CO2-enriched Conditions Research Articles

Responses of strawberry plantlets cultured in vitro under superbright red and blue light-emitting diodes (LEDs)

Unrooted strawberry cv. `Akihime' shoots with three leaves obtained from standard mixotrophic cultures were cultured in the ``Culture Pack''-rockwool system with sugar-free MS medium under CO2-enriched condition. To examine the effect of superbright red and blue light-emitting diodes (LEDs) on in vitro growth of plantlets, these cultures were placed in an incubator, ``LED PACK'', with either red LEDs, red LEDs1blue LEDs or blue LEDs light source. To clarify the optimum blue and red LED ratio, cultures were placed in ``LED PACK 3'' under LED light source with either 100, 90, 80, or 70% red + 0, 10, 20, 30% blue, respectively, and also under standard heterotrophic conditions. To determine the effects of irradiation level, cultures were grown under 90% red LEDs + 10% blue LEDs at 45, 60 or 75 μmol m−2 s−1 . Plantlet growth was best at 70% red + 30% blue LEDs. The optimal light intensity was 60 μmol m−2 s−1. Growth after transfer to soil was also best after in vitro culture with plantlets produced were 70% red LEDs + 30% blue LEDs.

Elevated carbon supply altered hypericin and hyperforin contents of St. John's wort (Hypericum perforatum L. cv `New Stem') grown in bioreactors

St. John's wort (Hypericum perforatum L.) is a traditional medicinal plant mainly used for the treatment of neurological disorders and depression. Recent investigation revealed that St John's wort can be grown efficiently in large-scale sterile bioreactors. The current investigation was carried out to assess the potential for increasing the concentration of medicinally important biomolecules (hypericin, pseudohypericin and hyperforin) by exposure to elevated levels of CO2 and/or sucrose in a balloon type bubble bioreactor system (BB-bioreactor). The biomass (fresh and dry mass) decreased with increasing sucrose concentration in the culture medium in both CO2 enriched (1500–1800 μmol mol−1) and non-enriched conditions. The results of chemical analyses indicated that the supply of excess carbon either as sucrose or CO2 did not increase the hypericin and pseudohypericin content of the tissues. However, hyperforin content was increased in plantlets grown at 45 g l−1 sucrose under CO2 non-enriched conditions. These findings provide evidence that the synthesis of the bioactive molecule in St. John's wort is likely dependent on several factors in addition to available nutrition and atmospheric conditions and indicate an alternate mechanism for regulation of the synthesis of hypericin/pseudohypericin and hyperforin.

In Situ Estimation of Carbon Balance of In Vitro Sweetpotato and Tomato Plantlets Cultured with Varying Initial Sucrose Concentrations in the Medium

The effects of initial sucrose (suc) concentrations in the medium (S0) on the carbon balance and growth of sweetpotato [Ipomoea batatas (L.) Lam. `Beniazuma'] and tomato (Lycopersicon esculentum Mill. `HanaQueen') plantlets were studied under controlled environmental conditions. Plantlets were cultured with 0, 7.5, 15, or 30 g·L-1 of S0 under high photosynthetic photon flux (160 to 200 μmol·m-2·s-1) and CO2 enriched (1400 to 2050 μmol·mol-1) conditions. Net photosynthetic rate per leaf area (Pl) decreased and dry weight per plantlet (Wd) increased with increasing S0, but did not differ significantly between S0 of 7.5 to 30 g·L-1 for sweetpotato or 15 to 30 g·L-1 for tomato. Carbon influxes and effluxes of the plantlets by metabolism of medium suc and/or photosynthesis, and respiration were estimated based on measurements of in situ and steady state CO2 exchange rates and sugar uptake during culture. At S0 from 7.5 to 30 g·L-1, photosynthesis was responsible for 82% to 92% and 60% to 67% of carbohydrate assimilation for sweetpotato and tomato, respectively. Estimated carbon balances of plantlets based on the estimated and actual increases of moles of carbon in plant tissue demonstrated that in situ estimation of carbon balance was reasonably accurate for sweetpotato at S0 of 0 to 15 g·L-1 and for tomato at S0 of 0 g·L-1 and that the actual contribution of photosynthesis for tomato at high S0 might be lower than the values estimated in the present experiment. Results showed that initial suc concentration affected the relative contribution of photosynthesis on their carbon balances and that the responses were species specific. The failure of validation at S0 in a range specific to each species suggested the need for further study on carbon metabolism of in vitro plantlets cultured with sugar in the medium.

