Over night temperature drop and light

I don't know if it applies to all plants, but using phalaenopsis as an example, it is a 10°-15°F drop in average growing temperature for a couple of weeks that results in spike initiation, not the day/night difference. Dr. Yin-Tung Wang, then at Texas A&M established that, and I put it to the test by tracking the average temperature over the course of a year.

I had 10°-15°F day/night variation in every month of the year, but by tracking the 14-day running average, I first saw the desired average reduction in early October, and sure enough, the plants started spiking some 6-8 weeks later.

As to the light question, it helps to think of light as a flow of energy into the plant, and use the flow rate (intensity) x time = "mass" of energy absorbed.

You're right that the sole use of artificial lighting makes it easier, and the area under the "on" part of the curve is simply the intensity times the duration. Natural light, however, follows a bell-curve profile, starting at zero, reaching a peak, and returning to zero. And, unless you're at the equator, both the peak height and duration constantly change. That "mass" of light is typically expressed in terms of moles of photons, and calculated on a per leaf area basis.

Professionals (i.e., folks who can afford the expensive meters) utilize "quantum meters" that can give the flux (µmol/square meter/second), and the summation of the photon flux over a complete time period.

 

Giles, interesting topic! Ray has given good info, but here is a couple additional points.

I don't know in details about the physiology of flower induction from temperature (I'm sure that different species use different cues). But the other aspect of diurnal temp range is the effect of temperature on the photosynthesis and respiration. These two aspects are influenced by temperature heavily. Generally both photosynthesis and respiration increases with temperature (obviously, if it is too hot, photosynthetic rate decreases with higher temp). But the relationship between the temperature and these physiological process is not linear, and the two processes respond differently to the temperature increase. During the day, the plants want to maximize the difference between the photosynthetic rate and respiration rate (in other words, net gain in carbon), and this gives the optimal temperature (different species have different optima). But at night, when photosynthesis (at least the light requiring part) is not working, plants want to minimize the loss due to the respiration. So lower temperature is better. But it can't be too low because there are some other enzymatic reactions are happening at night (for example, carbon assimilation in CAM plants), and low temp will disturb this process. So the night optimum temperature is more or less determined by the compromise between enzyme reaction and respiration. This is what I learned from the time when I tried to understand why diurnal range in temperature is beneficial for plants.

From this aspect, (some kind of weighted) average day temperature and average night temperature would be more relevant than the absolute min/max. But most of us just measures min/max because we don't have temperature loggers.

With the light, as Ray said, photosynthetic photon flux density (PPFD) is the standard. You can measure it with quantum PAR meters, but they are more expensive than cheap lux/footcandel light meters. But if you are dealing with sun light, you can make a conversion between lux and PPFD: Conversion - PPF to Foot-candles

Some researchers use data loggers to measure the PPFD at different time of the day, and calculate something called Day Light Integrals (DLI). HOBO data logger is a relatively affordable data logger (not cheap, though) which can use Apogee PAR sensor.

A cheaper method could be to make a data logger with Raspberry Pie, and connect a light sensor. Then you can get overall daily pattern.

Or if you can manually measure the light every hour or so to get the pattern.

Anyway, PPFD is relevant for the photosynthetic part, but for the flower induction, it is not so relevant. One of the light sensor in plants (phytochrome) is sensitive only to red and far red light. Also, I think that another light sensor called cryptochrome (sensitive to only blue light) is shown to be involved in flowering in some species. So for the flower induction, quantifying these specific wavelength is more relevant than PPFD.