Effect of Atmospheric CO(2) Enrichment on Growth, Nonstructural Carbohydrate Content, and Root Nodule Activity in Soybean

Elevated CO2 concentration enhance carbon and nitrogen metabolism and biomass accumulation of Ormosiahosiei

Elevated CO2 concentrations may inhibit photosynthesis due to nitrogen deficiency, but legumes may be able to overcome this limitation and continue to grow. Our study confirms this conjecture well. First, we placed the two-year-old potted saplings of Ormosia hosiei (O. hosiei) (a leguminous tree species) in the open-top chamber (OTC) with three CO2 concentrations of 400 (CK), 600 (E1), and 800 μmol·mol-1 (E2) to simulate the elevated CO2 concentration environment. After 146 days, the light saturation point (LSP), light compensation point (LCP), apparent quantum efficiency (AQE), and dark respiration rate (Rd) of O. hosiei were increased under increasing CO2 concentration and obtain the maximum ribulose diphosphate (RuBP) carboxylation rate (Vc max) and RuBP regenerated photosynthetic electron transfer rate (Jmax) were also significantly increased under E2 treatment (P < 0.05). This results in a significant increase of the maximum assimilation rate (Amax) under elevated CO2 concentrations. Sucrose phosphate synthase (SPS) activity in sucrose metabolism increased in the leaves, more soluble sugars, starches, and sucrose was produced, but sucrose content only in leaves increased at E2, and more carbon flows to the roots. The activity of the NH4+ assimilating enzymes glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) in the leaves of O. hosiei increases under elevated CO2 concentrations to promote nitrogen synthesis that reduces the content of ammonium nitrogen and increases the content of nitrate nitrogen. In addition, under E1 conditions, sucrose synthase (SS), direction of synthesis activity was highest and sucrose invertase (INV) activity was lowest, this means that the balance of C and N metabolism is maintained. While under E2 conditions SS activity decreased and INV activity increased, this increased C/N and nitrogen use efficiency. So, the elevated CO2 concentration promotes the accumulation of O. hosiei biomass, especially in the aboveground part, but did not have a significant effect on the accumulation of root biomass. This means that O. hosiei is able to cope under the elevated CO2 concentration without showing photosynthetic adaptation during the experimental period.

Assessing the growth dynamics of alfalfa varieties (Medicago sativa cv. Bilensoy 80 and Nimet) response to varied carbon dioxide (CO2) concentrations

Rising atmospheric CO2 levels drive greenhouse effects, elevating temperatures, and diminishing water accessibility in semi-arid regions, affecting agriculture. Alfalfa contributes to climate change mitigation by sequestering carbon, enhancing soil fertility and carbon storage, reducing synthetic nitrogen fertilizer use, preventing soil erosion, supplying high-quality livestock feed, and serving as a bioenergy source. This research examined the effects of elevated CO2 levels in climate change scenarios (600, 800, and 1000 ppm, with control at 400 ppm) on two alfalfa varieties, Medicago sativa cv. Nimet and Bilensoy-80. The experiments were conducted in specialized Climate Change Simulation Greenhouses, allowing control of CO2, water, and temperature variables. Results revealed a positive relationship between higher CO2 concentrations and increased photosynthesis (P ≤ 0.001), promoting the plant growth leaf area (P ≤ 0.001), yields and both leaf (P ≤ 0.05) and stem dry biomass (P ≤ 0.001). At 1000 ppm CO2, a saturation point was reached, halting further photosynthesis. This down-regulation was linked to decreased intercellular CO2 levels, which expedited chlorophyll and breakdown and potentially induced leaf senescence. High CO2 levels led to greater biomass, as anticipated. However, total protein levels, a forage quality indicator, initially decreased with high CO2 concentrations (up to 1000 ppm) due to an inverse relationship with shoot yield. Surprisingly, the 1000 ppm CO2 concentration mitigated this protein reduction in both alfalfa varieties.