Fructokinase and hexokinase from pollen grains of bell pepper (Capsicum annuum L.): possible role in pollen germination under conditions of high temperature and CO2 enrichment.

The processes of pollen grain development and germination depend on the uptake and metabolism of pollen sugars. In pepper (Capsicum annuum L.), initial sugar metabolism includes sucrose hydrolysis by invertase and subsequent phosphorylation of glucose and fructose by hexose kinases. The main objective of this study was to investigate changes in fructokinase (EC 2.7.1.4) and hexokinase (EC.2.7.1.1) activities in pepper flowers during their development, and to study the possible roles of these enzymes in determining pollen germination capacity under high temperature and under CO(2) enrichment, previously shown to modify sugar concentrations in pepper pollen (Aloni et al., 2001 Physiologia Plantarum 112: 505-512). Fructokinase (FK) activity was predominant in pepper pollen, and increased during pollen maturation. Pollen hexokinase (HK) activity was low and did not change throughout pollen development. High-temperature treatment (day/night, 32/26 degrees C) of pepper plants reduced the percentage of pollen that germinated compared with that under normal temperatures (26/22 degrees C), and concomitantly reduced the activity of FK in mature pollen. High temperature also reduced FK and HK activity in the anther. Under high ambient CO(2) (800 micro l l(-1)) pollen FK activity was enhanced. The results suggest that pollen and anther FK may play a role in the regulation of pollen germination, possibly by providing fructose-6-phosphate for glycolysis, or through conversion to UDP-glucose (UDPG) to support the biosynthesis of cell wall material for pollen tube growth. High temperature stress and CO(2) enrichment may influence pollen germination capacity by affecting these pathways.

Elevated CO2 concentrations increase leaf nitrate reduction by strengthening sink activity in soybean plants

An experiment was conducted to examine the effect of CO2 enrichment on the nitrate uptake, nitrate reduction activity, and translocation of assimilated-N from leaves at varying levels of nitrogen nutrition in soybean using 15N tracer technique. CO2 enrichment significantly increased the plant biomass, apparent leaf photosynthesis, sugar and starch contents of leaves, and reduced-N contents of the plant organs only when the plants were grown at high levels of nitrogen. A high supply of nitrogen enhanced plant growth and increased the reduced-N content of the plant organs, but its effect on the carbohydrate contents and photosynthetic rate were not significant. However, the combination of high CO2 and high nitrogen levels led to an additive effect on all these parameters. The nitrate reductase activity increased temporarily for a short period of time by CO2 enrichment and high nitrogen levels. 15N tracer studies indicated that the increase in the amount of reduced-N by CO2 enrichment was derived from nitrate-N and not from fixed-N of the plant. To examine the translocation of reduced-N from the leaf in more detail, another experiment was conducted by feeding the plants with 15NO3-N through a terminal leaflet of an upper trifoliated leaf under depodding and/or CO2 enrichment conditions. The export rate of 15N from the terminal leaflet to other plant parts decreased by depodding, but it increased by CO2 enrichment. CO2 enrichment increased the percentage of plant 15N in the stem and / or pods. Depodding increased the percentage of plant 15N in the leaf and stem. The results suggested that the increase in the leaf nitrate reduction activity by CO2 enrichment was due to the increase of the translocation of reduced-N from leaves through the strengthening of the sink activity of pods and / or stem for reduced-N.

