Problems in the methods of estimation of growth and maintenance respiration

According to Thornley, J.H.M. (Nature, 227, 304-305, 1970) and McCree, K.J. (Crop Sci., 14, 509-514, 1974), respiratory substances are used only for maintenance respiration when plants are exposed to the dark conditions for a long period of time (more than 2 d). The maintenance respiration is also affected by the nitrogen status in plant, because protein turnover is one of the major energy consumption sources under maintenance process. Therefore, to determine whether respiratory substances are used only for maintenance, ¹⁴C- [U] -sucrose or a mixture of ¹⁴C- [U] -amino acids was introduced to rice and soybean plants from the tip of leaf. Plants were grown under natural light conditions and under dark conditions for 4 d with 2 nitrogen levels (0.2 and 0.02 g N L^-1 soil). After the introduction of the ¹⁴C-compounds, the ¹⁴CO₂ respiratory rate was monitored during 24 h, then the ¹⁴C distribution to organic acids, free amino acids, proteins, sugars, and polysaccharides was analyzed. Following results were obtained.

1. When ¹⁴C-[U]-sucrose or a mixture of ¹⁴C-[U]-amino acids was introduced to the leaf of rice and soybean plants, the ¹⁴C release rate by respiration was not affected by the nitrogen and light treatments except when ¹⁴C-sucrose was introduced to soybean in the low N plot. The ¹⁴C release rate from the ¹⁴C-compounds introduced into leaf in the low N plot of soybean was higher in the dark treatment than in the natural light treatment.

2. ¹⁴C-distribution ratio after introduction of ¹⁴C-sucrose and a mixture of ¹⁴C-amino acids to the leaf was not significantly affected by the nitrogen treatment. When ¹⁴C-sucrose was introduced to rice leaf, the ¹⁴C-distribution ratio to sugars and proteins was higher and that to polysaccharides was lower in the natural light treatment than in the dark treatment. The ¹⁴C-distribution ratio was less aifected by the light or nitrogen treatment in case of soybean leaf.

3. Although it was assumed that maintenance metabolism was dominant in the lower leaf (counted from the bottom), the ¹⁴C-distribution ratio was similar to that of upper leaf.

4. Nitrogen content of leaf was not different between rice and soybean in the high N treatment, unlike the ¹⁴C-distribution ratio. In rice, the nitrogen content of leaf was about twice as high in the high N treatment compared with the low N treatment, while the ¹⁴C-distribution ratio in leaf was stable regardless of nitrogen treatment.

Based on the above results, it is suggested that since the ¹⁴C-distribution ratio into each chemical component did not change regardless of light treatment, nitrogen treatment, or leaf age, It was impossible to separate respiration into two components, such as growth and maintenance respiration. The results also indicated that current photosynthates and storage substances were not used only for growth and maintenance, respectively.     

Factors Affecting on Response of Broad Bean and Corn to Air Quality and Soil CO2 Flux Rates in Egypt

In response to worldwide increases in the burning of fossil fuels to meet energy demands for electric power generation and transportation, atmospheric CO₂ concentrations are currently rising at approximately 0.5% per year and ground-level O₃ values are increasing at a rate of 0.32% per year. Some plants showed positive increases in response to elevated atmospheric CO₂ concentrations, but are depressed when exposed to enhanced O₃ air pollution. The objective of this research was to examine relationships between alterations in leaf plant characteristics in response to air quality treatments and soil CO₂ flux activities during the growing season. Field studies were conducted in 2-m diameter?×?2-m height open-top chambers (OTC’s) at Sharkia Province during 2004 and 2005 involving the growth of broad bean (Vicia faba L. cv. Giza 40) and corn (Zea mays L. cv. 30 K8) in rotations using no-till management while being subjected full-season to five air quality treatments: charcoal-filtered (CF) air; CF + 150 µL CO₂ L^?1; non-filtered (NF) air; NF + 150 µL CO₂ L^?1 and ambient air (AA). Leaf photosynthesis (P_s), leaf area index (LAI), and vegetative carbohydrate contents were determined during pre- and post-anthesis in the two crops and soil CO₂ flux rates were monitored monthly during two growing seasons (2004–2005). Multiple and stepwise regression analyses were performed to establish linkages between plant canopy characteristics and soil CO₂ flux rates with results combined over growth stages and year for each crop. Increasing the atmospheric CO₂ concentration typically stimulated leaf P_s, soluble and total leaf carbohydrate contents, LAI values, and soil CO₂ flux rates throughout the growing season in both crop; however, the elevated O₃ treatments in NF air tended to lower these values compared to CF air. Soil CO₂ flux rates were significantly correlated with LAI, soluble and total sugar contents at P?≤?0.01 and with P_s rates at P?≤?0.05 in broad bean leaves, but with soluble and total sugar contents of leaves in corns at P?≤?0.01 only. Results of this study provided solid evidences linking the impact of changing air quality on plants factors processes and possible indirect effects on soil CO₂ flux activities throughout the growing season.     