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

Plant and seed germination responses to global change, with a focus on CO2: A review

Earth atmospheric CO2 concentration has risen by over 35% since 1750 and is presently increasing by about 2 parts per million (ppm) every year. Due to contributions from human activity, CO2 is projected to keep rising in the predictable future and to double sometime during this century if fossil fuels burning remains. As a result, air temperature is projected to rise from 2 to 5 °C by 2100. Following this rise in CO2, some ecosystems will face challenges in the next few decades as plants will live in warmer temperatures, higher evaporating demand and widespread changes in drought lengths and severity. To yield healthy crops and forests in changing climate surroundings, it is vital to define whether elevated CO2 disturbs seed germination and plant formation, but even more, the physiological traits conferring drought tolerance. Here, we review the current understanding on the role that CO2 plays on plant growth and seed germination, as well as its impact during the exposure of abiotic stresses like drought and salinity.

Xyloglucan endotransglycosylase/hydrolase increases tightly-bound xyloglucan and chain number but decreases chain length contributing to the defense response that Glycine max has to Heterodera glycines

The Glycine max xyloglucan endotransglycosylase/hydrolase (EC 2.4.1.207), GmXTH43, has been identified through RNA sequencing of RNA isolated through laser microdissection of Heterodera glycines-parasitized root cells (syncytia) undergoing the process of defense. Experiments reveal that genetically increasing XTH43 transcript abundance in the H. glycines-susceptible genotype G. max[Williams 82/PI 518671] decreases parasitism. Experiments presented here show decreasing XTH43 transcript abundance through RNA interference (RNAi) in the H. glycines-resistant G. max[Peking/PI 548402] increases susceptibility, but it is unclear what role XTH43 performs. The experiments presented here show XTH43 overexpression decreases the relative length of xyloglucan (XyG) chains, however, there is an increase in the amount of those shorter chains. In contrast, XTH43 RNAi increases XyG chain length. The experiments show that XTH43 has the capability to function, when increased in its expression, to limit XyG chain extension. This outcome would likely impair the ability of the cell wall to expand. Consequently, XTH43 could provide an enzymatically-driven capability to the cell that would allow it to limit the ability of parasitic nematodes like H. glycines to develop a feeding structure that, otherwise, would facilitate parasitism. The experiments presented here provide experimentally-based proof that XTHs can function in ways that could be viewed as being able to limit the expansion of the cell wall.

Using microcontrollers and sensors to build an inexpensive CO2 control system for growth chambers

PREMISE

A CO2 control system is important for investigating how elevated CO2 affects plant growth. Our automatic CO2 monitoring and control system offers an inexpensive and flexible way to make CO2-enriched environments.

METHOD AND RESULTS

Using microcontrollers paired with non-dispersive infrared CO2 sensors, relays, and valves, we developed a low-cost system for monitoring and controlling CO2 levels in growth chambers.

CONCLUSIONS

Compared with existing commercially available CO2 control systems, Arduino-based microcontrollers offer affordable access to the data logging of CO2 levels in growth chambers, thereby reducing budget limitations for creating growth conditions with highly controlled CO2 concentrations.

Soybean Seed Vigor: Uniformity and Growth as Key Factors to Improve Yield

Emergence uniformity and the time required for emergence are essential factors for obtaining highly productive potential in plants. Factors such as sowing depth and soil moisture affect uniformity and emergence, but little is known about the impacts of seed vigor. Thus, we determined the impacts of seed vigor on uniformity and growth as well as development and yield in soybean (Glycine max [L.] Merrill) plants. The treatments consisted of four vigor levels (89%, 57%, 47%, and 43%) obtained by accelerated aging, in a randomized block design, with five replicates at three sites. Seeds with the highest vigor level showed higher uniformity and faster emergence. Dominated plants through their phenotypic plasticity modify stem diameter and internode length. The plants that emerged earlier had a larger leaf area in the three stages (V1, V4, and R2), allowing them to accumulate more photoassimilates in the initial stages. Consequently, these plants exhibited an increase in yield components, especially in the number of fertile nodes and the number of pods per plant, making them more productive.

