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Nia30(145) transformants with very low nitrate reductase activity provide an in vivo screen to identify processes that are regulated by nitrate. Nia30(145) resembles nitrate-limited wild-type plants with respect to growth rate and protein and amino acid content but accumulates large amounts of nitrate when it is grown on high nitrate. The transcripts for nitrate reductase (NR), nitrite reductase, cytosolic glutamine synthetase, and glutamate synthase increased; NR and nitrite reductase activity increased in leaves and roots; and glutamine synthetase activity increased in roots. The transcripts for phosphoenolpyruvate carboxylase, cytosolic pyruvate kinase, citrate synthase, and NADP-isocitrate dehydrogenase increased; phosphoenolpyruvate carboxylase activity increased; and malate, citrate, isocitrate, and alpha-oxoglutarate accumulated in leaves and roots. There was a decrease of the ADP-glucose pyrophosphorylase transcript and activity, and starch decreased in the leaves and roots. After adding 12 mM nitrate to nitrate-limited Nia30(145), the transcripts for NR and phosphoenolpyruvate carboxylase increased, and the transcripts for ADP-glucose pyrophosphorylase decreased within 2 and 4 hr, respectively. Starch was remobilized at almost the same rate as in wildtype plants, even though growth was not stimulated in Nia30(145). It is proposed that nitrate acts as a signal to initiate coordinated changes in carbon and nitrogen metabolism.

Maize (Zea mays L.) plants were grown to the nine-leaf stage. Despite a saturating N supply, the youngest mature leaves (seventh position on the stem) contained little NO3- reserve. Droughted plants (deprived of nutrient solution) showed changes in foliar enzyme activities, mRNA accumulation, photosynthesis, and carbohydrate and amino acid contents. Total leaf water potential and CO2 assimilation rates, measured 3 h into the photoperiod, decreased 3 d after the onset of drought. Starch, glucose, fructose, and amino acids, but not sucrose (Suc), accumulated in the leaves of droughted plants. Maximal extractable phosphoenolpyruvate carboxylase activities increased slightly during water deficit, whereas the sensitivity of this enzyme to the inhibitor malate decreased. Maximal extractable Suc phosphate synthase activities decreased as a result of water stress, and there was an increase in the sensitivity to the inhibitor orthophosphate. A correlation between maximal extractable foliar nitrate reductase (NR) activity and the rate of CO2 assimilation was observed. The NR activation state and maximal extractable NR activity declined rapidly in response to drought. Photosynthesis and NR activity recovered rapidly when nutrient solution was restored at this point. The decrease in maximal extractable NR activity was accompanied by a decrease in NR transcripts, whereas Suc phosphate synthase and phosphoenolpyruvate carboxylase mRNAs were much less affected. The coordination of N and C metabolism is retained during drought conditions via modulation of the activities of Suc phosphate synthase and NR commensurate with the prevailing rate of photosynthesis

Fructan is a general term used for any carbohydrate in which one or more fractosyl-fructose link constitutes the majority of osidic bonds. This review focuses on the fate of inulin-type fructans (namely native chicory inulin, oligofructose produced by the partial enzymatic hydrolysis of chicory inulin, and synthetic fructans produced by enzymatic synthesis from sucrose) in the gastrointestinal tract, as well as on their systemic physiological effects on mineral absorption, carbohydrate and lipid metabolism, hormone balance, and nitrogen homeostasis. The scientific evidence for the functional claims of inulin-type fructans is discussed, as well as their potential application in risk reduction of disease, namely constipation, infectious diarrhea, cancer, osteoporosis, atherosclerotic cardiovascular disease, obesity, and non-insulin dependent diabetes

While ecologists involved in management or policy often are advised to learn to deal with uncertainty, there are a number of components of global environmental change of which we are certain–certain that they are going on, and certain that they are human—caused. Some of these are largely ecological changes, and all have important ecological consequences. Three of the well—documented global changes are: increasing concentrations of carbon dioxide in the atmosphere; alterations in the biogeochemistry of the global nitrogen cycle; and ongoing land use/land cover change. Human activity–now primarily fossil fuel combustion– has increased carbon dioxide concentrations from °280 to 355 mL/L since 1800; the increase is unique, at least in the past 160 000 yr, and several lines of evidence demonstrate unequivocally that it is human—caused. This increase is likely to have climatic consequences–and certainly it has direct effects on biota in all Earth's terrestrial ecosystems. The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily for nitrogen fertilizer, also by legume crops and as a by product of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophiction of the biosphere, and has substantial regional effects on biological diversity in the most affected areas. Finally, human land use/land cover change has transformed one—their to one—half of Earth's ice—free surface. This in and of itself probably represents the most important component of global change now and will for some decades to come; it has profound effects on biological diversity on land and on ecosystems downwind and downstream of affected areas. Overall, any clear dichotomy between pristine ecosystems and human—altered areas that may have existed in the past has vanished, and ecological research should account for this reality. These three and other equally certain components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused in turn by the extraordinary growth in size and resource use of the human population. On a broad scale, there is little uncertainty about any of these components of change or their causes. However, much of the public believes the causes–even the existence–of global change to be uncertain and contentious topics. By speaking out effectively, we can help to shift the focus of public discussion towards what can and should be done about global environmental change.

