Garden Tips How Plants Use Light To Grow
The way that all plants grow is by photosynthesis, the process whereby carbon dioxide (CO2) is converted into the organic material that makes up the plant (carbon, proteins, sugars and so on) using energy derived from sunlight. This energy is absorbed through special proteins containing chlorophyll molecules that reside in photosynthetic cell membranes called chloroplasts.
Now the point about chlorophyll is that it only absorbs light from particular parts of the spectrum, mostly the blue and red parts. It is especially poor at absorbing light from the green part of the spectrum which is why chlorophyll itself and anything containing it (such as plant leaves, grass and so on) appears green, since that area of sunlight is reflected rather than absorbed. In fact, chlorophyll is so effective at absorbing light outside the green part of the spectrum that these colors are only revealed when the chlorophyll decays – hence the reason we have to wait till Autumn to see the reds, browns and yellows that are present in leaves but masked by the chlorophyll.
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There are in fact two types of chlorophyll, rather unimaginatively termed Chlorophyll a and Chlorophyll b (note the lower case designation). These differ slightly in chemical composition and their absorption peaks. So type “a” has a wider gap between its peaks, which sit in the violet and red portions of the spectrum, whereas type “b” has its peaks closer together in the blue and yellow zones. Both of course avoid the trough of green that sits in the middle between blue and yellow, and by targeting adjacent areas (violet/blue and yellow/red) they extend the range of useable light, with absorption in the violet/blue region being especially beneficial in low light conditions. If you’re not familiar with the layout of the spectrum, this graphic should help clear things up….
Photosynthetically Available Radiation (also called Photosynthetically Active Radiation and abbreviated to PAR) defines the spectral band between 400 and 700 nanometres (nm) as illustrated above where plants find light suitable for photosynthesis. The standard unit of measure for PAR is µmol (micromoles) per square meter per second which gives the photosynthetic photon flux density (PPF or PPFD) – essentially how much light is hitting a given area. Where lumens per watt are used to determine the efficacy of regular lighting (i.e. intended for humans), PPF holds sway in horticulture.
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