Diurnal changes in the ratio of underwater red to far red light in relation to aquatic plant photoperiodism

(1) Diurnal measurements on the penetration of red (660 nm), far red (730 nm) and photosynthetically active radiation (400-700 nm) in three 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. (2) 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. (3) 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. (4) 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.