Cortical parenchyma wall width regulates root metabolic cost and maize performance under suboptimal water availability.

We describe how increased root cortical parenchyma wall width (CPW) can improve tolerance to drought stress in maize by reducing the metabolic costs of soil exploration. Significant variation (1.0-5.0 µm) for CPW was observed in maize germplasm. The functional-structural model RootSlice predicts that increasing CPW from 2 µm to 4 µm is associated with a ~15% reduction in root cortical cytoplasmic volume, respiration rate, and nitrogen content. Analysis of genotypes with contrasting CPW grown with and without water stress in the field confirms that increased CPW is correlated with an ~32-42% decrease in root respiration. Under water stress in the field, increased CPW is correlated with 125% increased stomatal conductance, 325% increased leaf CO2 assimilation rate, 73-78% increased shoot biomass, and 92-108% increased yield. CPW was correlated with leaf mesophyll midrib parenchyma wall width, indicating pleiotropy. Genome-wide association study analysis identified candidate genes underlying CPW. OpenSimRoot modeling predicts that a reduction in root respiration due to increased CPW would also benefit maize growth under suboptimal nitrogen, which requires empirical testing. We propose CPW as a new phene that has utility under edaphic stress meriting further investigation.

Substrate and adenylate limit subtropical tree fine-root respiration under soil warming.

How root respiration acclimates to global warming remains unclear, especially in subtropical forests that play a key role in the global carbon budget. In a large-scale in situ soil warming experiment, the occurrence of, and mechanisms controlling over, the acclimation of fine-root respiration of Cunninghamia lanceolata during the fourth year of warming were investigated. Specific respiration rates (at reference temperature of 20°C; SRR20 ) were measured with exogenous glucose addition, uncoupler addition, or no addition, and root morphological and chemical traits were also measured. Warming decreased SRR20 by 18.4% only during summer, indicating partial thermal acclimation of fine-root respiration under warming. Warming did not change fine-root N concentration, showing no possible enzyme limitation on respiration. Warming decreased root soluble sugar/starch ratio in summer, and glucose addition increased respiration only under warming, indicating a warming-induced substrate limitation on respiration. Uncoupler addition also stimulated respiration only under warming, showing a warming-induced adenylate limitation on respiration. These findings suggest that thermal acclimation of root respiration in subtropical forests, which is at least partially constrained by substrate and adenylate use, is conducive to reducing ecosystem carbon emissions and mitigating the positive feedback between atmospheric CO2 and climate warming.

Responses of key root traits in the genus Oryza to soil flooding mimicked by stagnant, deoxygenated nutrient solution.

Excess water can induce flooding stress resulting in yield loss, even in wetland crops such as rice (Oryza). However, traits from species of wild Oryza have already been used to improve tolerance to abiotic stress in cultivated rice. This study aimed to establish root responses to sudden soil flooding among eight wild relatives of rice with different habitat preferences benchmarked against three genotypes of O. sativa. Plants were raised hydroponically, mimicking drained or flooded soils, to assess the plasticity of adventitious roots. Traits included were apparent permeance (PA) to O2 of the outer part of the roots, radial water loss, tissue porosity, apoplastic barriers in the exodermis, and root anatomical traits. These were analysed using a plasticity index and hierarchical clustering based on principal component analysis. For example, O. brachyantha, a wetland species, possessed very low tissue porosity compared with other wetland species, whereas dryland species O. latifolia and O. granulata exhibited significantly lower plasticity compared with wetland species and clustered in their own group. Most species clustered according to growing conditions based on PA, radial water loss, root porosity, and key anatomical traits, indicating strong anatomical and physiological responses to sudden soil flooding.

What determines root-sprouting ability: Injury or phytohormones?

