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.
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