Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

Nitrogen in cell walls of sclerophyllous leaves accounts for little of the variation in photosynthetic nitrogen-use efficiency

Photosynthetic rate per unit nitrogen generally declines as leaf mass per unit area (LMA) increases. To determine how much of this decline was associated with allocating a greater proportion of leaf nitrogen into cell wall material, we compared two groups of plants. The first group consisted of two species from each of eight genera, all of which were perennial evergreens growing in the Australian National Botanic Gardens (ANBG). The second group consisted of seven Eucalyptus species growing in a greenhouse. The percentage of leaf biomass in cell walls was independent of variation in LMA within any genus, but varied from 25 to 65% between genera. The nitrogen concentration of cell wall material was 0.4 times leaf nitrogen concentration for all species apart from Eucalyptus , which was 0.6 times leaf nitrogen concentration. Between 10 and 30% of leaf nitrogen was recovered in the cell wall fraction, but this was independent of LMA. No trade-off was observed between nitrogen associated with cell walls and the nitrogen allocated to ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco). Variation in photosynthetic rate per unit nitrogen could not be explained by variation in cell wall nitrogen.     

Predictive models in spatially and temporally variable freshwater systems

Abstract This paper discusses the relationships between scaling and predictability in ecosystems. The logical basis of ecosystem modelling is explored using ideas first developed in complexity theory and analogies with the behaviour of complex adaptive systems. Any ecological model is a scale-dependent entity and both empirical and dynamic models of freshwater systems have their strengths and weaknesses. The logical basis of modelling using functional groups is explored. I conclude that such an approach can be justified and that such models have predictive power. Any predictive model of freshwater systems must take the major scales of external (atmospheric and catchment) forcing into account as well as the scales of key processes in the ecosystem itself. The importance of so-called ‘pink noise’ spectra, which arise both from external forcing and the internal dynamics of dynamic systems, is noted. The key scales of pattern and process in freshwater ecosystems are discussed in relation to the properties of the major functional groups. In order to have predictive power, I conclude that models of freshwater systems must include sediment exchanges and the properties of aquatic macrophytes as well as water column interactions and the pelagic components. When viewed at the scale of functional groups and the major biogeochemical processes, freshwater ecosystems may not be as complex as is often assumed.     

The size and composition of soil seed-banks in remnant patches of three structural rainforest types in North Queensland

The size and species composition of the soil seed-bank in a remnant patch of each of three structurally and floristically distinct rainforests (Complex Mesophyll Vine Forest, Complex Notophyll Vine Forest and Semi-Evergreen Vine Thicket) were assessed. Seeds of 94 species germinated from 12 surface soil samples collected from each site. All three seed-banks were composed mostly of herbs characteristic of roadsides and agricultural land, and pioneer rainforest trees and shrubs. Agglomerative classifications indicated that the seed-bank samples from each rainforest remnant had a characteristic species composition and could be distinguished reliably from seed-bank samples drawn from other sites. Seeds of species present in the standing forest were poorly represented in the seed-banks except for one long-lived pioneer tree, Dendrocnide photinophylla, at one site. The seed-bank from the seasonally dry vine thicket was significantly larger (4000 seeds m^-2) than those from the two moister sites (400–600 seeds m^-2, contained more seeds of roadside and agricultural herbs, and fewer seeds of rainforest pioneer and secondary shrubs and trees. We suggest three explanations for the different seed-bank structure observed in the seasonally dry forest site. First, with increased deciduousness in rainforests, seed-banks are increasingly subject to invasion and domination by seeds of rapidly maturing herbs. Second, long-lived seeds that germinate in canopy gaps would be less likely to accumulate under deciduous forests because they would he exposed annually to conditions suitable for germination. Third, chronic disturbance by cattle and pigs produces sites suitable for the establishment of rapidly maturing herbs, and possibly disperses their seeds into the forest.     

