Light doesn’t only drive photosynthesis. Changes in the quantities of certain colors of light serve as signals to the plant to grow or stop growing; to flower or not to ripen fruit. The molecules that sense or harvest the light are called photopigments because they have color. We are most familiar with chlorophyll, beta-carotene and anthrocyanins (that color plants red and purple). In addition to these there are Phytochromes, Cryptochromes and Phototropins that are the “eyes” of the plant by which it can “see” what it has to do. First, let’s look at the different colors I will refer to by name and where they sit in the spectrum of visible light: http://physics.tutorvista.com/waves/wavelength-of-light.html What we refer to as blue light in the hobby is actually three colors: blue and violet. If I refer to blue light below I am referring to light of approximately 455–492 nm. Another important concept is that different colors of light are associated with different quantities of energy. Violet photons (particles of light) carry more energy than red photons. What this means is that a red lamp of 1 W will produce more photons than a blue lamps of 1 W. Plants aren’t sensitive to the energy of the photons when it comes to photosynthesis. For them what matters if the number of photons that they can absorb; but for us who want to provide our plants with the most useful photons the color and energy of the photons of that color is important. But what wavelengths are needed for photosynthesis? The classic Elodea action spectra (hence forth: ELAS) is given in Figure 1 and shows that, Watt per Watt of energy, which light colors stimulate photosynthesis the most efficiently . These are the violet (400-450 nm) and the orange-red (620-680 nm) wavelengths. As a consequence the ELAS in Figure 1 has to be modified to account for the number of photons per unit energy. This is shown in Figure 1 with the line labeled Photon Yield Flux (hence forth ELPYF) which is the expression of the ELAS in terms of photosynthetic efficiency per photon. Elodea is almost half as sensitive to red light compared to blue light. Let us now consider the molecules that are absorbing the light. The chlorophyll a and b molecules are the principal molecules that harvest photons of light and, in a protein complex called a photosystem, turn the energy of those photons into energy to fix carbon dioxide as sugars, ATP and NADPH (the two energy carriers involved). A third photopigment, beta-carotene, can also harvest photons and transfer the energy to the photosystems but its effectiveness at transferring the energy to the photosystems is poor (ranging from 8 to 31%) [2-3]. It has a negligible role in plant photosynthesis. Its primary role is an anti-oxidant in the photosystem that can absorb high energy blue/violet/UV photons that might damage the chlorophyll pigments. Chlorophyll a, b and beta-carotene are present in plants at stable ratios of 2:1:1 [4-9]. This ratio does not change in response to light conditions . There is no reason to believe that aquarium plants can adapt to use yellow and green light. That plant can perform photosynthesis with yellow light is true, but you would need three times as much yellow light as you would violet light. On the other hand, the action spectra of algae peak in the yellow range so providing lots of yellow light favors algae (http://www.life.illinois.edu/govindjee/photosynBook/Chapter11.pdf) . The beta-carotene that is oxidized bay violet and ultraviolet light plays an important role in plant physiology. It is used by two proteins, Cryptochrome and Phototropin, to sense changes in light intensity so the plant can respond to it appropriately. They initiate stomatal opening, cell elongation (plant growth), the production of new chloroplasts as well as anthrocyanin production. Cryptochrome and Phototropin have a broad action spectrum spanning 350 to 500 nm (peaking at 430 nm). The action of Cryptochrome requires simultaneous stimulation of Phytochrome. Phytochrome has several physiological roles distinct to that of Cryptochrome. These are stem elongation and the initiation of flowering and germination as well as the ripening of fruit (where they control anthrocyanin production). Phytochrome activation requires a high red (650-700 nm) to far red (700+ nm) photon ratio. In the presence of red light Phytochrome is switched on, and in the presence of far red light it is switched off. Phytochrome has an action spectrum peak at 730 and 660 nm . However, in green (and red) leaved plants with high concentrations of chlorophyll the plant’s Phytochrome molecules are most sensitive to light of 628 nm . For plants to grow they need adequate stimulation of Cryptochrome, Phototropin and Phytochrome and that