SPACE BUCKETS

Adding UV LEDs (with about 100 to 400nm wavelength) are typically only used for light sensitive protein reactions effects, not as photosynthesis drivers per se. The pure UV-A grows I've done did result in slow grow and stunted plants.

  
The Electromagnetic Spectrum (source)
 

If I wanted to keep a tiny, important plant alive for a long duration I would be using pure UV-A. But, the effects of UV-A on a plant can be unpredictable and needs to be tested by cultivar. The main UV light sensitive protein known about currently is the UVR8 protein which is a 280-315nm UV-B receptor, not a UV-A receptor.

The theoretical maximum PPE of a 375nm UV-A LED is 3.13 umol/joule, and the relative low photosynthesis rate is going to make them a no-go in LED lighting except for photomorphogenesis effects. Making red lettuce cultivars more red by increasing anthocyanin production, or trying to increase trichome and cannabinoid production in cannabis plants, may be reasons to use UV light.

Anecdotally, certain selective photomorphogenesis experiments I've done with UVA compared to blue leads me to believe that there may be at least one unknown UVA light sensitive protein either as a primary receptor, or my SWAG (scientific wild-ass guess) is a UVA light sensitive protein that can express itself differently in different plant parts, affecting the protein phototropin/cryptochrome signal transduction pathways locally.

For example the hypocotyl (the stem before the first set of true leaves) can react much differently than the epicotyl (the stem after the first set of true leaves) in some plants like pole beans in my 470nm vs 405nm experiments.

Safety Considerations

• UV-A light (315–400nm) is fairly safe (it can be dangerous when you stick your eye close to a light source that appears dim yet has a high radiant flux) and at the time of this writing, only UV-A LEDs are used in LED grow lights if UV light is used.
• The UV-B (280–315nm) light sources I've seen in grow lights are still tube based because UV-B LEDs are still inefficient (5-10% range).
• UV-C (100–280nm) should be considered dangerous, and in testing I have damaged a number of plants with higher amounts of UV-C.