Elevation alters carbon and nutrient concentrations and stoichiometry in Quercus aquifolioides in southwestern China

Elevation is a complex environmental factor altering temperature, light, moisture and soil nutrient availability, and thus may affect plant growth and physiology. Such effects of elevation may also depend on seasons. Along an elevational gradient of the Balang Mountain, southwestern China, we sampled soil and 2-year old leaves, 2-year old shoots, stem sapwood and fine roots (diameter<5mm) of Quercus aquifolioides at 2843, 2978, 3159, 3327, 3441 and 3589m a.s.l. in both summer and winter. In summer, the concentrations of tissue non-structural carbohydrates (NSC) did not decrease with increasing elevation, suggesting that the carbon supply is sufficient for plant growth at high altitude in the growing season. The concentration of NSC in fine roots decreased with elevation in winter, and the mean concentration of NSC across tissues in a whole plant showed no significant difference between the two sampling seasons, suggesting that the direction of NSC reallocation among plant tissues changed with season. During the growing season, NSC transferred from leaves to other tissues, and in winter NSC stored in roots transferred from roots to aboveground tissues. Available soil N increased with elevation, but total N concentrations in plant tissues did not show any clear elevational pattern. Both available soil P and total P concentrations in all plant tissues decreased with increasing elevation. Thus, tissue N:P ratio increased with elevation, suggesting that P may become a limiting element for plant growth at high elevation. The present study suggests that the upper limit of Q. aquifolioides on Balang Mountain may be co-determined by winter root NSC storage and P availability. Our results contribute to better understanding of the mechanisms for plants' upper limit formation.

Minirhizotron imaging reveals that nodulation of field-grown soybean is enhanced by free-air CO2 enrichment only when combined with drought stress

The rate of N2 fixation by a leguminous plant is a product of the activity of individual nodules and the number of nodules. Initiation of new nodules and N2 fixation per nodule are highly sensitive to environmental conditions. However, the effects of global environmental change on nodulation in the field are largely unknown. It is also unclear whether legumes regulate nodulation in response to environment solely by varying root production or also by varying nodule density per unit of root length. This study utilised minirhizotron imaging as a novel in situ method for assessing the number, size and distribution of nodules in field-grown soybean (Glycine max (L.) Merr.) exposed to elevated atmospheric CO2 ([CO2]) and reduced precipitation. We found that nodule numbers were 134-229% greater in soybeans grown at elevated [CO2] in combination with reduced precipitation, and this response was driven by greater nodule density per unit of root length. The benefits of additional nodules were probably offset by an unfavourable distribution of nodules in shallow, dry soil in reduced precipitation treatment under elevated [CO2] but not ambient [CO2]. In fact, significant decreases in seed and leaf nitrogen concentration also occurred only in elevated [CO2] with reduced precipitation. This study demonstrates the potential of minirhizotron imaging to reveal previously uncharacterised changes in nodule production and distribution in response to global environmental change.

Additional cause for reduced JA-Ile in the root of a Lotus japonicus phyB mutant

Light is critical for supplying carbon for use in the energetically expensive process of nitrogen-fixing symbiosis between legumes and rhizobia. We recently showed that root nodule formation in phyB mutants [which have a constitutive shade avoidance syndrome (SAS) phenotype] was suppressed in white light, and that nodulation in wild-type is controlled by sensing the R/FR ratio through jasmonic acid (JA) signaling. We concluded that the cause of reduced root nodule formation in phyB mutants was the inhibition of JA-Ile production in root. Here we show that the shoot JA-Ile level of phyB mutants is higher than that of the wild-type strain MG20, suggesting that translocation of JA-Ile from shoot to root is impeded in the mutant. These results indicate that root nodule formation in phyB mutants is suppressed both by decreased JA-Ile production, caused by reduced JAR1 activity in root, and by reduced JA-Ile translocation from shoot to root.

