Carbon use in root respiration as affected by elevated atmospheric O2

The use of fossil fuel is predicted to cause an increase of the atmospheric CO₂ concentration, which will affect the global pattern of temperature and precipitation. It is therefore essential to incorporate effects of temperature and water supply on the carbon requirement for root respiration of plants to predict effects of elevated [CO₂] on the carbon budget of natural and managed systems.There is insufficient information to support the contentention that an increase in the concentration of CO₂ in the atmosphere will enhance the CO₂ concentration in the soil to an extent that is likely to affect root respiration. Moreover, there is no convincing evidence for a direct effect of elevated atmospheric [CO₂] on the rate of root respiration per unit root mass or the fraction of carbon required for root respiration. However, there are likely to be indirect effects of elevated [CO₂] on the carbon requirement of plants in natural systems.Firstly, it is very likely that the carbon requirement of root respiration relative to that fixed in photosynthesis will increase when elevated [CO₂] induces a decrease in nutrient status of the plants. Although earlier papers have emphasized that elevated [CO₂] favours investment of biomass in roots relative to that in leaves, these are in fact indirect effects. The increase in root weight ratio is due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. This will decrease the specific rate of root respiration, but increase the carbon requirement as a fraction of the carbon fixed in photosynthesis. It is likely that these effects will be minor in systems where the nutrient supply is very high, e.g. in many managed arable systems, and increase with decreasing soil fertility, i.e. in many natural systems.Secondly, a decrease in rainfall in some parts of the world may cause a shortage in water supply which favours the carbon partitioning to roots. Water stress is likely to reduce rates of root respiration per unit root mass, but enhance the fraction of total assimilates required for root respiration, due to greater allocation of biomass to roots.Increased temperatures are unlikely to affect the specific rate of root respiration in all species. Broadly generalized, the effect of temperature on biomass allocation is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both higher and lower temperatures. The root respiration of some species acclimates to growth temperature, so that the effect of global temperature rise is entirely accounted for by the effect of temperature on biomass allocation. The specific rate of root respiration of other species will increase with global warming. In response to global warming the carbon requirement of roots is likely to decrease in temperate regions, when temperatures are suboptimal for the roots' capacity to acquire water. Here global warming will induce a smaller biomass allocation to the roots. Conversely, the carbon requirements are more likely to increase in mediterranean environments, where temperatures are often supraoptimal and a rise in temperature will induce greater allocation of biomass to the roots.     

Evaluation of the effects of elevated CO2 concentrations on the growth of cassava storage roots by destructive harvests and ground penetrating radar scanning approaches

Cassava (Manihot esculenta Crantz) production will need to be improved to meet future food demands in Sub-Saharan Africa. The selection of high-yielding cassava cultivars requires a better understanding of storage root development. Additionally, since future production will happen under increasing atmospheric CO₂ concentrations ([CO₂]), cultivar selection should include responsiveness to elevated [CO₂]. Five farmer-preferred African cassava cultivars were grown for three and a half months in a Free Air CO₂ Enrichment experiment in central Illinois. Compared to ambient [CO₂] (~400 ppm), cassava storage roots grown under elevated [CO₂] (~600 ppm) had a higher biomass with some cultivars having lower storage root water content. The elevated [CO₂] stimulation in storage root biomass ranged from 33% to 86% across the five cultivars tested documenting the importance of this trait in developing new cultivars. In addition to the destructive harvests to obtain storage root parameters, we explored ground penetrating radar as a nondestructive method to determine storage root growth across the growing season.     

Production, turnover and mycorrhizal colonization of root systems of three Populus species grown under elevated CO2 (POPFACE)

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO₂ treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO₂ increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO₂. Response of the mycorrhizal community to elevated CO₂ was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO₂ conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long‐term C storage in the soil.     

