FREEZING STRESS AND OSMOTIC DEHYDRATION IN FUCUS-DISTICHUS (PHAEOPHYTA) - EVIDENCE FOR PHYSIOLOGICAL SIMILARITY

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 responses to both osmotic dehydration and freezing stress and that these responses resemble thos e in the literature for the effect of desiccation in air. Both stresse s inhibited light-limited (P(subsat)) and light-saturated (P(max)) pho tosynthesis measured immediately after plants were reimmersed in seawa ter. The degree of initial inhibition and subsequent recovery of photo synthesis were proportional to the severity of the dehydration or free zing treatment. P(subsat) and P(max) recovered completely from osmotic dehydration for 3 h in 200 parts per thousand and 3 hr at -10-degrees -C, but recovery was only partial following 3 h in 300 parts per thous and or 3 h at -15-degrees-C. In most cases, recovery was complete with in 2 h following dehydration, with little further recovery occurring b etween 2 and 24 h posttreatment. No time-dependent recovery occurred f ollowing severe freezing. Observations using the vital stain fluoresce in diacetate suggested that the lack of complete recovery might be due to severe damage or death of a proportion of cells in the thallus. Th ere were no clear effects of either osmotic dehydration or freezing on dark respiration (R(d)), although R(d) was stimulated in all emersed treatments (frozen plants and 5-degrees-C controls) immediately follow ing reimmersion. Measurement of chlorophyll fluorescence induction kin etics indicated that both osmotic dehydration and freezing reduced the ratio of variable to maximum florescence (F(v)/F(m)), indicating a de crease in the quantum efficiency of photosystem I. Based on these data , we suggest that there are common cellular and physiological componen ts involved in the response of fucoid algae to a range of water stress es. This hypothesis was supported by experiments that showed that osmo acclimation in hyperosmotic seawater (51 parts per thousand) for 2 wee ks increased the ability of F. distichus to recover from freezing at - 15-degrees-C. During acclimation, mannitol content increased under hyp erosmotic conditions and decreased under hypoosmotic conditions. Chang es in plasma membrane integrity, determined by fresh weight: dry weigh t ratio, and amino acid release following freezing indicated an increa sing gradient of freezing tolerance from low to high salinity. However , none of these physiological changes fully explained the marked incre ase in the freezing tolerance of photosynthesis observed in plants acc limated under hyperosmotic conditions.