Carbon Metabolism in Seagrasses: II. PATTERNS OF PHOTOS YNTHETIC CO2 INCORPORATION

Patterns of initial photosynthetic CO₂ incorporation were determinedfor some seagrasses and were related to activities of primarycarbon fixing enzymes, carbonic anhydrase activities, and ¹³Cvalues. According to the incorporation patterns, Cymodocea nodosa wasa C4 species while Thalassia hemprichli and Thalassodendronciliatum were C₃ plants. Halophila stipulacea showed an unusualincorporation pattern which could be viewed as intermediatebetween typical C₃ and C₄ pathways. The activity ratios of ribulose-l,5-bisphosphate carboxylase (RUBPcase) to phosphoenolpyruvatecarboxylase (PEPcase) were about 3 for Thalassodendron ciliatumand 1 for Cymodocea nodosa and Halophila stipulacea. The lattervalue, which is intermediate to ratios found in terrestrialC₃ and C₄ plants, may correlate with the incorporation patternsfound for Halophila stipulacea. Since the C₄ seagrass lackedthe Kranz anatomy, it may, in addition, point to a flexibleincorporation potential for these plants. The high ¹³C values found in these and other seagrasses didnot correlate with their photosynthetic pathways as in terrestrialplants. This discrepancy is probably due to a ‘closedsystem’ type of photosynthesis in which CO₂ is efficientlyutilized. The C₃ species which utilize CO₂ enzymatically must convertexogenous HCO-₃ to CO₂ internally. Even though carbonic anhydraseactivities were very low, conversion rates seemed to be sufficientfor high rates of photosynthesis. Since enzymatic fixation ratesapproached photosynthetic rates even at CO₂ saturation, thelimitation for these seagrasses to express their high photosyntheticpotential is most probably the HCO^–₃ uptake system.     

Carbon Metabolism in Seagrasses: III. ACTIVITIES OF CARBON-FIXING ENZYMES IN RELATION TO INTERNAL SALT CONCENTRATIONS

The internal salt content and distribution in photosynthetictissues as well as the effect of NaCl on photosynthetic carbonfixation enzymes was investigated in two seagrass species fromthe Red Sea. Concentrations of both Na⁺ and Cl^– were lower in the chloroplast-richepidermis than in underlying cell layers in Halophila stipulacea.In Halodule uninervis, the concentration of Na⁺ was lower inthe epidermis than in the underlying cells, while K⁺ was evenlydistributed between cell layers. The epidermal concentrationsof Na+ were estimated to be 0.17 and 0.10 M for Halophila stipulaceaand Halodule uninervis, respectively, which were about to the average leaf concentrations. Epidermal Cl^– concentrationof Halophila stipulacea was estimated to be 0.08 M, a valueonly about of the overall leaf concentration. Phosphoenolpyruvate carboxylase (PEPcase) extracted from leavesof these seagrasses showed increased activity at 0.05–0.3M NaCl in vitro. Ribulose-l, 5-bisphosphate carboxylase (RuBPcase)activity, on the other hand, was inhibited by NaCl at all testedconcentrations. At epidermal NaCl concentrations, PEPcase activitywas thus stimulated while RuBPcase was inhibited. The reducedRuBPcase activity at such concentrations compared to salt-freeconditions was still sufficient to account for observed photosyntheticrates. We conclude that these seagrasses have adapted to a saline environmentboth by maintaining relatively low ion concentrations in theepidermis where photosynthesis occurs and by having carbon-fixingenzymes capable of functioning in the presence of salt.     

Ultrastructure of spermiogenesis and spermatozoa in Prorhynchus sp. (Platyhelminthes,Lecithoepitheliata, Prorhynchidae)

Summary

Mature sperm of Prorhynchus sp. have an elongated nucleus, multiple mitochondria and dense bodies, and two free axonemes which are located in grooves of the main shaft for much of their length. The axonemes are subterminally inserted and have the typical 9+ ‘1’ arrangement unique to Platyhelminthes and synapomorphic for taxa of Trepaxonemata. The testis follicles examined had small numbers of developing spermatids and very few mature sperm were present. During spermiogenesis, spermatids remain joined in clusters by distinctive bridges. In each spermatid two centrioles (with an intercentriolar body between them) give rise to free axonemes which grow out in opposite directions from each other. Indistinct ciliary rootlets are present. The axonemes are carried distally from the main spermatid mass on an elongating process and turn back towards the main spermatid mass. Nucleus, mitochondria and dense bodies move into the shaft, and the spermatid elongates before detaching from others in the cluster. This is the first detailed study of sperm and spermiogenesis in Lecithoepitheliata. Mature sperm are distinctly different from those of prolecithophorans, to which they are reputedly related, the latter having aflagellate sperm without dense bodies.     