Glutamine synthetase in legumes: recent advances in enzyme structure and functional genomics

Glutamine synthetase (GS) is the key enzyme involved in the assimilation of ammonia derived either from nitrate reduction, N(2) fixation, photorespiration or asparagine breakdown. A small gene family is encoding for different cytosolic (GS1) or plastidic (GS2) isoforms in legumes. We summarize here the recent advances carried out concerning the quaternary structure of GS, as well as the functional relationship existing between GS2 and processes such as nodulation, photorespiration and water stress, in this latter case by means of proline production. Functional genomic analysis using GS2-minus mutant reveals the key role of GS2 in the metabolic control of the plants and, more particularly, in carbon metabolism.

Photorespiratory metabolism and nodule function: behavior of Lotus japonicus mutants deficient in plastid glutamine synthetase

Two photorespiratory mutants of Lotus japonicus deficient in plastid glutamine synthetase (GS(2)) were examined for their capacity to establish symbiotic association with Mesorhizobium loti bacteria. Biosynthetic glutamine synthetase (GS) activity was reduced by around 40% in crude nodule extracts from mutant plants as compared with the wild type (WT). Western blot analysis further confirmed the lack of GS(2) polypeptide in mutant nodules. The decrease in GS activity affected the nodular carbon metabolism under high CO(2) (suppressed photorespiration) conditions, although mutant plants were able to form nodules and fix atmospheric nitrogen. However, when WT and mutant plants were transferred to an ordinary air atmosphere (photorespiratory active conditions) the nodulation process and nitrogen fixation were substantially affected, particularly in mutant plants. The number and fresh weight of mutant nodules as well as acetylene reduction activity showed a strong inhibition compared with WT plants. Optical microscopy studies from mutant plant nodules revealed the anticipated senescence phenotype linked to an important reduction in starch and sucrose levels. These results show that, in Lotus japonicus, photorespiration and, particularly, GS(2) deficiency result in profound limitations in carbon metabolism that affect the nodulation process and nitrogen fixation.

Lotus japonicus nodulation is photomorphogenetically controlled by sensing the red/far red (R/FR) ratio through jasmonic acid (JA) signaling

Light is critical for supplying carbon to the energetically expensive, nitrogen-fixing symbiosis between legumes and rhizobia. Here, we show that phytochrome B (phyB) is part of the monitoring system to detect suboptimal light conditions, which normally suppress Lotus japonicus nodule development after Mesorhizobium loti inoculation. We found that the number of nodules produced by L. japonicus phyB mutants is significantly reduced compared with the number produced of WT Miyakojima MG20. To explore causes other than photoassimilate production, the possibility that local control by the root genotype occurred was investigated by grafting experiments. The results showed that the shoot and not the root genotype is responsible for root nodule formation. To explore systemic control mechanisms exclusive of photoassimilation, we moved WT MG20 plants from white light to conditions that differed in their ratios of low or high red/far red (R/FR) light. In low R/FR light, the number of MG20 root nodules dramatically decreased compared with plants grown in high R/FR, although photoassimilate content was higher for plants grown under low R/FR. Also, the expression of jasmonic acid (JA) -responsive genes decreased in both low R/FR light-grown WT and white light-grown phyB mutant plants, and it correlated with decreased jasmonoyl-isoleucine content in the phyB mutant. Moreover, both infection thread formation and root nodule formation were positively influenced by JA treatment of WT plants grown in low R/FR light and white light-grown phyB mutants. Together, these results indicate that root nodule formation is photomorphogenetically controlled by sensing the R/FR ratio through JA signaling.

Successful nodulation of Casuarina by Frankia in axenic conditions

AIMS

In order to depict the fine interactions that lead to nodulation, absolute microbiological control of the symbiotic partners is required, i.e. the ability to obtain in vitro axenic nodulation, a condition that has never been fulfilled with the Casuarina-Frankia symbiosis. The effects of culture conditions on plant growth and nodule formation by Casuarina cunninghamiana were investigated.