PREMISE: Root-sprouting (RS) is an evolutionarily independent alternative to axillary stem branching for a plant to attain its architecture. Root-sprouting plants are better adapted to disturbance than non-RS plants, and their vigor is frequently boosted by biomass removal. Nevertheless, RS plants are rarer than plants that are not root-sprouters, possibly because they must overcome developmental barriers such as intrinsic phytohormonal balance or because RS ability is conditioned by injury to the plant body. The objective of this study was to identify whether phytohormones or injury enable RS. METHODS: In a greenhouse experiment, growth variables, root respiration, and phytohormones were analyzed in two closely related clonal herbs that differ in RS ability (spontaneously RS Inula britannica and rhizomatous non-RS I. salicina) with and without severe biomass removal. RESULTS: As previously reported, I. britannica is a root-sprouter, but injury did not boost its RS ability. Root respiration did not differ between the two species and decreased continuously with time irrespectively of injury, but their phytohormone profiles differed significantly. In RS species, the auxins-to-cytokinins ratio was low, and injury further decreased it. CONCLUSIONS: This first attempt to test drivers behind different plant growth forms suggests that intrinsic phytohormone regulation, especially the auxins-to-cytokinins ratio, might be behind RS ability. Injury, causing a phytohormonal imbalance, seems to be less important in spontaneously RS species than expected for RS species in general.

Maize (Zea mays L.) responses to salt stress in terms of root anatomy, respiration and antioxidative enzyme activity.

BACKGROUND: Soil salt stress is a problem in the world, which turns into one of the main limiting factors hindering maize production. Salinity significantly affects root physiological processes in maize plants. There are few studies, however, that analyses the response of maize to salt stress in terms of the development of root anatomy and respiration. RESULTS: We found that the leaf relative water content, photosynthetic characteristics, and catalase activity exhibited a significantly decrease of salt stress treatments. However, salt stress treatments caused the superoxide dismutase activity, peroxidase activity, malondialdehyde content, Na+ uptake and translocation rate to be higher than that of control treatments. The detrimental effect of salt stress on YY7 variety was more pronounced than that of JNY658. Under salt stress, the number of root cortical aerenchyma in salt-tolerant JNY658 plants was significantly higher than that of control, as well as a larger cortical cell size and a lower root cortical cell file number, all of which help to maintain higher biomass. The total respiration rate of two varieties exposed to salt stress was lower than that of control treatment, while the alternate oxidative respiration rate was higher, and the root response of JNY658 plants was significant. Under salt stress, the roots net Na+ and K+ efflux rates of two varieties were higher than those of the control treatment, where the strength of net Na+ efflux rate from the roots of JNY658 plants and the net K+ efflux rate from roots of YY7 plants was remarkable. The increase in efflux rates reduced the Na+ toxicity of the root and helped to maintain its ion balance. CONCLUSION: These results demonstrated that salt-tolerant maize varieties incur a relatively low metabolic cost required to establish a higher root cortical aerenchyma, larger cortical cell size and lower root cortical cell file number, significantly reduced the total respiration rate, and that it also increased the alternate oxidative respiration rate, thereby counteracting the detrimental effect of oxidative damage on root respiration of root growth. In addition, Na+ uptake on the root surface decreased, the translocation of Na+ to the rest of the plant was constrained and the level of Na+ accumulation in leaves significantly reduced under salt stress, thus preempting salt-stress induced impediments to the formation of shoot biomass.

Functional phenomics and genetics of the root economics space in winter wheat using high-throughput phenotyping of respiration and architecture.

The root economics space is a useful framework for plant ecology but is rarely considered for crop ecophysiology. In order to understand root trait integration in winter wheat, we combined functional phenomics with trait economic theory, utilizing genetic variation, high-throughput phenotyping, and multivariate analyses. We phenotyped a diversity panel of 276 genotypes for root respiration and architectural traits using a novel high-throughput method for CO2 flux and the open-source software RhizoVision Explorer to analyze scanned images. We uncovered substantial variation in specific root respiration (SRR) and specific root length (SRL), which were primary indicators of root metabolic and structural costs. Multiple linear regression analysis indicated that lateral root tips had the greatest SRR, and the residuals from this model were used as a new trait. Specific root respiration was negatively correlated with plant mass. Network analysis, using a Gaussian graphical model, identified root weight, SRL, diameter, and SRR as hub traits. Univariate and multivariate genetic analyses identified genetic regions associated with SRR, SRL, and root branching frequency, and proposed gene candidates. Combining functional phenomics and root economics is a promising approach to improving our understanding of crop ecophysiology. We identified root traits and genomic regions that could be harnessed to breed more efficient crops for sustainable agroecosystems.

