grandidentata in the field at ambient and twice ambient CO2 in open productivity(1-4) This observation has led to the suggestion that, by bottom root boxes filled with organic matter poor native soil. Nitrogen taking up CO2, the terrestrial biosphere might mitigate the potential was added to all root boxes at a rate equivalent to net N mineralization greenhouse warming associated with anthropogenic CO2 emissions(5). in local dry oak forests. Nitrogen added during August was enriched Whiting and Chanton(6) have found, however, that for wetlands of with N-25 to trace the flux of N within the plant-soil system. Above- and varying productivity around the world, higher net primary production is belowground growth, CO2 assimilation, and leaf N content were associated with higher emissions of methane-another important measured non- destructively over 142 d. After final destructive harvest, greenhouse gas. Here we present measurements of methane emissions roots, stems, and leaves were analyzed for total N and N-15. There was from a marsh that has been exposed to twice the present ambient no CO2 treatment effect on leaf area, root length, or net assimilation concentration of atmospheric CO2. We find that over a one-week period, prior to the completion of N addition. Following the N addition, leaf N the CO2-enriched sites had significantly higher emissions of methane content increased in both CO2 treatments, but net assimilation showed than the control sites. Our results suggest that future increases in a sustained increase only in elevated CO2 grown plants. Root relative atmospheric CO2 concentration may lead to significant increases in extension rate was greater at elevated CO2, both before and after the N methane emissions from wetlands. addition. Although final root biomass was greater at elevated CO2, there was no CO2 effect on plant N uptake or allocation. While low soil N KEYWORDS: COMMUNITIES, ELEVATED CO2, ESTUARINE availability severely inhibited CO2 responses, high CO2 grown plants MARSH, FIELD, GROWTH, PLANTS, PRODUCTIVITY, RESPONSES, were more responsive to N. This differential behavior must be RICE PADDIES, WETLANDS considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests. 482 Dahlman, R.C. 1993. Co2 and plants - revisited. Vegetatio 104:339- KEYWORDS: CARBON DIOXIDE, ENRICHMENT, NITROGEN, 355. PHOTOSYNTHESIS, QUERCUS-ALBA, SEEDLING GROWTH, TREES The decade-long USA research program on the direct effects of CO2 enrichment on vegetation has achieved important milestones and has 480 produced a number of interesting and exciting findings. Research Cushman, J.C., and H.J. Bohnert. 1997. Molecular genetics of beginning in 1980 focused on field experiments to determine whether Crassulacean acid metabolism. Plant Physiology 113(3):667-676. phenomena observed in the laboratory indeed occurred in natural environments. The answer is yes. Data obtained from numerous field Most higher plants assimilate atmospheric CO2 through the C-3 studies show mixed response of crop and native species to CO2 pathway of photosynthesis using ributose-1,5-bisphosphate enrichment however. Nearly all experiments demonstrate that plants carboxylase/oxygenase (Rubisco). However, when CO2 availability is exhibit positive gain when grown at elevated CO2; although the reduced by environmental stress conditions, the incomplete magnitude varies greatly. Most crop responses range from 30 to 50 % discrimination of CO2 over O-2 by Rubisco leads to increased increase in yield. Results from long-term experiments with woody photorespiration, a process that reduces the efficiency of C-3 species and ecosystems are even more variable. Huge growth responses photosynthesis. To overcome the wasteful process of photorespiration, (100 to nearly 300 % increase relative to controls) are reported from approximately 10% of higher plant species have evolved two alternate several tree experiments and the salt-marsh ecosystem experiment. Other strategies for photosynthetic CO2 assimilation, C-3 photosynthesis and results from experiments with woody species and the tundra ecosystem Crassulacean acid metabolism. Both of these biochemical pathways suggest little no effect of CO2 on physiology, growth or productivity. employ a ''CO2 pump'' to elevate intracellular CO2 concentrations in the Numerous studies of the physiology of the CO2 effect are continuing in vicinity of Rubisco, suppressing photorespiration and therefore attempts to understand controlling mechanisms and to explain the improving the competitiveness of these plants under conditions of high variable growth responses. Particular emphasis needs to be given to light intensity, high temperature, or low water