METHODS AND RESULTS

Axenic (capped tubes with different substrates), and nonaxenic cultures (Gibson tubes, pot cultures) were tested. In axenic conditions, C. cunninghamiana, inoculated with Frankia, had poor growth and did not form nodules at 6 weeks. Plants cultivated in Gibson tubes reached the four axillary shoots stage within 6 weeks and formed nodules 4 weeks after inoculation. Sand-pot cultures allowed us to relate the plant development stage at inoculation with nodulation.

CONCLUSIONS

The sterile replacement of the cap by a plastic bag increased plant growth and enabled nodule formation 6 weeks after inoculation. The new system of plant culture allows the axenic nodule formation 6 weeks after inoculation. Nodulation behaviour is related to plant development and confinement.

SIGNIFICANCE AND IMPACT OF THE STUDY

This axenic plant nodulation system is of major interest in analysing the roles of Frankia genes in nodulation pathways.

Effects of atmospheric CO(2) enrichment on plant constituents related to animal and human health

Atmospheric CO(2) enrichment is known to significantly enhance the growth and development of nearly all plants, implying a potential for elevated levels of CO(2) to alter the concentrations of plant constituents related to animal and human health. Our review of this subject indicates that increases in the air's CO(2) content typically lead to reductions in the nitrogen and protein concentrations of animal-sustaining forage and human-sustaining cereal grains when soil nitrogen levels are sub-optimal. When plants are supplied with all the nitrogen they can use, however, no such reductions are observed. CO(2)-enriched plants growing in the natural environment also tend to overcome initial reductions in plant mineral concentrations as time progresses, possibly due to development of larger root systems and consequent enhanced abilities to locate and absorb mineral nutrients. Atmospheric CO(2) enrichment additionally appears to reduce oxidative stresses in plants; and it has been shown to increase the concentration of vitamin C in certain fruits and vegetables. Elevated CO(2) has also been demonstrated to increase the biomass of plants grown for medicinal purposes while simultaneously increasing the concentrations of the disease-fighting substances produced within them. It is likely, therefore, that the ongoing rise in the air's CO(2) content will continue to increase food production around the world, while maintaining the nutritive quality of that food and enhancing the production of certain disease-inhibiting plant compounds.

Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem

Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.

A Short-Term Decrease in Nitrogenase Activity (C2H2 Reduction) Is Induced by Exposure of Soybean Shoots to Their CO2 Compensation Point

Photosynthesis and nitrogenase acetylene-reducing activity (ARA) were measured in soybeans (Glycine max [L.] Merr.) in which the shoots were exposed for 48 h to 60 [mu]L L-1 CO2, a value corresponding to their CO2 compensation point. Six hours after the beginning of the light period at low CO2, the ARA started to decrease, reaching a rate of 50% of the control rate in 14 to 24 h and 20% of the control rate in 34 to 38 h after the beginning of the CO2 treatment. At these times, there was no net photosynthesis, and the transpiration rate was 20% lower than that in the control plants. An increase in the partial pressure of O2 around the nodules alleviated this inhibition of ARA. The maximal ARA achieved at 40 kPaO2 was 3 times higher than that at 20 kPa O2 and similar to the maximal ARA of the control plants. It was argued that the decrease in ARA of soybean exposed to the CO2 compensation point was due to a decrease in the nodule's permeability to O2 diffusion.

Plant responses to atmospheric CO2 enrichment with emphasis on roots and the rhizosphere

