How Plants Respond to Light


Mar 12, 2020 | 0 comments

Plant Growth Stages

The biological stages for flowering crops are (1) seed germination or clone/cutting establishment (EST), (2) vegetative growth (VEG), and (3) reproductive/flower growth (FLOW). Recommended PPFD light intensities increase as the plant enters each stage. Light intensive crops such as cannabis, tomato, cucumber, and pepper can require up to 1000 µmol PAR/m2·s during flowering stage.

Graph showing recommended light intensity by growth stage

Spectrum Comparison

Plants respond to light in two different ways.  PHOTOSYNTHESIS uses light energy to grow and store chemical energy and PHOTOMORPHOGENESIS uses light to trigger developmental responses.  Photomorphogenesis combined with other changing environmental parameters are what allow plants to be highly adaptive to their environment.  Photosynthesis is not as sensitive to light wavelength, as long as the emitted light is within the PAR range of photons (400-700 nm). HPS photons work for photosynthesis just like LED photons. However, they are not as closely matched to the average plant photosynthetic response curve and they require more energy to create.

Recommended DLI

Each crop has a total integrated amount of photons it should receive each day for optimal growth.  This is known as the Daily Light Integral (DLI) and is provided in units of mol PAR/m2·.  Recommended DLI will differ depending on crop and growth stage. Light intensive crops can require levels as high as 43 mol PAR/m2·d.

Light Calculations

The photoperiod is the number of hours the plants are exposed to light each day.  For crops that are triggered into the reproductive stage due to decreasing photoperiod, the grower must decrease the amount of time the lights are on to transition from vegetative (VEG) growth to flower growth (FLOW).

If the recommended DLI and photoperiod are known for the crop, the appropriate light intensity can be calculated. Similarly, if the DLI and PPFD are known, the photoperiod can be calculated.  See the equations below for how these three parameters (PPFD, DLI, and photoperiod) are interconverted.



Useful References


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