Combined effects of elevated CO2 and soil drought on carbon and nitrogen allocation of the desert shrub Caragana intermedia

Impacts of either elevated CO₂ or drought stress on plant growth have been studied extensively, but interactive effects of these on plant carbon and nitrogen allocation is inadequately understood yet. In this study the response of the dominant desert shrub, Caragana intermedia Kuanget H.c.Fu, to the interaction of elevated CO₂ (700 ± 20 μmol mol⁻¹) and soil drought were determined in two large environmental growth chambers (18 m²). Elevated CO₂ increased the allocation of biomass and carbon into roots and the ratio of carbon to nitrogen (C:N) as well as the leaf soluble sugar content, but decreased the allocation of biomass and carbon into leaves, leaf nitrogen and leaf soluble protein concentrations. Elevated CO₂ significantly decreased the partitioning of nitrogen into leaves, but increased that into roots, especially under soil drought. Elevated CO₂ significantly decreased the carbon isotope discrimination (Δ) in leaves, but increased them in roots, and the ratio of Δ values between root and leaf, indicating an increased allocation into below-ground parts. It is concluded that stimulation of plant growth by CO₂ enrichment may be negated under soil drought, and under the future environment, elevated CO₂ may partially offset the negative effects of enhanced drought by regulating the partitioning of carbon and nitrogen.     

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO<Subscript>2</Subscript> and varying soil resource availability

Rising atmospheric [CO₂] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO₂] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO₂. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO₂]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO₂]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO₂]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO₂] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO₂], rather than changes in substrate chemistry.     

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂] over a large gradient, including low [CO₂], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole‐plant net assimilation (A) allocated to night‐time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂] releases allocation constraints, thereby allowing greater investment into long‐term survival in the form of NSC and SMs.     

Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO₂, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO₂ enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol^?1) and enriched (700 μmol mol^?1) levels of atmospheric CO₂. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO₂‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO₂ effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO₂ treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO₂ enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO₂‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO₂ enrichment.     

Aluminium tolerance in triticale,wheat and rye as measured by root growth characteristics and aluminium concentration

Summary Screening large populations of plant species for Al tolerance requires simple and rapid tests. In this study, root characteristics of 12 cultivars of triticale (X Triticosecale, Witt Mack), wheat (Triticum aestivum L.), and rye (Secale cereale L.) were measured in nutrient solution with 0 or 6 ppm Al added. Aluminum injury to roots of triticale and wheat was characterized by decreases in root length, increases in the number of roots, and in Al-sensitive Redcoat and Arthur wheats by decrease in root weight. Root length and number of roots were correlated in triticale (r=−0.73^*) and in wheat (r=−0.85^*). Root length was also correlated with root weight in wheat (r=0.65^*); there was no relationship between the number of roots and weight. Differences in Al tolerance of cultivars of the three species were greater when the solution was adjusted to pH 4.8 only on the first day of the experiment than when pH was maintained at pH 4.8 throughout the growing period. Triticale and rye cultivars low in ability to increase solution pH gradually overcame Al toxicity by increasing the nutrient solution pH between 12 and 22 days. Aluminum sensitive triticale and wheat accumulated more Al in roots than tolerant cultivars when the solution pH was not adjusted daily; but no differences in Al accumulation were obtained between wheat cultivars at constant pH value. This study indicated that root length and number of roots can be reliably used for screening triticales for Al tolerance within 12 days of exposure to Al. Root length, Al concentration, and dry weight after 22 days of Al treatment were also reliable criteria for evaluating differential Al tolerances among triticale cultivars.     

