Diurnal Changes in the Ratio of Underwater Red to Far Red Light in Relation to Aquatic Plant Photoperiodism

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Abstract
Diurnal measurements on the penetration of red (660 nm), far red (730 nm) and photosynthetically active radiation (400-700 nm) in 3 Scottish lakes revealed that twilight (dawn and dusk) is characterized by a distinct decrease in the underwater red:far red quantum ratio (R:FR) compared with the mid-day value. Except in the clearest waters, dawn to dusk changes in R:FR were not proportional to those observed above the lake surface, but were larger and due primarily to decreased attenuation of far red radiation. Absolute R:FR values at any depth varied between the lakes at both twilight and mid-day as a function of both depth and chlorophyll a concentration in the water. Diurnal variation in R:FR, as perceived by the red-far red photoreversible plant pigment, phytochrome, is considered as a possible mechanism by which aquatic plants might detect the length of photoperiod.