Phytochrome-mediated germination of very sensitive oospores.

The light receptor and its mode of operation were studied in photosensitive oospores of Nitella furcata subsp. megacarpa (Allen emend. Wood). Brief pulses of light activated maximal germination of post-secondary dormant oospores removed from lake sediments. Fluence response data at 12 wavelengths were used to construct an action spectrum for germination. The shape of the action spectrum with its maximum at 669 nm provides evidence for the involvement of phytochrome. Germination was induced with photon fluences that established as little as 0.01% of the phytochrome in the far red-absorbing form, which suggests that phytochrome was operating in the very low-fluence response mode. The functioning of phytochrome in the very low-fluence response mode in Nitella is similar to that in higher plants.