CO2-Enrichment and photosynthetic photon flux affect the growth of in vitro-cultured apple plantlets

Micro-cuttings (shoots with two small leaves) of cultivar M9 apple were cultured in-vitro for 40 d under CO2-enriched and non-enriched (i.e., ambient air) conditions, and at a PPF of 40 or 100 μmol m-2 s-1 Afterward, shoot length, number of leaves, leaf area, chlorophyll content, shoot and root fresh weights, and % survival were recorded. Those plant-lets grown under CO2- and PPF-enriched treatments were healthy and vigorous, and showed higher values for their growth parameters. In contrast, those grown without supplemental CO2 or PPF often showed hyperhydricity. We also demonstrated that CO2 enrichment and a relatively high PPF during in-vitro culture promoted normal photosynthesis and growth after ex-vitro transplantation.

Effect of free‐air CO2 enrichment (FACE) on CO2 exchange at the flood‐water surface in a rice paddy field

Summary Diurnal and seasonal changes in CO2 exchange between flooding water and the atmosphere are reported in a rice (Oryza sativa) paddy field under ambient and free‐air CO2 enrichment (FACE) conditions. Measurements of CO2 exchange were made in a randomized complete‐block design experiment with two CO2 mole fractions (ambient; FACE, ambient +200 mmol mol−1) and four blocks. Evolution of CO2 from the flooding water occurred throughout the entire day under ambient CO2, although the rate decreased in the daytime. By contrast, under FACE, CO2 evolution from the flooding water was observed at night, and CO2 absorption from the atmosphere was observed in the daytime. The time course of CO2 exchange was similar to that of solar radiation. The diurnal pattern is similar to that for photosynthesis of terrestrial plant communities. This suggests that photosynthesis of the free‐floating weeds and algae in the flooding water is related to the CO2 exchange between the flooding water and the atmosphere. In addition, the amplitude of the diurnal cycle of CO2 exchange was larger under the FACE than under ambient conditions.

A Forced Ventilation Micropropagation System for Photoautotrophic Production of Sweetpotato Plug Plantlets in a Scaled-up Culture Vessel: II. Carbohydrate Status

Sweetpotato [Ipomoea batatas (L.) Lam., `Beniazuma'] plantlets were grown photoautotrophically (without sugar) for 12 days in an improved forced ventilation system designed with air distribution pipes for uniform spatial distributions of carbon dioxide (CO2) concentration. Enriched CO2 conditions and photosynthetic photon flux (PPF) were provided at 1500 μmol·mol-1 and 150 μmol·m-2·s-1, respectively. The forced (F) ventilation treatments were designated high (FH), medium (FM), and low (FL), corresponding to ventilation rates of 23 mL·s-1 (1.40 inch3/s), 17 mL·s-1 (1.04 inch3/s), and 10 mL·s-1 (0.61 inch3/s), respectively, on day 12. The natural (N) ventilation treatment was extremely low (NE) at 0.4 mL·s-1 (0.02 inch3/s), relative to the forced ventilation treatments. Total soluble sugar (TSS) and starch content were determined on day 12. Total soluble sugars (sucrose, glucose, fructose) of FH plantlets were lowest in leaf tissue and highest in stem tissue as compared to other ventilation treatments. Starch concentration was higher in leaf tissue of FH or FM plantlets as compared to that of FL or NE plantlets. Plantlets subjected to FH or FM treatments exhibited significantly higher net photosynthetic rates (NPR) than those of the other treatments; and on day 12, NPR was almost five times higher in the FH or FM treatment than the FL or NE treatments. Carbohydrate concentration of plantlets was also influenced by the position of the plantlets in the vessel. Within the forced ventilation vessels, leaf TSS of FH and FM plantlets was similar regardless of whether plantlets were located near the inlet or outlet of CO2 enriched air. However, under FH or FM conditions, leaf starch concentration was higher in plantlets located closest to the CO2 inlet as compared to the outlet.

