How does one determine which part of the light spectrum plants are lacking when grown under lights? What are the deficiency symptoms to look for?
You really do not have any visible signs that tell you which spectrum is missing. Chlorophyll cells are activated by light in the blue and red spectrum whereas green and yellow light is reflected (very general description). This means that plants grown under blue/red lights tend to grow compact, have green foliage, and lush growth (provided other factors such as temperatures, moisture, and fertilizer are present). Tall skinny plants with poor color is an indication of inadequate light. It does not tell you which specific color, but you can atleast dermine that if the room is bright with plenty of (visible) light, it is probably light of the different spectrum (more green/yellow as opposed to blue/red).