Jeffrey R. Seemann

Elevated CO2 increases productivity and invasive species success in an arid ecosystem.

Arid ecosystems, which occupy about 20% of the earths 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 CO 2 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.

Maximal biomass of Arabidopsis thaliana using a simple, low-maintenance hydroponic method and favorable environmental conditions.

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.

Sucrose cycling, Rubisco expression, and prediction of photosynthetic acclimation to elevated atmospheric CO2

Photosynthetic acclimation to elevated CO2 cannot presently be predicted due to our limited understanding of the molecular mechanisms and metabolic signals that regulate photosynthetic gene expression. We have examined acclimation by comparing changes in the leaf content of RuBP carboxylase/oxygenase (Rubisco) with changes in the transcripts of Rubisco subunit genes and with leaf carbohydrate metabolism. When grown at 1000 mm3 dm–3 CO2, 12 of 16 crop species at peak vegetative growth had a 15–44% decrease in leaf Rubisco protein, but with no specific association with changes in transcript levels measured at midday. Species with only modest reductions in Rubisco content (10–20%) often had a large reduction in Rubisco small subunit gene mRNAs (> 30%), with no reduction in large subunit gene mRNAs. However, species with a very large reduction in Rubisco content generally had only small reductions in transcript mRNAs. Photosynthetic acclimation also was not specifically associated with a change in the level of any particular carbohydrate measured at midday. However, a threshold relationship was found between the reduction in Rubisco content at high CO2 and absolute levels of soluble acid invertase activity measured in plants grown at ambient or high CO2. This relationship was valid for 15 of the 16 species examined. There also occurred a similar, albeit less robust, threshold relationship between the leaf hexose/sucrose ratio at high CO2 and a reduced photosynthetic capacity ≥ 20%. These data indicate that carbohydrate repression of photosynthetic gene expression at elevated CO2 may involve leaf sucrose cycling through acid invertase and hexokinase.

Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure.

With increasing interest in the effects of elevated atmospheric CO2 on plant growth and the global carbon balance, there is a need for greater understanding of how plants respond to variations in atmospheric partial pressure of CO2. Our research shows that elevated CO2 produces significant fine structural changes in major cellular organelles that appear to be an important component of the metabolic responses of plants to this global change. Nine species (representing seven plant families) in several experimental facilities with different CO2-dosing technologies were examined. Growth in elevated CO2 increased numbers of mitochondria per unit cell area by 1.3–2.4 times the number in control plants grown in lower CO2 and produced a statistically significant increase in the amount of chloroplast stroma (nonappressed) thylakoid membranes compared with those in lower CO2 treatments. There was no observable change in size of the mitochondria. However, in contrast to the CO2 effect on mitochondrial number, elevated CO2 promoted a decrease in the rate of mass-based dark respiration. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO2 concentrations.

Effects of elevated atmospheric CO2 concentration on leaf dark respiration of Xanthium strumarium in light and in darkness

Leaf dark respiration (R) is an important component of plant carbon balance, but the effects of rising atmospheric CO2 on leaf R during illumination are largely unknown. We studied the effects of elevated CO2 on leaf R in light (RL) and in darkness (RD) in Xanthium strumarium at different developmental stages. Leaf RL was estimated by using the Kok method, whereas leaf RD was measured as the rate of CO2 efflux at zero light. Leaf RL and RD were significantly higher at elevated than at ambient CO2 throughout the growing period. Elevated CO2 increased the ratio of leaf RL to net photosynthesis at saturated light (Amax) when plants were young and also after flowering, but the ratio of leaf RD to Amax was unaffected by CO2 levels. Leaf RN was significantly higher at the beginning but significantly lower at the end of the growing period in elevated CO2-grown plants. The ratio of leaf RL to RD was used to estimate the effect of light on leaf R during the day. We found that light inhibited leaf R at both CO2 concentrations but to a lesser degree for elevated (17–24%) than for ambient (29–35%) CO2-grown plants, presumably because elevated CO2-grown plants had a higher demand for energy and carbon skeletons than ambient CO2-grown plants in light. Our results suggest that using the CO2 efflux rate, determined by shading leaves during the day, as a measure for leaf R is likely to underestimate carbon loss from elevated CO2-grown plants.

ELEVATED ATMOSPHERIC CO2 DOES NOT CONSERVE SOIL WATER IN THE MOJAVE DESERT

Numerous studies, including those of desert plants, have shown reduced stomatal conductance under elevated atmospheric CO2. As a consequence, soil water has been postulated to increase. Soil water was measured for .4 yr at the Nevada Desert Free Air CO2 Enrichment (FACE) Facility to determine if elevated atmospheric CO2 conserves soil water for a desert scrub community in the Mojave Desert. We measured soil water in the top 0.2 and 0.5 m of soil with time domain reflectometry and to 1.85 m with a neutron probe for the three treatments at Desert FACE: elevated CO2 (550 mmol/mol), blower control (ambient CO2), and non-ring treatments. The treatment main effect was not significant in any analyses of variance. Although the treatment 3 date interaction was significant for soil water in the top 0.5 m of soil, the expected greater soil water for elevated CO 2 vs. ambient CO2 only occurred on one sampling date. In contrast, soil water for that same depth was significantly lower under elevated CO 2 on six dates. Thus, we infer that increased water use from increased primary productivity (and therefore leaf area) under elevated CO 2 offset the decreased water use from reduced stomatal conductance, and hence soil water was not conserved under elevated CO2 in the Mojave Desert, unlike other ecosystems.

