A little write-up I found for you. Sounds legit, lol.
Written by Penguin:All plants have three critical photosythnetic response points with respect to irradiance levels. These are the light compensation point, the light saturation point, and the point of photoinhibition.
In darkness, photosynthesis does not occur and plant metabolism uses energy derived from respiration, for net energy loss by the plant. As irradiance increases, photosynthesis occurs - but at a rate lower than respriation, and therefore continuing net energy loss.
At a certain critical level of irradiance, called the light compensation point (LCP), the rate of photosynthesis exactly matches the rate of respiration, so there is zero net energy change. LCP varies by cultivar and conditions. As irradiance increases above the LCP, net energy gain occurs as the rate of photosynthesis outpaces the rate of respiration.
However, at a certain point (level of irradiance), increasing irradiance no longer results in an increasing rate of photosynthesis. This is known as the light saturation point (LSP), and varies by cultivar and conditions. This is the "optimum" light level that produces the maximum possible rate of photosynthesis. Beyond this point, any additional irradiance is not used by the plant, but also does not cause any harm to the plant. In the context of a growroom, once the LSP has been reached, any additional wattage is simply wasted (and ends up as heat).
As irradiance continues to increase, eventually it will reach a point where it becomes counterproductive. This is known as the point of photoinhibition. Prolonged/chronic photoinhibition causes degradation (photo-oxidation) of chlorophylls ("leaf bleaching"), and damages protiens involved in photosystem II, further impairing the ability of the plant to use light. This damage is permanent - damaged leaves will not recover if/when irradiance levels are reduced below the point of photoinhibition.
The exact levels of irradiance determining LCP, LSP, and photoinhibition can be directly measured, and have been for many plants. Environmental factors including CO2 concentration, temperature, relative humidty, nutrient availability, water status, and general plant health are known to affect the values. I am not aware of any direct measurements for any variety of cannabis, in any conditions. Generally, sativas can be expected to have higher values for all three points than indicas - sativas require, utilize, and tolerate more light than indicas.
Assuming that all other conditions are optimum, one would not expect any plants to be adapted to use or even tolerate light at irradiance levels beyond what is provided by the sun at noon, at the equator, on the equinox, with 'perfect' atomspheric conditions. This maximum solar irradiance is approximately 1000 w/m^2 (all wavelengths). Horticultural HPS produces approximately 0.35 PAR watts per input watt, and so exceeds maximum solar PAR irradiance by a fair margin at approximately 3000-3500 (input) w/m^2. In the grow room environment, it's always noon on the equinox, and there never any clouds, so at 3500 w/sf the total w/m^2/day far exceeds what outdoor plants get at the equator, and can very reasonably be expected to cause chronic photoinhibition.
There's a pretty broad consensus among experienced
growers that, given good environmental conditions, around 55 watts of horticultural HPS per square foot of canopy is "optimum." That's not the same as a direct measurement, but is a pretty good indication that increasing the light does not improve results - i.e., it's a pretty good ballpark for LSP.
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