Page 21 - Griffin Gazette Issue 2 - 2021
P. 21
A single photon is a single packet of light, which has both By plugging PPFD and hours of light into this equation,
wavelength (color) and energy. Moles (mol) and micromoles DLI can be consistently and quickly calculated. Some
(µmol) are counts of particles and are not unique to controllers and datalogging equipment will even perform this
photons. One mole is approximately 6.022 * 10^23 or calculation automatically, when provided with a light sensor.
602,200,000,000,000,000,000,000 pieces of whatever Different plant species have different DLI requirements for
you are counting. In the case of light, that would be 6.022 * optimal growth, usually ranging between 12 – 25 mols per
10^23 photons. One micromole is one millionth of one mole, day. This information is readily available for a number of
which is still a huge number (602,200,000,000,000,000). crops in published culture guides.
Most plant species require at least 10-40 µmol/m2/s for Now that we know how light is measured and why it
photosynthetic activity to equal cellular respiration; this is matters, we can discuss brightness. Many household and
known as the compensation point. Above this point, plants commercial fixtures provide information on electrical usage
can grow and develop normally; maintained at this point, in terms of wattage. Watts can be used to calculate efficacy
plants will neither grow nor wither and die. Measuring in terms of light produced per watt consumed. Watts by
light levels should be done on an instantaneous basis themselves do not tell us much at all about a light, other
(photosynthetic photon flux, PPFD) and cumulative basis than how much electricity it will consume while it runs.
(daily light integral, DLI). PPFD is measured in µmol/m2/s, This does not relate directly to how bright a light appears.
and the daily light integral sums up these numbers over the Many lighting producers also provide electrical efficacy
course of the day to report the number of mols/m2/day.
information, in terms of photons / Joule; one Watt is one
Measuring light levels should be done on an instantaneous Joule per second. If the efficacy of a light is known in lumens
basis (photosynthetic photon flux, PPFD) and cumulative or photons per watt produced, illuminance (brightness) can
basis (daily light integral, DLI). PPFD is measured in µmol/ be predicted. For traditional incandescent or some LED
m2/s, and the daily light integral sums up these numbers light fixtures, efficacy is known, so manufacturers are able
over the course of the day to report the number of mols/ to make the statement that, for example, a 40W or 100W
m2/day. For indoor environments, while using artificial light will produce a certain amount of light. When we change
lights, calculating DLI is fairly simple. Knowing the number the type of light technology used, we must also change our
of hours an artificial light is on, and the PPFD of a light at a understanding of light produced per watt consumed.
certain height, the following equation can be used:
(PPFD*60 sec*60 min*#Hours) / 10^6=DLI
PETUNIAS DAHLIA BEGONIA MUMS
PPFD: 200-250 µmol/m /s PPFD: 250-350 µmol/m /s (TUBEROUS) PPFD: 250-350 µmol/m /s
2
2
2
range range PPFD: 100-200 µmol/m /s range
2
DLI: 12-17 mols/day ideal DLI: 12-14 mols/day ideal range DLI: 10-14 mols/day ideal
DLI: 6-8 mols/day ideal
Photoperiod: 13-16 hours Photoperiod: Variable Photoperiod: Variable
Photoperiod: Variable
Special Considerations? Special Considerations? Special Considerations?
Petunias flower earlier Under short days (<12 Special Considerations? Short-day crop. Vegetative
when grown under long hours), tuber production Short-days promote growth occurs under
days. Some varieties will and flowering are favored, flowering (<12 hours), while long photoperiods (16-
flower under both short while long days favor long days promote tuber 18 hours) with flower
and long days, but not foliage and shoot growth, production. initiation occurring as
all. Be familiar with the but Dahlias will flower days begin to shorten.
requirements of your under both short and long Nighttime interruption is
particular cultivar. day conditions. often employed during
mum propagation to keep
cuttings vegetative.