Comparative studies on the photosynthesis of higher plants

The rate of sugar formation from aspartate-¹⁴C(U) and alanine-L-¹⁴C was examined under various light intensities in three C₄-plants. The results obtained were as follows.

The rates of sugar formation from aspartate-¹⁴C(U) became larger in the following order, Paspalum urvillei, Egragrostis ferrunginea, and Zoysia japonica. This order agreed well with the order of their photosynthetic rates measured by gas analyzer. In all the C₄-plants, there were three steps in the sugar formation curve from aspartate-¹⁴C(U). At first, sugar linearly increased with an increase in the light intensity up to 20 klux. Second, from 20 to 40 klux, it hardly increased with an increase in the light intensity. Third, above 40 klux, it increased linearly again. On the other hand, the plateau did not exist between 20 and 40 klux in the sugar formation curves from alanine-l-¹⁴C in any of the cases, and sugar continued to increase with an increase in the light intensity up to 80 klux.

At low light intensities, the amount of CO₂ released from aspartate-¹⁴C(U) and alanine-l-¹⁴C correlated well with the magnitude of the dark respiration in the C₄-plants. At a high light intensity, however, CO₂ release closely correlated with the thickness of mesophyll layers surrounding the vascular bundle sheath. The thicker the mesophyll layers were, the smaller the release of CO₂ became. From this evidence, we conclude that the mesophyll layers play a vital role in refixation of the internal CO₂ in the light.     

Roles of glycine betaine in mitigating deleterious effect of salt stress on lettuce (Lactuca sativa L.)

This study was conducted to evaluate the roles of glycine betaine (GB) in mitigating deleterious effect of salt stress on lettuce. Lettuce plants were subjected to two salinity (0 and 100 mmol l^?1 NaCl) and four GB levels (0, 5, 10, 25 mmol l^?1). Salinity resulted in a remarkable decrease in growth parameters, relative leaf water content and stomatal conductance. Plants subjected to salt stress exhibited an increase in membrane permeability (MP), lipid peroxidation (MDA), leaf chlorophyll reading value, H₂O₂ and sugar content. Exogenous foliar applications of GB reduced MP, MDA and H₂O₂ content in salt-stressed lettuce plants. Salt stress increased Na and generally decreased other nutrient elements. GB reduced Na accumulation, but significantly increased other element contents under salinity conditions. The study showed that gibberellic acid (GA) and salicylic acid (SA) content in salt-stressed plants were lower than those of nonstressed plants. However, salinity conditions generally increased the abscisic acid content. GB treatments elevated the concentrations of GA, SA and indole acetic acid (IAA) at especially 10 and 25 mmol l^?1 GB under salt stress conditions. It could be concluded that exogenous GB applications could ameliorate the harmful effects of salt stress in lettuce.     

Method for 14C‐labeling maize field plots and assessment of label uniformity within plots

Abstract

There is increasing interest in use of isotopic tracers to study nutrient liberation and transformation in plant tissues and soils. We developed a technique for pulse‐labeling plants in the field with ¹⁴C. Spatial distribution of radioactivity was measured in plots of maize (Zea mays L.) plants exposed to ¹⁴CO₂. Two clear polyvinyl chambers measuring 1 m wide × 2 m long × 1 m high were used to ¹⁴C‐ label maize plants in conventional tillage and no‐tillage treatments. A closed loop in‐line with a pump allowed injection of ¹⁴CO₂ and unlabeled CO₂, and subsampling through an infrared gas analyzer. Cooling and mixing of the air within the chambers was achieved through the use of a free‐standing automobile radiator with fan placed in the center of each plot. The specific activities of leaf tips differed by an order of magnitude among maize plants within the plot. Tillage and time after labeling within the first 48 h had no significant effect on specific activity of maize plants. Plant activity significantly differed by row. The row closest to the inlet and along the edge of the chamber was significantly lower in several plots. Despite differences among leaf tip specific activities, total aboveground activity was uniform within the plot. Plant allometry and plant sampling immediately after labeling would help in correcting for within chamber variability in future field labeling studies.     

