The effects of elevated atmospheric CO2 on lipid metabolism in leaves from mature wheat (Triticum aestivum cv. Hereward) plants

Winter wheat (Triticum aestivum L. cv. Hereward) plants were grown for 35 d either at 350 μ mol mol^–1 CO₂ or at 650 μ mol mol^–1 CO₂. Lipid synthesis was studied in these plants by incubating the 5th leaf on the main stem with [1–¹⁴C]acetate. Increased CO₂ concentrations did not significantly affect the total incorporation of radiolabel into lipids of whole leaf tissue, but altered the distribution for individual lipid classes. Most noticeable amongst acyl lipids was the reduction in labelling of diacylglycerol and a corresponding increase in the proportion of phosphatidylcholine labelling. In the basal regions, there were similar changes and, in addition, phosphatidylglycerol labelling was particularly increased following growth in an enriched CO₂ atmosphere. The stimulation of labelling of the mitochondrial-specific lipid, diphosphatidylglycerol, prompted an examination of the mitochondrial population in wheat plants. Mitochondria were localized in intact wheat sections by immunolabelling for the mitochondrial-specific chaperonin probe. Growth in elevated CO₂ doubled the number of mitochondria compared to growth in ambient CO₂. Fatty acid labelling was also significantly influenced following growth at elevated CO₂ concentrations. Most noticeable were the changes in 16C:18C ratios for the membrane lipids, phosphatidylcholine, phosphatidylglycerol and monogalactosyldiacylglycerol. These data imply a change in the apportioning of newly synthesized fatty acids between the 'eukaryotic' and 'prokaryotic' pathways of metabolism under elevated CO₂.     

Modifications of the carbon and nitrogen allocations in the plant (Triticum aestivum L.) soil system in response to increased atmospheric CO2 concentration

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO₂ concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ¹⁴CO₂ concentration of 350 or 700 L L^–1 at two levels of soil nitrogen fertilization. Doubling CO₂ availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO₂ enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO₂ increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO₂ availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO₂ enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-¹⁴C came from respiration, leaving only 10 to 15% in the soil as organic ¹⁴C. However, when total root-derived ¹⁴C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO₂, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO₂ concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO₂ increase.Abbreviations N soil fertilization without nitrogen - N soil fertilization with nitrogen     

The photosynthesis – leaf nitrogen relationship at ambient and elevated atmospheric carbon dioxide: a meta-analysis

Estimation of leaf photosynthetic rate (A) from leaf nitrogen content (N) is both conceptually and numerically important in models of plant, ecosystem, and biosphere responses to global change. The relationship between A and N has been studied extensively at ambient CO₂ but much less at elevated CO₂. This study was designed to (i) assess whether the A–N relationship was more similar for species within than between community and vegetation types, and (ii) examine how growth at elevated CO₂ affects the A–N relationship. Data were obtained for 39 C3 species grown at ambient CO₂ and 10 C3 species grown at ambient and elevated CO₂. A regression model was applied to each species as well as to species pooled within different community and vegetation types. Cluster analysis of the regression coefficients indicated that species measured at ambient CO₂ did not separate into distinct groups matching community or vegetation type. Instead, most community and vegetation types shared the same general parameter space for regression coefficients. Growth at elevated CO₂ increased photosynthetic nitrogen use efficiency for pines and deciduous trees. When species were pooled by vegetation type, the A–N relationship for deciduous trees expressed on a leaf-mass basis was not altered by elevated CO₂, while the intercept increased for pines. When regression coefficients were averaged to give mean responses for different vegetation types, elevated CO₂ increased the intercept and the slope for deciduous trees but increased only the intercept for pines. There were no statistical differences between the pines and deciduous trees for the effect of CO₂. Generalizations about the effect of elevated CO₂ on the A–N relationship, and differences between pines and deciduous trees will be enhanced as more data become available.     

The regulation of nitrogen accumulation in the grain of wheat plants (Triticum aestivum)

Nitrogen accumulation in the ear of wheat plants ( Triticum aestivum L. cv. Klein Chamaco) during ear growth was studied under 4 experimental conditions. Plants were grown in pots with Perlite or soil, and fertilized with nutrient solutions. In one experiment the plants were grown in a greenhouse and supplied with high (16m M ) or low (1.6 m M ) N in the nutrient solutions until anthesis, and then with or without nitrogen supply until ripening. In a second experiment the plants were grown with high N supply until anthesis, and then for half of the plants light intensity was decreased by 50%, and at the same time. N supply was terminated for half of the plants within each light treatment. A third experiment was similar to the previous one, but was carried out in a growth cabinet under 20% of the maximal irradiance in the greenhouse. In a fourth experiment half the ear was excised at anthesis in half of the plants, and these plants were then supplied with or without nitrogen.
In all experiments there was a linear relation between the rate of N accumulation and the rate of ear growth. A wide range of final individual grain weights and N concentration was observed among the experiments. The same maximum N concentration was observed for all grain sizes, although the N concentration could be different between grains of the same size. The grain N concentration correlated with the rate of N accumulation per unit of ear weight increase during ear growth. It is suggested that in wheat plants there is a dependence of nitrogen transport on carbon transport to the ear, and to the ear, and that the final grain N concentration is determined by the N/C ratio exported from the vegetative tissues.     