Empirical records provide incontestable evidence of global changes: foremost among these changes is the rising concentration of CO(2) in the earth's atmosphere. Plant growth is nearly always stimulated by elevation of CO(2). Photosynthesis increases, more plant biomass accumulates per unit of water consumed, and economic yield is enhanced. The profitable use of supplemental CO(2) over years of greenhouse practice points to the value of CO(2) for plant production. Plant responses to CO(2) are known to interact with other environmental factors, e.g. light, temperature, soil water, and humidity. Important stresses including drought, temperature, salinity, and air pollution have been shown to be ameliorated when CO(2) levels are elevated. In the agricultural context, the growing season has been shortened for some crops with the application of more CO(2); less water use has generally, but not always, been observed and is under further study; experimental studies have shown that economic yield for most crops increases by about 33% for a doubling of ambient CO(2) concentration. However, there are some reports of negligible or negative effects. Plant species respond differently to CO(2) enrichment, therefore, clearly competitive shifts within natural communities could occur. Though of less importance in managed agro-ecosystems, competition between crops and weeds could also be altered. Tissue composition can vary as CO(2) increases (e.g. higher C: N ratios) leading to changes in herbivory, but tests of crop products (consumed by man) from elevated CO(2) experiments have generally not revealed significant differences in their quality. However, any CO(2)-induced change in plant chemical or structural make-up could lead to alterations in the plant's interaction with any number of environmental factors-physicochemical or biological. Host-pathogen relationships, defense against physical stressors, and the capacity to overcome resource shortages could be impacted by rises in CO(2). Root biomass is known to increase but, with few exceptions, detailed studies of root growth and function are lacking. Potential enhancement of root growth could translate into greater rhizodeposition, which, in turn, could lead to shifts in the rhizosphere itself. Some of the direct effects of CO(2) on vegetation have been reasonably well-studied, but for others work has been inadequate. Among these neglected areas are plant roots and the rhizosphere. Therefore, experiments on root and rhizosphere response in plants grown in CO(2)-enriched atmospheres will be reviewed and, where possible, collectively integrated. To this will be added data which have recently been collected by us. Having looked at the available data base, we will offer a series of hypotheses which we consider as priority targets for future research.

Growth Kinetics, Carbohydrate, and Leaf Phosphate Content of Clover (Trifolium subterraneum L.) after Transfer to a High CO(2) Atmosphere or to High Light and Ambient Air

Intact air-grown (photosynthetic photon flux density, 400 microeinsteins per square meter per second) clover plants (Trifolium subterraneum L.) were transfered to high CO(2) (4000 microliters CO(2) per liter; photosynthetic photon flux density, 400 microeinsteins per square meter per second) or to high light (340 microliters CO(2) per liter; photosynthetic photon flux density, 800 microeinsteins per square meter per second) to similarly stimulate photosynthetic net CO(2) uptake. The daily increment of net CO(2) uptake declined transiently in high CO(2), but not in high light, below the values in air/standard light. After about 3 days in high CO(2), the daily increment of net CO(2) uptake increased but did not reach the high light values. Nightly CO(2) release increased immediately in high light, whereas there was a 3-day lag phase in high CO(2). During this time, starch accumulated to a high level, and leaf deterioration was observed only in high CO(2). After 12 days, starch was two- to threefold higher in high CO(2) than in high light, whereas sucrose was similar. Leaf carbohydrates were determined during the first and fourth day in high CO(2). Starch increased rapidly throughout the day. Early in the day, sucrose was low and similar in high CO(2) and ambient air (same light). Later, sucrose increased considerably in high CO(2). The findings that (a) much more photosynthetic carbon was partitioned into the leaf starch pool in high CO(2) than in high light, although net CO(2) uptake was similar, and that (b) rapid starch formation occurred in high CO(2) even when leaf sucrose was only slightly elevated suggest that low sink capacity was not the main constraint in high CO(2). It is proposed that carbon partitioning between starch (chloroplast) and sucrose (cytosol) was perturbed by high CO(2) because of the lack of photorespiration. Total phosphate pools were determined in leaves. Concentrations based on fresh weight of orthophosphate, soluble esterified phosphate, and total phosphate markedly declined during 13 days of exposure of the plants to high CO(2) but changed little in high light/ambient air. During this time, the ratio of orthophosphate to soluble esterified phosphate decreased considerably in high CO(2) and increased slightly in high light/ambient air. It appears that phosphate uptake and growth were similarly stimulated by high light, whereas the coordination was weak in high CO(2).

