Daily amount of light

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The daily amount of light (engl. Daily light integral , DLI) describes the number of photosynthetically active photon (photons in the photosynthetically active radiation area) incident over a period of 24 hours in a field. The daily amount of light is particularly useful for specifying lighting conditions for plants. This parameter is directly related to the growth, development and harvest quality of the plants.

The growth of a plant is determined by the number of photons it receives every day. Less intense light over a long period of time leads to the same number of photons as twice the light intensity, but only over half as long.

The daily amount of light DLI is measured in moles per square meter (m −2 ) per day (d −1 ) or: mol · m −2 · d −1 . The daily amount of light is the sum of the PPFD values ​​( Photosynthetically Active Photon Flux Density ) for a day.

Associated terms

photosynthetically active (engl. radiation photosynthetically active radiation , PAR)
PAR indicates the wavelengths of light in which photosynthetic radiation is active. Photons with a wavelength between 400 and 700 nanometers (nm) can be used by plants for photosynthesis. A large part of this radiation is also in the range that is visible to humans.
photosynthetic photon flux (PPF)
PPF is expressed in micromoles (μmol) per second (s) or μmol / s. Measures how many photosynthetic photons are emitted from a light source in one second. PPF enables a quick and reliable comparison of different light sources.
photosynthetic photon flux density (PPFD)
Measures how many photosynthetic photons hit one square meter in one second. Measured in micromoles (μmol) per second (s) per square meter (m²) or μmol / s / m². Intense light has a high PPFD value, less intense light has a lower one.
Photosynthetic photon
A photon is a single light particle and can have different wavelengths. A photon that has a wavelength between 400 and 700 nanometers can contribute to photosynthesis and is called a photosynthetic photon.

Usual daily amount of light

The daily amount of light outside varies depending on the geographical latitude, the season and the cloudiness. In some places, values ​​over 70 mol · m −2 · d −1 can be reached on very bright summer days . For plants that grow in the shade of higher plants, such as plants on the forest floor, the daily amount of light can be less than 1 mol · m −2 · d −1 even in summer . In greenhouses, 30–70 percent of the light is absorbed or reflected from outside. This is why the daily amount of light in greenhouses is seldom higher than 30 mol · m −2 · d −1 . Values ​​between 10 and 30 mol · m −2 · d −1 are normal in climatic chambers .

Effects on plants

The DLI affects many properties of plants. Even if not all plants react in the same way to certain lighting conditions, some similarities can be determined.

Too high a DLI value will damage the plant and too low a value will reduce its growth potential.

Exposing plants to light for too long can affect their growth cycle and ability to flower.

The light intensity also plays a role. If the light intensity is too high, the soil from which the plant draws water and moisture can dry out and therefore promote dehydration of the plant. When a plant is exposed to too much light, some of the light's energy is converted into heat. The plant uses water to cool itself and to reduce this heat again. Over a long period of time, the plant's water reserves are used up and it can no longer use water for vital processes such as photosynthesis. If the plant can no longer dissipate this heat because it has no more water to cool it or the heat is too great for the plant to dissipate, it can be damaged.

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