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The effects of osmotic dehydration and freezing on photosynthesis were studied in the brown alga Fucus distichus L. The data indicated that F. distichus exhibits similar physiological response to both osmotic dehydration and freezing stress and that these responses resemble those in the literature for the effect of desiccation in air. Both stresses inhibited light-limited (P[subsubsat]) and light-saturated (P[submax]) photosynthesis measured Immediately after plants were reimmersed in seawater. The degree of initial inhibition and subsequent recovery of photosynthesis were proportional to the severity of the dehydration or freezing treatment. P[subsubsat] and P[submax] recovered completely from osmotic dehydration for 3 h in 200% and 3 hr at -10°C, but recovery was only partial following 3 h in 300% or 3 h at -15°C. In most cases, recovery was complete within 2 h following dehydration, with little further recovery occurring between 2 and 24 h posttreatment. No time-dependent recovery occurred following severe freezing. Observations using the vital stain fluorescein diacetate suggested that the lack of complete recovery might be due to severe damage or death of a proportion of cells in the thallus. There were no clear effects of either osmotic dehydration or freezing on dark respiration (R[subd]), although R[subd] was stimulated in all emersed treatments (frozen plants and 5°C controls) immediately follwing reimmersion. Measurement of chlorophyll fluorescence induction kinetics indicated that both osmotic dehydration and freezing reduced the ratio of variable to maximum florescence (F[subv]/F[subm]), indicating a decrease in the quantum efficiency of photosystem I. Based on these data, we suggest that there are common cellular and physiological components involved in the response of fucoid algae to a range of water stresses. This hypothesis was supported by experiments involved in the response of fucoid algae to a range of water stresses. This hypothesis was supported by experiments that showed that osmoacclimation in hyperosmotic seawater (51%) for 2 weeks increased the ability of F. distichus to recover from freezing at -15°C. During acclimation mannitol content increased under hyperosmotic conditions and decreased under hypoosmotic conditions. Changes in plasma membrane integrity, determined by fresh weight: dryweight ratio, and amino acid release following freezing indicated an increasing gradient of freezing tolerance from low to high salnity. However, none of these physiological changes fully explained the marked increse in the freezing tolerance of photosynthesis observed in plants acclinated under hyperosmotic conditions. [ABSTRACT FROM AUTHOR]

The settlement patterns of spores of Polysiphonia lanosa (L.) tandy on Ascophyllum nodosum (L.) Le Jolis and Focus vesiculosus L. were studied using a flow tank. Settlement sites were defined as "sheltered" or 'exposed'. Surface area calculations revealed non-random settlement on A. nodosum, with higher than expected spore frequencies on the thallus and lateral pits and lowere than expected frequencies on the vesicles. Settlement on F. vesiculosus was random and significantly lowere than on A. nodosum. On the shore, survival of sporelings from September (post-sporulation) to May (pre-sporulation) was highly non-random on both basiphytes. On A, nodosum, lateral pits ('sheltered') showed the highest survival frequency. Here the proportion of surviving sporelings increased over the study period, whereas the proportion on open thallus area ('exposed') decreased. On F. vesiculosus also, preferential survival accurred on 'sheltered' sites such as vesicle/thallus interfaces and wounds. Between September and May, all P. lanosa sporelings were lost from 'exposed' areas (thallus surface and vesicles). Overall, frequencies of surviving sporelings were much greater on A, nodosum than on F. vesiculous. These results are discussed with reference to basiphyte morphology, epiphyte removal mechanisms and the survival strategy of P. lanosa. [ABSTRACT FROM AUTHOR]