Root Respiration Rate Research Articles

Advantages of NH4 + on growth, nitrogen uptake and root respiration of Phragmites australis

We investigated growth, N nutrition, and root respiration in Phragmites australis (Cav.) Trin. ex Steud. grown under conditions with different N sources, and evaluated the advantages of NH4 + nutrition in relation to adaptation to anaerobic soil conditions. Hydroponics culture was carried out for 2 months under two treatment conditions with different N sources, NH4 + and NO3 −. The relative growth rate (RGR) of the roots, shoot and whole plant, net N uptake rate (NNUR), and root respiration rate were examined. Shoot RGR, shoot to root (S/R) ratio, and NNUR were obviously higher with the NH4 + treatment. High S/R ratio of plants grown in the NH4 + treatment contributed to repression of whole-root oxygen consumption. In consequence, NNUR per root respiration rate was higher with the NH4 + treatment, which clearly suggested efficient oxygen consumption in the roots. In conclusion, higher S/R ratio due to higher NNUR enable to efficiently use oxygen for N nutrition through the repression of whole-root oxygen consumption, which is consequently achieved by NH4 + nutrition. Therefore, we suggest that NH4 + nutrition is indispensable for hydrophytic species growing in anaerobic soil because it enables both sufficient N nutrition and efficient oxygen consumption.

Ethylene production and ethylene effects on respiration rate of postharvest sugarbeet roots

Ethylene elevates respiration, is induced by wounding, and contributes to wound-induced respiration in most postharvest plant products. Ethylene production and its effects on respiration rate, however, have not been determined during storage of sugarbeet ( Beta vulgaris L.) root, even though any elevation in respiration due to ethylene would increase storage losses and reduce postharvest quality. To determine the effect of ethylene on sugarbeet root storage respiration rate, sugarbeet root ethylene production was quantified, and the effects of exogenous ethylene, an ethylene biosynthesis inhibitor, and ethylene response inhibitors on root respiration rate were determined using uninjured, severely injured, and conventionally harvested roots. Ethylene production was low (0.045–0.047 pmol kg −1 s −1) in uninjured and conventionally harvested and piled roots. Consequently, ethylene concentrations in commercial piles 0–67 d after piling were low, ranging from <0.001 to 0.054 μL L −1. Exogenous ethylene at concentrations of 0.020–14 μL L −1 increased root respiration. The increase in respiration rate, however, was transient at ethylene concentrations ≤0.11 μL L −1 suggesting that any ethylene effects on respiration rate in commercial piles would be short term. Severe injury induced ethylene production an average of 3.7-fold and increased respiration rate 3–4 d after injury. Wound-induced ethylene production, however, was not directly responsible for wound-induced respiration since elimination of wound-induced ethylene production by the ethylene synthesis inhibitor aminoethoxyvinylglycine had no effect on wound-induced respiration. The ethylene response inhibitors 1-methylcyclopropene (1-MCP) and silver thiosulfate reduced wound-induced respiration 3–4 d after injury when applied after wounding. A portion of the increase in respiration due to wounding, therefore, required ethylene perception. However, when applied prior to wounding, 1-MCP elevated wound-induced respiration 3–4 d after injury, suggesting that blockage of ethylene receptors prior to injury was ineffective at eliminating ethylene perception after wounding, possibly due to the synthesis of new receptors after the injury. Moreover, 1-MCP effects on root respiration rate occurred only when roots were severely injured; 1-MCP had no effect on respiration rate of uninjured or conventionally harvested roots. Postharvest sugarbeet roots, therefore, produce ethylene, increase ethylene production in response to wounding, and respond to exogenous ethylene with an increase in respiration rate, but ethylene production and ethylene effects on root respiration rate are likely to be small under commercial storage conditions and of limited economic significance.

