Light is the single most important environmental variable concerning plant growth.
SPECTRAL LIGHT QUALITY
Figure 2
Average plant response to photosynthetically active radiation (PAR).
LIGHT INTENSITY
Fiagure 3
Photosynthetic light response curves.
LIGHT DURATION
Light duration (photoperiod) is the length of time a plant is exposed to light during a 24-hour period. The length of a photoperiod can influence the overall light intensity that a plant receives in 24 hours, which in turn influences overall growth. This is described as daily light integral (DLI), which is defined as the cumulative PPFD delivered during 24 hours, and is expressed in mol/m2/d. Photoperiod also influences the transition from vegetative to reproductive growth in several plant species. However, it is actually the dark period (skotoperiod) and not the photoperiod that determines when certain species will transition to reproductive growth. The photoreceptor phytochrome is mainly responsible for signaling the transition to reproductive growth in photoperiodic crops . Short day (long night) plants flower when the phytochrome perceives an uninterrupted long night (generally ≥ 12 hours). Long day (short night) plants flower during short nights (generally ≤ 12 hours). Alternatively, several plant species are day neutral, where photoperiod does not influence flowering.
The end of the flower cycle may be toward the end of the growth cycle, but that does not mean your yield’s quality is set in stone. It’s no secret that light spectrum can affect everything from a plant’s smell to its appearance and tweaking light exposure at the tail-end of flowering can have dramatic effects.
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When the sun goes down, plants don’t just stop photosynthesis. They transition into a different biological process: metabolism. The gradual light change of the setting sun signals that it’s time to switch processes. Same deal with the rising sun.