Amino acid metabolism associated with N-mobilization from the flag leaf of wheat (Triticum aestivum L.) during grain development

Field-grown winter wheat (Triticum aestivum L. cv. Castell) was used to study changes in the free amino acid pools of different plant parts and related enzyme activities in the flag leaf throughout the grain-filling period in three consecutive growing seasons. Amino acid analysis data indicated that, during senescence, the nitrogen flow in the flag leaf was directed towards the synthesis of glutamine as a specific nitrogen transport form. Of the enzymes involved, total glutamine synthetase (GS; EC 6.3.1.2) and especially ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1) activities declined continuously as senescence progressed. Unlike (chloroplastic) GS₂, (cytosolic) GS₁ was shown to be very persistent suggesting a special role for this isoenzyme in the N-reallocation process. Glutamate-oxaloacetate transaminase (GOT; EC 2.6.1.1), glutamate-pyruvate transaminase (GPT; EC 2.6.1.2) and isocitrate dehydrogenase (IDH; EC 1.1.1.42) showed a characteristic biphasic activity profile after anthesis. It is proposed that these enzymes, for each of which at least two isoenzymes were demonstrated, are involved in glutamate synthesis at the later stages of leaf senescence. Ammonium levels were fairly constant throughout the flag leafs life span, an ultimate rise often following peak values of glutamate dehydrogenase (GDH; EC 1.4.1.4) activity. The enzymology of flag leaf amino acid metabolism during grain development is further discussed in relation to observations of NH₃-volatilization from naturally senescing wheat plants.     

Effects of foliar applied benzyladenine on grain yield and grain protein in wheat (Triticum aestivum L.)

The effects of foliar applications of nitrogen and benzyladenine (BA) on grain yield and grain protein of wheat grown under field conditions were studied over 2 years with 5 cultivars at 2 locations. Nitrogen (N) at 20 kg.ha^–1, and BA at 100 or 800 mg.l^–1 were applied alone or combined at pre and post-anthesis; applications of BA at 8 mg.l^–1 were also made on individual ears in order to study the effect on cell number. Weekly determinations of the chlorophyll content of the flag leaf were conducted after anthesis to study leaf senescence. At harvest, yield, yield components and grain protein percentage were determined. N and BA applications delayed chlorophyll loss in the flag leaf, but modified neither yield nor yield components. Foliarly applied BA increased grain protein in four of the five cultivars tested. It is concluded that delay of the senescence induced by BA might allow more energy to be available for N uptake by the crop leading to an increase in grain protein.Research supported by a CAFPTA grant 1656/86 and by CONICET, PID 30017700/85.CONICETComisión de Investigaciones Cientificas de la Provincia de Buenos AiresInstituto de Fisiologia Vegetal     

The effect of elevated atmospheric carbon dioxide levels on soil bacterial communities

The effect of elevated carbon dioxide levels on total bacterial communities was studied in a series of controlled and replicated model terrestrial ecosystems over a period of 38 weeks. The bacterial community was profiled using Denaturing Gradient Gel Electrophoresis (DGGE) analysis of bacterial 16S rRNA gene fragments amplified by the Polymerase Chain Reaction from DNA extracted directly from soil. Bacterial community DGGE profiles provided three major findings: (i) there was a high degree of profile similarity after ≈ 12 weeks (one plant generation); (ii) whilst overall DGGE profile was maintained over the 38 weeks (three plant generations), the banding patterns became more diverse with time; (iii) DGGE data provided no evidence for a shift in bacterial community structure resulting from exposure of the ecosystem to an increased atmospheric CO₂ level.     

Effects of elevated carbon dioxide and ozone on the growth and yield of spring wheat (Triticum aestivum L.)