Soil respiration fluxes in a temperate mixed forest: seasonality and temperature sensitivities differ among microbial and root-rhizosphere respiration

Although soil respiration, a major CO(2) flux in terrestrial ecosystems, is known to be highly variable with time, the response of its component fluxes to temperature and phenology is less clear. Therefore, we partitioned soil respiration (SR) into microbial (MR) and root-rhizosphere respiration (RR) using small root exclusion treatments in a mixed mountain forest in Switzerland. In addition, fine root respiration (FRR) was determined with measurements of excised roots. RR and FRR were strongly related to each other (R(2) = 0.92, n = 7), with RR contributing about 46% and FRR about 32% to total SR. RR rates increased more strongly with temperature (Q(10) = 3.2) than MR rates (Q(10) = 2.3). Since the contribution of RR to SR was found to be higher during growing (50%) than during dormant periods (40%), we separated the 2-year data set into phenophases. During the growing period of 2007, the temperature sensitivity of RR (Q(10) = 2.5, R(2) = 0.62) was similar to that of MR (Q(10) = 2.2, R(2) = 0.57). However, during the dormant period of 2006/2007, RR was not related to soil temperature (R(2) = 0.44, n.s.), in contrast to MR (Q(10) = 7.2; R(2) = 0.92). To better understand the influence of plant activity on root respiration, we related RR and FRR rates to photosynthetic active radiation (both R(2) = 0.67, n = 7, P = 0.025), suggesting increased root respiration rates during times with high photosynthesis. During foliage green-up in spring 2008, i.e., from bud break to full leaf expansion, RR increased by a factor of 5, while soil temperature increased only by about 5 degrees C, leading to an extraordinary high Q(10) of 10.6; meanwhile, the contribution of RR to SR increased from 29 to 47%. This clearly shows that root respiration and its apparent temperature sensitivity highly depend on plant phenology and thus on canopy assimilation and carbon allocation belowground.

Effect of lowering the root/shoot ratio by pruning roots on water use efficiency and grain yield of winter wheat

A pot and a field experiment were conducted to assess the effects of root/shoot ratio (R/S) on the water use efficiency (WUE) and grain yield of winter wheat. The R/S was regulated by pruning the roots during the stem elongation stage, resulting in reduced root systems of the plants. At the heading stage, the root dry weight of root-pruned plants was less than that of intact-root plants, but their R/S was similar to that of intact-root plants under both experimental conditions. After tiller pruning, the R/S of root-pruned plants was significantly lower than that of intact-root plants ( p < 0.05). Root pruning reduced the rate of leaf transpiration and lowered the number of tillers per plant ( p < 0.05) during the vegetative stage. As a result, root-pruned wheat showed reduced water use when compared to intact-root plants before heading ( p < 0.05). At anthesis, there was no significant difference in transpiration between plants with intact roots and those with pruned roots in the pots. However, under field conditions, transpiration of root-pruned plants was significantly higher than that of intact-root plants at anthesis. Additionally, at anthesis root-pruned plants had a higher rate of leaf photosynthesis and lower rate of root respiration, which resulted in a significantly higher grain yield at maturity when compared to plants with intact roots. Under both experimental conditions, there were no significant differences in shoot dry weight per plant between root-pruned and intact-root plants grown in monoculture. When root-pruned plants were grown with intact-root plants, the root-pruned wheat was less productive and had a lower relative shoot dry weight (0.78 and 0.86, respectively) than the intact-root plants (1.24 and 1.16, respectively). These results suggest that plants with pruned roots had a lower ability to compete and to acquire and use the same resources in the mixture when compared with intact-root plants. Root pruning improved the WUE of winter wheat under both experimental conditions. This suggests that appropriate management for the root system/tillers in wheat crops can be used to increase grain yield and water use efficiency. Specifically, lowering the R/S improved the grain yield and WUE of winter wheat significantly by lowering its competitive ability and improving root efficiency. Therefore, drought-resistance breeding to improve the grain yield and WUE, at least for wheat, should be made by targeted selection of less competitive progeny with a small R/S for cultivation in arid and semiarid areas.

Seasonal changes of soil respiration in Betula platyphylla forest in Changbai Mountain, China

