Effect of Atmospheric CO2 Enrichment on Root Growth and Carbohydrate Allocation of Phaseolus spp

A glasshouse experiment was conducted with plants of Phaseolus grown in liquid culture. Root growth parameters (biomass, diameter, length, growth rate, zone of cell division), root rheological components (wall extensibility, water potential yield threshold, water potential), shoot growth, carbon allocation, and abscisic acid (ABA) concentration were measured in Phaseolus acutifolius A. Gray at ambient (550 μmol mol-1) and elevated (700 μmol mol-1) atmospheric CO2 concentrations. For contrast, measurements of above- and belowground growth were conducted on Phaseolus vulgaris L. in the same treatments. Under nonlimiting conditions of water and nutrients, elevated CO2 increased root and shoot growth of P. acutifolius but not P. vulgaris. While root mass was increased by nearly 60% in P. acutifolius, there was no effect of atmospheric CO2 on any of the rheological components measured. In contrast, starch and ABA accumulated in roots of P. acutifolius. The concentration of starch in roots of P. acutifolius increased by 10-fold, while root concentrations of ABA doubled. From the data it is concluded that CO2 enrichment is favorable for root growth in some species in that more carbon is allocated to belowground growth. In addition, ABA may play a role in growth responses and/or allocation of photosynthates at elevated CO2 in P. acutifolius.     

Effects of CO(2) Enrichment and Carbohydrate Content on the Dark Respiration of Soybeans

During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO₂ per liter air, was 1.45 milligrams CO₂ evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO₂ per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO₂ enriched compared with CO₂ normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO₂ enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO₂ enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO₂ adapted plants, but not in the control plants.

Mitochondria, isolated simultaneously from the leaves of CO₂ enriched and control plants, showed no difference in NADH or malate-glutamate dependent O₂ uptake, and there were no observed differences in the specific activities of NAD⁺ linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O₂ uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO₂ enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status.

     

Effects of Atmospheric CO(2) Enrichment on Photosynthesis, Respiration, and Growth of Sour Orange Trees

Numerous net photosynthetic and dark respiratory measurements were made over a period of 4 years on leaves of 24 sour orange (Citrus aurantium) trees; 8 of them growing in ambient air at a mean CO₂ concentration of 400 microliters per liter, and 16 growing in air enriched with CO₂ to concentrations approaching 1000 microliters per liter. Over this CO₂ concentration range, net photosynthesis increased linearly with CO₂ by more than 200%, whereas dark respiration decreased linearly to only 20% of its initial value. These results, together with those of a comprehensive fine-root biomass determination and two independent aboveground trunk and branch volume inventories, suggest that a doubling of the air's current mean CO₂ concentration of 360 microliters per liter would enhance the growth of the trees by a factor of 3.8.     

Three Phases of Plant Response to Atmospheric CO(2) Enrichment

Several years of research on seven different plants (five terrestrial and two aquatic species) suggest that the beneficial effects of atmospheric CO₂ enrichment may be divided into three distinct growth response phases. First is a well-watered optimum-growth-rate phase where a 300 parts per million increase in the CO₂ content of the air generally increases plant productivity by approximately 30%. Next comes a nonlethal water-stressed phase where the same increase in atmospheric CO₂ is more than half again as effective in increasing plant productivity. Finally, there is a water-stressed phase normally indicative of impending death, where atmospheric CO₂ enrichment may actually prevent plants from succumbing to the rigors of the environment and enable them to maintain essential life processes, as life ebbs from corresponding ambient-treatment plants.     

