IMPORTANT UPDATE!!
there is a drop off in effectiveness of light and it is tied to many factors.
the most important being co2. at levels above 1200 ppm rubisco activase is inhibited:
Very high CO2 reduces photosynthesis, dark respiration and yield in wheat
Reuveni J, Bugbee B.
Ann Bot. 1997 Oct;80(4):539-46.
Although terrestrial CO2 concentrations, [CO2] are not expected to reach 1000 micromoles mol-1 for many decades, CO2 levels in closed systems such as growth chambers and glasshouses, can easily exceed this concentration. CO2 levels in life support systems in space can exceed 10000 micromoles mol-1 (1%). Here we studied the effect of six CO2 concentrations, from ambient up to 10000 micromoles mol-1, on seed yield, growth and gas exchange of two wheat cultivars (USU-Apogee and Veery-l0). Elevating [CO2] from 350 to 1000 micromoles mol-1 increased seed yield (by 33%), vegetative biomass (by 25%) and number of heads m-2 (by 34%) of wheat plants. Elevation of [CO2] from 1000 to 10000 micromoles mol-1 decreased seed yield (by 37%), harvest index (by 14%), mass per seed (by 9%) and number of seeds per head (by 29%). This very high [CO2] had a negligible, non-significant effect on vegetative biomass, number of heads m-2 and seed mass per head. A sharp decrease in seed yield, harvest index and seeds per head occurred by elevating [CO2] from 1000 to 2600 micromoles mol-1. Further elevation of [CO2] from 2600 to 10000 micromoles mol-1 caused a further but smaller decrease. The effect of CO2 on both wheat cultivars was similar for all growth parameters. Similarly there were no differences in the response to high [CO2] between wheat grown hydroponically in growth chambers under fluorescent lights and those grown in soilless media in a glasshouse under sunlight and high pressure sodium lamps. There was no correlation between high [CO2] and ethylene production by flag leaves or by wheat heads. Therefore, the reduction in seed set in wheat plants is not mediated by ethylene. The photosynthetic rate of whole wheat plants was 8% lower and dark respiration of the wheat heads 25% lower when exposed to 2600 micromoles mol-1 CO2 compared to ambient [CO2]. It is concluded that the reduction in the seed set can be mainly explained by the reduction in the dark respiration in wheat heads, when most of the respiration is functional and is needed for seed development. "
CO2 crop growth enhancement and toxicity in wheat and rice
Bugbee B, Spanarkel B, Johnson S, Monje O, Koerner G.
Adv Space Res. 1994 Nov;14(11):257-67.
Abstract
The effects of elevated CO2 on plant growth are reviewed and the implications for crop yields in regenerative systems are discussed. There is considerable theoretical and experimental evidence indicating that the beneficial effects of CO2 are saturated at about 0.12% CO2 in air. However, CO2 can easily rise above 1% of the total gas in a closed system, and we have thus studied continuous exposure to CO2 levels as high as 2%. Elevating CO2 from 340 to 1200 micromoles mol-1 can increase the seed yield of wheat and rice by 30 to 40%; unfortunately, further CO2 elevation to 2500 micromoles mol-1 (0.25%) has consistently reduced yield by 25% compared to plants grown at 1200 micromoles mol-1; fortunately, there was only an additional 10% decrease in yield as the CO2 level was further elevated to 2% (20,000 micromoles mol-1). Yield increases in both rice and wheat were primarily the result of increased number of heads per m2, with minor effects on seed number per head and seed size. Yield increases were greatest in the highest photosynthetic photon flux. We used photosynthetic gas exchange to analyze CO2 effects on radiation interception, canopy quantum yield, and canopy carbon use efficiency. We were surprised to find that radiation interception during early growth was not improved by elevated CO2. As expected, CO2 increased quantum yield, but there was also a small increase in carbon use efficiency. Super-optimal CO2 levels did not reduce vegetative growth, but decreased seed set and thus yield. The reduced seed set is not visually apparent until final yield is measured. The physiological mechanism underlying CO2 toxicity is not yet known, but elevated CO2 levels (0.1 to 1% CO2) increase ethylene synthesis in some plants and ethylene is a potent inhibitor of seed set in wheat. "
basically what you can gather from this is people have been wasting co2 for years
the recommended amount of co2 enrichment stands at 1000-1200 ppm
with recommend temps at 85-89 farenheit
rh should be no more than 55%
so the threshold for not exactly photo inhibition, but the rate at which photosynthesis is being utilized to the maximum would be 4500-5000 footcandles, on the bottom leaves.
this insures the entire plant is utilizing all the co2 to maximum efficiency and the only problem now is if over 15000 fc,(which is necessary to ensure light penetration does in fact lead to 5000 fc on the bottom leaves) will in fact cause photo inhibition, bleaching the leaves and destroying chloroplasts.
a simple question turned so complex but ended up needed the answer to the original query
also here is an important conversion:
1,200 ppm CO2 = 0.12% CO2 = 1,200 micromole mol^-1 CO2.
~1,200 ppm CO2 is the limit to benefits from CO2. And ~1,000-1,200 ppm CO2 is the saturation point for most C3 species.