Expression of genes involved in symbiotic carbon and nitrogen transport in Pinus taeda mycorrhizal roots exposed to CO2 enrichment and nitrogen fertilization

As atmospheric carbon dioxide (CO₂) concentrations rise, one important mechanism by which plants can gain greater access to necessary soil nutrients is through greater investment in their mycorrhizal symbionts. In this study, we tested the hypotheses that (1) plants increase C allocation to ectomycorrhizal fungi (EMF) under elevated CO₂ conditions, (2) N fertilization decreases C allocation to EMF, and (3) EMF activity at the site of symbiotic C and nutrient exchange is enhanced with CO₂ enrichment. To test these hypotheses, we examined expression levels of Pinus taeda genes encoding monosaccharide transport (MST) and ammonium transport (AMT) proteins thought to be involved in symbiotic C and N movement, respectively, from mycorrhizal root tips exposed to CO₂ and N fertilization. We also examined EMF ribosomal RNA expression (18S rRNA) to determine EMF activity. There was a trend toward lower relative MST expression with increased CO₂. AMT expression levels showed no significant differences between control and treatment plots. EMF 18S rRNA expression was increased in CO₂-enriched plots and there was a marginally significant positive interactive effect of CO₂ and N fertilization on expression (p = 0.09 and 0.10, respectively). These results are consistent with greater C allocation to EMF and greater EMF metabolic activity under elevated CO₂ conditions, although selective allocation of C to particular EMF species and greater fungal biomass on roots are plausible alternative hypotheses.     

Nitrate and ammonium uptake for single-and mixed-species communities grown at elevated CO2

Sustained increases in plant production in elevated CO₂ depend on adequate belowground resources. Mechanisms for acquiring additional soil resources include increased root allocation and changes in root morphology or physiology. CO₂ research to date has focused almost exclusively on changes in biomass and allocation. We examined physiological changes in nitrate and ammonium uptake in elevated CO₂, hypothesizing that uptake rates would increase with the amount of available CO₂. We combined our physiological estimates of nitrogen uptake with measurements of root biomass to assess whole root-system rates of nitrogen uptake. Surprisingly, physiological rates of ammonium uptake were unchanged with CO₂, and rates of nitrate uptake actually decreased significantly (P<0.005). Root boomass increased 23% in elevated CO₂ (P<0.005), but almost all of this increase came in fertilized replicates. Rates of root-system nitrogen uptake in elevated CO₂ increased for ammonium in nutrient-rich soil (P<0.05) and were unchanged for nitrate (P>0.80). Root-system rates of nitrogen uptake were more strongly correlated with physiological uptake rates than with root biomass in unamended soil, but the reverse was true in fertilized replicates. We discuss nitrogen uptake and changes in root biomass in the context of root nutrient concentrations (which were generally unchanged with CO₂) and standing pools of belowground plant nitrogen. In research to date, there appears to be a fairly general increase in root biomass with elevated CO₂, and little evidence of up-regulation in root physiology.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

Root to shoot ratio of crops as influenced by CO2

Crops of tomorrow are likely to grow under higher levels of atmospheric CO₂. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO₂ concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO₂ concentration will affect roots and shoots of crop plants proportionally. Since elevated CO₂ can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO₂ on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability. Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO₂ and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO₂ enrichment will add substantially to the global change response data base.Abbreviations R:S root to shoot ratio, dry weight basis     

Aluminium effects on growth and Al,Ca, Mg,K and P levels in triticale,wheat and rye

Summary The effects of A1 on the growth and mineral composition of different cultivars of triticale (X Triticosecale, Wittmack), wheat (Triticum aestivum L.) and rye (Secale cereale L.) growing in 1/5 strength Steinberg solutions containing 0 or 6 ppm A1 were evaluated after 32 days. Aluminum increased the concentrations of P and K in the roots and K in the tops of most of the cultivars tested. A1 tolerant triticale retained a lower concentration of Mg in the roots and tops than the A1 sensitive triticale, when subjected to A1 stress. In addition, A1 treatments resulted in smaller increases in root P for the A1 tolerant triticale than for the A1 sensitive cultivars.The concentration of root Ca and P of the A1 tolerant wheat cultivars were significantly below that of the more sensitive plants. Aluminum tolerance in rye appeared to be associated with lower Ca and higher Mg concentrations in the tops. The accumulation of P and A1 in the roots was characteristic of sensitivity in triticale, wheat and rye.     

