Over the past five years, with the support of Floriculture
Industry Research and Scholarship Trust (FIRST), we conducted a series of
experiments to develop guidelines for using the daily light integral (DLI) to
gauge the light environment in greenhouses. DLI is a term that refers to the
total quantity of light delivered to a surface. An analogy will help to clarify
the technique and usefulness of measuring DLI. If you wanted to know how much
rain fell yesterday, you would place a bucket outdoors to capture all the
raindrops delivered to that particular spot. Similarly, if you wanted to know
how much light was available for plant growth yesterday, you would place a
light sensor on the greenhouse bench and capture all the light particles
(photons) delivered to that particular spot. This value would be the light
quantity, or DLI.
Current methods
We have survived to this point with basically two methods of
measuring light. Most commonly, growers use foot-candle meters to estimate the
maximum light delivered to a crop inside a greenhouse or to estimate the
greenhouse light transmission by comparing outside and inside measurements.
While there is nothing wrong with this, its usefulness — its ability to
describe the greenhouse light environment — is very limited. A single
light intensity measurement cannot take the day length (or the number of hours
of sunlight) into account. For example, if a crop is said to receive 2,000
foot-candles in December and 2,000 foot-candles in June, the total amount of
light (the DLI) delivered to the June crop is considerably higher than the
December crop, since the June crop receives 15-16 hours of sunlight, while the
December crop receives only 8-9 hours.
More recently, growers have used outdoor weather stations
connected to climate control computers. These stations can report the DLI
delivered outside the greenhouse; however, they cannot estimate the light
delivered to the greenhouse crop, especially if a retractable shade curtain is
used.
Our dilemma is that we never know how much light is actually
being delivered to the greenhouse crop. DLI is not a new concept for plant
scientists; however, few growers have ever used this measurement because the
equipment for measuring DLI has been cost prohibitive. That has changed, thus
we are writing this article now, because DLI measurements can be a reality for
commercial greenhouse growers. But first, let's discuss the value and use for
DLI measurements.
Using DLI
Plants are "light counters." Photosynthesis is
driven by the amount of light intercepted by the plant canopy and the amount of
carbon dioxide that can be absorbed through the stomata. Since DLI is the sum
of the light delivered during the course of one day, there is an excellent
relationship between plant growth and DLI. So, if you measure the DLI, you have
an excellent estimate of the plant growth to be expected. Á
The FIRST Web site (www.firstinfloriculture.org/research_
reports.htm and click on Light Management in Greenhouses) contains a detailed
report on the use of DLI, so only a brief overview will be presented here. The
first challenge in using a new unit of measure is getting used to the numbers
and units. The unit of measure for DLI is "moles per day" or (more
technically correct) "mol·m-2·d-1," which means moles
per square meter per day.
Measuring DLI
The value of DLI measurements has been evident for many years;
however, commercial growers never had the ability to measure DLI due to the
high equipment cost. In 2003, Spectrum Technologies, Plainfield, Ill.,
developed a portable datalogger (Greenhouse Weather Tracker) that measures DLI.
The meter can be easily positioned within any greenhouse crop. A digital
display provides the current DLI for that particular day as well as a 30-day
archive for previous days. The display also shows the current light intensity
(micromoles per second or µmol·m-2·s-1) and current air
temperature. The minimum and maximum daily temperatures are also archived. DIF
measurements will be displayed in future versions. Á
We have been making DLI measurements for several years in
research and commercial greenhouses. The measurements never cease to amaze me.
We usually rely so heavily on our eyes to evaluate the greenhouse light
environment; however, the human eye is a lousy light sensor. This is because
our eyes are so effective at adjusting to the ambient light conditions. It is
remarkable that we can see well under a full moon (0.01
µmol·m-2·s-1) or during a sunny day (2,000
µmol·m-2·s-1). The following are some of the conditions
that provide surprising DLI measurements (although they seem like common sense
once you've made the measurement):
* Shadows
cast during a sunny day are much darker than the same shadow cast on an
overcast day. Similarly, if you have a sensor positioned in the shadow of a
greenhouse wall on a sunny day, the light intensity may actually increase as a
cloud moves overhead, since the cloud reflects the light into the shaded area.
* In
east/west-oriented greenhouses, the benches immediately north of the gutters
can have very low DLI on sunny days (less than 5 moles per day in South
Carolina in October).
* The
DLI measured at the truss (where a hanging basket would be positioned) is much
higher (20-50 percent) than the DLI measured on the floor or bench, even
without any hanging baskets.
* The
outdoor DLI increases 100 percent (or doubles in magnitude) from December to
June.
* The
DLI delivered to the bench inside the greenhouse is often only 50 percent of
the outdoor DLI, even without hanging baskets or shade curtains.
* The
DLI delivered to the greenhouse bench may be lower in May than in March if a
lot of shade is applied to the greenhouse.
* Excessive
hanging baskets or excessively large hanging baskets can reduce the DLI
delivered to the greenhouse bench by more than 40 percent.
* White
hanging baskets intercept 10-30 percent less light than green hanging baskets.
* Even
though the glazing material may allow 90-percent light transmission, the
complete greenhouse structure may allow only Á 35-70 percent, depending
on the angle of the sunlight, the amount of infrastructure, overhead curtains,
dust, condensation, etc.
* Even
though the shade curtain may allow 50 percent transmission, the entire shade
structure may allow only 40 percent transmission when the retractable curtains
are pulled.
* The
DLI delivered inside a retractable roof greenhouse is less than the outside DLI
due to shadows cast by the structure, even when the roof is open. This is
particularly true in hinged roof systems, compared to accordion-style
retractable roofs.
As you can see, there is a lot to be learned about the light
environment inside a greenhouse. Everything seems straight forward until you
make some measurements, then you realize there's more variation and differences
than you've ever imagined.
The next step
EC and pH measurements became commonplace in the 1980s. This
tool has dramatically improved a grower's ability to reliably produce crop
after crop. Graphical tracking became a commonplace in the 1990s. This tool
took much of the guesswork out of height control. DLI meters have the potential
to be the next major tool for growers to add to their repertoire. DLI
measurements made inside the greenhouse will provide growers with further
insight into greenhouse crop management. It will take time, but I believe that
over the next decade growers will become familiar with the DLI concept. This
will improve our ability to accurately communicate to each other about light
and will improve our ability to reliably grow quality crops.
Author's Note: The DLI work discussed in this article is a
result of a project funded by FIRST. A more detailed discussion of DLI can be
found in the Research Report provided on the FIRST website
(www.firstinfloriculture.org).