Papiliochromis gen.n., a New Genus of South American Cichlid Fish (Teleostei, Perciformes)

A new, monotypic, genus is proposed for the fish hitherto known as Apistogramma ramirezi Myers & Harry, 1948. It is probably related to Geophagus Heckel and Biotodoma Eigenmann & Kennedy. It differs considerably in behaviour and morphology from other species assigned to the genus Apistogramma Regan, to which it is considered to be not very closely related.     

Ecosystem CO2 production during winter in a Swedish subarctic region: the relative importance of climate and vegetation type

General circulation models consistently predict that regional warming will be most rapid in the Arctic, that this warming will be predominantly in the winter season, and that it will often be accompanied by increasing snowfall. Paradoxically, despite the strong cold season emphasis in these predictions, we know relatively little about the plot and landscape‐level controls on tundra biogeochemical cycling in wintertime as compared to summertime. We investigated the relative influence of vegetation type and climate on CO₂ production rates and total wintertime CO₂ release in the Scandinavian subarctic. Ecosystem respiration rates and a wide range of associated environmental and substrate pool size variables were measured in the two most common vegetation types of the region (birch understorey and heath tundra) at four paired sites along a 50 km transect through a strong snow depth gradient in northern Sweden. Both climate and vegetation type were strong interactive controls on ecosystem CO₂ production rates during winter. Of all variables tested, soil temperature explained by far the largest amount of variation in respiration rates (41–75%). Our results indicate that vegetation type only exerted an influence on respiration when snow depth was below a certain threshold (～1 m). Thus, tall vegetation that enhanced snow accumulation within that threshold resulted in more effective thermal insulation from severe air temperatures, thereby significantly increasing respiratory activity. At the end of winter, within several days of snowmelt, gross ecosystem photosynthesis rates were of a similar magnitude to ecosystem respiration, resulting in significant net carbon gain in some instances. Finally, climate and vegetation type were also strong interactive controls on total wintertime respiration, suggesting that spatial variations in maximum snowdepth may be a primary determinant of regional patterns of wintertime CO₂ release. Together, our results have important implications for predictions of how the distribution of tundra vegetation types and the carbon balances of arctic ecosystems will respond to climate change during winter because they indicate a threshold (～1 m) above which there would be little effect of increased snow accumulation on wintertime biogeochemical cycling.     

Significance of cold-season respiration and photosynthesis in a subarctic heath ecosystem in Northern Sweden

While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R²-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO₂ flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO₂ uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m⁻² to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem.     

The ABSCISIC ACID-INSENSITIVE 3 (ABI3) gene is expressed during vegetative quiescence processes in Arabidopsis

The ABSCISIC ACID-INSENSITIVE 3 ( ABI3 ) gene of Arabidopsis thaliana (L.) Heynh is known to play an important role during seed maturation and dormancy. Here, we present evidence suggesting an additional role for ABI3 during vegetative quiescence processes. During growth in the dark, ABI3 is expressed in the apex of the seedlings after cell division is arrested. The 2S seed storage protein gene, a target gene of ABI3 in seeds, is also induced in the arrested apex under similar darkness conditions. In addition, β -glucuronidase expression under the control of the ABI3 promoter is abolished by treatments that provoke leaf development in the dark [sucrose and abscisic acid (ABA) biosynthesis inhibitors] and induced by treatments that prevent leaf development (darkness and ABA). Furthermore, ABI3 expression is absent in apices of dark-grown de-etiolated ( det 1 ) and abi3 mutants, both known to develop leaves or leaf primordia in the dark. The fact that the expression of the ABI3 gene is only observed in a fraction of the analysed plants suggests that ABI3 is probably only one of the components of a molecular network underlying quiescence. In addition to the expression of ABI3 in apices of dark-grown seedlings, the ABI3 promoter confers expression in other vegetative organs as well, such as the stipules and the abscission zones of the siliques. In conclusion, apart from its role in seed development, ABI3 might have additional functions.     

Wood properties of Populus and Betula in long‐term exposure to elevated CO2 and O3

We studied the interactive effects of elevated concentrations of CO₂ and O₃ on radial growth and wood properties of four trembling aspen (Populus tremuloides Michx.) clones and paper birch (Betula papyrifera Marsh.) saplings. The material for the study was collected from the Aspen FACE (free‐air CO₂ enrichment) experiment in Rhinelander (WI, USA). Trees had been exposed to four treatments [control, elevated CO₂ (560 ppm), elevated O₃ (1.5 times ambient) and combined CO₂ + O₃] during growing seasons 1998–2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO₂‐ and O₃‐exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO₂, radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O₃ decreased radial growth and the diameters of vessels and fibres. Clone‐specific decrease in wood density and cell wall thickness was observed under elevated CO₂. In birch, the treatments had no major impacts on wood anatomy or wood density. Our study indicates that short‐term impact studies conducted with young seedlings may not give a realistic view of long‐term ecosystem responses.