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Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C4 plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C3 and C4 woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (Ks ) was 16%–98% lower in the C4 than C3 shrubs from the American west. In the Asian species, the C3 Nitraria schoberi had similar and Halimodendron halodendron higher Ks values compared with three C4 species. Leaf specific conductivity (KL ; hydraulic conductivity per leaf area) was 60%–98% lower in the C4 than C3 species, demonstrating that the presence of the C4 pathway alters the relationship between leaf area and the ability of the xylem to transport water. C4 species produced similar or smaller vessels than the C3 shrubs except in Calligonum, and most C4 shrubs exhibited higher wood densities than the C3 species. Together, smaller conduit size and higher wood density indicate that in most cases, the C4 shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C4 shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C4 photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO2 variation, may also affect the evolution of wood structure and function. [ABSTRACT FROM AUTHOR]

Archived wheat grain and straw samples from the unfertilised plot of the Broadbalk Continuous Wheat Experiment at Rothamsted, England, were used to investigate the trends in stable C isotope ratios since 1845. Grain δ13C was higher than straw δ13C. Both grain and straw δ13C have decreased by approximately 2.5–2.8‰ over the last 153 years, and since the 1960s the decrease has been more rapid than the decrease of the δ13C of atmospheric CO2. The C isotope discrimination (Δ), and hence the ratio of CO2 concentration in leaf intercellular space to that in the ambient air (ci/ca), remained relatively stable or decreased slightly between 1845 and the mid-1960s, then increased considerably. The period with increasing Δ and ci/ca corresponds to the introduction of the modern short-straw cultivars of wheat. When grown on the unfertilised plot in the same seasons, a modern wheat cultivar had slightly higher Δ values than an old cultivar. The ci derived from both grain and straw Δ have increased by 33–37% over the last 153 years, with the increase being more rapid after than before the mid-1960s. However, there was no clear trend in the yields of straw and grain, primarily because of the limitations of N and other nutrients, and also because of large year-to-year variations. [ABSTRACT FROM AUTHOR]

It is uncertain whether elevated atmospheric CO2 will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO2 may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO2 to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO2 (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 µg N g–1). Ambient and twice-ambient CO2 concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with 14CO2 and measured soil 14CO2 respiration and the 14C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used 15N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO2 significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO2. CO2 enrichment had no significant effect on the allometric distribution of biomass or 14C among plant components, total 14C allocation belowground, or cumulative (6-day) 14CO2 soil respiration. Elevated CO2 significantly increased microbial 14C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial 14C contents at elevated CO2 was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO2, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO2. Despite significantly greater rates of N immobilization under elevated CO2, standing pools of microbial N were not affected by elevated CO2, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO2. [ABSTRACT FROM AUTHOR]