Zygotes of the brown alga Fucus distichus L. Powell accumulate a sulfated polysaccharide (fucoidin) in the cell wall at the site of rhizoid formation. Previous work indicated that zygotes grown in seawater minus sulfate do not sulfate the preformed fucan (an unsulfated fucoidin) but form rhizoids. Under these conditions, we determined whether sulfation of the fucan is required for its localization in the rhizoid wall. This was accomplished by developing a specific stain for both the fucan and fucoidin. Using a precipitin assay, we demonstrated in vitro that the lectin ricin (RCAz) specifically complexes with both the sulfated and desulfated polysaccharide. No precipitate is observed when either is incubated in 0.1 M o-galactose or when RCAI is mixed with laminarin or alginic acid, the other major polysaccharides in Fucus. RCA~ conjugated with fluorescein isothiocyanate (FITC) is also shown to bind specifically to fucoidin using a filter paper (DE81) assay. When added to zygotes, RCArFITC binds only to the site of fucoidin localization, i.e., the rhizoid cell wall. However, RCArFITC is not observed in the rhizoid wall of zygotes grown in the absence of sulfate. This observation is not due to the inability of RCAI-FITC to bind to the fucan in vivo. Chemically desulfated cell walls that contained fucoidin in the rhizoid wall bind RCAI-FITC only in the rhizoid region. Also, the concentration of fucose-containing polymers and polysaccharides that form precipitates with RCA~ is the same in embryos grown in the presence or absence of sulfate. If sulfate is added back to cultures of zygotes grown without sulfate, fucoidin is detected at the rhizoid tip by RCA[-FITC several hours later. These results support the conclusion that the enzymatic sulfation of the fucan is a modification of the polysaccharide required for its localization and/or assembly into a specific region of the cell wall. KEY WORDS Fucus distichusricin 9 fluorescence microscopy fucoidin sulfation cell wall cytoplasmic localization rhizoidThe localization of subcellular components into specific regions of a variety of plant and animal cells is a common and important aspect of cellular differentiation (c.f. reference 17). However, the mechanism by which macromolecules are sequestered into localized regions or structures of cells is not well understood. The rhizoid of the Fucus zygote provides a model system to investigate the regulation of intracellular localization. Previous reports indicated that a new and different sulfated polysaccharide is deposited in only that region of the zygote cell wall that forms the rhizoid (4, 20).

Evidence from exposure-response studies and a turbulent transfer model demonstrate that plant response is differential to concentration, duration, temporal pattern, and time of day of exposure. Reductions in productivity of crops and trees as seedlings are greater when plants are exposed to higher daytime ozone (O 3 ) concentrations (0800-2000 hr standard time) or for longer durations. Primary evidence on the greater role of concentration comes from exposure-response experiments where plants are exposed to a series of pollutant concentrations in open-top chambers under field conditions. These studies demonstrate that the integrated exposure indices that give preferential weight to higher concentrations are better predictors of response than mean or peak indices. Evidence suggesting that midrange O 3 concentrations (0.05-0.09 parts per million, ppm) play a greater role than higher concentrations (>0.09 ppm) in biological response could not be justified. The time of day when O 3 concentrations and atmospheric and stomatal conductances of gas exchange are optimal is a key to understanding plant response because plants respond only to O 3 entering the leaf via stomata. A turbulent transfer model that describes the resistance of pollutant gas exchange from the atmosphere to the boundary layer of a forest canopy, as a function of micrometeorological variables, is developed IMPLICATIONS Tropospheric O 3 , the primary photochemical oxidant in ambient air, is the most important pollutant affecting plant productivity in the United States. O 3 concentrations and atmospheric and stomatal properties that govern the diffusion of O 3 from the atmosphere to the leaf interior via stomata display diurnal and seasonal variations over the growing season. The assignment of ecological effects of O 3 implied by exposure-response and micrometeorological studies to a specific time of day and range of O 3 concentrations has implications for the process of setting a secondary O 3 standard.

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