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Many of the genes that control photosynthesis are carried in the chloroplast. These genes differ among species. However, evidence has yet to be reported revealing the involvement of organelle genes in the initial stages of plant speciation. To elucidate the molecular basis of aquatic plant speciation, we focused on the unique plant species Chara braunii C. C. Gmel. that inhabits both shallow and deep freshwater habitats and exhibits habitat-based dimorphism of chloroplast DNA (cpDNA). Here, we examined the 'shallow' and 'deep' subpopulations of C. braunii using two nuclear DNA (nDNA) markers and cpDNA. Genetic differentiation between the two subpopulations was measured in both nDNA and cpDNA regions, although phylogenetic analyses suggested nuclear gene flow between subpopulations. Neutrality tests based on Tajima's D demonstrated diversifying selection acting on organelle DNA regions. Furthermore, both 'shallow' and 'deep' haplotypes of cpDNA detected in cultures originating from bottom soils of three deep environments suggested that migration of oospores (dormant zygotes) between the two habitats occurs irrespective of the complete habitat-based dimorphism of cpDNA from field-collected vegetative thalli. Therefore, the two subpopulations are highly selected by their different aquatic habitats and show prezygotic isolation, which represents an initial process of speciation affected by ecologically based divergent selection of organelle genes. [ABSTRACT FROM AUTHOR]

The effects of salinity on growth, maturation and photosynthesis were examined in the filamentous algaRhizocloniumsp. (Cladophoraceae, Chlorophyta) growing in a brackish water habitat in a canal draining into Tokyo Bay, Japan. In this habitatRhizocloniumsp. was exposed to a wide salinity range, both daily, 5−23‰ during November 1996, and hourly, 6−24‰ during the spring tide day. From the results of culture experiments, growth and maturation ofRhizocloniumsp. occurred in the wide salinity range of 10−40‰ at 20 µmol photons m−2s−1 at 20°C, but did not occur at salinity of 0‰. Light saturation on the photosynthesis-irradiance curve at 20°C at 20‰ was reached at 100 µmol photons m−2s−1, which is characteristic for shade-adapted algae. On the photosynthesis-salinity curve at 20°C at saturated irradiance (160 µmol photons m−2s−1), the net photosynthetic rate increased with increasing salinity up to 30‰ but decreased at 40‰. On the photosynthesis-salinity curve at 20°C at 20 µmol photons m−2s−1 (at nearin situirradiance), the photosynthetic rates were almost the same in the salinity range from 0 to 40‰. Therefore, this species is able to grow, reproduce and photosynthesize with a relative efficiency in a wide salinity range, which shows that it is well adapted to a brackish water environment. [ABSTRACT FROM AUTHOR]

Physiological properties of the temperate hermatypic coral Acroporapruinosa Brook with symbiotic algae (zooxanthellae) on the southerncoast of the Izu Peninsula, Shizuoka Prefecture, central Japan,were compared between summer and winter. Photosynthesis and respirationrates of the coral with symbiotic zooxanthellae were measured insummer and winter under controlled temperatures and irradianceswith a differential gas-volumeter (Productmeter). Net photosyntheticrate under all irradiances was higher in winter than in summer at thelower range of temperature (12−20°C), while lower thanin summer at the higher range of temperature (20−30°C).The optimum temperature for net photosynthesis was apt to fall withthe decrease of irradiance both in summer and winter, whereas itwas higher in summer than in winter under each irradiance. At 25/50/100 µmolphotons m-2 s-1, it was nearly the sea­watertemperature in each season. Dark respiration rate was higher inwinter than in summer, especially in the range from 20−30°C.In both seasons the optimum temperature for gross photosynthesiswas 28°C under 400 µmol photons m-2 s-1 andlowered with decreasing irradiance up to 22°C under 25 µmolphotons m-2 s-1 in summer, while 20°Cunder the same irradiance in winter. The optimum temperature forproduction/respiration (P/R) ratio was higherin summer than in winter under each irradiance. Results indicatedthat metabolism of coral and zooxanthellae is adapted to ambienttemperature condition under nearly natural irradiance in each season. [ABSTRACT FROM AUTHOR]

Summary Intertidal seaweeds are periodically exposed during low tide and thus experience extreme levels of desiccation. The physiological activity of seaweeds changes during this water loss process. This study examined how desiccation affects the photosynthesis and respiration of seaweeds from different intertidal levels, and whether the ability to retain photosynthesis and respiration rates during desiccation varies among these species. Photosynthesis and respiration rates of 12 species of seaweeds were measured under various levels of desiccation, using an infrared CO2 gas analyzer. High levels of drought negatively affected photosynthesis, while most species showed initial rises in photosynthetic rates. The ability to retain photosynthesis and respiration activities under desiccation conditions varied among species. These physiological responses were not related to the intertidal level at which these species occur, but to their ability to prevent water loss. The species with lower rates of water loss had slower declines in the rate of photosynthesis and respiration. [ABSTRACT FROM AUTHOR]

SUMMARY Characteristics of photosynthesis and respiration of bladelets were compared between Ecklonia cava Kjellman sporophytes growing in a warmer temperate locality (Tei, Kochi Pref., southern Japan) and in a cooler temperate locality (Nabeta, Shizuoka Pref., central Japan). Photosynthesis and respiration were measured with a differential gas-volumeter (Productmeter). In photosynthesis–light curves at 20°C, the rate of net photosynthesis was almost the same at light intensities lower than 25 μmol m–2 s–1 and the light-saturation occurred at 200–400 μmol m–2s–1 in plants of both localities. The light-saturated net photosynthetic rates were higher in winter and spring than in summer and autumn in both plants. The optimum temperature for net photosynthesis at 400 μmol m–2 s–1 was 27°C throughout the year in the Tei plant and 25–27°C in the Nabeta plant. The decrease of net photosynthetic rates in the supraoptimal temperature range up to 29°C was sharper in winter and spring than in summer and autumn in both plants, being smaller in the Tei plant than in the Nabeta plant in all seasons. The dark respiration rate always increased with water temperature rise in both plants. No clear differences were found in the dark respiration rate between Tei and Nabeta plants except that when measured against dry weight, the Tei plant showed a slightly lower rate as compared with the Nabeta plant. [ABSTRACT FROM AUTHOR]