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Low PPFD irradiance may limit photosynthesis at the leaf surface. High PPFD irradiance in excess of photosynthetic needs at the leaf surface may be a threat to plant metabolism. A 2010 study found that the most productive PPFD for cannabis is 1500–2000 μmol∙m-2∙s-1 with environmental conditions at 25–30°C with CO2 concentration elevated to 750 ppm.

Under these indoor conditions photosynthesis in cannabis was optimized. Lighting with intensities of 1500–2000 μmol∙m-2∙s-1 can draw a lot of electricity and be very expensive to run. Most growers will use light that is less intense and produces a good yield. Seedlings, clones, and mother plants can be successful with just 200 to 400 μmol∙m-2∙s-1 PPFD. For plants in the vegetative growth phase of cannabis 400 to 600 μmol∙m-2∙s-1 PPFD can be sufficient. Flowering plants will benefit from more intense light with 600 to 900 μmol∙m-2∙s-1 PPFD. Daily light integrals (DLI) are another important measure of photosynthetic light that describes the number of photosynthetically active photons that reach a particular area in a 24 hour period. DLI is expressed as mol∙m-2∙s-1, which is moles per square meter per second.

Where PPFD expresses light intensity in a second, DLI expresses the sum of those measurements in a 24 hour period (86,400 seconds per 24 hours). In the northern latitudes of North America days will be shorter and nights will be longer, so the DLI will be lower than in southern latitudes where days are longer. This has big implications for greenhouse growers working with short day plants like cannabis. Cannabis begins to flower when light exposure periods are less than 12 hours. Day lengths can naturally drop under 12 hours earlier in the year in the north than in the south. Many greenhouse growers in the north will have to use daylight extension lighting to make sure their cannabis plants have sufficient vegetative growth before they move on to flowering. Plants use pigments to harvest light to fuel photosynthesis. Chlorophyll is the most common and abundant plant pigment. Chlorophyll mostly captures blue and red light wavelengths. Plants also have accessory pigments like carotenes and xanthophylls to help harvest additional light from the full spectrum of wavelengths. If light wavelengths are too short (like UV-C) they can damage pigment molecules in a plant. If a light wavelength is too long it won’t carry enough energy particles to provide the energy needed for the photosynthetic process to activate. Red light falls between 600 nm and 700 nm on the electromagnetic spectrum and is absorbed by the chlorophyll b pigment. The brightest red light occurs around 660 nm on the spectrum and will be the most efficient red wavelength for the plant to absorb. Red light overall is as good as stimulating photosynthesis as blue light. Growing plants under only red light produces undesirable elongation, so some amount of blue light is needed. The ratio of red light to far-red light also influences leaf growth and stem elongation. When the amount of far-red light increases relative to the amount of red light, stem growth is elongated. When using LED lights with little to no far-red light, plants can have very compact overall growth. Red light plays a major role in plant flowering by activating phytochrome pigments. When low intensities of red light (1 μmol∙m-2∙s-1) interrupt darkness, they can inhibit flowering in short day plants or stimulate flowering in long day plants. As cannabis is a short day plant, lighting for the vegetative growth phase may emit red and far-red light.

Blue light falls between 400 nm and 500 nm on the electromagnetic spectrum and is absorbed by the chlorophyll a pigment. In photosynthesis, blue light is less productive than green and red light. A minimal amount of blue light is needed since it prevents irregular stem elongation and leaf shrinking. Plants grown with blue light tend to be shorter, thicker, and have darker green leaves. Plants that are deprived of blue and UV light can have purple leaves because short wavelength radiation stimulates production of leaf coloration compounds. Blue light is also important for regulating opening of the stomatal pores on the underside of the leaves. Some water is lost when they open so it is important for them to be well regulated. Typical photoperiodic lighting intensity for blue light is 1–2 µmol∙m-2∙s-1. At this intensity blue light will not influence flowering in long day or short day plants. At intensities greater than 20 µmol∙m-2∙s-1 blue light can inhibit flowering in short day plants like cannabis, and promote flowering in long day plants.

There are several different types of grow lights for cannabis growers to select from. LED lights have become very popular and are becoming more affordable alternatives to high intensity discharge lamps. In order to select proper lighting, growers need to consider PAR, PPFD, DLI, and other factors that describe a light’s electrical use, lifespan, and color temperature. Watts are a unit to measure the electric currency a bulb draws, a watt is equivalent to one joule per second.

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