Spring wheat cv. Minaret was grown under three carbon dioxide(CO₂) and two ozone (O₃) concentrations from seedling emergenceto maturity in open-top chambers. Under elevated CO₂ concentrations,the green leaf area index of the main shoot was increased, largelydue to an increase in green leaf area duration. Biomass increasedlinearly in response to increasing CO₂ (ambient, 550 and 680ppm). At anthesis, stem and ear dry weights and plant heightwere increased by up to 174%, 5% and 9 cm, respectively, andbiomass at maturity was 23% greater in the 680 ppm treatmentas compared to the ambient control. Grain numbers per spikeletand per ear were increased by 0.2 and 5 grains, respectively,and this, coupled with a higher number of ears bearing tillers,increased grain yield by up to 33%. Exposure to a 7 h daily mean O₃ concentration of 60 ppb inducedpremature leaf senescence during early vegetative growth (leaves1–7) under ambient CO₂ concentrations. Damage to the mainshoot and possible seedling mortality during the first 3 weeksof exposure altered canopy structure and increased the proportionof tillers 1 and 2 which survived to produce ears at maturitywas increased; as a result, grain yield was not significantlyaffected. In contrast to the older leaves, the flag leaf (leaf8) sustained no visible O₃ damage, and mean grain yield perear was not affected. Interactions between elevated CO₂ andO₃ influenced the severity of visible leaf damage (leaves 1–7),with elevated CO₂ apparently protecting against O₃-induced prematuresenescence during early vegetative growth. The data suggestthat the flag leaf of Minaret, a major source of assimilateduring grain fill, may be relatively insensitive to O₃ exposure.Possible mechanisms involved in damage and/or recovery are discussed. Key words: Carbon dioxide, ozone, spring wheat (cv. Minaret), leaf damage, tiller, yield     

Increasing in ROS levels and callose deposition in peduncle vascular bundles of wheat (Triticum aestivum L.) grown under nitrogen deficiency

Nitrogen availability is closely related to crop senescence and productivity, but its associated effect on reserve remobilization is not yet fully understood. In this study, we observed that nitrogen deficiency (N^?) led to significant decreases in the activities of superoxide dismutase (SOD) (P<0.05), guaiacol peroxidase (P<0.05), and catalase (P<0.05) as well as a higher concentration of reactive oxygen species (ROS) (P<0.05) in wheat (Triticum aestivum L.) peduncles during the middle grain-filling compared with the application of 225 kg N ha^?1 (N⁺). Callose concentration showed the same trend of temporal changes as ROS. Histochemical staining revealed that both ROS and callose predominantly occurred in vascular bundles of peduncles. Ultimately, grain filling rates and grain weight in N^? wheat were reduced compared with N⁺ plant. These data suggest that the grain yield decline in N^? wheat may be at least partially attributed to the higher callose deposition in peduncle vascular bundles and ROS level is closely associated with the increase in the callose deposition in wheat peduncle vascular bundles.     

The effect of temperature and CO2 on seed quality development in wheat (Triticum aestivum L.)

Winter wheat (Triticum aestivum L.) cv. Hereward was grown inthe field in two double-walled polyethylene-covered tunnelswithin each of which a temperature gradient was superimposedon diurnal and seasonal fluctuations in temperature. The meantemperature between anthesis and harvest maturity varied from14.3 to 18.4C among plots within these tunnels. The CO₂ concentrationwas controlled at different values in each tunnel; seasonalmean concentrations were 380 and 684 µmol CO₂ mol^–1air. Crops were also grown outside the tunnels at ambient temperaturesand CO₂. Samples of seeds were harvested sequentially from eachplot between anthesis and harvest maturity. Seed germinationand seed survival during subsequent air-dry storage were determinedfor each sample. The onset of both ability to germinate anddesiccation tolerance (ability to germinate after rapid desiccationto 10–15% moisture content and subsequent rehydration)coincided in all environments. Full germination capacity (>97%, determined at 10C) was reached 4–18 d before theend of the seed-filling phase (mass maturity) in most cases.There was little or no decline in germination capacity duringsubsequent seed development and maturation. Differences in seedquality were evident, however, throughout seed development andmaturation when seed survival curves during subsequent storagewere compared. Potential longevity in air-dry storage (assessedby the value K₁ of the seed viability equation) improved consistentlyboth before and after mass maturity. There was a significantpositive relation between the rate of increase in potentiallongevity (dK₁Idt) and temperature (the minimum temperaturefor seed quality development was 4.8 C), but neither CO₂ concentrationnor production within the polyethylene tunnels affected thisrelation. Key words: Wheat, Triticum aestivum L., seed development, seed longevity, carbon dioxide, temperature     

Carbon balance assessment of a Belgian winter wheat crop (Triticum aestivum L.)