A stdudy was conducted to determine the seasonal changes of soil respiration and the contribution of root respiration to soil respiration in Betula platyphylla forest in Changbai Mountain from May to September in 2004. Results indicated that the total soil respiration, root-severed soil respiration and the root respiration followed a similar seasonal trend, with a high rate in summer due to wet and high temperature and a low rate in spring and autumn due to lower temperature. The mean rates of total soil respiration, root-severed soil respiration and root respiration were 4.44, 2.30 and 2.14 µmol·m−2·s−1, respectively during the growing season, and they were all exponentially correlated with temperature. Soil respiration rate had a linear correlation with soil volumetric moisture. The Q10 values for total soil respiration, root-severed soil respiration and root respiration were 2.82, 2.59 and 3.16, respectively. The contribution rate of root respiration to the total soil respiration was between 29.3% and 58.7% during the growing season, indicating that root is a major component of soil respiration. The annual mean rates of total soil respiration, root-severed soil respiration and root respiration were 1.96, 1.08, and 0.87 µmol·m−2·s−1, or 741.73, 408.71, and 329.24 g·m−2·a−1, respectively. Root respiration contributed 44.4% to the annual total soil respiration. The relationship proposed for soil respiration with soil temperature was useful for understanding and predicting potential changes in Changbai Mountain B. platyphylla forest ecosystem in response to forest management and climate change.

Nitrogen deficiency increases the residence time of respiratory carbon in the respiratory substrate supply system of perennial ryegrass

Plant respiration draws on substrate pools of different functional/biochemical identity. Little is known about the effect of nitrogen deficiency on those pools' sizes, half-lives and relative contribution to respiration, and consequently, of carbon residence time in respiratory metabolism. Here we studied how nitrogen fertilization affects the respiratory carbon supply system of shoots and roots of Lolium perenne, a perennial grass. Plants grown at two nitrogen supply levels in continuous light were labelled with (13)CO(2)/(12)CO(2) for intervals ranging from 1 h to 1 month. The rate and isotopic composition of shoot, root and plant respiration were measured, and the time-courses of tracer incorporation into respired CO(2) were analysed by compartmental modelling. Nitrogen deficiency reduced specific respiration rate by 30%, but increased the size of the respiratory supply system by 30%. In consequence, mean residence time of respiratory carbon increased with nitrogen deficiency (4.6 d at high nitrogen and 9.2 d at low nitrogen supply). To a large extent, this was due to a greater involvement of stores with a long half-life in respiratory carbon metabolism of nitrogen-deficient plants. At both nitrogen supply levels, stores supplying root respiration were primarily located in the shoot, probably in the form of fructans.

Seed and seedling vigour traits in groundnut (Arachis hypogaea L.)

Experiments were conducted with groundnut (Arachis hypogaea L.) germplasm accessions and cultivars to identify agronomic desirable traits associated with seed and seedling vigour. Seed vigour was assessed under optimum conditions in dormant (ICGS 11) and non-dormant (GG 2) seeded cultivars. Wide genetic variations were recorded in the parameters such as germination rate, germination speed, co-efficient of germination, vigour index, respiration rate and number of secondary roots. Genetic variations were observed in germinability, and root, hypocotyl and epicotyl lengths, opening of cotyledons, and growth of secondary roots in germplasm accessions and cultivars evaluated for tolerance to drought and high temperature stresses, under simulated conditions. Promising cultivars viz. TAG 24, Girnar 1, J 11 and KRG 1, all belonging to the Spanish market type, suitable for sowing in drought prone areas were identified. Germplasm accessions viz. NRCG 12752 (SHANTUNG) and 12642 (57-C-3-4-4-1-1-1) tolerant to high temperature during germination and early seedling growth were also identified. Utilisation of reserve food material, evaluated in 10 cultivars, belonging to three different seed-weight groups and Virginia and Spanish market types, indicated that both medium and higher seed-weight groups are efficient in utilisation of reserve food material. The lower seed-weight group was found inefficient in supply of reserve food material stored in the cotyledons to establish vigorous seedlings. A significant inverse relationship (r = -0.71, n = 15) between specific leaf area (SLA) and total seedling biomass was established, i.e., the lower the SLA (thicker leaf) the higher the biomass, and the higher the SLA (thinner leaf) the lower the biomass. Thus the parameters such as 100-seed weight, efficient utilisation of reserve food material, development of secondary roots and lower SLA are desirable agronomic traits in groundnut cultivation, especially in raindependent systems. In addition, the tolerant lines/cultivars could be utilised as donor parents in improving groundnut cultivars for higher seed vigour through breeding.

