Tomatoville® Gardening Forums - View Single Post - Semi-tech --light spectrum for tomatoes

artis · December 13, 2015

Many of the things that Hotwire posted here in Tomatoville regarding grow lights were incorrect. His biggest mix-up was that he incorrectly assumed that PAR (Photosynthetically Active Radiation) and photoreceptor response was the same thing.

PAR only controls photosynthesis (conversion of light into chemical energy, for example formation of sugars from carbon dioxide and water). Photoreceptors are completely different and they control plant's response to light, such as short/long stems, movement towards light and flowering. Basically, PAR is like high energy food that goes into plant's "stomach" and photoreceptors are plant's "eyes".

https://en.wikipedia.org/wiki/Photos...tive_radiation
https://en.wikipedia.org/wiki/Photoreceptor_protein

In any case, Hotwire's claim that only 6500K fluorescent lights are good for plants in terms of PAR is incorrect. It may be true that bright 6500K light helps to grow short and stocky seedlings but that would be mainly due to photoreceptor response, not PAR.

By the way, here is a more realistic graph to describe photosynthesis efficiency for a typical plant such as tomato (from Wikipedia). Please note that red light is the most efficient for photosynthesis:

December 13, 2015	#17
artis Tomatovillian™ Join Date: Jun 2006 Location: central NJ z6/7 Posts: 73	Many of the things that Hotwire posted here in Tomatoville regarding grow lights were incorrect. His biggest mix-up was that he incorrectly assumed that PAR (Photosynthetically Active Radiation) and photoreceptor response was the same thing. PAR only controls photosynthesis (conversion of light into chemical energy, for example formation of sugars from carbon dioxide and water). Photoreceptors are completely different and they control plant's response to light, such as short/long stems, movement towards light and flowering. Basically, PAR is like high energy food that goes into plant's "stomach" and photoreceptors are plant's "eyes". https://en.wikipedia.org/wiki/Photos...tive_radiation https://en.wikipedia.org/wiki/Photoreceptor_protein In any case, Hotwire's claim that only 6500K fluorescent lights are good for plants in terms of PAR is incorrect. It may be true that bright 6500K light helps to grow short and stocky seedlings but that would be mainly due to photoreceptor response, not PAR. By the way, here is a more realistic graph to describe photosynthesis efficiency for a typical plant such as tomato (from Wikipedia). Please note that red light is the most efficient for photosynthesis: