you will have double the amount
of yield. For every temperature, there is a saturation point:
a point where adding more light will no longer add to extra
photosynthesis. The saturation point is lower at a high
temperature, but the efficiency of the applied light is much
higher at an optimal temperature. Hence, you need to grow
at the right temperature to get optimum effect from your
light, 86°F in this example.
Remember the limiting factors of photosynthesis? The
moment you see the curve tapering off, you have reached
a limiting factor. In this case, temperature and PPFD were
variable, while CO2 is a constant. Adding CO2 will give you
a longer linear curve, so a much higher photosynthetic rate.
Q: Should I use the same PPFD during the vegetative stage of my
short day crop?
A: Using the same PPFD in the vegetative and flowering phase
will result in your crop getting 50% more light (higher DLI)
in the vegetative phase when you light it 18 hours in veg, and
12 hours in flowering. Reducing your PPFD in veg by 33% will
result in the same DLI. So, if you flower with 1000 µmol m-2
s-1 for 12 hours, giving your crop 667 µmol m-2 s-1 for 18 hours
will result in the same amount of light per day.
Q: How about supplemental lighting in greenhouses? How much
do I need?
A: That depends on the DLI of the sunlight throughout the
season you grow, and your crop. The DLI you get from
natural sunlight depends on your geographical position.
Purdue University published a good overview of DLI during
different seasons in the USA:
Source: http://bit.ly/purdue-DLI
However, that is not the DLI your crop will receive in the
greenhouse:
• During a clear sky summer day of full sun you will
probably shade your plants, because the PPFD is too
high, reducing the DLI of the sunlight.
• Your greenhouse construction takes away light.
Transmission losses can be as high as 25%, or more.
Secondly, you need to know the optimal DLI for your crop,
and whether you are going to give this in a long day, or a
short day. For a short day crop, the time that you can light
your crop is limited. The light level will need to be higher
than for a long day crop, which you can light for a long time
to compensate low sunlight DLI. 3
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