Effects of Atmospheric CO(2) Enrichment on the Growth and Mineral Nutrition of Quercus alba Seedlings in Nutrient-Poor Soil

One-year-old dormant white oak (Quercus alba L.) seedlings were planted in a nutrient-deficient forest soil and grown for 40 weeks in growth chambers at ambient (362 microliters per liter) or elevated (690 microliters per liter) levels of CO₂. Although all of the seedlings became severely N deficient, CO₂ enrichment enhanced growth by 85%, with the greatest enhancement in root systems. The growth enhancement did not increase the total water use per plant, so water-use efficiency was significantly greater in elevated CO₂. Total uptake of N, S, and B was not affected by CO₂, therefore, tissue concentrations of these nutrients were significantly lower in elevated CO₂. An increase in nutrient-use efficiency with respect to N was apparent in that a greater proportion of the limited N pool in the CO₂-enriched plants was in fine roots and leaves. The uptake of other nutrients increased with CO₂ concentration, and P and K uptake increased in proportion to growth. Increased uptake of P by plants in elevated CO₂ may have been a result of greater proliferation of fine roots and associated mycorrhizae and rhizosphere bacteria stimulating P mineralization. The results demonstrate that a growth response to CO₂ enrichment is possible in nutrient-limited systems, and that the mechanisms of response may include either increased nutrient supply or decreased physiological demand.     

Ethylene Contamination of CO(2) Cylinders: Effects on Plant Growth in CO(2) Enrichment Studies

The mechanical extensibilities of stage IVb Phycomyces were measured before and after a humidified wind stimulus. We find that when the humidity of the wind is greater than that of the ambient air, there is an increase in the mechanical extensibility of the cell wall. We also find that a step decrease in wind humidity results in a decrease in the mechanical extensibility of the cell wall.     

Photosynthesis and Growth of Water Hyacinth under CO(2) Enrichment

Water hyacinth (Eichhornia crassipes [Mart.] Solms) plants were grown in environmental chambers at ambient and enriched CO₂ levels (330 and 600 microliters CO₂ per liter). Daughter plants (ramets) produced in the enriched CO₂ gained 39% greater dry weight than those at ambient CO₂, but the original mother plants did not. The CO₂ enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO₂ increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO₂ level. After 14 days at the elevated CO₂, leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO₂, ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO₂ level the enriched CO₂ plants had only half the net photosynthetic rate of their counterparts grown at ambient CO₂, they showed similar internal CO₂ concentrations. This suggested that the decreased supply of CO₂ to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO₂. Photorespiration and dark respiration were lower, such that the CO₂ compensation point was not altered. The photosynthetic light and CO₂ saturation points were not greatly changed, nor was the O₂ inhibition of photosynthesis (measured at 330 microliters CO₂ per liter). It appears that with CO₂ enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.     

Effects of CO(2) Concentration on Rubisco Activity, Amount, and Photosynthesis in Soybean Leaves

Growth at an elevated CO₂ concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO₂ per liter when measured across a range of intercellular CO₂ concentrations and irradiance. Soybean plants grown at elevated CO₂ concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO₂ per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO₂ per square meter per second expressed on a leaflet area basis) to short-term (~1 hour) exposures to a range of CO₂ concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO₂ per minute per milligram of protein) to growth CO₂ concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO₂ concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO₂ is required for activation of rubisco, results indicated that within the range of CO₂ concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO₂.     

Influence of 5-Methyltryptophan-Resistant Bradyrhizobium japonicum on Soybean Root Nodule Indole-3-Acetic Acid Content

Bradyrhizobium japonicum mutants resistant to 5-methyltryptophan were isolated. Some of these mutants were found to accumulate indole-3-acetic acid (IAA) and tryptophan in culture. In greenhouse studies, nodules from control plants inoculated with wild-type bradyrhizobia contained 0.04, 0.10, and 0.58 μg of free, ester-linked, and peptidyl IAA g (fresh weight) of nodules⁻¹, respectively. Nodules from plants inoculated with 5-methyltryptophan-resistant bradyrhizobia contained 0.94, 1.30, and 10.6 μg of free, ester-linked, and peptidyl IAA g (fresh weight) of nodules⁻¹, respectively. This manyfold increase in nodule IAA content indicates that the Bradyrhizobium inoculum can have a considerable influence on the endogenous IAA level of the nodule. Further, these data imply that much of the IAA that accumulated in the high-IAA-containing nodules was of bacterial rather than plant origin. These high-IAA-producing 5-methyltryptophan-resistant bacteria were poor symbiotic nitrogen fixers. Plants inoculated with these bacteria had a lower nodule mass and fixed less nitrogen per gram of nodule than did plants inoculated with wild-type bacteria.     