availability. This CO2 physiological measures of interactions involving the CO2 effect and pump consists of a primary carboxylating enzyme, phosphoenolpyruvate other environmental influences, and to the wide-ranging observations of carboxylase. In C-4 plants, this CO2-concentrating mechanism is photosynthesis acclimation to CO2. Prospects for future research are achieved by the coordination of two carboxylating reactions that are identified. spatially separated into mesophyll and bundle-sheath cell types (for review, see R.T. Furbank, W.C. Taylor [1995] Plant Cell 7:797-802; KEYWORDS: ACCLIMATION, ATMOSPHERIC CARBON-DIOXIDE, M.S.B. Ku, Y. Kano-Murakami, M. Matsuoka [1996] Plant Physiol ELEVATED LEVELS, ENRICHMENT, GROWTH, INHIBITION, 111:949-957). In contrast, Crassulacean acid metabolism plants perform PHOTOSYNTHESIS, SEEDLINGS, SHORT- TERM, TEMPERATURE both carboxylation reactions within one cell type, but the two reactions are separated in time. Both pathways involve cell- specific changes in the expression of many genes that are not present in C-3 plants. 483 Dale, H., and M.C. Press. 1998. Elevated atmospheric CO2 influences KEYWORDS: ABSCISIC- ACID, C-3 PHOTOSYNTHESIS, CAM, the interaction between the parasitic angiosperm Orobanche minor and COMMON ICE PLANT, DIFFERENTIAL EXPRESSION, INDUCTION, its host Trifolium repens. New Phytologist 140(1):65-73. MESEMBRYANTHEMUM-CRYSTALLINUM L, NADP-MALIC ENZYME, PHOSPHOENOLPYRUVATE CARBOXYLASE, SALT The influence of the root holoparasitic angiosperm Orobanche minor STRESS Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO2 concentrations (350 and 550 mu mol mol(-1)). Infected plants 481 accumulated less biomass than their uninfected counterparts, although Dacey, J.W.H., B.G. Drake, and M.J. Klug. 1994. Stimulation of early in the association there was a transient stimulation of growth. methane emission by carbon-dioxide enrichment of marsh vegetation. Infection also influenced biomass allocation both between tissues Nature 370(6484):47-49. (infected plants had lower root:shoot ratios) and within tissues: infected roots were considerably thicker before the point of parasite attachment THERE is substantial evidence that many plants respond to increased and thinner below. Higher concentrations of starch were also found in concentrations of atmospheric carbon dioxide by increasing their roots above the point of attachment, particularly for plants grown in elevated CO2. Elevated CO2 stimulated the growth of T. repens only difference in the degree of photoinhibition between leaves exposed to during the early stages of development. There was a significant high light in 10 or 210 mmol O-2/mol. But photoinhibition was more interaction between infection and CO2 on growth, with infected plants pronounced at 10 mmol O-2/mol and 0 mu mol CO2/mol than at 210 showing a greater response, such that elevated CO2 partly alleviated the mmol O-2/mol and 50 pmol CO2/mol, i.e. when both photorespiration effects of the parasite on host growth. Elevated CO2 did not affect total and CO2 refixation are suppressed. Despite this photoprotective role of O. minor biomass per host, the number of individual parasites supported photorespiration, photoinhibition was enhanced by decreasing CO2 by each host, or their time of attachment to the host root system. concentration in bean leaves, especially at elevated (30-35 degrees Photosynthesis was stimulated by elevated CO2 but was unaffected by C)temperatures. (C) 1997 Elsevier Science Ireland Ltd. O. minor. There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO2 in either infected or uninfected KEYWORDS: ASSIMILATION, CHLOROPHYLL FLUORESCENCE, plants. The data are discussed with regard to the influence of elevated CHLOROPLAST PROTEIN, DROUGHT, LIGHT, PHOTOCHEMICAL CO2 on other parasitic angiosperm-host associations and factors which EFFICIENCY, PHOTORESPIRATION, PHOTOSYNTHETIC control plant responses to elevated CO2. ELECTRON FLOW, PHOTOSYSTEM, WATER-STRESS KEYWORDS: CARBON DIOXIDE, GROWTH, METABOLISM, N2 FIXATION, NITROGEN, PHOTOSYNTHETIC ACCLIMATION, 487 SORGHUM, STRIGA-HERMONTHICA, TEMPERATURE, WHITE Darrah, P.R. 1996. Rhizodeposition under ambient and elevated CO2 CLOVER levels. Plant and Soil 187(2):265-275. As global CO2 levels rise, can soils store more carbon and so buffer 484 atmospheric CO2 levels? Answering this question requires a knowledge Dalen, L.S., O. Johnsen, and G. Ogner. 1997. Frost hardiness of the rates of C inputs to soil and of CO2 outputs via decomposition. development in young Picea abies seedlings under simulated autumn Below-ground inputs from roots are a major component of the C flow conditions in a phytotron - effects