In oceanic, coastal, and estuarine environments, an average of 25 to 41 percent of the dissolved inorganic nitrogen (NH4+ and NO3-) taken up by phytoplankton is released as dissolved organic nitrogen (DON). Release rates for DON in oceanic systems range from 4 to 26 nanogram-atoms of nitrogen per liter per hour. Failure to account for the production of DON during nitrogen-15 uptake experiments results in an underestimate of gross nitrogen uptake rates and thus an underestimate of new and regenerated production. In these studies, traditional nitrogen-15 techniques were found to underestimate new and regenerated production by up to 74 and 50 percent, respectively. Total DON turnover times, estimated from DON release resulting from both NH4+ and NO3(-) uptake, were 10 +/- 1, 18 +/- 14, and 4 days for oceanic, coastal, and estuarine sites, respectively

In eastern Canada, the use of fertilizer N has been identified as the most energy-consuming component of maize (Zea mays L.) grain production. As the economic and environmental costs of excessive N fertilization rise, there is an increased emphasis on selection of hybrids with greater N use efficiency (NUE; defined as the ratio of the amount of ¹⁵N recovered in grain or stover dry matter to the amount of fertilizer ¹⁵N applied to the soil in this study). Using an ¹⁵N-labelling approach, a field study was conducted on a tile-drained Brandon loam soil (Typic Endoaquoll) on the Central Experimental Farm at Ottawa, Canada (45°22' N, 75°43 /W) in 1993 and 1994. Fertilizer N uptake and partitioning within the plant in relation to dry matter changes were monitored during development of a current stay-green maize hybrid and an older early-senescing hybrid grown with three fertilizer N levels (0, 100, 200 kg N ha⁻¹). Dry matter, N concentration and ¹⁵N atom% enrichment of plant components were determined at five growth stages. The current stay-green hybrid, 'Pioneer 3902' had greater NUE than the old early-senescing hybrid, 'Pride 5', which was associated with 24% more dry matter production and 20% more N uptake during grain fill for Pioneer 3902. There was no indication of greater allocation of N to the grain in Pioneer 3902. Our data suggest that prolonged maintenance of green leaf area for photosynthate production during grain fill and the ability to take up available soil N later in grain filling are characteristics of maize hybrids with greater NUE.

Annual C inputs from plant production in terrestrial ecosystems only meet the maintenance energy requirements of soil microorganisms, allowing for little or no net annual increase in their biomass. Because microbial growth within soil is limited by C availability, we reasoned that plant production should, in part, control the biomass of soil microorganisms. We also reasoned that soil texture should further modify the influence of plant production on soil C availability because fine-textured soils typically support more microbial biomass than coarse-textured soils. To test these ideas, we quantified the relationship between aboveground net primary production (ANPP) and soil microbial biomass in late-successional ecosystems distributed along a continent-wide gradient in North America. We also measured labile pools of C and N within the soil because they represent potential substrate for microbial activity. Ecosystems ranged from a Douglas-fir forest in the western United States to the grasslands of the mid-continent to the hardwood forest in the eastern U.S. Estimates of ANPP obtained from the literature ranged from 82 to 1460 g@?m^-^2@?yr^-^1. Microbial biomass C and N were estimated by the fumigation-incubation technique. Labile soil pools of C and N and first-order rate constants for microbial respiration and net N mineralization were estimated using a long-term (32 wk) laboratory incubation. Regression analyses were used to relate ANPP and soil texture with microbial biomass and labile soil C and N pools. Microbial biomass carbon ranged from 2 g/m^2 in the desert grassland to 134 g/m^2 in the tallgrass prairie; microbial N displayed a similar trend among ecosystems. Labile C pools, derived from a first-order rate equation, ranged from 115 g/m^2 in the desert grassland to 491 g/m^2 in the southern hardwood forest. First-order rate constants for microbial respiration (k) fell within a narrow range of values (0.180 to 0.357 wk^-^1), suggesting that labile C pools were chemically similar among this diverse set of ecosystems. Potential net N mineralization rates over the 32-wk incubation were linear in most ecosystems with first-order responses only in the alpine tundra, tallgrass prairie, and forests. Microbial biomass C displayed a positive, linear relationship with ANPP (r^2 = 0.51), but was not significantly related to soil texture. Labile C also was linearly related to ANPP (r^2 = 0.32) and to soil texture (r^2 = 0.33). Results indicate that microbial biomass and labile organic matter pools change predictably across broad gradients of ANPP, supporting the idea that microbial growth in soil is constrained by C availability.

Ferredoxin-dependent glutamate synthase (Fd-GOGAT) plays a major role in photorespiration in Arabidopsis, as has been determined by the characterization of mutants deficient in Fd-GOGAT enzyme activity (gls). Despite genetic evidence for a single Fd-GOGAT locus and gene, we discovered that Arabidopsis contains two expressed genes for Fd-GOGAT (GLU1 and GLU2). Physical and genetic mapping of the gls1 locus and GLU genes indicates that GLU1 is linked to the gls1 locus, whereas GLU2 maps to a different chromosome. Contrasting patterns of GLU1 and GLU2 expression explain why a mutation in only one of the two genes for Fd-GOGAT leads to a photorespiratory phenotype in the gls1 mutants. GLU1 mRNA was expressed at the highest levels in leaves, and its mRNA levels were specifically induced by light or sucrose. In contrast, GLU2 mRNA was expressed at lower constitutive levels in leaves and preferentially accumulated in roots. Although these results suggest a major role for GLU1 in photorespiration, the sucrose induction of GLU1 mRNA in leaves also suggests a role in primary nitrogen assimilation. This possibility is supported by the finding that chlorophyll levels of a gls mutant are significantly lower than those of the wild type when grown under conditions that suppress photorespiration. Both the mutant analysis and gene regulation studies suggest that GLU1 plays a major role in photorespiration and also plays a role in primary nitrogen assimilation in leaves, whereas the GLU2 gene may play a major role in primary nitrogen assimilation in roots