Metabolism of 2-carboxyarabinitol 1-phosphate and regulation of ribulose-1,5-bisphosphate carboxylase activity

Metabolism of 2′-carboxy-D-arabinitol 1-phosphate (CA1P) is an important component in the light-dependent regulation of ribulose-1,5-bisphosphate carboxylase (Rubisco) activity and whole leaf photosynthetic CO2 assimilation in many species, and functions as one mechanism for regulating Rubisco activity when photosynthesis is light-limited. Species differ in their capacity to accumulate CA1P, ranging from those which can synthesize levels of this compound approaching or in excess of the Rubisco catalytic site concentration, to those which apparently lack the capacity for CA1P synthesis. CA1P is structurally related to the six carbon transition state intermediate of the carboxylation reaction and binds tightly to the carbamylated catalytic site of Rubisco, making that site unavailable for catalysis. Under steady-state, the concentration of CA1P in the leaf is highest at low photon flux density (PFD) or in the dark. Degradation of CA1P and recovery of Rubisco activity requires light and is stimulated by increasing PFD. The initial degradation reaction is catalyzed by an enzyme located in the chloroplast stroma, CA1P phosphatase, which yields carboxyarabinitol (CA) and inorganic phosphate as its products. The pathway of CA metabolism in the plant remains to be determined. Synthesis of CA1P occurs in the dark, and in Phaseolus vulgaris this process has been shown to be stimulated by low PFD. The pathway of CA1P synthesis and its relationship to the degradative pathway remains unknown at the present time. The discovery of the existence of this previously unknown carbon pathway in photosynthesis indicates that we still have much to learn concerning the regulation of Rubisco activity and photosynthesis.

Long-term kinetics of the light-dependent regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in plants with and without 2-carboxyarabinitol 1-phosphate

The light-dependent modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was studied in two species: Phaseolus vulgaris L., which has high levels of the inhibitor of Rubisco activity, carboxyarabinitol 1-phosphate (CA1P), in the dark, and Chenopodium album L., which has little CA1P. In both species, the ratio of initial to fully-activated Rubisco activity declined by 40–50% within 60 min of a reduction in light from high a photosynthetic photon flux density (PPFD; >700 μmol · m−2 · s−1) to a low PPFD (65 ± 15 μmol · m−2 · s−1) or to darkness, indicating that decarbamylation of Rubisco is substantially involved in the initial regulatory response of Rubisco to a reduction in PPFD, even in species with potentially extensive CA1P inhibition. Total Rubisco activity was unaffected by PPFD in C. album, and prolonged exposure (2–6 h) to low light or darkness was accompanied by a slow decline in the activity ratio of this species. This indicates that the carbamylation state of Rubisco from C. album gradually declines for hours after the large initial drop in the first 60 min following light reduction. In P. vulgaris, the total activity of Rubisco declined by 10–30% within 1 h after a reduction in PPFD to below 100 μmol · m−2 · s−1, indicating CA1P-binding contributes significantly to the reduction of Rubisco capacity during this period, but to a lesser extent than decarbamylation. With continued exposure of P. vulgaris leaves to very low PPFDs (< 30 μmol · m−2 · s−1), the total activity of Rubisco declined steadily so that after 6–6.5 h of exposure to very low light or darkness, it was only 10–20% of the high-light value. These results indicate that while decarbamylation is more prominent in the initial regulatory response of Rubisco to a reduction in PPFD in P. vulgaris, binding of CA1P increases over time and after a few hours dominates the regulation of Rubisco activity in darkness and at very low PPFDs.

Growth, cell walls, and UDP-Glc dehydrogenase activity of Arabidopsis thaliana grown in elevated carbon dioxide

Summary The impact of elevated CO 2 (1000 μmol/mol) was assessed on the common weed, Arabidopsis thaliana (Landsberg erecta), which is used as a model plant system. Elevated CO 2 stimulated relative growth rate (RGR) and leaf area gain of Arabidopsis beginning from the cotyledon stage and continuing through the juvenile stage. This early advantage in growth enabled the plants grown in elevated CO 2 to gain more DW despite similar RGRs throughout the latter stages of development. The greater accumulation of DW in leaves grown in elevated CO 2 resulted in a lower specific leaf area (SLA). However, the amount of cell wall investment per unit of leaf area, specific “wall” area (SWA), was similar indicating that elevated CO 2 did not affect the distribution of cell carbon to the cell wall of leaves beyond that needed for cell and leaf expansion. Furthermore, cell wall composition changed with time due to developmental changes and was not affected by elevated CO 2 . Associated with the increase in RGR by elevated CO 2 was a concomitant increase in the activity of UDP-Glc dehydrogenase (E.C. 1.1.1.22), a key enzyme in the nucleotide-sugar interconversion pathway necessary for biosynthesis of many cell-wall polysaccharides.

Intracellular localization of CA1P and CA1P phosphatase activity in leaves of Phaseolus vulgaris L.

CA1P and CA1P phosphatase occur in the chloroplasts of leaf mesophyll cells of many species. However, whether either may occur exclusively in the chloroplast has not yet been established. To examine their intracellular distribution, mature, dark-or light-treated leaves of Phaseolus vulgaris were frozen, lyophilized and then centrifuged in density gradients of heptane and tetrachloroethylene. After gradient fractionation, both CA1P and CA1P phosphatase activity co-segregated with chloroplast material. Distribution analyses using sub-cellular compartment markers indicated that both CA1P and CA1P phosphatase do occur exclusively in leaf chloroplasts.