Dehydration accelerates respiration in postharvest sugarbeet roots

Sugarbeet ( Beta vulgaris L.) roots lose water during storage and often become severely dehydrated after prolonged storage and at the outer regions of storage piles which have greater wind and sun exposure. Sucrose loss is known to be elevated in dehydrated roots, although the metabolic processes responsible for this loss are unknown. To identify processes that contribute to sucrose loss in dehydrated roots, respiration rate, cellular electrolyte leakage, and sucrolytic enzyme activities were determined in roots of two varieties (VDH 66156 and Beta 4797R) during 4 weeks of 10 °C storage at high (85%) and low (40%) relative humidities. Roots stored at 40% relative humidity dehydrated significantly and lost almost 50% of their weight after 4 weeks of storage. Electrolyte leakage increased in these roots, indicating that dehydration damaged cellular membranes. Respiration rate generally increased in roots stored at 40% relative humidity compared to roots stored at 85% relative humidity. The increase in respiration rate was positively correlated with root weight loss and electrolyte leakage. Respiration rate was most closely associated with electrolyte leakage, however, suggesting that elevations in respiration rate were not due to dehydration, but to the membrane damage that occurred in response to dehydration. Activities of the sucrose-degrading enzymes, sucrose synthase, alkaline invertase and soluble acid invertase, were unaltered by dehydration. Alterations in sucrolytic enzyme activities, therefore, were not needed to provide for the increased demand for respiratory substrates in dehydrated roots. These results suggest that storage at low relative humidity alters the postharvest physiology of sugarbeet roots by increasing the rate of weight loss, reducing membrane integrity, and accelerating root respiration rate.

Physiological expression of emotional reactions in sheep

With the aim of judging emotional valence from an animal's perspective, multiple physiological variables were recorded in sheep when they were exposed to situations likely to induce negative and positive emotional states. Fourteen sheep were conditioned for several weeks to anticipate the delivery of standard feed. In three experimental trials carried out thereafter, the animals' expectations regarding feed quality were either fulfilled by offering the familiar standard feed (control), frustrated by giving unpalatable wooden pellets (negative treatment) or surpassed by delivering enriched feed (positive treatment). Heart rate, root mean square successive difference (RMSSD), respiration rate, body-surface humidity, body-surface temperature and percentage of eye white were recorded prior to the delivery of feed (anticipation phase) and during the delivery (feeding phase) of either standard feed, wooden pellets or enriched feed. Data were analysed using linear mixed-effects models. Heart rate, respiration rate and variability of body-surface humidity were high during the delivery of wooden pellets and low during the feeding phases with standard and enriched feed; RMSSD showed an inverse pattern. In addition, heart rate was increased during the first feeding phase after the one with standard feed, independent of its presumed valence, whereas differential reactions were observed in the second feeding phase after standard feed. The results show that presumed negative and positive emotional states in sheep differ in their physiological reactions. Despite a need for validation in additional situations, the combination of heart rate, respiration rate, RMSSD and body-surface humidity appeared to be most useful for assessing physiological correlates of negative and positive emotional reactions in sheep.

Growth and functional responses of different cultivars of Lotus corniculatus to aluminum and low pH stress

Aluminum toxicity is an important stress factor in acid soils. Growth, respiration and permeability properties of root cells were studied in five cultivars of Lotus corniculatus subjected to aluminum (Al) or low pH stress. The cultivars showed significant differences in root elongation under stress conditions, which correlated with changes in membrane potential (E(M)) of root cortical cells. A pH drop from 5.5 to 4.0 resulted in significant membrane depolarization and root growth inhibition. The strongest inhibition was observed in cv. São Gabriel (33.6%) and least in cv. UFRGS (25.8%). Application of an extremely high Al concentration (2mM) stopped the root growth in cv. INIA Draco, while inhibition in cv. UFRGS reached only 75%. The E(M) values of cortical cells of Lotus roots varied between -115 and -144mV. Treatment with 250microM of AlCl(3) (pH 4) resulted in rapid membrane depolarization. The extent of the membrane depolarization ranged between 51mV (cv. UFGRS) and 16mV (cv. INIA Draco). The membrane depolarization was followed by a loss of K(+) from Al-treated roots (2mM Al) and resulted in a decrease of the diffusion potential (E(D)). The total amount of K(+) in Al-treated roots dropped from 31.4 to 16.8micromolg(-1) FW in sensitive cv. INIA Draco, or from 26.1 to 22.7micromolg(-1) FW in tolerant cv. UFGRS. The rate of root respiration under control conditions as well as under Al treatment was higher in cv. INIA Draco than in cv. UFRGS. Al-induced inhibition of root respiration was 21-34% of the control.

