Daily light integral

[2][3] 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.

[4] If the photosynthetic light intensity stays the same for the entire 24-hour period, DLI in mol m−2 d−1 can be estimated from the instantaneous PPFD 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.

In the past, biologists have used lux or energy meters to quantify light intensity.

They switched to using PPFD when it was realized that the flux of photons in the 400-700 nm range is the important factor in driving the photosynthetic process.

This is a convenient time scale when measuring short-term changes in photosynthesis in gas exchange systems, but falls short when the light climate for plant growth has to be characterized.

Daily light integral includes both the diurnal variation and day length, and can also be reported as a mean value per month or over an entire experiment.

It has been shown to be better related to plant growth and morphology than PPFD at any moment or day length alone.

Outdoors, DLI values vary depending on latitude, time of year, and cloud cover.

Per unit leaf area there is more RuBisCO and a higher photosynthetic rate under light-saturated conditions.

Expressed per unit leaf dry mass, however, photosynthetic capacity decreases.

Plants growing at high light invest less of their biomass in leaves and stems, and more in roots.

Measuring DLI over a growing season and comparing it to results can help determine which varieties of plants will thrive in a specific location.