Turnover time of the non‐structural carbohydrate pool influences δ18O of leaf cellulose

Certainty regarding the degree to which organic molecules exchange oxygen with local water during plant cellulose synthesis (p_ex) is necessary for cellulose oxygen isotope (δ¹⁸O_cell)‐based applications in environmental and ecological studies. However, the currently accepted notion that p_ex is a constant of ca. 0.42 appears inconsistent with biochemical theory, which predicts that marked variation may be present in p_ex, in relation to variation in the turnover time (τ) of the carbohydrate pool available for cellulose synthesis. The above prediction was tested in the present study with the analysis of data collected from leaves of Ricinus communis grown in controlled environmental conditions that varied in light intensity and vapour pressure deficit. The results revealed the existence of considerable variation in both p_ex and τ across plants in the various growth environments. Moreover, despite uncertainties in estimates of the proportion of source water in the synthesis water (p_x) and of the biochemical fractionation factor (ε_o), our experiment yielded strong evidence that p_ex exhibits a significant, positive relationship with τ, consistent with biochemical theory. The observed variation in p_ex in association with τ has important implications for the interpretation of δ¹⁸O_cell data in environmental/ecological studies.     

Transformation of blueberry without antibiotic selection

Transformation of the blueberry cultivar North Country (Vaccinium corymbosum × V. angustifolium) was achieved using the disarmed Agrobacterium tumefaciens isolate LBA4404, containing a binary vector with an intron-containing β-glucuronidase (GUS) marker gene. Plant regeneration was carried out in the absence of antibiotics due to their toxicity to blueberry explants. Agrobacterium contamination was controlled by dipping the explants in antibiotics and rinsing in sterile distilled water. Once regeneration had been achieved, the plantlets were placed on to medium containing the antibiotic ticaricillin at 250 mg litre^-1 to control and try to eliminate any remaining Agrobacterium. Selection of regenerating explants expressing GUS was achieved by growing the plant material for 2 days on a medium containing 4-methyl umbelliferyl glucuronide (MUG), and examining the medium under UV light to detect fluorescent activity. From the 19 explants showing signs of regeneration, seven produced fluorescent patches on MUG medium. From the selected explants, five plantlets were found to express the GUS gene as detected by fluorometric and histochemical analysis. PCR was used to confirm the presence of the GUS and/or NPTII marker genes after 2 years in culture. Bacterial contamination isolated from plant material (which appeared free of contamination) was examined for GUS activity and analysed using PCR with GUS and NPTII specific primers, but no positive results were obtained.     

Trypanosoma gerrhonoti n. sp., and Extrinsic Development of Lizard Trypanosomes in California Sandflies

Trypanosoma gerrhonoti n. sp. is described from southern alligator lizards Gerrhonotus multicarinatus of central California. It is the 2nd trypanosome species described from western North America. Extrinsic development of T. gerrhonoti and T. scelopori of western fence lizards was observed in phlebotomine sandflies Lutzomyia vexatrix occidentis, the probable natural vector. Both trypanosomes establish dense anterior midgut infections composed of long, thin epimastigotes.     

Analysis of cytosolic isocitrate dehydrogenase and glutathione reductase 1 in photoperiod‐influenced responses to ozone using Arabidopsis knockout mutants

Oxidative stress caused by ozone (O₃) affects plant development, but the roles of specific redox‐homeostatic enzymes in O₃ responses are still unclear. While growth day length may affect oxidative stress outcomes, the potential influence of day length context on equal‐time exposures to O₃ is not known. In Arabidopsis Col‐0, day length affected the outcome of O₃ exposure. In short‐days (SD), few lesions were elicited by treatments that caused extensive lesions in long days (LD). Lesion formation was not associated with significant perturbation of glutathione, ascorbate, NADP(H) or NAD(H). To investigate roles of two genes potentially underpinning this redox stability, O₃ responses of mutants for cytosolic NADP‐isocitrate dehydrogenase (icdh) and glutathione reductase 1 (gr1) were analysed. Loss of ICDH function did not affect O₃‐induced lesions, but slightly increased glutathione oxidation, induction of other cytosolic NADPH‐producing enzymes and pathogenesis‐related gene 1 (PR1). In gr1, O₃‐triggered lesions, salicylic acid accumulation, and induction of PR1 were all decreased relative to Col‐0 despite enhanced accumulation of glutathione. Thus, even at identical irradiance and equal‐time exposures, day length strongly influences phenotypes triggered by oxidants of atmospheric origin, while in addition to its antioxidant function, the GR‐glutathione system seems to play novel signalling roles during O₃ exposure.