The Effect of High CO2 and Low O2 Concentrations in Simulated Landfill Gas on the Growth and Nodule Activity of Leucaena leucocephala

When roots of Leucaena leucocephala seedlings (White Popinac,a tropical legume tree belongs to the Family Mimosaceae) werefumigated with simulated landfill gas (CO₂ above 10% and O₂from 10% to atmospheric level), the stem elongation rate andstomatal conductance were inhibited at the absence of any apparentleaf water deficit. When compared with a treatment where rootsystem was flooded, the effect of gas fumigation on the shootphysiology was relatively mild and appeared later. On the otherhand, nodule activity (measured as rate of acetylene reductionactivity, ARA) was much more severely inhibited by gas fumigation.Although nodule dry weight and carbohydrate storage in noduleswere reduced, the inhibition was not likely a result of theshortage of carbohydrate reserve in the nodules. This was becausethe ARA of untreated fresh nodules was also inhibited immediatelyfollowing exposure to the simulated landfill gas. In furtherexperiments where CO₂ and O₂ were manipulated separately, althougha reduction of O₂ concentration to half of the atmospheric levelmight account for up to 30% loss of ARA with considerable variation,the high CO₂ alone showed a much more severe inhibition. ThisCO₂-induced inhibition was not reversible one hour after thehigh CO₂ gas was removed. There was some recovery of activity5 day after plants were fumigated, suggesting that the legumeplant can maintain some nitrogen-fixation activity under theinfluence of landfill gas. (Received April 10, 1995; Accepted August 22, 1995)     

Effects of Light and Elevated Atmospheric CO(2) on the Ribulose Bisphosphate Carboxylase Activity and Ribulose Bisphosphate Level of Soybean Leaves

Soybean (Glycine max L. Merr. cv Bragg) was grown throughout its life cycle at 330, 450, and 800 microliters CO₂ per liter in outdoor controlled-environment chambers under solar irradiance. Leaf ribulose-1,5-bisphosphate carboxylase (RuBPCase) activities and ribulose-1,5-bisphosphate (RuBP) levels were measured at selected times after planting. Growth under the high CO₂ levels reduced the extractable RuBPCase activity by up to 22%, but increased the daytime RuBP levels by up to 20%.

Diurnal measurements of RuBPCase (expressed in micromoles CO₂ per milligram chlorophyll per hour) showed that the enzyme values were low (230) when sampled before sunrise, even when activated in vitro with saturating HCO₃⁻ and Mg²⁺, but increased to 590 during the day as the solar quantum irradiance (photosynthetically active radiation or PAR, in micromoles per square meter per second) rose to 600. The nonactivated RuBPCase values, which averaged 20% lower than the corresponding HCO₃⁻ and Mg²⁺-activated values, increased in a similar manner with increasing solar PAR. The per cent RuBPCase activation (the ratio of nonactivated to maximum-activated values) increased from 40% before dawn to 80% during the day. Leaf RuBP levels (expressed in nanomoles per milligram chlorophyll) were close to zero before sunrise but increased to a maximum of 220 as the solar PAR rose beyond 1200. In a chamber kept dark throughout the morning, leaf RuBPCase activities and RuBP levels remained at the predawn values. Upon removal of the cover at noon, the HCO₃⁻ and Mg²⁺-activated RuBPCase values and the RuBP levels rose to 465 and 122, respectively, after only 5 minutes of leaf exposure to solar PAR at 1500.

These results indicate that, in soybean leaves, light may exert a regulatory effect on extractable RuBPCase in addition to the well-established activation by CO₂ and Mg²⁺.