Tomato growth and mineral composition as influenced by nitrogen form and light intensity

Abstract

Tomato plants were grown in sand culture with NH⁺ ₄, and NO^? ₃, forms of N and three levels of light. Plants supplied with NH⁺ ₄, nutrition under high light intensity had symptoms of stunting, leaf roll, wilting, interveinal chlorosis of the older leaves, and one third the dry weight of N0₃‐fed plants. In contrast, growth of plants receiving NH⁺ ₄, nutrition under shade appeared normal although dry weight was reduced. NH₄‐N nutrition suppressed K, Ca and Mg accumulation in tissues and increased P contents as compared to NO₃‐N nutrition.     

Distribution of 14C in Pulse-Labelled Spring Barley. Effect of Light Intensity and Length of Photoperiod Before Labelling

Abstract

The distribution of photoassimilated C in spring barley plants was determined at different times after the onset of light and at different light intensities during assimilation. The plants were grown in pots in a greenhouse, and at late tillering and late elongation, ¹⁴CO₂ pulse-labellings of 2 h duration were carried out 1.5, 4 or 8.5 h after the onset of light. At the labelling started after a 4 h photoperiod, two light intensities was included (80 and 170 W m^?2).

To analyse samples low in ¹⁴C, a ¹⁴CO₂-trapping system interfaced with a Leco high-temperature induction furnace was developed. The ¹⁴CO₂ was trapped directly in the scintillation vial in 5 mL of liquid Carbosorb, enabling subsequent liquid ¹⁴C-scintillation counting to take place without subsampling.

The proportion of photosynthate translocated below ground tended to be higher early in the morning than later in the day. Labelling 1.5 h after the onset of light, 19.8 and 7.6% was translocated below ground at late tillering and late elongation, respectively. Corresponding values found at later labellings were 15.4–16.0 and 6.0–6.4%.

Higher proportions tended to be translocated below ground when plants were exposed to low light intensity. Exposing plants to low light intensity caused below ground translocation to be 15.4 and 6.0% of the ¹⁴C recovered at late tillering and late elongation, respectively, compared with 12.1 and 5.2% after exposure to a higher light intensity. Further experiments are needed to substantiate the observations of this study. The results suggest that the distribution of photoassimilates varies during the daytime and light intensity during the labelling.     

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.     

La(NO3)3 对盐胁迫下黑麦草幼苗生长及抗逆生理特性的影响

为探讨稀土元素镧(La)对牧草盐胁迫伤害的缓解作用, 采用水培法研究了叶面喷施20 mg·L^-1La(NO₃)₃ 对NaCl 胁迫下黑麦草幼苗生长及其抗逆生理特性的影响。结果表明: 盐胁迫显著抑制黑麦草幼苗的生长, 提高叶片电解质渗漏率及丙二醛(MDA)、O₂^- 和H₂O₂ 含量, 其作用随盐浓度的增大而增强。超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)活性和抗坏血酸(AsA)、谷胱甘肽(GSH)、可溶性蛋白质、脯氨酸含量随盐浓度增大呈先升后降趋势, 可溶性糖和Na⁺/K⁺比逐渐增大, 质膜H⁺-ATP 酶活性逐渐降低, 过氧化物酶(POD)活性及POD 同功酶数量表达增强。喷施La(NO₃)₃ 处理可降低盐胁迫下黑麦草幼苗叶片的O₂^- 和H₂O₂ 含量, 提高SOD、CAT、POD、APX 和质膜H⁺-ATP 酶的活性及POD 同功酶的表达, 使AsA、GSH、可溶性蛋白质、可溶性糖和游离脯氨酸含量及幼苗生物量增加, Na⁺/K⁺比降低。表明La(NO₃)₃ 可通过提高抗氧化系统的活性和积累渗透溶质减轻盐胁迫伤害, 从而提高黑麦草的耐盐性。     