Photosynthesis, plant growth and N allocation in transgenic rice plants with decreased Rubisco under CO2 enrichment.

Ribulose-1,5-bisphosphate carboxylase (Rubisco) efficiency for CO2-saturated photosynthesis was examined in leaves of rice (Oryza sativa L.). The amount of Rubisco in a leaf was calculated to be 30-55% in excess for the light-saturated rate of photosynthesis at 100 Pa CO2. Long-term exposure to CO2 enrichment decreased the amount of Rubisco protein. However, N was not reallocated from decreased Rubisco to other components limiting photosynthesis, and the decrease in Rubisco was simply due to a decrease in total leaf-N content by CO2 enrichment. Thus, rice plants did not optimize N allocation into Rubisco at elevated CO2. Transgenic rice plants with decreased Rubisco were obtained by transformation with the rbcS antisense gene. The transformant with 65% wild-type Rubisco was selected as a plant with optimal Rubisco content for CO2-saturated photosynthesis at the level of a single leaf. This selected transgenic plant had 20% lower rates of photosynthesis at normal CO2 (36 Pa), but 5-15% higher rates of photosynthesis at elevated CO2 (100 Pa) for a given leaf N content. However, such transgenic plants did not necessarily show greater production of biomass even under conditions of CO2 enrichment. Although they had a higher N-use efficiency for plant growth under such conditions during the middle stage of growth, the growth rate was lower during the early stage of growth. Thus, improvement of N-use efficiency by a single leaf did not necessarily lead to greater production of biomass by the whole plant.

Effects of NaCl salinity and CO 2 enrichment on pepino ( Solanum muricatum Ait.) : I. Growth and yield

One-month old, rooted, semi-hardwood cutting plants of pepino cv. Xotus in sand-potted culture were treated with 200 ml Hoagland nutrient solution with or without additional 25 mM NaCl twice a week for 2 months, and exposed to 350 ± 10, 700 ± 10 or 1050 ± 10 ppm CO 2 in controlled environment chambers during the last month of the experiment. NaCl salinity in the rhizosphere reduced growth of all the organs, but raised stem dry weight ratio and root dry weight ratio. In contrast, atmospheric CO 2 enrichment increased plant and fruit growth. Leaf dry weight ratio and fruit dry weight ratio rose, but stem dry weight ratio and root dry weight ratio decreased at high CO 2 levels. Daily expansion rate of leaf area, growth rate of side-shoot length, rate of plant dry mass production, and increased rate of fresh fruit weight decreased due to NaCl stress, but increased with CO 2 enrichment. Side-shoot diameter rose, whereas specific leaf area, leaf area ratio, and side-shoot dry weight ratio declined under both NaCl-stressed and CO 2-enriched conditions. In comparison with the 350 ppm CO 2 treatment without NaCl salinity in the rhizosphere, net assimilation rate and relative growth rate of plants were reduced by 8–13% and 16–32% due to NaCl salinity, and enhanced by 22–23% and 42–64% at 700 ppm CO 2, and by 36–44% and 64–101% at 1050 pm CO 2, respectively. The simultaneous treatments of NaCl salinity and high CO 2 resulted in indefinite effects on vegetative and reproductive growth as well as on dry mass production of different plant organs. Nevertheless, the negative impacts of NaCl stress on plant growth and fruit yield diminished at high CO 2 levels. Atmospheric CO 2 enrichment increased the tolerance of pepino to NaCl salinity in the root medium.