The carbon balance of a winter wheat crop in Lonzée, Belgium, was assessed from measurements carried out at different spatial and temporal scales between November 2004 and August 2005. From eddy covariance measurements, the net ecosystem exchange was found to be ?0.63 kg C m^?2 and resulted from the difference between gross primary productivity (GPP) (?1.58 kg C m^?2) and total ecosystem respiration (TER) (0.95 kg C m^?2). The impact of the u_* threshold value on these fluxes was assessed and found to be very small. GPP assessment was partially validated by comparison with an estimation scaled up from leaf scale assimilation measurements. Soil respiration (SR), extrapolated from chamber measurements, was 0.52 kg C m^?2. Net primary productivity, assessed from crop sampling, was ?0.83 kg C m^?2. By combining these fluxes, the autotrophic and heterotrophic components of respiration were deduced. Autotrophic respiration dominated both TER and SR. The evolution of these fluxes was analysed in relation to wheat development.     

Mapping QTLs for nitrogen uptake in relation to the early growth of wheat (Triticum aestivum L.)

The objective of this study was to map QTLs for N uptake (NUP) in wheat, and to investigate factors influencing NUP. Two independent field trials with low N (LN) and high N (HN) treatments were conducted in the growing seasons of 2002–2003 (trial 1) and 2003–2004 (trial 2) to measure NUP per plant (N accumulated in the aerial part at maturity stage) of a doubled haploid (DH) population consisting of 120 DH lines derived from winter wheat varieties Hanxuan 10 and Lumai 14. A hydroponic culture with all nutrients supplied sufficiently was conducted to investigate shoot dry weight (SDW), root dry weight (RDW), tiller number (TN) and NUP (total plant N uptake) per plant of this mapping population at seedling stage. SDW, RDW, TN and NUP investigated in the hydroponic culture were significantly and positively correlated with each other, and with NUP under both LN and HN conditions in the field trials. Nine and eight QTLs for NUP were detected under LN and HN conditions in the field trials, respectively. Four to five QTLs for SDW, RDW, TN and NUP were detected in the hydroponic culture. One SDW QTL, three RDW QTLs, two TN QTLs detected in the hydroponic culture were linked with QTLs for NUP under LN or HN condition in the field trials. The positive correlation and genetic linkage for the traits between the field trials and the hydroponic culture demonstrated that greater seedling vigor of root and shoot is an important factor influencing N uptake in wheat. Diaoguo An and Junying Su: These authors contributed equally to this work. Section Editor: H.J. Kronzucker     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Responses of stomatal conductance to light, humidity and temperature in winter wheat and barley grown at three concentrations of carbon dioxide in the field

Responses of leaf stomatal conductance to light, humidity and temperature were characterized for winter wheat and barely grown at ambient (about 350 μmol mol^?1 in the daytime), ambient + 175 and ambient + 350 μmol mol^?1 concentrations of carbon dioxide in open‐topped chambers in field plots over a three year period. Stomatal responses to environment were determined by direct manipulation of single environmental factors, and those results were compared with responses derived from natural day to day variation in mid‐day stomatal conductance. The purpose of these experiments was to determine the magnitude of reduction in stomatal conductance at elevated [CO₂], and to assess whether the relative response of conductance to elevated [CO₂] was constant across light, humidity and temperature conditions. The results indicated that light, humidity and temperature all significantly affected the relative decrease in stomatal conductance at elevated [CO₂]. The relative decrease in conductance with elevated [CO₂] was greater at low light, low water vapour pressure difference, and high temperature in both species. For measurements made at saturating light near mid‐day, the ratio of mid‐day stomatal conductances at doubled [CO₂] to that at ambient [CO₂] ranged from 0.42 to 0.86, with a mean of 0.66 in barley, and from 0.33 to 0.80, with a mean of 0.56 in wheat. Day‐to‐day variation in the relative effect of elevated [CO₂] on conductance was correlated with the relative stimulation of [CO₂] assimilation rate and with temperature. Some limitations of multiple linear regression, multiplicative, and ‘Ball–Berry' models as summaries of the data are discussed. In barley, a better fit to the models occurred in individual years than for the combined data, and in wheat a better fit to the models occurred when data from near the end of the season were removed.     

