Photoperiodism in Delesseria Sanguinea (Ceramiales, Rhodophyta) 2. Daylengths are shorter underwater.

An explanation was sought for the fact that both gametophytes and tetrasporophytes of Delesseria sanguinea start their reproductive processes well before their (different) critical daylengths occur. The trigger date for males was estimated from the growth rate of the reproductive bladelets in culture and the field and the mean found to be 27 August, 27 days before their critical astronomical 12 h day. For tetrasporophytes, similarly estimated, the date was 9 October, 20 days before their critical 9.5 h day. Estimated dates were not significantly influenced by shading by cliffs or timing of neap tides while deep water plants were unexpectedly less advanced, perhaps because of the absence of canopy. Day-extensions of low light of blue, green and red resulted in a graded reaction over a range of irradiances up to 2.6 μmol photons m-2 s-1 in males, with a 50% response to green of 0.7 and blue and red of 0.4 μmol photons m-2 s-1. Female sensitivity was similar. Males were much less sensitive to a night-break than day extension, the 50% response to 2 h being at over 3 μmol photons m-2 s-1. Tetrasporophytes were more sensitive than gametophytes, both to day-extensions and night-breaks, 1.0 and 0.6 μmol photons m-2 s-1 (the lowest irradiances tested) of green respectively totally inhibiting reproduction. With both blue and green day-extensions, males were more sensitive after days of green light than after blue. Measured solar irradiance at the critical times of day on the trigger dates reduced by estimated reflection, water attenuation and canopy effect was 1 .32 and 0.74 μmol photons m-2 s-1 for the gametophyte and tetrasporophyte dates respectively, close to their sensitivities and allowing the conclusion that daylengths are shorter underwater for Delesseria. [ABSTRACT FROM AUTHOR]