by /u/SuperAngryGuy of /r/HandsOnComplexity

The light that we're interested in for photosynthesis is about 400nm (UV-A) to 700nm (deep red) and is known as Photosynthetically Active Radiation (PAR). It actually extends a little lower than 400nm but it's very inefficient.

Phloem transports the products of photosynthesis (source)

The whole concept of photosynthesis is a plant takes in water, carbon dioxide and light to make sugar and oxygen. It's all about making sugar which is transported through the plant via the phloem network.

This is expressed in the simplified equation of 6CO2 (six carbon dioxide molecules from the air) + 6H2O (six water molecules from the roots) powered by light = C6H12O6 (one sugar molecule that the plant uses for energy) + 6O2 (six oxygen molecules given off as a gas).

Roots have the ability to uptake nutrients selectively through transport proteins. They can also alter the pH of its immediate surroundings by releasing organic acids. There are two transport structures in a land plant:

  • Xylem: one way, powered by the transpiration process. Nutrients that are nonmobile can only be translocated through the xylem which is why nonmobile deficiencies show up in new growth.
  • Phloem: two way powered by the pressure flow hypothesis. Can transport sugars, amino acids, some proteins, mobile nutrients. Mobile nutrient deficiencies can show up in old growth because enzymes (proteins) can release them from the leaves in to the phloem to go to new growth areas of the plant.

There is very weak evidence that land plants can uptake sugars and carbohydrates through their roots and have these sugars and carbohydrates translocated to the upper plant. Also, no fresh air means a low photosynthesis rate in a small volume since the carbon dioxide in the air is rapidly consumed unless CO2 enhancement is used. For that purpose you could use solutions such as a tank/regulator, or just being in the same room with the plants. As a reference, a typical exhaled breath is 4500-5000 ppm CO2.

The dreaded charts

There's four charts that people often get confused: chlorophyll and other pigments dissolved in a solvent, leaf absorption, action spectra and relative quantum yield. If you're going off a chart that has sharp peaks and talk about very specific wavelengths needed for photosynthesis optimization, then you're probably using the wrong chart (pigments dissolved in a solvent). If you're using a chart with a really deep dip in the green/yellow/orange area then it's likely for algae or aquatic plants.

The relative quantum yield chart is what we want to use since it is a measure of how much sugar is produced. This is the correct chart for land plants and are the average of dozens of plants. Keep in mind that this is only for monochromatic light which below you'll see why may be problematic. These are relative charts and not absolute charts.

For leaves grown in the field (solid) and in a growth chamber (dashed), normalized leaf yield relative to quantum of energy absorbed (original source)

Although red light is generally most efficient in photosynthesis, green light is also actively used in photosynthesis. In fact, with a bright white light source it can be the case that adding more green rather than red or blue is how to increase photosynthesis efficiency. This is because green can reach in to deeper chloroplasts in the leaves.

Be careful of improper use of pigment charts

LED grow light manufacturers tend to use the solvent absorption charts which are wildly off in the green/yellow/orange area to boost their claims of very high yields per watt. It's all BS marketing. Look at the spectrum of HPS vs quantum yield charts and you'll see that it has a very high efficiency and not the 10% ballpark efficiency that is often claimed. For example, a 600 and 1000 watt HPS puts out around 215 and 358 PAR watts perspectively (this is 35.8% PAR efficient).

LED grow light manufacturers also often use the incorrect chlorophyll dissolved in a solvent or algae charts to back their claims that specific wavelengths are needed for photosynthesis.

The correct chart to use is the McCree curve based on an average of 22 different plants which shows 550nm green is more efficient than 450nm blue (blue gets absorbed by some other pigments in addition to chlorophyll) and is the chart used in plant photobiology. The McCree curve is only valid at about 15-150 umol/m2/sec of monochromatic light and is most certainly not the be-all and end all-in in lighting spectrum charts. But, it's a good starting point and much more honest.

If you find a chart with a deep dip in the green area then it's for some sort of algae or bacteria, not green terrestrial plants. If you find a chart with a bunch of chlorophyll and other pigment peaks then it's only valid as an extract in vitro (in the test tube or cuvette) and not in vivo (the living leaf itself). The pigment peaks can differ depending on the solvent used and the charts do not tell how much there is of a particular pigment so take them with a grain of salt. They are only valid for the particular set up used.

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