Soil resource heterogeneity in the form of aggregated litter alters maize productivity

Soils are spatially heterogeneous. To better understand the effects of sub-plant scale soil resource heterogeneity on primary productivity we aggregated plant litter (Trifolium pratense L. shoots and Secale cereale L. leaves) into 8, 24, 32, and 72 patches and uniformly distributed in 50-L containers (0.15 m2) of soil and grew Zea mays L. throughout two seasons. On average aggregated T. pratense litter enhanced final aboveground maize biomass by 14% relative to uniformly distributed T. pratense litter. This effect may be related to the reduction in root carbon allocation observed as lower apparent root respiration rates compared to uniform litter distribution. In contrast, the spatial distribution of S. cereale litter did not affect productivity. The common experimental approach of uniformly distributing resources to understand their influence on soil-plant processes likely oversimplifies field conditions because our results indicate that the spatial distribution of plant litter alone can influence productivity and plant carbon allocation.

Stand level estimation of root respiration for two subtropical plantations based on in situ measurement of specific root respiration

In this study, the stand level root respiration was estimated for two monoculture plantations: Acacia crassicarpa and Eucalyptus urophylla, based on in situ measurement of specific root respiration using simplified root chamber method. The respiration rates of fine roots (<5 mm) were significantly higher than those of coarse roots (>5 mm) for both A. crassicarpa and E. urophylla species. The root respiration of A. crassicarpa showed a clear seasonal pattern with a higher value in the wet season. For E. urophylla, the seasonal pattern was observed for fine roots but not for coarse roots. After determining the biomass of fine roots and coarse roots and their specific rates of respiration at different time points, root respiration at the stand level ( R a) was estimated using a direct up-scaling model. We found that the R a accounted for 14% and 19% of total soil respiration ( R s) for A. crassicarpa and E. urophylla, respectively. The fine ( R Tf) and coarse ( R Tc) root respiration at the stand level accounted for about 47% and 53% of the R a for A. crassicarpa, and accounted for 58% and 42% for E. urophylla. This suggests that coarse root respiration cannot be ignored when estimating the root respiration at the stand level. Our results showed that the Q 10 values were more accurate in representing the temperature dependence when the confounding effect of soil moisture was considered. This study introduces an alternative approach to estimate stand level root respiration, but its reliability is largely dependent on the accuracy of root biomass quantification.

Fine root morphological traits determine variation in root respiration of Quercus serrata

Fine root respiration is a significant component of carbon cycling in forest ecosystems. Although fine roots differ functionally from coarse roots, these root types have been distinguished based on arbitrary diameter cut-offs (e.g., 2 or 5 mm). Fine root morphology is directly related to physiological function, but few attempts have been made to understand the relationships between morphology and respiration of fine roots. To examine relationships between respiration rates and morphological traits of fine roots (0.15-1.4 mm in diameter) of mature Quercus serrata Murr., we measured respiration of small fine root segments in the field with a portable closed static chamber system. We found a significant power relationship between mean root diameter and respiration rate. Respiration rates of roots<0.4 mm in mean diameter were high and variable, ranging from 3.8 to 11.3 nmol CO2 g(-1) s(-1), compared with those of larger diameter roots (0.4-1.4 mm), which ranged from 1.8 to 3.0 nmol CO2 g(-1) s(-1). Fine root respiration rate was positively correlated with specific root length (SRL) as well as with root nitrogen (N) concentration. For roots<0.4 mm in diameter, SRL had a wider range (11.3-80.4 m g(-1)) and was more strongly correlated with respiration rate than diameter. Our results indicate that a more detailed classification of fine roots<2.0 mm is needed to represent the heterogeneity of root respiration and to evaluate root biomass and root morphological traits.

