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[APD] Minimum Light Threshold

Rex proposed a minimum amount of light below which a plant would die:

I feel this is due to a little explored or understood "Minimum Light Threshold", the idea behind this is that every plant requires a certain amount of light energy that is not clearly defined in the watt per gallon rule.....There are plants that do well in deep shade and others that require full sun. Light is a nutrient. Some plants require more of some nutrients that others. And all plants need a certain amount of light energy to function. This amount varies from plant to plant.

He went on to ask if he was totally nuts in thinking this way. Well, according to Aquarium Plants by Christel Kasselmann in the section on light pages 4 to 13 she speaks of "optimum light curves" for plants and cites a work done by a German, K. Sauer in 1989. In this work the author studied Anubias barteri var. nana and Bacopa caroliniana and classified the former as a shade plant and the later as a sun plant. 

To me, this seems almost intuitive, but at least some real data exists to verify it. So yes, a minimum light threshold does exist. How it relates to watts per gallon and the size of the tank I can't comment on.

Kasselmann goes on to say that if further research were to be performed probably most aquarium plants would be classified as sun plants with the exception of only a very few, such as several Cryptocoryne and Anubias species.

Jerry Smith in Bloomingdale, NJ
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