Effects of different nitrogen forms on potato growth and development

The objective of this study was to test if the effects of different nitrogen forms on potato growth depend on the plant growth stage. Plants from different potato cultivars were treated with different forms of nitrogen before tuber initiation and after tuber formation. A nitrification inhibitor was used to prevent the transformation of ammonium (NH₄⁺) to nitrate (NO₃^?). Plant growth, tuber formation, leaf area, leaf chlorophyll content, and tuber yield were assessed. The results obtained over 2 years indicate that plants treated with NO₃-nitrogen (N) before or at tuber initiation produced more tubers per plant than those treated with NH₄-N. However, plants treated with NH₄-N develop tubers earlier. Additionally, after tuber formation, plants treated with NH₄-N had better shoot growth than plants treated with NO₃-N. A larger leaf area with higher leaf chlorophyll content resulted in greater dry matter accumulation and higher tuber yield at harvest for plants treated with NH₄-N.     

Physiological changes in soybean treated with ozone and molybdenum

Abstract

An open‐top field chamber experiment was conducted to evaluate the impact of Molybdenum (Mo) addition to soil on the physiological changes in soybean (Glycine max L. Merrill) exposed to ozone (O₃). Plants grown with Mo (0, 1.0, or 2.0 mg kg"¹ soil dry weight) were exposed to O₃ (O, 0.06, or 0.12 μmol mol^‐1) in open‐top field chambers for 12 h d^‐1 for 21 d with a N‐free fertilizer, during the sensitive growth stage (R2). The rate of photosynthesis (PN), specific root nodule nitrogenase activity (SNA), leaf nitrogen (N), chlorophyll (chl‐a, chl‐b) and biomass of soybean were measured. The increase in O₃ levels significantly reduced PN, SNA, leaf‐N, chl‐a, chl‐b, and biomass. Addition of Mo increased leaf‐N, shoot, root, and nodule dry weights but did not change PN, SNA, or chlorophyll. The addition of Mo (2 mg kg ^‐1) helped in significantly increasing PN and chlorophyll in the presence of 0.06 umol mol^‐1 O₃ but no change was observed in the presence of 0.12 μmol mol^‐1 O₃.     

Carbon dioxide assimilation in urea‐treated wheat leaves

Application of 10 mM urea to the flag leaf of wheat plants enhanced in vivo urease activity several fold. Photosynthetic rate was also increased considerably. There were significant differences in the leaf internal carbon dioxide (CO₂) concentrations between the urea‐treated and untreated leaves. The finding that carbon (¹⁴C) was detected in the ethanol extract of the leaves fed with ¹⁴C‐urea suggests that CO₂ released from urea is re‐fixed by the leaves.     

Difference in system of current photosynthesized carbon distribution to carbon and nitrogen compounds between rice and soybean

¹⁴CO₂ was assimilated during 10 min in leaf of rice and soybean under 21 kPa O2 (21% O₂ treatment) and 2 kPa O₂ (2% O₂ treatment) at the vegetative growth stage and flowering stage. The ¹⁴C distribution ratio to respired CO₂ and crude chemical components (sugars, polysaccharides, amino acids, organic acids, and proteins) was determined. In this paper, since emphasis was placed on the ¹⁴C distribution mechanism to carbon compounds and nitrogen compounds, the terms carbon metabolism pool (C-pool) composed of sugars and polysaccharides, and nitrogen metabolism pool (N-pool) composed of organic acids, amino acids and proteins were used. The results obtained were as follows.

¹⁴C distribution ratio to N-pool at 0 min after ¹⁴C assimilation was higher in soybean than in rice regardless of the treatments and stages, and that at 30 min after ¹⁴C assimilation under light condition markedly decreased both in rice and soybean. Therefore, especially in soybean, a large amount of photosynthesized ¹⁴C was once distributed to the N-pool, then ¹⁴C compounds in the N-pool were reconstructed into the C-pool. During this reconstruction process, ¹⁴C compounds in the N-pool were actively respired.

¹⁴C distribution to N-pool at 0 min after ¹⁴C assimilation changed slightly or did not change by the N treatment. ¹⁴C distribution to N-pool in the - N treatment of soybean (13–29 mg N g^-1 content in leaves) was higher than that in the + N treatment of rice (31–48 mg N g^-1 content in leaves). Photosynthesized carbon distribution to N-pool in rice decreased with growth, while it remained constant in soybean. Accordingly, in soybean, photosynthesized carbon was predominantly distributed to the N-pool through photorespiration and/or Calvin cycle (supplying triose-P), which was less affected by nitrogen nutrient and aging. Thus, the mechanism of photosynthesized carbon distribution to carbon and nitrogen compounds was basically regulated by inherited characters of each plant more than by the nitrogen status of leaves.

