STIMULATION OF LIGHT‐SATURATED PHOTOSYNTHESIS IN LAMINARIA (PHAEOPHYTA) BY BLUE LIGHT 1

The light‐saturated rate of photosynthesis in blue light was 50‐100% higher than that in red light for young sporophytes of Laminaria digitata (Huds.) Lamour., although photosynthetic rates were slightly higher in red than in blue light at low irradiances. Short exposures to low irradiances (e.g. 2 min at 20 μmol · m −2 · s −1 ) of blue light also stimulated the subsequent photosynthesis of Laminaria sporophytes in saturating irradiances of red light but had little effect on photosynthesis in low irradiances of red light. The full stimulatory effect of short exposures to blue light was observed within 5 min of the blue treatment and persisted for at least 15 min in red light or in darkness. Thereafter, the effect began to decline, but some stimulation was still detectable 45 min after the blue treatment. The degree of stimulation was proportional to the logarithm of the photon exposure to blue light over the range 0.15‐2.4 mmol · m −2 , and the effectiveness of an exposure to 0.6 mmol · m −2 at different wavelengths was high at 402‐475 nm (with a peak at 460‐475 nm) but declined sharply at 475‐497 nm and was minimal at 544‐701 nm. Blue light appears, therefore, to exert a direct effect on the dark reaction of photosynthesis in brown algae, possibly by activating carbon‐fixing enzymes or by stimulating the uptake or transport of inorganic carbon in the plants.