     

The Effect of CO2 Enrichment on the Growth of Nodulated and Non-Nodulated Isogenic Types of Soybean Raised Under Two Nitrogen Concentrations

To find the effects of CO₂ enrichment on plant development and photosynthetic capacity of nodulated (line A62-1) and non-nodulated (line A62-2) isogenic lines of soybean (Glycine max Merr.), we examined the interactions among two CO₂ treatments (36±3 Pa = AC and 70±5 Pa = EC), and two nitrogen concentrations [0 g(N) m⁻²(land area) = 0N; 30 g(N) m⁻²(land area) = 30N]. Nodules were found in both CO₂ treatments in 0N of A62-1 where the number and dry mass of nodules increased from AC to EC. While the allocation of dry mass to root and shoot and the amount of N in each organ did not differ between the growth CO₂ concentrations, there was larger N allocation to roots in 0N than in 30N for A62-2. The CO₂-dependence of net photosynthetic rate (P _N) for A62-1 was unaffected by both CO₂ and N treatments. In contrast, the CO₂-dependence of P _N was lower in 0N than in 30N for A62-2, but it was independent of CO₂ treatment. P _N per unit N content was unaffected by CO₂ concentrations. The leaf area of both soybean lines grown in 30N increased in EC. But in 0N, only the nodulated A62-1 showed an increase in leaf area in EC. Nitrogen use efficiency of plants, NUE [(total dry mass of the plant)/(amount of N accumulated in the plant)] in 30N was unaffected by CO₂ treatments. In 0N, NUE in EC was lower than in AC in A62-1, and was higher than that at AC in A62-2. Hence, the larger amount and/or rate of N fixation with the increase of the sink-size of symbiotic microorganisms supplied adequate N to the plant under EC. In EC, N deficiency caused the down-regulation of the soybean plant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogenase Activity and Nodule Gas Permeability Response to Rhizospheric NH(3) in Soybean

This study was conducted on soybean (Glycine max L. Merr.) nodules to determine if exogenous NH₃ exerts a controlling influence over nitrogenase activity through changes in nodule gas permeability (P), and if decreasing carbohydrate availability, as a result of low-light treatment, increases the sensitivity of root nodules to NH₃. Nodulated root systems of intact plants were exposed to one of several NH₃ concentrations ranging from 0 to 821 microliters per liter for an 8-hour period. Treatments were conducted under high-light (2300 micromoles per square meter per second) or low-light (800 micromoles per square meter per second) conditions. Increasing the NH₃ concentration and length of exposure of NH₃ caused a progressive decline in acetylene reduction activity (ARA). There was generally a greater reduction in ARA under the low-light treatment compared to the high-light treatment at a particular NH₃ concentration. The NH₃ concentration necessary to decrease P was greater than that needed to decrease ARA, and there was no evidence of a causal relationship between P and ARA in response to NH₃.     

Interactive Effects of CO(2) and O(2) in Soil on Root and Top Growth of Barley and Peas

Barley and pea plants were grown under several regimens of different compositions of soil atmosphere, the O₂ concentration varying from 0 to 21% and the CO₂ concentration from 0 to 8%. In absence of CO₂, the effect of O₂ on root length in barley was characterized by equal root lengths within the range of 21 to 7% O₂ and a steep decline between 7 and 0%. In peas, while showing the same general response, the decline occurred between 14 and 7% O₂. Root numbers of the seminal roots of barley decreased already with reduction in O₂ concentration from 21 to 14%. Dry matter production was affected somewhat differently by O₂ and CO₂ concentration. Dry matter production in barley was reduced at 14% O₂ while root length decreased between 7 and 0%. In peas, dry matter production was favored by low CO₂ concentrations except where there was no oxygen. At 21% O₂, increasing CO₂ concentrations did not seem to affect root length up to concentrations of 2% CO₂. At 8% CO₂, root length was decreased. The inter-active effects of CO₂ and O₂ are characterized by a reduced susceptibility to CO₂ at O₂ values below 7%, and a very deleterious effect of 8% CO₂ at 7% O₂.     