means supplying light in the UV-Violet range (380-430 nm) and orange range (620-630 nm). The criteria that Greystoke and I chose to evaluate the various aquarium lamps by are the following: 400-450 nm (blue-violet light) and 650-700 nm (orange-red light) to determine which lamps possess the most photosynthetically stimulating light. The ELPYF was used to convert the lamp spectra into the number of effective photons per Watt. As a lamp with mostly yellow light can still have a high ELPYF the amount of red + blue photons in the above ranges were added up to determine the lamps with the most light that is useful to plants. What is important to keep in mind is that Elodea is an obligate aquatic plant while many aquarium plants, such as amazon swords, are facultative aquatic plants that can survive out of water in boggy soil. In fact, the latter is their preferred habitat. Whether they have an action spectra such as that of Elodea or of terrestrial plants that display more sensitivity to red light is unknown at this time (if anyone has action spectrum data of an facultative aquatic plant please let me know!). The chlorophyll/beta-carotene ratios [2-10] are unhelpful as the plant can modify the absorption spectrum of the pigments in the photosystems to adapt to light input in the red and blue ranges. The stable ratios can be misleading. Still, and importantly, there is no evidence that green plants can adapt to yellow light for photosynthesis and grow well. They need the blue and red light. 380-430 nm (violet light) and 620-630 nm (orange) light was used to estimate Cryptochrome/Phototropin and Phytochrome stimulation. Whether there is a minimum and maximum value, over which no additional photon flux will increase growth, is not known at this time. As a consequence our analytical scheme is not sensitive to this issue and could give some lamps a higher rating than they deserve. All the lamps in question produce several times less far red light than red light so the inhibition of Phytochrome is negligible. I defer to Greystoke to present and explain his Excel spreadsheet and how he did the math… I must be explicit here: we do not claim to know which is THE best lamp, only to have some idea of what makes the best LAMPS for growing aquarium plants. Our ranking may not reflect reality 100%. To determine which is THE best lamp would require experimentation. References 1. Data extracted from Figure 8.8 of Life, Science of Biology by William K. Purves, David Sadava, Gordon H. Orians, H. Craig Heller (6th Edition) Sinaur. If anyone has the original reference (and the original data) please let me know! 2. Faller, Peter, Andy Pascal, and A. William Rutherford. "β-Carotene redox reactions in photosystem II: electron transfer pathway." Biochemistry 40.21 (2001): 6431-6440. 3. Telfer, Alison. "What is β–carotene doing in the photosystem II reaction centre?." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 357.1426 (2002): 1431-1440. 4. Shukla, O. P., S. Dubey, and U. N. Rai. "Preferential accumulation of cadmium and chromium: toxicity in Bacopa monnieri L. under mixed metal treatments." Bulletin of environmental contamination and toxicology 78.3 (2007): 252-257. 5. Vajpayee, P., et al. "Chromium-induced physiologic changes in Vallisneria spiralis L. and its role in phytoremediation of tannery effluent." Bulletin of environmental contamination and toxicology 67.2 (2001): 246-256. 6. Sivaci, Aysel, et al. "Removal of cadmium by Myriophyllum heterophyllum Michx. and Potamogeton crispus L. and its effect on pigments and total phenolic compounds." Archives of environmental contamination and toxicology 54.4 (2008): 612-618. 7. Maleva, Maria G., et al. "Ecophysiological tolerance ofElodea canadensis to nickel exposure." Chemosphere 77.3 (2009): 392-398. 8. Lafabrie, C., et al. "Comparison of morphology and photo-physiology with metal/metalloid contamination inVallisneria neotropicalis." Journal of hazardous materials 191.1 (2011): 356-365. 9. Hussner, Andreas, Deborah Hofstra, and Peter Jahns. "Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submergedLagarosiphon majorunder natural light conditions." Flora-Morphology, Distribution, Functional Ecology of Plants 206.10 (2011): 904-909. 10. Voeste, D., Levine, L. H., Levine, H. G., & Blüm, V. (2003). Pigment composition and concentrations within the plant (Ceratophyllum demersum) component of the STS-89 CEBAS Mini-Module spaceflight experiment. Advances in Space Research, 31(1), 211-214. 11. Rabinowitch, Eugene. "Photosynthesis." Annual Review of Physical Chemistry 2.1 (1951): 361-382. 12. http://www.photobiology.info/Shinkle.html 13. Jose, A. M., and E. Schäfer. "Distorted phytochrome action spectra in green plants." Planta 138.1 (1978): 25-28.