ピートバッグ栽培におけるイチゴの収量，果実品質と養水分吸収に及ぼすＣＯ２施用と栽植密度の影響

The effects of CO2 enrichment (700-900 ppm in the day time) and planting density on yield, fruit quality and water and mineral uptake by plants were investigated with strawberry (Fragaria × ananassa Duch. cv. Nyoho) grown in peat bag culture (width : 30 cm, length : 80 cm, media : 18 L) . Under CO2 enriched condition, total amount of uptake in each mineral increased with vigorous plant growth and increase in yield. Therefore, concentration of minerals and EC in drainage were quite low and only 3-10, 10 and 7-12% of supplied N, P and K were wasted with drainage, respectively. Total yield per plant increased by 50 to 70% with CO2 enrichment. Yield per peat bag (equal to area basis) increased with increasing planting density under CO2 enriched condition, however, that in control (200-400 ppm in the day time) increased little with increase in planting density. Average fruit weight and fruit total soluble solid content decreased with increasing planting density. Supplemental CO2 enrichment may serve to increase the yield of strawberry grown in peat bag culture where little CO2 supply from the soil could be expected, and eight plants in a 80 cm long peat bag may be a reasonable planting density to obtain both high yield and fruit quality.

Photosynthesis and Plant Growth at Elevated Levels of CO2

In this review, we discuss the effects of elevated CO2 levels on photosynthesis in relation to the whole plant growth in terrestrial higher C3 plants. Short-term CO2 enrichment stimulates the rate of photosynthesis. Plant mass is also enhanced by CO2 enrichment. However, the effects of long-term CO2 enrichment on photosynthesis are variable. Generally, the prolonged exposure to CO2 enrichment reduces the initial stimulation of photosynthesis in many species, and frequently suppresses photosynthesis. These responses are attributed to secondary responses related to either excess carbohydrate accumulation or decreased N content rather than direct responses to CO2. Accumulation of carbohydrates in leaves may lead to the repression of photosynthetic gene expression and excess starch seems to hinder CO2 diffusion. Therefore, the species which have the sink organs for carbohydrate accumulation do not show the suppression of photosynthesis. The suppression of photosynthesis by CO2 enrichment is always associated with decreases in leaf N and Rubisco contents. These decreases are not due to dilution of N caused by a relative increase in the plant mass but are the result of a decrease in N allocation to leaves at the level of the whole plant, and the decrease in Rubisco content is not selective. Leaf senescence and plant development are also accelerated by CO2 enrichment. However, they are independent of each other in some species. Thus, various responses to CO2 observed at the level of a single leaf result from manifold responses at the level of the whole plant grown under conditions of CO2 enrichment.

Direct rooting and hardening of tea microshoots in the field

Micropropagation of tea (Camellia sinensis (L.) O. Kuntze) has been widely attempted but commercial exploitation of this method is limited by heavy losses during the hardening procedures. In the present study, optimization of time of harvesting (spring and early summer) of microshoots, shoot size, soil pH (4.0–6.4), plant growth regulator treatment (IBA; 500 mg l-1, 30 min) CO2 (9.09/10×10−5 mol l-1 to 10.22/10×10-5 mol l-1 and 20/11×10−5 mol l-1 to 80/13×10−7 mol l-1) enrichment and light (15 μ mol m-2 s-1) conditions in specially designed hardening chambers, made a significant impact on the percent of success for hardening. Following the standardized procedure, up to 71.6% root induction and 73% survival could be achieved. Successful field transfer was also accomplished.

The physiology of Cymbidium plantlets cultured in vitro under conditions of high carbon dioxide and low photosynthetic photon flux density

SummaryCymbidium plantlets were grown in vitro under conditions of high CO2 and low photosynthetic photon flux density using the Miracle Packt culture system. Shoots and roots of plantlets showed differential growth characteristics. Shoot growth was not different in plantlets cultured under CO2-enriched (CDE) and non-enriched (NCDE) conditions. Root growth was promoted in plantlets cultured under CDE in the presence or absence of 2% sucrose (S) with rockwool (R) as the supporting material. Growth was poor in plantlets cultured in 1% agar. Root growth was best in plantlets cultured under CDE R+S. Sucrose is still an important component for root growth under CDE conditions even though CO2 can be used as an alternative carbon source. Photosynthetic measurements (CO2 uptake and total Rubisco activity) showed the presence of active and operational photosynthetic machinery in plantlets cultured under CDE and NCDE conditions. The apparent lack of photoautotrophy (as evident from the lack of starch grains in chloroplasts) in plantlets cultured under NCDE conditions is not the result of a lesser potential for photoautotrophy; rather it is a consequence of sub-optimal CO2 concentrations within the culture vessels.