Photosynthesis and photosynthate partitioning in n(2)-fixing soybeans

Leaf area, chlorophyll content, net CO(2) photoassimilation, and the partitioning of fixed carbon between leaf sucrose and starch and soluble protein were examined in Glycine max (L) Merr. cv Williams grown under three different nitrogen regimes. One group (Nod+/+) was inoculated with Bradyrhizobium and watered daily with a nutrient solution containing 6 millimolar NH(4)NO(3). A second set (Nod+/-) was inoculated and had N(2) fixation as its sole source of nitrogen. A third group (Nod(-)) was not inoculated and was watered daily with a nutrient solution containing 6 millimolar NH(4)NO(3). The mean net micromole CO(2) uptake per square decimeter per hour of the most recently matured source leaves was similar among the three groups of plants, being about 310. Mean leaf area of the source leaves, monitored for net photosynthesis was also similar. However, the mean milligram of chlorophyll per square decimeter of Nod+/- test leaves was about 50% lower than the other groups' leaves and indicated nitrogen deficiency. Thus, Nod+/- utilized their chlorophyll more efficiently for photosynthetic CO(2) uptake than the plants of the other treatments. The ratio of foliar carbohydrate:protein content was high in Nod+/- but low in the plants from the other two treatments. This inverse relationship between foliar protein and carbohydrate content suggests that more fixed carbon is diverted to the synthesis of protein when nitrogen availability is high. It was also found that Nod+/- sequestered more storage protein in their paraveinal mesophyll than plants of the other treatments. This study indicates that when inorganic nitrogen regimes are used to control photosynthate partitioning, then both leaf carbohydrate and leaf protein must be considered as end products of carbon assimilate allocation.

Carbohydrate partitioning and the capacity of apparent nitrogen fixation of soybean plants grown outdoors

Patterns of leaf carbohydrate partitioning and nodule activity in soybean plants grown under natural conditions and the irradiance level required to produce sufficient carbohydrate to obtain maximum rates of apparent N(2)-fixation (acetylene reduction) were measured. Soybean plants, grown outdoors, maintained constant levels of leaf soluble sugars while leaf starch pools varied diurnally. When root temperature was kept at 25 degrees C and shoot temperature was allowed to vary with ambient temperature, the plants maintained constant rates of apparent N(2)-fixation and root+nodule respiration. Results from a second experiment, in which the entire plant was kept at 25 degrees C, were similar to those of the first experiment. Shoot carbon exchange rate of plants from the second experiment was light saturated at photosynthetic photon flux densities between 400 and 600 micromoles per square meter per second. When plants were subjected to an extended 40-hour dark period to deplete carbohydrate reserves, apparent N(2)-fixation was unaffected during the first 10 hours of darkness, decreased rapidly between 10 and 16 hours, and plateaued at one-third the initial level thereafter. After the extended dark period, plants were exposed to photosynthetic photon flux density from 200 to 1000 micromoles per square meter per second for 10 hours. Photosynthetic photon flux densities of 200 micromoles per square meter per second and greater resulted in maximum leaf soluble sugar content and nodule activity. Leaf starch content increased with irradiance levels up to 600 micromoles per square meter per second with no further increase at higher irradiance levels. Results presented here indicate that maximum nodule activity occurs at irradiance levels that do not saturate the plant's photosynthetic apparatus. This response would allow for maximum N(2)-fixation to occur in a nodulated legume during periods of inclement weather.

Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO(2) Levels

The effects of water stress and CO(2) enrichment on photosynthesis, assimilate export, and sucrose-P synthase activity were examined in field grown soybean plants. In general, leaves of plants grown in CO(2)-enriched atmospheres (300 microliters per liter above unenriched control, which was 349 +/- 12 microliters per liter between 0500 and 1900 hours EST over the entire season) had higher carbon exchange rates (CER) compared to plants grown at ambient CO(2), but similar rates of export and similar activities of sucrose-P synthase. On most sample dates, essentially all of the extra carbon fixed as a result of CO(2) enrichment was partitioned into starch. CO(2)-enriched plants had lower transpiration rates and therefore had a higher water use efficiency (milligrams CO(2) fixed per gram H(2)O transpired) per unit leaf area compared to nonenriched plants. Water stress reduced CER in nonenriched plants to a greater extent than in CO(2)-enriched plants. As CER declined, stomatal resistance increased, but this was not the primary cause of the decrease in assimilation because internal CO(2) concentration remained relatively constant. Export of assimilates was less affected by water stress than was CER. When CERs were low as a result of the imposed stress, export was supported by mobilization of reserves (mainly starch). Export rate and leaf sucrose concentration were related in a curvilinear manner. When sucrose concentration was above about 12 milligrams per square decimeter, obtained with nonstressed plants at high CO(2), there was no significant increase in export rate. Assimilate export rate was also correlated positively with SPS activity and the quantitative relationship varied with CER. Thus, export rate was a function of both CER and carbon partitioning.

Nitrate and Nitrite Reduction in Relation to Nitrogenase Activity in Soybean Nodules and Rhizobium japonicum Bacteroids

Soybean (Glycine max L. cv Williams) seeds were sown in pots containing a 1:1 perlite-vermiculite mixture and grown under greenhouse conditions. Nodules were initiated with a nitrate reductase expressing strain of Rhizobium japonicum, USDA 110, or with nitrate reductase nonexpressing mutants (NR(-) 108, NR(-) 303) derived from USDA 110. Nodules initiated with either type of strain were normal in appearance and demonstrated nitrogenase activity (acetylene reduction). The in vivo nitrate reductase activity of N(2)-grown nodules initiated with nitrate reductase-negative mutant strains was less than 10% of the activity shown by nodules initiated with the wild-type strain. Regardless of the bacterial strain used for inoculation, the nodule cytosol and the cell-free extracts of the leaves contained both nitrate reductase and nitrite reductase activities. The wild-type bacteroids contained nitrate reductase but not nitrite reductase activity while the bacteroids of strains NR(-) 108 and NR(-) 303 contained neither nitrate reductase nor nitrite reductase activities.Addition of 20 millimolar KNO(3) to bacteroids of the wild-type strain caused a decrease in nitrogenase activity by more than 50%, but the nitrate reductase-negative strains were insensitive to nitrate. The nitrogenase activity of detached nodules initiated with the nitrate reductase-negative mutant strains was less affected by the KNO(3) treatment as compared to the wild-type strain; however, the results were less conclusive than those obtained with the isolated bacteroids.The addition of either KNO(3) or KNO(2) to detached nodules (wild type) suspended in a semisolid agar nutrient medium caused an inhibition of nitrogenase activity of 50% and 65% as compared to the minus N controls, and provided direct evidence for a localized effect of nitrate and nitrite at the nodule level. Addition of 0.1 millimolar sucrose stimulated nitrogenase activity in the presence or absence of nitrate or nitrite. The sucrose treatment also helped to decrease the level of nitrite accumulated within the nodules.

Proteolytic Activity in Soybean Root Nodules : Activity in Host Cell Cytosol and Bacteroids throughout Physiological Development and Senescence

Root nodules were harvested from chamber-grown soybean (Glycine max L. Merrill cv Woodworth) plants throughout development. Apparent nitrogenase activity (acetylene reduction) peaked before seeds began to develop, but a significant amount of activity remained as the seeds matured. Nodule senescence was defined as the period in which residual nitrogenase activity was lost. During this time, soluble protein and leghemoglobin levels in the host cell cytosol decreased, and proteolytic activity against azocasein increased. Degradative changes were not detected in bacteroids during nodule senescence. Total soluble bacteroid protein per gram of nodule remained constant, and an increase in proteolytic activity in bacteroid extracts was not observed. These results are consistent with the view that soybean nodule bacteroids are capable of redifferentiation into free-living bacteria upon deterioration of the legume-rhizobia symbiosis.