Plants grown under elevated atmospheric [CO 2 ] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO 2 ]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO 2 . This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO 2 , although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO 2 , plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.Key words: carbon dioxide; dilution; elevated CO 2 ; graphical vector analysis; nitrogen; plants; root uptake; tissue concentrations. Growth of plants at atmospheric concentrations of carbon dioxide (CO 2 ) greater than the current ambient can greatly affect plant tissue chemistry (Poorter et al. 1997;Loladze 2002). One of the most commonly seen effects is a decrease in the dry mass concentration of N (N m ). Cotrufo et al. (1998), synthesizing data from a broad range of studies, found mean decreases in N m of 14% in aboveground tissues and 9% in roots. This compares closely with the findings of other data syntheses, which have found elevated [CO 2 ] mediated decreases in N m Received 25 Feb. 2008 Accepted 11 Jun. 2008 Supported by the Cullen Fund of Southwestern University to D. R. Taub.* Author for correspondence. Yin (2002) found that the effect of elevated [CO 2 ] was greatest for woody deciduous species and that decreases in N m under elevated CO 2 were most pronounced at high light levels, high temperatures and large pot sizes. Yin (2002) and Taub et al. (2008) both found that the effect of [CO 2 ] on N m was reduced by N fertilization. Several studies have reported that the effect of elevated [CO 2 ] on N m is less for nitrogen-fixing species than for other types of plants (Cotrufo et al. 1998; Jablonski et al. 2002;Taub et al. 2008). Ainsworth and Long (2005) and Taub et al. (2008) found that the effect of elevated CO 2 on N m increased under ozone stress (although see Taub et al. 2008 for divergent results for soybean).Although it has been well established that elevated [CO 2 ] typically decreases N m , the mechanisms by which this occurs are n...

Meta-analysis techniques were used to examine the effect of elevated atmospheric carbon dioxide [CO 2 ] on the protein concentrations of major food crops, incorporating 228 experimental observations on barley, rice, wheat, soybean and potato. Each crop had lower protein concentrations when grown at elevated (540-958 lmol mol À1 ) compared with ambient (315-400 lmol mol À1 ) CO 2 . For wheat, barley and rice, the reduction in grain protein concentration was $ 10-15% of the value at ambient CO 2 . For potato, the reduction in tuber protein concentration was 14%. For soybean, there was a much smaller, although statistically significant reduction of protein concentration of 1.4%. The magnitude of the CO 2 effect on wheat grains was smaller under high soil N conditions than under low soil N. Protein concentrations in potato tubers were reduced more for plants grown at high than at low concentrations of ozone. For soybean, the ozone effect was the reverse, as elevated CO 2 increased the protein concentration of soybean grown at high ozone concentrations. The magnitude of the CO 2 effect also varied depending on experimental methodology. For both wheat and soybean, studies performed in opentop chambers produced a larger CO 2 effect than those performed using other types of experimental facilities. There was also indication of a possible pot artifact as, for both wheat and soybean, studies performed in open-top chambers showed a significantly greater CO 2 effect when plants were rooted in pots rather than in the ground. Studies on wheat also showed a greater CO 2 effect when protein concentration was measured in whole grains rather than flour. While the magnitude of the effect of elevated CO 2 varied depending on the experimental procedures, a reduction in protein concentration was consistently found for most crops. These findings suggest that the increasing CO 2 concentrations of the 21st century are likely to decrease the protein concentration of many human plant foods.

A general understanding of biological invasions will provide insights into fundamental ecological and evolutionary problems and contribute to more efficient and effective prediction, prevention and control of invasions. We review recent papers that have proposed conceptual frameworks for invasion biology. These papers offer important advances and signal a maturation of the field, but a broad synthesis is still lacking. Conceptual frameworks for invasion do not require invocation of unique concepts, but rather should reflect the unifying principles of ecology and evolutionary biology. A conceptual framework should incorporate multicausality, include interactions between causal factors and account for lags between various stages. We emphasize the centrality of demography in invasions, and distinguish between explaining three of the most important characteristics by which we recognize invasions: rapid local population increase, monocultures or community dominance, and range expansion. As a contribution towards developing a conceptual synthesis of invasions based on these criteria, we outline a framework that explicitly incorporates consideration of the fundamental ecological and evolutionary processes involved. The development of a more inclusive and mechanistic conceptual framework for invasion should facilitate quantitative and testable evaluation of causal factors, and can potentially lead to a better understanding of the biology of invasions.

We present evidence that plant growth at elevated atmospheric CO 2 increases the high-temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO 2 (~360 mmol mol -1 ) and elevated CO 2 (550-1000 mmol mol -1 ) in three separate growth facilities, including the Nevada Desert Free-Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO 2 treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO 2 -grown plants maintained PSII efficiency (F v /F m ) to significantly higher temperatures than ambient-grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F v /F m in elevated versus ambient CO 2 -grown leaves following heat stress was due to both a higher F m and a lower F o , and that F v /F m differences between elevated and ambient CO 2 -grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO 2 -grown plants had a critical temperature for the rapid rise in F o that averaged 2·9 °C higher than leaves from ambient CO 2 -grown plants, and maintained a higher maximal rate of net CO 2 assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high-temperature damage and that rising atmospheric CO 2 content will drive temperature increases in many already stressful environments, this CO 2 -induced increase in plant high-temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.

I compared the C(4) grass flora and climatic records for 32 sites in the United States. Consistent with previous studies, I found that the proportion of the grass flora that uses the NADP malic enzyme (NADP-ME) variant of C(4) photosynthesis greatly increases with increasing annual precipitation, while the proportion using the NAD malic enzyme (NAD-ME) variant (and also the less common phosphoenolpyruvate carboxykinase [PCK] variant) decreases. However the association of grass subfamilies with annual precipitation was even stronger than for the C(4) decarboxylation variants. Analysis of the patterns of distribution by partial correlation analysis showed that the correlations between the frequency of various C(4) types and rainfall were solely due to the association of the C(4) types with particular grass subfamilies. In contrast, there was a strong correlation of the frequency of the different subfamilies with annual precipitation that was independent of the influence of the different C(4) variants. It therefore appears that other, as yet unidentified, characteristics that differ among grass subfamilies may be responsible for their differences in distribution across natural precipitation gradients.