ＮＦＴ栽培におけるイチゴの養水分吸収に及ぼすＣＯ２施用の影響

This experiment was carried out to investigate the effects of CO2 enrichment (700-800 ppm in the day time) on the absorption of water and mineral nutrients in strawberry (Fragaria × ananassa Duch. cv. Nyoho) grown by nutrient film technique. The concentration of major elements in the nutrient solution were NO3- : 5, NH4+ : 1, H2PO4- : 1, K+ : 3, Ca2+ : 1, Mg2+: 1 mmol·L-1. Vigorous growth and high yield were obtained by CO2 enrichment. In control (ambient CO2, 200-400 ppm), the uptake concentration (each mineral absorbed per unit volume of water absorbed, n/w) was almost equal to the concentration of the solution, throughout growing period. However, under CO2 enriched condition, n/w and total uptake amount of minerals increased by 30-60% compared to control, especially, from November to February when the temperature and light intensity were low. The n/w of minerals were also 50-100% higher than the concentration of the nutrient solution. Total-N concentration in leaves was markedly decreased by CO2 enrichment, while there was little difference in P, K, Ca and Mg. To keep the concentration of nutrients in the solution, the concentration of nutrients should be kept 50-100% higher than those previously reported, when strawberry plants are grown with CO2 enrichment.

Photoautotrophic in vitro Multiplication of the Orchid Dendrobium under CO<sub>2</sub> Enrichment

An attempt to reduce the production cost on tissue cultured plants, photoautotrophic culture of a high value orchid Dendrobium was established under CO2-enriched conditions. The shoot length and the number of leaves were almost equal in plantlets grown on medium with 2 % sucrose or without sucrose and under normal or enhanced (40 g m-3) CO2 concentration, whereas the fresh and dry masses were higher in cultures grown in sucrose containing media or under CO2 enrichment. Development of roots was observed only on media without sucrose, but CO2 enrichment did not have significant effects on in vitro rootings.

Natural pasture community response to enriched carbon dioxide atmosphere

We examined the response of a pasture community in southern Quebec (Canada) to long-term exposure of enriched atmospheric CO2 conditions. The study was conducted using open-top growth chambers directly placed on top of the natural pasture community. To investigate the change in the overall species composition in time and space, we used canonical correspondence analysis, a direct ordination method. Over the three years, the overall community responded significantly to enriched CO2. The analyses show that, after three years, CO2 was the most important environmental variable affecting the species composition. Initially the presence of the wall of the chambers influenced the composition but CO2 became more important by the third year. Soil and air temperatures only slightly influenced the community composition. The first two axes of the canonical correspondence analysis explained a large proportion of the variation in the three years and these trends appeared to increase with time. Species such as Agropyron repens appeared to be positively influenced by the presence of the wall (slightly warmer conditions). However, the analyses suggest that Phleum pratense and Trifolium repens, for example, were favored by the increase in atmospheric CO2. The variation in species composition in enriched versus ambient CO2 chambers suggests that the effect of the environmental factors, particularly CO2, were important in affecting the rate and pattern of succession. Furthermore, the temporal increase in importance of the variable CO2 in the present analyses indicates that there might be a time-lag in response to atmospheric enrichment.