Impact of elevated ozone concentration on growth,physiology, and yield of wheat (Triticum aestivum L.): a meta‐analysis

We quantitatively evaluated the effects of elevated concentration of ozone (O₃) on growth, leaf chemistry, gas exchange, grain yield, and grain quality relative to carbon‐filtered air (CF) by means of meta‐analysis of published data. Our database consisted of 53 peer‐reviewed studies published between 1980 and 2007, taking into account wheat type, O₃ fumigation method, rooting environment, O₃ concentration ([O₃]), developmental stage, and additional treatments such as drought and elevated carbon dioxide concentration ([CO₂]). The results suggested that elevated [O₃] decreased wheat grain yield by 29% (CI: 24–34%) and aboveground biomass by 18% (CI: 13–24%), where CI is the 95% confidence interval. Even in studies where the [O₃] range was between 31 and 59 ppb (average 43 ppb), there was a significant decrease in the grain yield (18%) and biomass (16%) relative to CF. Despite the increase in the grain protein content (6.8%), elevated [O₃] significantly decreased the grain protein yield (?18%). Relative to CF, elevated [O₃] significantly decreased photosynthetic rates (?20%), Rubisco activity (?19%), stomatal conductance (?22%), and chlorophyll content (?40%). For the whole plant, rising [O₃] induced a larger decrease in belowground (?27%) biomass than in aboveground (?18%) biomass. There was no significant response difference between spring wheat and winter wheat. Wheat grown in the field showed larger decreases in leaf photosynthesis parameters than wheat grown in < 5 L pots. Open‐top chamber fumigation induced a larger reduction than indoor growth chambers, when plants were exposed to elevated [O₃]. The detrimental effect was progressively greater as the average daily [O₃] increased, with very few exceptions. The impact of O₃ increased with developmental stages, with the largest detrimental impact during grain filling. Both drought and elevated [CO₂] significantly ameliorated the detrimental effects of elevated [O₃], which could be explained by a significant decrease in O₃ uptake resulting from decreased stomatal conductance.     

Lifetime and temporal occurrence of ectomycorrhizae on ponderosa pine (Pinus ponderosa Laws.) seedlings grown under varied atmospheric CO2 and nitrogen levels

Climate change (elevated atmospheric CO2, and altered air temperatures, precipitation amounts and seasonal patterns) may affect ecosystem processes by altering carbon allocation in plants, and carbon flux from plants to soil. Mycorrhizal fungi, as carbon sinks, are among the first soil biota to receive carbon from plants, and thereby influence carbon release from plants to soil. One step in this carbon release is via fine root and mycorrhizal turnover. It is necessary to know the lifetime and temporal occurrence of roots and mycorrhizae to determine the capacity of the soil ecosystem to sequester carbon assimilated aboveground. In this study, ponderosa pine (Pinus ponderosa Laws) seedlings were grown under three levels of atmospheric CO2 (ambient, 525 and 700 mol CO2 mol-1) and three levels of annual nitrogen additions (0,100 and 200 kg N ha-1) in open-top chambers. At a two-month frequency during 18 months, we observed ectomycorrhizal root tips observed using minirhizotron tubes and camera. The numbers of new mycorrhizal root tips, the numbers of tips that disappeared between two consecutive recording events, and the standing crop of tips at each event were determined. There were more mycorrhizal tips of all three types seen during the summer compared with other times of the year. When only the standing crop of mycorrhizal tips was considered, effects of the CO2 and N addition treatments on carbon allocation to mycorrhizal tips was weakly evident. However, when the three types of tips were considered collectively, tips numbers flux of carbon through mycorrhizae was greatest in the: (1) high CO2 treatment compared with the other CO2 treatments, and (2) intermediate N addition treatment compared with the other N addition treatments. A survival analysis on the entire 18 month cohort of tips was done to calculate the median lifetime of the mycorrhizal root tips. Average median lifetime of the mycorrhizal tips was 139 days and was not affected by nitrogen and CO2 treatments.     

Suppression of cell wall stiffening along coleoptiles of wheat (Triticum aestivum L.) seedlings grown under osmotic stress conditions

Effects of polyethylene glycol (PEG)-induced osmotic stress on the mechanical properties of cell walls and the levels of their components were investigated along intact wheat (Triticum aestivum L.) coleoptiles. Stress-relaxation analysis showed that the cell walls of stressed coleoptiles were loosened as compared with those of unstressed ones not only in the apical but in the basal regions. The amounts of wall-bound ferulic acid (FA) and diferulic acid (DFA) of stressed coleoptiles were substantially lower than those of unstressed ones in all regions. The cellulose and hemicellulose contents increased toward the coleoptile base. Osmotic stress reduced the cellulose content in the basal region but it slightly affected the hemicellulose content. The molecular weight of hemicellulose in the apical region of stressed coleoptiles was higher than that of unstressed ones, while that in the basal region was almost the same in both coleoptiles. FA, DFA and cellulose contents correlated with the cell wall mechanical property. The amount and molecular weight of hemicellulose, however, did not correlate. These results suggest that the reduced levels of FA and DFA in all regions and cellulose in the basal region of wheat coleoptiles are involved in maintaining the cell wall extensibility under osmotic stress.