Soil warming and CO2 enrichment induce biomass shifts in alpine tree line vegetation

Responses of alpine tree line ecosystems to increasing atmospheric CO₂ concentrations and global warming are poorly understood. We used an experiment at the Swiss tree line to investigate changes in vegetation biomass after 9 years of free air CO₂ enrichment (+200 ppm; 2001–2009) and 6 years of soil warming (+4 °C; 2007–2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above‐ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO₂ enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m^?2) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO₂ led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understorey above‐ground mass was not altered by soil warming or elevated CO₂. However, Vaccinium myrtillus mass increased with both treatments, graminoid mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (?40% for all roots <2 mm in diameter at 0–20 cm soil depth) but no change with CO₂ enrichment. Our findings suggest that enhanced overall productivity and shifts in biomass allocation will occur at the tree line, particularly with global warming. However, individual species and functional groups will respond differently to these environmental changes, with consequences for ecosystem structure and functioning.     

Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2 and temperature regimes

Increases in the concentration of atmospheric carbon dioxide may have a fertilizing effect on plant growth by increasing photosynthetic rates and therefore may offset potential growth decreases caused by the stress associated with higher temperatures and lower precipitation. However, plant growth is determined both by rates of net photosynthesis and by proportional allocation of fixed carbon to autotrophic tissue and heterotrophic tissue. Although CO₂ fertilization may enhance growth by increasing leaf-level assimilation rates, reallocation of biomass from leaves to stems and roots in response to higher concentrations of CO₂ and higher temperatures may reduce whole-plant assimilation and offset photosynthetic gains. We measured growth parameters, photosynthesis, respiration, and biomass allocation of Pinus ponderosa seedlings grown for 2 months in 2×2 factorial treatments of 350 or 650 bar CO₂ and 10/25° C or 15/30° C night/day temperatures. After 1 month in treatment conditions, total seedling biomass was higher in elevated CO₂, and temperature significantly enhanced the positive CO₂ effect. However, after 2 months the effect of CO₂ on total biomass decreased and relative growth rates did not differ among CO₂ and temperature treatments over the 2-month growth period even though photosynthetic rates increased 7% in high CO₂ treatments and decreased 10% in high temperature treatments. Additionally, CO₂ enhancement decreased root respiration and high temperatures increased shoot respiration. Based on CO₂ exchange rates, CO₂ fertilization should have increased relative growth rates (RGR) and high temperatures should have decreased RGR. Higher photosynthetic rates caused by CO₂ fertilization appear to have been mitigated during the second month of exposure to treatment conditions by a 3% decrease in allocation of biomass to leaves and a 9% increase in root:shoot ratio. It was not clear why diminished photosynthetic rates and increased respiration rates at high temperatures did not result in lower RGR. Significant diametrical and potentially compensatory responses of CO₂ exchange and biomass allocation and the lack of differences in RGR of ponderosa pine after 2 months of exposure of high CO₂ indicate that the effects of CO₂ fertilization and temperature on whole-plant growth are determined by complex shifts in biomass allocation and gas exchange that may, for some species, maintain constant growth rates as climate and atmospheric CO₂ concentrations change. These complex responses must be considered together to predict plant growth reactions to global atmospheric change, and the potential of forest ecosystems to sequester larger amounts of carbon in the future.     