High mineral contents explain the low construction cost of leaves, stems and fruits of tomato plants

The construction cost of plant tissues is used in crop models to convert the products of photosynthesis intobiomass. As for other greenhouse crops, tomato tissues are specific in that they have a high mineral content. The consequences of this accumulation of minerals on the construction cost of the tissues and the possible interactions with the physiological age of the organs and with the CO2 concentration in the atmosphere was examined. For that purpose, threemethods of estimating the construction cost were used and compared. Large quantities of minerals accumulated in the tissues of tomato plants (ranging from 0.05 in fruits to 0.26 g g-1 DM in leaves). The subsequent dilution of the organic matter explained why the estimated construction cost of the dry matter (organic matter+minerals) was fairly low in comparison to that of other crop species. The construction cost was higher in fruits than in vegetative organs, partly because of a lower mineral content. It decreased by 7–12% from top to bottom of the canopy, following the increase in the physiological age of the tissues. This ontogenic drift was partly explained by the accumulation of minerals in the older organs. In the conditions of CO2 enrichment of a commercial greenhouse, no effect of CO2 concentration on the mineral content and on the construction cost of tissues was observed. Such a variability of the construction cost of tomato plant tissues due to the accumulation of minerals or to the ontogeny questions the use of standard values in crop models.

Effect of CO2 enrichment on carbon and nitrogen interaction in wheat and soybean

Effect of CO2 enrichment on the carbon-nitrogen balance in whole plant and the acclimation of photosynthesis was studied in wheat (spring wheat) and soybean (A62-1 [nodulated] and A62-2 [non-nodulated]) with a combination of two nitrogen application rates (0 g N land area m-2 and 30 g N land area m-2) and two temperature treatments (30/20°C (day/night) and 26/16°C). Results were as follows. 1. Carbon (dry matter)-nitrogen balance of whole plant throughout growth was remarkably different between wheat and soybean, as follows: 1) in wheat, the relationship between the amount of dry matter (DMt) and amount of nitrogen absorbed (Nt) in whole plant was expressed by an exponential regression, in which the regression coefficient was affected by only the nitrogen application rate, and not by CO2 and temperature treatments, and 2) in soybean the DMt-Nt relationship was basically expressed by a linear regression, in which the regression coefficient was only slightly affected by the nitrogen treatment (at 0N, DMt-Nt balance finally converged to a linear regression). Thus, carbon-nitrogen interaction in wheat was strongly affected by the underground environment (nitrogen nutrition), but not by the above ground environment (CO2 enrichment and temperature), while that in soybean was less affected by both under and above ground environments. 2. The photosynthetic response curve to CO2 concentration in wheat and soybean was less affected by the CO2 enrichment treatment, while that in wheat and soybean (A62-2) was affected by the nitrogen treatment, indicating that nitrogen nutrition is a more important factor for the regulation of photosynthesis regardless of the CO2 enrichment. 3. Carbon isotope discrimination (..:1) in soybean was similar to that in wheat under ambient CO2, while lower than that in wheat under CO2 enrichment, suggesting that the carbon metabolism is considerably different between wheat and soybean under the CO2 enrichment conditions.

Modulation of carbon and nitrogen metabolism, and of nitrate reductase, in untransformed and transformed Nicotiana plumbaginifolia during CO 2 enrichment of plants grown in pots and in hydroponic culture

Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or β-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO2-enriched atmosphere. Whereas CO2 enrichment (1000 μl · l−1) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modified when plants were grown in hydroponic culture in high CO2 compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO2, were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the pot-grown tobacco, growth at elevated CO2 levels caused a concomitant decrease in the N content of the leaves involving losses in NO− 3 and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO2 enrichment when the plants were grown in hydroponic culture, suggesting that an increased flux through nitrogen assimilation was possible in the 35S-NR plants. Under CO2 enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The differences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the difference in total extractable NR activity was more marked following CO2 enrichment. Foliar NR message levels were decreased by CO2 enrichment in all growth conditions but this was much more pronounced in pot-grown material than in that grown hydroponically. Since β-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO2 enrichment (to test the effects of CO2 enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was specifically accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO2 enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible effectors include NO3 −, glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO2 enrichment has a direct effect on NR mRNA stability.