Contribution of Root and Microbial Respiration to Soil CO 2 Efflux and Their Environmental Controls in a Humid Temperate Grassland of Japan

Soil CO 2 efflux, root mass, and root production were investigated in a humid temperate grassland of Japan over a growing season (Apr. to Sep.) of 2005 to reveal seasonal changes of soil CO 2 efflux, to separate the respective contributions of root and microbial respiration to the total soil CO 2 efflux, and to determine the environmental factors that control soil respiration. Minimal microbial respiration rate was estimated based on the linear regression equations between soil CO 2 efflux and root mass at different experimental sites. Soil CO 2 efflux, ranging from 4.99 to 16.29 μmol CO 2 m −2 s −1, depended on the seasonal changes in soil temperature. The root mass at 0–10 cm soil depth was 0.82 and 1.27 kg m −2 in Apr. and Sep., respectively. The root mass at 0–10 cm soil depth comprised 60% of the total root mass at 0–50 cm soil depth. The root productivity at 0–30 cm depth varied from 8 to 180 g m −2 month −1. Microbial and root respiration rates ranged from 1.35 to 5.51 and 2.72 to 12.06 μmol CO 2 m −2 s −1, respectively. The contribution of root respiration to the total soil CO 2 efflux averaged 53%, ranging from 33% to 72%. The microbial respiration rate was exponentially related to soil temperature at 10 cm depth ( R 2 = 0.9400, P = 0.002, n = 6), and the root respiration rate was linearly related to the root production at 0–30 cm depth ( R 2 = 0.6561, P = 0.042, n = 6).

Midday depression in root respiration of Quercus crispula and Chamaecyparis obtusa: its implication for estimating carbon cycling in forest ecosystems

AbstractWe observed the phenomenon of midday depression in the rate of tree root respiration. Diurnal changes in the root respiration rate of Quercus crispula and Chamaecyparis obtusa were measured under intact conditions using a closed chamber method and a soil respiration measurement system (LI‐6400 with a root respiration chamber) in a forest in the foothills of Mt. Fuji. After the measurement of intact root respiration in the field, the root was excised and taken to the laboratory, and the temperature dependence on the respiration rate of the detached root was measured using an open‐flow gas exchange system with an infrared gas analyzer (LI‐6252). The measurement was conducted in September 2003, August and November 2005, and June 2006. Whereas the root respiration rate of both species under intact conditions increased with increasing soil and root temperatures from dawn to early morning, the respiration rate decreased around midday from 10:00 to 15:00 despite an increment of soil and root temperatures. There was no clear relationship between the intact root respiration rate and root temperature in either species, although the detached root respiration rate of both increased exponentially with the temperature. The amount of the CO2 efflux estimated using the temperature dependence of detached root respiration tended to underestimate the actual measurement value (intact respiration rate) by 20–50% in both species. These results indicate that evaluating midday depression in root respiration would be important for a more accurate estimation of the carbon cycle or net ecosystem production in forests.

Adjustment of Forest Ecosystem Root Respiration as Temperature Warms

Adjustment of ecosystem root respiration to warmer climatic conditions can alter the autotrophic portion of soil respiration and influence the amount of carbon available for biomass production. We examined 44 published values of annual forest root respiration and found an increase in ecosystem root respiration with increasing mean annual temperature (MAT), but the rate of this cross-ecosystem increase (Q(10)= 1.6) is less than published values for short-term responses of root respiration to temperature within ecosystems (Q(10)= 2-3). When specific root respiration rates and root biomass values were examined, there was a clear trend for decreasing root metabolic capacity (respiration rate at a standard temperature) with increasing MAT. There also were tradeoffs between root metabolic capacity and root system biomass, such that there were no instances of high growing season respiration rates and high root biomass occurring together. We also examined specific root respiration rates at three soil warming experiments at Harvard Forest, USA, and found decreases in metabolic capacity for roots from the heated plots. This decline could be due to either physiological acclimation or to the effects of co-occurring drier soils on the measurement date. Regardless of the cause, these findings clearly suggest that modeling efforts that allow root respiration to increase exponentially with temperature, with Q(10) values of 2 or more, may over-predict root contributions to ecosystem CO2 efflux for future climates and underestimate the amount of C available for other uses, including net primary productivity.