By the 2% O₂ treatment, ¹⁴C distribution to N-pool decreased in both crops regardless of N treatment, indicating that photorespiration plays an important role in the supply of the preliminarily photosynthesized carbon compounds to N-pool. In the 2% O₂ treatment, ¹⁴C distribution to N-pool was higher in soybean than in rice, indicating that triose-P transported from chloroplast was preferentially distributed to N-pool in the case of soybean.     

Effect of elevated atmospheric CO2 concentration on growth and physiology of wheat and sorghum under cadmium stress

ABSTRACT

Elevated concentrations of carbon dioxide (e[CO₂]) affect plant growth and physiological characteristics, including metal accumulation, and the activity of anti-oxidant enzymes. These effects were investigated in cadmium (Cd) tolerant wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor (L.) Moench.) cultivars. Plants were grown at the ambient and elevated CO₂ levels, with four concentrations of Cd (0, 10, 20 and 40 mg kg^?1) added to the soil. After 60 days, subsamples were tested for chlorophylls and carotenoids, protein, enzyme activities and morphological characteristics.

Results showed that e[CO₂] increased plant height, leaf area, and the dry weight of shoots and roots (P < 0.01). In addition, it decreased the Cd concentration in the shoots and roots of wheat, and increased the same concentrations for sorghum. With increasing Cd, the activities of the anti-oxidants, SOD and GSH-px increased in wheat. The differences in enzyme activity parallel the changes in Cd concentration in the plants of both species.     

Stem labeling results in different patterns of 14C rhizorespiration and 15N distribution in plants compared to natural assimilation pathways

To investigate C and N rhizodeposition, plants can be ¹³C‐¹⁵N double‐labeled with glucose and urea using a stem‐feeding method (wick method). However, it is unclear how the ¹³C applied as glucose is released into the soil as rhizorespiration in comparison with the ¹³C applied as CO₂ using a natural uptake pathway. In the present study, we therefore compared the short‐term fate of ¹⁴C and ¹⁵N in white lupine and pea plants applied either by the wick method or the natural pathways of C and N assimilation. Plants were pulse‐labeled in ¹⁴CO₂‐enriched atmosphere and ¹⁵N urea was applied to the roots (atmosphere–soil) following the natural assimilation pathways, or plants were simultaneously labeled with ¹⁴C and ¹⁵N by applying a ¹⁴C glucose–¹⁵N urea solution into the stem using the wick method. Plant development, soil microbial biomass, total rhizorespiration, and distribution of N in plants were not affected by the labeling method used but by plant species. However, the ¹⁵N : N ratio in plant parts was significantly (p < 0.05) affected by the labeling method, indicating more homogeneous ¹⁵N enrichment of plants labeled via root uptake. After ¹⁴CO₂ atmosphere labeling of plants, the cumulated ¹⁴CO₂ release from roots and soil showed the common saturation dynamics. In contrast, after ¹⁴C‐glucose labeling by the wick method, the cumulated ¹⁴CO₂ release increased linearly. These results show that ¹⁴C applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short‐term ¹⁴CO₂ pulse. This is partly due to a slower ¹⁴C uptake and partly due to slow distribution within the plant. Consequently, ¹⁴C‐glucose application by the wick method is no pulse‐labeling approach. However, the advantages of the wick method for ¹³C‐¹⁵N double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.     

Foliar Injuries Caused by Ozone in Betula pubescens Ehrh. and Phleum pratense L. as Influenced by Climatic Conditions Before and During O3 Exposure

Seedlings of Betula pubescens Ehrh. (mountain birch) and Phleum pratense L. (timothy) were grown for 42 days under full light or 50% shade in the field at 12°C, and at comparable photosynthetic active radiation (PAR) levels in a greenhouse at 18°C. Plants from the four pretreatments were exposed to 78 nmol mol^-1 (ppb) O₃ (8 h day^-1) under two temperatures (15 and 25°C), two relative air humidities (50 and 80% RH) or two CO₂     