Effect of respiratory homeostasis on plant growth in cultivars of wheat and rice

Some plants have the ability to maintain similar respiratory rates (measured at the growth temperature), even when grown at different temperatures, a phenomenon referred to as respiratory homeostasis. The underlying mechanisms and ecological importance of this respiratory homeostasis are not understood. In order to understand this, root respiration and plant growth were investigated in two wheat cultivars (Triticum aestivum L. cv. Stiletto and cv. Patterson) with a high degree of homeostasis, and in one wheat cultivar (T. aestivum L. cv. Brookton) and one rice cultivar (Oryza sativa L. cv. Amaroo) with a low degree of homeostasis. The degree of homeostasis (H) is defined as a quantitative value, which occurs between 0 (no acclimation) and 1 (full acclimation). These plants were grown hydroponically at constant 15 or 25 °C. A good correlation was observed between the rate of root respiration and the relative growth rates (RGR) of whole plant, shoot or root. The plants with high H showed a tendency to maintain their RGR, irrespective of growth temperature, whereas the plants with low H grown at 15 °C showed lower RGR than those grown at 25 °C. Among several parameters of growth analysis, variation in net assimilation rate per shoot mass (NAR_m) appeared to be responsible for the variation in RGR and rates of root respiration in the four cultivars. The plants with high H maintained their NAR_m at low growth temperature, but the plants with low H grown at 15 °C showed lower NAR_m than those grown at 25 °C. It is concluded that respiratory homeostasis in roots would help to maintain growth rate at low temperature due to a smaller decrease in net carbon gain at low temperature. Alternatively, growth rate per se may control the demand of respiratory ATP, root respiration rates and sink demands of photosynthesis. The contribution of nitrogen uptake to total respiratory costs was also estimated, and the effects of a nitrogen leak out of the roots and the efficiency of respiration on those costs are discussed.     

Growth and water-use efficiency of 10 Triticum aestivum cultivars at different water availability in relation to allocation of biomass

In environments where the amount of water is limiting growth, water-use efficiency (biomass production per unit water use) is an important trait. We studied the relationships of plant growth and water use efficiency with the pattern of biomass allocation, using 10 wheat cultivars, grown at two soil moisture levels in a growth chamber. Allocation pattern and relative growth rate were not correlated, whereas allocation pattern and water use efficiency were. Variation in transpiration per plant resulted from variation in the rate of transpiration per unit leaf area or root weight, rather than from differences in leaf area or root weight per plant. Transpiration per unit leaf area or root weight was lower when the leaf area or root weight per unit plant weight was larger. Also, the efficiency of water use at the plant and leaf levels was higher for plants with a higher leaf area per unit plant weight, and it was not correlated with the plant's growth rate. Differences in water-use efficiency at the leaf level were related to variation in stomatal conductance, rather than in the rate of photosynthesis. A high photosynthetic water-use efficiency was associated with a low efficiency of nitrogen use for photosynthesis.     

Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO2 enrichment

Prunella vulgaris was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO₂ (ambient, 350 μl l⁻¹, and elevated, 600 μl l⁻¹) to test whether a plants response to elevated CO₂ is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots, we also tested whether elevated CO₂ affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO₂; the biomass of the root system, for example, increased by a factor of 2. The colonization of AMF inside the root remained constant, indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO₂ was up to 5 times that at ambient CO₂, indicating that elevated CO₂ promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO₂ but this resulted in either an increase or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO₂ would result in increased P acquistion by the plant. To test this we supplied phosphorus, in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO₂ when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however, play a significant role in the movement of fixed carbon to the soil and increase soil aggregation. Received: 28 March 1998 / Accepted: 27 August 1998     

Seasonal soil‐surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

Elevated atmospheric CO₂ concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO₂ concentration were higher than those at ambient CO₂ concentration, and whether this was related to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol^?1) CO₂ concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m^?2 s^?1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO₂ concentrations at four depths. For the second year of growth, the annual carbon flux from the plots was 1671 g y^?1 and 1895 g y^?1 at ambient and elevated CO₂ concentrations, respectively, although this was not a significant difference. Higher f at elevated CO₂ concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.     

Development and function ofAzospirillum-inoculated roots

Summary The surface distribution ofAzospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs.Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains ofAzospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO^– ₃, K⁺ and H₂PO^– ₄ uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher inAzospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.