Leaf- and cell-level carbon cycling responses to a nitrogen and phosphorus gradient in two Arctic tundra species.

PREMISE OF THE STUDY: Consequences of global climate change are detectable in the historically nitrogen- and phosphorus-limited Arctic tundra landscape and have implications for the terrestrial carbon cycle. Warmer temperatures and elevated soil nutrient availability associated with increased microbial activity may influence rates of photosynthesis and respiration. • METHODS: This study examined leaf-level gas exchange, cellular ultrastructure, and related leaf traits in two dominant tundra species, Betula nana, a woody shrub, and Eriophorum vaginatum, a tussock sedge, under a 3-yr-old treatment gradient of nitrogen (N) and phosphorus (P) fertilization in the North Slope of Alaska. • KEY RESULTS: Respiration increased with N and P addition-the highest rates corresponding to the highest concentrations of leaf N in both species. The inhibition of respiration by light ("Kok effect") significantly reduced respiration rates in both species (P < 0.001), ranged from 12-63% (mean 34%), and generally decreased with fertilization for both species. However, in both species, observed rates of photosynthesis did not increase, and photosynthetic nitrogen use efficiency generally decreased under increasing fertilization. Chloroplast and mitochondrial size and density were highly sensitive to N and P fertilization (P < 0.001), though species interactions indicated divergent cellular organizational strategies. • CONCLUSIONS: Results from this study demonstrate a species-specific decoupling of respiration and photosynthesis under N and P fertilization, implying an alteration of the carbon balance of the tundra ecosystem under future conditions.

Clinal variation in stomatal characteristics of an Arctic sedge, Eriophorum vaginatum (Cyperaceae).

PREMISE OF THE STUDY: In a large reciprocal transplant experiment, Eriophorum vaginatum tussocks transplanted along a latitudinal gradient in Alaska's interior exhibited genetic differentiation and phenotypic plasticity for vegetative traits. Using the same tussocks 30 yr later, we used estimates of growing season temperature at each site to ask whether there was a climatic cline for stomatal density, size, and conductance. METHODS: We created impressions of the abaxial leaf surfaces of the transplanted individuals for viewing under a microscope and measured stomatal density (SD) and length (SL) for 224 individuals. We used SD and SL to estimate stomatal conductance (C). Separate one-way analyses of variance were performed to quantify the effect of population genetic differences and latitudinal environmental variation on stomatal characteristics. KEY RESULTS: Our data suggest that stomatal size was influenced by both genetics and environment and that plasticity for stomatal density produced highest densities at the coolest sites. Stomatal conductance increased with decreasing temperature of site from which the populations originated. CONCLUSIONS: Our results demonstrate a cline in stomatal conductance in E. vaginatum, with some ability of populations to plastically produce an appropriate phenotypic response in a new environment. Because the species is a dominant species in many arctic plant communities, its ability to produce an appropriate stomatal phenotype and to optimize water use efficiency by decreasing stomatal conductance in warmer environments could affect both community composition and total primary productivity in future, warmer climates.

Selective microspore abortion correlated with aneuploidy: an indication of meiotic drive.

Selective megaspore abortion (monomegaspory) probably arose once in seed plants and occurs routinely in more than 70% of angiosperm species, representing one of the key characters of a heterosporous life history. In contrast, selective microspore abortion leading to pollen dispersal as pseudomonads (here termed monomicrospory) apparently arose at least twice independently within angiosperms, though it occurs in a limited number of taxa. Remarkably, similar examples of monomicrospory occur in members of two distantly related angiosperm families: the sedge family (Cyperaceae) and the epacrid subfamily (Styphelioideae) of the eudicot family Ericaceae. In sedges, monomicrospory is derived directly from normal tetrads, whereas epacrid pseudomonads apparently evolved via an intermediate stage, in which variable sterility occurs in a single tetrad. Our comparison of these two examples of selective microspore abortion highlights a correlation with aneuploidy, indicating that non-random chromosome segregation caused by monomicrospory could drive chromosomal mutations to rapid fixation through meiotic drive.

AFLP markers and cytotaxonomic analysis reveal hybridisation in the genus Schoenus (Cyperaceae).

Molecular, cytological, and morphological data support the existence of a hybrid population between Schoenus nigricans and Schoenus ferrugineus. This population was found in northeastern Italy, where S. nigricans is central with respect to its natural range and S. ferrugineus is marginal, being most common in the Alps and in central and northern Europe. Molecular marker data show that the putative hybrid population is genetically intermediate between nearby populations of the parent species. Cytological evidence confirmed the hybrid nature of this population, as does the almost complete sterility of plants within the population. Although no seeds were produced by the hybrid population, some possibly fertile pollen grains were produced; this suggests that the possibility of introgression between the two species through the hybrids cannot completely be excluded.