Daily light integral
Daily light integral (DLI) describes the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period.
The daily light integral (DLI) is a function of photosynthetic light intensity and duration (day) and is usually expressed as moles of light (mol) per square meter (m−2) per day (d−1), or: mol·m−2·d−1. In other words, DLI measures the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day.
DLI is usually calculated by measuring the Photosynthetic Photon Flux Density (PPFD) in μmol·m−2·s−1 (number of photons in the PAR range received in a square meter per second) as it changes throughout the day, and then using that to calculate total estimated number of photons in the PAR range received over a 24-hour period for a specific area. In other words, DLI describes the sum of the per second PPFD measurements during a 24-hour period.
If PPF stays the same for the entire 24-hour period, DLI in mol m−2 d−1 can be estimated from the instantaneous PPF from the following equation: μmol m−2 s−1 multiplied by 86,400 (number of seconds in a day) and divided by 106 (number of μmol in a mol). Thus, 1 μmol m−2 s−1 = 0.0864 mol m−2 d−1 if light intensity stays the same for the entire 24 hour period.
Outdoors, DLI values vary depending on latitude, time of year, and cloud cover, and ranges from 5-60 mol·m−2·d−1 in the United States. For plants growing in the shade of taller plants, such as on the forest floor, DLI may be less than 1 mol·m−2·d−1, even in summer. In greenhouses, 30-70% of the light will be absorbed or reflected by the glass and other greenhouse structures. DLI levels in greenhouses therefore rarely exceed 30 mol·m−2·d−1.. In growth chambers, values between 10 and 30 mol·m−2·d−1 are most common.
Effects on plants
Up to a certain light level DLI is strongly and positively related to the photosynthesis of a leaf, the relative growth rate of a whole plant and the productivity of a crop. Each type of plant has a different DLI optimum for growth. DLI also changes leaf morphology, allocation of biomass to leaves, stems and roots morphology, and the timing of flowering. DLI is directly correlated with plant quality, and a minimum amount of DLI is required for marketable horticultural plants. Measuring DLI over a growing season and comparing it to results can help determine which varieties of plants will thrive in a specific location.
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