Effect of carbon dioxide on sorghum yield,root growth,and water use

The concentration of atmospheric carbon dioxide (CO₂) is rising. The effect of higher than ambient levels of CO₂ on plants grown in the sub-humid central Great Plains of the U.S.A. has not been investigated. Therefore, an experiment was conducted at Manhattan, Kansas, to study the effect of elevated levels of CO₂ on grain sorghum [Sorghum bicolor (L.) Moench]. During the summer of 1984, the sorghum was grown in rhizotrons in which root and shoot growth could be monitored throughout the growth cycle. The tops of the plants were enclosed in plastic chambers, which contained one of four concentrations of CO₂ : 330 (ambient), 485, 660, and 795 μl 1⁻¹.Enriched CO₂ delayed the boot, half bloom, and soft dough stages. Sorghum grown at elevated concentrations of CO₂ yielded more roots and shoots than plants grown with 330 μl 1⁻¹. At all soil-profile depths, root numbers and weights were higher at elevated CO₂ than at ambient CO₂. However, water use per unit dry matter of leaf, stem, root, and grain was decreased 13, 30, 31, and 29%, respectively, in plants grown at 795 μl 1⁻¹ CO₂ compared to plants at 330 μl 1⁻¹ CO₂. Although elevated CO₂ levels increased the stomatal resistance and leaf temperature, an increase in leaf area indices resulted in a lower canopy resistance.     

INFLUENCE OF A MYCORRHIZAL FUNGUS AND/OR RHIZOBIUM ON GROWTH AND BIOMASS PARTITIONING OF SUBTERRANEAN CLOVER EXPOSED TO OZONE

The influence of soilborne symbionts such as rhizobia or mycorrhizal fungi on plant response to ozone (O₃) has not been well defined. Leguminous plants in the field are infected by both types of organisms, which influence plant nutrition and growth. We studied the effects of infection with Rhizobium leguminosarum biovar trifolii and/or Gigaspora margarita on response of subterranean clover (Trifolium subterraneum L. cv Mt. Barker) to O₃. Exposures were conducted in greenhouse CSTR chambers using four O₃ concentrations [charcoal-filtered (CF), 50, 100, or 150 ppb; 6 h day^-1, 5 day wk^-1 for 12 weeks] as main plots (replicated). Four inoculum types were subplot treatments, i.e., inoculated with one, both, or neither microorganisms. At 2-wk intervals, plants were exposed to ¹⁴CO₂ and harvested 24 h later for determination of biomass and ¹⁴C content of shoots and roots. Ozone at 100 or 150 ppb suppressed clover growth during the experiment. Inoculation with G. margarita alone suppressed clover growth by the last two harvests, whereas R. leguminosarum alone enhanced growth during this time period. When both symbionts were present, the plants grew similarly to the noninoculated controls. Shoot/root ratios were increased by 100 or 150 ppb O₃ compared to that for CF-treated plants. Shoot/root ratios were greater for all inoculated plants compared to noninoculated controls. Under low O₃ stress (CF or 50 ppb), plants inoculated with both R. leguminosarum and G. margarita transported a greater proportion of recent photosynthate (¹⁴C) to roots than did noninoculated plants; we attribute this to metabolic requirements of the microorganisms. At the highest level of O₃ stress (150 ppb), this did not occur, probably because little photosynthate was available and the shoots retained most of it for repair of injury. Statistically significant interactions occurred between O₃ and inoculum types for shoot and total biomass. When averaged across harvests, 50 ppb O₃ suppressed biomass in the plants inoculated with G. margarita alone. Apparently, the mycorrhizal fungus is such a significant C drain that even a small amount of O₃ stress suppresses plant growth under these conditions.     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.     

Experimental studies on SO2 injuries in higher plants III: Inhibitory effect of suite ion on 14 CO 2 fixation

Photosynthesis decreases reversibly in plants exposed to SO₂. Photosynthesis recovers when the exposure to SO₂ is discontinued. Inactivation of a photosynthetic enzyme, ribulose-1,5-diphosphate carboxylase, by sulfonation of its SH groups was investigated as a cause of the reversible reduction of photosynthesis. The relationship between the sulfite ion concentration in the reaction mixture and ¹⁴CO₂ fixation catalized by the enzyme which was prepared from alfalfa leaves was explored by using radioactive NaHCO₃. About 50% and 85% inhibitions of ¹⁴CO₂ fixation were observed at 3 × 10^?3 M and 3 × 10^?2 M concentration of sulfite ion in the reaction mixture, respectively. The accumulation of 3 × 10^?4 M sulfite ion on the reaction site of the enzyme involved in the plants which were exposed to SO₂ could considerably reduce the CO₂ assimilation of the plant.