Fungal Growth Stimulation by CO(2) and Root Exudates in Vesicular-Arbuscular Mycorrhizal Symbiosis

Transformed roots of carrot were used to determine the effects of root metabolites on hyphal development from spores of the vesicular-arbuscular mycorrhizal fungus Gigaspora margarita. Hyphal growth of this obligately biotrophic symbiont was greatly stimulated by a synergistic interaction between volatile and exudated factors produced by roots. Root volatiles alone provided little stimulation, and root exudates alone had no effect. For the first time, carbon dioxide was demonstrated to be a critical root volatile involved in the enhancement of hyphal growth. C-labeled root volatiles were fixed by the fungus and thus strongly suggested that CO(2) served as an essential carbon source.     

Effects of High Atmospheric CO(2) and Sink Size on Rates of Photosynthesis of a Soybean Cultivar

The effect of sink strength on photosynthetic rates under conditions of long-term exposure to high CO₂ has been investigated in soybean. Soybean plants (Merr. cv. Fiskeby V) were grown in growth chambers containing 350 microliters CO₂ per liter air until pod set. At that time, plants were trimmed to three trifoliolate leaves and either 21 pods (high sink treatment) or 6 pods (low sink treatment). Trimmed plants were either left in 350 microliters CO₂ per liter of air or placed in 1000 microliters CO₂ per liter of air (high CO₂ treatment) until pod maturity. Whole plant net photosynthetic rates of all plants were measured twice weekly, both at 350 microliters CO₂ per liter of air and 1000 microliters CO₂ per liter of air. Plants were also harvested at this time for dry weight measurements. Photosynthetic rates of high sink plants at both measurement CO₂ concentrations were consistently higher than those of low sink plants, and those of plants given the 350 microliter CO₂ per liter of air treatment were higher at both measurement CO₂ concentrations than those of plants given the 1000 microliters CO₂ per liter of air treatment. When plants were measured under treatment CO₂ levels, however, rates were higher in 1,000 microliter plants than 350 microliter CO₂ plants. Dry weights of all plant parts were higher in the 1,000 microliters CO₂ per liter air treatment than in the 350 microliters CO₂ per liter air treatment, and were higher in the low sink than in the high sink treatments.     

Growth and Specific Nodule Activity of Soybean during Application and Recovery of a Leaf Moisture Stress

Soybean plants growing at day/night temperatures of 30/18, 26/18, and 22/18 C were subjected to a single drying and recovery cycle during an 18- to 19-day period in the early to midpod development stage. Leaf total electrochemical water potential was reduced to about −24 bars during the 4-day drying cycle at all temperatures, but recovered to control levels upon rewatering. The changes in dry matter accumulation in whole plants and plant parts, specific activity of nodules as measured by acetylene reduction, and levels of adenosine phosphates in nodules were measured periodically during stress and recovery.     

Effect of Restricted Root Growth on Carbohydrate Metabolism and Whole Plant Growth of Cucumis sativus L

The effects of varied rooting volumes on root growth and source leaf carbohydrate metabolism were studied in greenhouse-grown cucumber (Cucumis sativus L cv Calypso) plants. Plants were grown for 7 weeks in container volumes that ranged from 0.4 to 5.9 liters. Plants grown in the smaller containers exhibited less leaf expansion, lower root and shoot weight, and fewer lateral stems than plants grown in the 5.9 liter containers. Shoot/root ratio was not altered by the container volume, suggesting coordination of root and shoot growth due to rooting volume. Source leaf carbon exchange rates, assimilate export rates, and starch accumulation rates for plants grown in 0.4 liter containers were approximately one-half or less in comparison to those for plants grown in 5.9 liter containers. Starch concentrations per unit leaf area were maintained at high levels in source leaves of plants grown in 0.4 liter containers over the entire day/night cycle. Lower extractable galactinol synthase activities and higher galactinol concentrations occurred in leaves of plants grown in 0.4 liter container volumes. The reduced sink demand, induced by restricted root growth, may have led to increased starch concentrations and to a reduction in stachyose biosynthesis in cucumber source leaves.