The effect of long-term (weeks to months) CO2 enhancement on (a) the gas-exchange characteristics, (b) the content and activation state of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) leaf nitrogen, chlorophyll, and dry weight per area were studied in five C3 species (Chenopodium album, Phaseolus vulgaris, Solanum tuberosum, Solanum melongena, and Brassica oleracea) grown at CO2 partial pressures of 300 or 900 to 1000 microbars. Long-term exposure to elevated CO2 affected the CO2 response of photosynthesis in one of three ways: (a) the initial slope of the CO2 response was unaffected, but the photosynthetic rate at high CO2 increased (S. tuberosum); (b) the initial slope decreased but the C02-saturated rate of photosynthesis was little affected (C. album, P. vulgaris); (c) both the initial slope and the C02-saturated rate of photosynthesis decreased (B. oleracea, S. melongena). In all five species, growth at high CO2 increased the extent to which photosynthesis was stimulated following a decrease in the partial pressure of 02 or an increase in measurement CO2 above 600 microbars. This stimulation indicates that a limitation on photosynthesis by the capacity to regenerate orthophosphate was reduced or absent after acclimation to high CO2. Leaf nitrogen per area either increased (S. tuberosum, S. melongena) or was little changed by CO2 enhancement. The content of rubisco was lower in only two of the five species, yet its activation state was 19% to 48% lower in all five species following longterm exposure to high CO2. These results indicate that during growth in C02-enriched air, leaf rubisco content remains in excess of that required to support the observed photosynthetic rates.During short-term (minutes to hours) exposure to elevated partial pressure of C02, the rate of light-saturated CO2 assimilation (A2) in many C3 plants is primarily limited by the capacity to regenerate Pi from phosphorylated photosynthetic intermediates (10, 17,22,23,26). A Pi-regeneration limitation of photosynthesis is characterized by a lack of sensitivity ofA to changes in ambient partial pressure of CO2 and/ or 02 (10,

Arid ecosystems, which occupy about 20% of the earth's terrestrial surface area, have been predicted to be one of the most responsive ecosystem types to elevated atmospheric CO2 and associated global climate change. Here we show, using free-air CO2 enrichment (FACE) technology in an intact Mojave Desert ecosystem, that new shoot production of a dominant perennial shrub is doubled by a 50% increase in atmospheric CO2 concentration in a high rainfall year. However, elevated CO2 does not enhance production in a drought year. We also found that above-ground production and seed rain of an invasive annual grass increases more at elevated CO2 than in several species of native annuals. Consequently, elevated CO2 might enhance the long-term success and dominance of exotic annual grasses in the region. This shift in species composition in favour of exotic annual grasses, driven by global change, has the potential to accelerate the fire cycle, reduce biodiversity and alter ecosystem function in the deserts of western North America.

There have been many recent exciting advances in our understanding of the cellular processes that underlie photosynthetic acclimation to rising atmospheric CO 2 concentration. Of particular interest have been the molecular processes that modulate photosynthetic gene expression in response to elevated CO 2 and the biochemical processes that link changes in atmospheric CO 2 concentration to the production of a metabolic signal. Central to this acclimation response is a reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein content. Studies indicate that this reduction results from species-dependent variation in the differential use and temporal control of molecular processes. We present a model for the control of Rubisco protein accumulation that emphasizes the role of subunit message translation as well as the abundance of subunit messages as components of the acclimation response. Many studies indicate that photosynthetic acclimation to elevated CO 2 results from adjustments in leaf carbohydrate signalling. The repression of photosynthetic gene expression is considered to occur primarily by hexokinase functioning as a hexose flux sensor that ultimately affects transcription. Leaf hexoses may be produced as potential sources of signals primarily by sucrose cycling and secondarily by starch hydrolysis. An increased rate of sucrose cycling is suggested to occur at elevated CO 2 by enhanced provision of sucrose to leaf acid invertases. Additionally, sink limitations that accentuate photosynthetic acclimation may result from a relative decrease in the export of leaf sucrose and subsequent increase in cellular sucrose levels and sucrose cycling.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0-150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl(-) concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K(+) concentration, although to a lesser degree. Increases in leaf Na(+) concentration occurred only at the higher external NaCl concentrations (≧100 mM). Increases in leaf Cl(-) were primarily balanced by increases in K(+) and Na(+). X-ray microanalysis of leaf cells from salinized plants showed that Cl(-) concentration was high in both the cell vacuole and chloroplast-cytoplasm (250-300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO2 concentration (C i) was reduced by up to 30%. Salinization of plants was found to alter the δ(13)C value of leaves of Phaseolus by up to 5‰ and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO2 concentration. Salt stress also brought about a reduction in photosynthetic CO2 fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl(-) concentrations. The quantum yield for net CO2 uptake was also reduced by salt stress.

The advantages of Arabidopsis thaliana (L.) Heynh. for genetic studies are well known, but its diminutive stature and associated low biomass at maturity make it a challenging species for complementary physiological and biochemical studies. Hydroponic culture can significantly increase plant growth and produce uniform, stress-free root and shoot material that can be harvested throughout the life span of the plant. However, many shy away from the use of hydroponic culture because of the perceived difficulties in set-up and maintenance. Although other methods for the hydroponic culture of Arabidopsis have been reported (Rodecap et al., 1994; Delhaize and Randall, 1995; Hirai et al., 1995), they suffer from various shortcomings, including poor aeration, loss of root material, overcrowding, excess manipulation, and less-than-favorable environmental conditions. In this paper we describe an easy, low-maintenance method of hydroponic culture for Arabidopsis that combines the use of rockwool culture for uniform seedling establishment and a closed system of solution culture for the duration of plant growth. In addition, some consideration is given to temperature and light conditions that favor biomass production.The most difficult part of hydroponic culture for Arabidopsis is to rstablish a good root system, because young seedlings are prone to hypoxic stress from water logging. Rockwool (GrodanHP, Agro Dynamics, East Brunswick, NJ) provides an excellent, well-aerated rooting environment that is a far superior medium for reliable and uniform seedling establishment compared with other media we have tried, including cheesecloth, blue blotter paper, brown germination paper, filter paper, fiberglass matting, agar, and soil-or vermiculite-filled straws. Rockwool is a mixture of igneous rock and limestone that is heated and spun into mats. Even when saturated, rockwool holds about 15% air space.