Short-term and long-term root respiratory acclimation to elevated temperatures associated with root thermotolerance for two Agrostis grass species

This study was designed to investigate whether thermotolerant roots exhibit respiratory acclimation to elevated temperatures. Root respiratory acclimation traits in response to increasing temperatures were compared between two Agrostis species contrasting in heat tolerance: thermal A. scabra and heat-sensitive A. stolonifera. Roots of both species were exposed to 17, 27, or 37 °C. Root RGR declined with increasing temperatures from 17 °C to 37 °C in both species; however, root growth of A. scabra maintained a significantly higher RGR than A. stolonifera at 27 °C or 37°C. A. scabra exhibited a significantly higher respiration acclimation potential to elevated temperatures, both in the short term (60 min) and in the long term (7–28 d) as compared with A. stolonifera, when temperatures increased from 17 °C to 27 °C or from 27 °C to 37 °C. Thermal A. scabra also maintained a significantly lower maintenance cost than A. stolonifera as temperatures increased to 27 °C or 37 °C. The results suggested that root thermotolerance of thermal A. scabra was associated with both short-term and long-term respiratory acclimation to changes in temperatures. The superior ability of adjusting the rate of root respiration to compensate for increases in carbon demand during short- or long-term temperature increases in the heat-tolerant A. scabra may result in the reduction in carbon expenditure or costs for maintenance, leading to extended root survivability in high temperature soils.

Mycorrhizal respiration: implications for global scaling relationships

Most plant species form mycorrhizas, yet these are neglected by plant physiologists. One consequence of this neglect is reduced ability to predict plant respiration, because respiration rate (R) in mycorrhizal roots might be higher than in non-mycorrhizal roots owing to increased substrate availability associated with enhanced nutrient uptake, coupled with increased respiratory product demand. Other predictions include that mycorrhizal colonization will affect scaling of R with tissue nitrogen concentrations; that mycorrhizal and non-mycorrhizal root R differ in their response to nutrient supply; and that the impact of colonization on R is related to fungal biomass. Failure to examine properly the role of colonization in determining root R means that current interpretations of root and soil respiration data might be flawed.

Relating coarse root respiration to root diameter in clonal Eucalyptus stands in the Republic of the Congo

Root respiration is an important component of the carbon balance of a forest ecosystem. We measured CO2 efflux of excised fine roots and intact coarse roots in 3-, 4- and 13-year-old Eucalyptus stands in the region of Pointe-Noire, Republic of the Congo. A transportable and adaptable closed chamber gas exchange system directly measured CO2 efflux of roots from 0.5 to 32 mm in diameter. Fluxes were corrected for measurement system leaks and normalized to a reference temperature of 30 degrees C. Mean fine root respiration rates at the reference temperature varied between 8.5 and 10.8 micromol CO2 kg(-1) s(-1) depending on the stand. Coarse root respiration was strongly negatively correlated to root diameter. We propose a model based on a radial gradient of respiratory activity within the root to simulate the exponential decrease in respiration with diameter. Although many sources of uncertainty in the measurements remain, as discussed in this paper, these results provide a basis for scaling up organ-level root respiration measurements to the tree and stand levels.

An optimality-based model of the coupled soil moisture and root dynamics

Abstract. The main processes determining soil moisture dynamics are infiltration, percolation, evaporation and root water uptake. Modelling soil moisture dynamics therefore requires an interdisciplinary approach that links hydrological, atmospheric and biological processes. Previous approaches treat either root water uptake rates or root distributions and transpiration rates as given, and calculate the soil moisture dynamics based on the theory of flow in unsaturated media. The present study introduces a different approach to linking soil water and vegetation dynamics, based on vegetation optimality. Assuming that plants have evolved mechanisms that minimise costs related to the maintenance of the root system while meeting their demand for water, we develop a model that dynamically adjusts the vertical root distribution in the soil profile to meet this objective. The model was used to compute the soil moisture dynamics, root water uptake and fine root respiration in a tropical savanna over 12 months, and the results were compared with observations at the site and with a model based on a fixed root distribution. The optimality-based model reproduced the main features of the observations such as a shift of roots from the shallow soil in the wet season to the deeper soil in the dry season and substantial root water uptake during the dry season. At the same time, simulated fine root respiration rates never exceeded the upper envelope determined by the observed soil respiration. The model based on a fixed root distribution, in contrast, failed to explain the magnitude of water use during parts of the dry season and largely over-estimated root respiration rates. The observed surface soil moisture dynamics were also better reproduced by the optimality-based model than the model based on a prescribed root distribution. The optimality-based approach has the potential to reduce the number of unknowns in a model (e.g. the vertical root distribution), which makes it a valuable alternative to more empirically-based approaches, especially for simulating possible responses to environmental change.