Lateral roots, in addition to adventitious roots, form a barrier to radial oxygen loss in Zea nicaraguensis and a chromosome segment introgression line in maize.

Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O2 loss (ROL), but it was not known if this was also the case for lateral roots. We tested the hypothesis that lateral roots arising from adventitious roots can form a ROL barrier, using root-sleeving electrodes and O2 microsensors to assess ROL of Zea nicaraguensis, the maize (Zea mays ssp. mays) introgression line with a locus for ROL barrier formation (introgression line (IL) #468) from Z. nicaraguensis and a maize inbred line (Mi29). Lateral roots of Z. nicaraguensis and IL #468 both formed a ROL barrier under stagnant, deoxygenated conditions, whereas Mi29 did not. Lateral roots of Z. nicaraguensis had higher tissue O2 status than for IL #468 and Mi29. The ROL barrier was visible as suberin in the root hypodermis/exodermis. Modelling showed that laterals roots can grow to a maximum length of 74 mm with a ROL barrier, but only to 33 mm without a barrier. Presence of a ROL barrier in lateral roots requires reconsideration of the role of these roots as sites of O2 loss, which for some species now appears to be less than hitherto thought.

Effects of root morphology, respiration and carboxylate exudation on carbon economy in two non-mycorrhizal lupines under phosphorus deficiency.

Under phosphorus (P) deficiency, Lupinus albus develops cluster roots that allow efficient P acquisition, while L. angustifolius without cluster roots also grows well. Both species are non-mycorrhizal. We quantitatively examined the carbon budgets to investigate the different strategies of these species. Biomass allocation, respiratory rates, protein amounts and carboxylate exudation rates were examined in hydroponically-grown plants treated with low (1 µM; P1) or high (100 µM; P100) P. At P1, L. albus formed cluster roots, and L. angustifolius increased biomass allocation to the roots. The respiratory rates of the roots were faster in L. albus than in L. angustifolius. The protein amounts of the non-phosphorylating alternative oxidase and uncoupling protein were greater in the cluster roots of L. albus at P1 than in the roots at P100, but similar between the P treatments in L. angustifolius roots. At P1, L. albus exuded carboxylates at a faster rate than L. angustifolius. The carbon budgets at P1 were surprisingly similar between the two species, which is attributed to the contrasting root growth and development strategies. L. albus developed cluster roots with rapid respiratory and carboxylate exudation rates, while L. angustifolius developed a larger root system with slow respiratory and exudation rates.

Microbial seasonality promotes soil respiratory carbon emission in natural ecosystems: A modeling study.

Seasonality is a key feature of the biosphere and the seasonal dynamics of soil carbon (C) emissions represent a fundamental mechanism regulating the terrestrial-climate interaction. We applied a microbial explicit model-CLM-Microbe-to evaluate the impacts of microbial seasonality on soil C cycling in terrestrial ecosystems. The CLM-Microbe model was validated in simulating belowground respiratory fluxes, that is, microbial respiration, root respiration, and soil respiration at the site level. On average, the CLM-Microbe model explained 72% (n = 19, p < 0.0001), 65% (n = 19, p < 0.0001), and 71% (n = 18, p < 0.0001) of the variation in microbial respiration, root respiration, and soil respiration, respectively. We then compared the model simulations of soil respiratory fluxes and soil organic C content in top 1 m between the CLM-Microbe model with (CLM-Microbe) and without (CLM-Microbe_wos) seasonal dynamics of soil microbial biomass in natural biomes. Removing soil microbial seasonality reduced model performance in simulating microbial respiration and soil respiration, but led to slight differences in simulating root respiration. Compared with the CLM-Microbe, the CLM-Microbe_wos underestimated the annual flux of microbial respiration by 0.6%-32% and annual flux of soil respiration by 0.4%-29% in natural biomes. Correspondingly, the CLM-Microbe_wos estimated higher soil organic C content in top 1 m (0.2%-7%) except for the sites in Arctic and boreal regions. Our findings suggest that soil microbial seasonality enhances soil respiratory C emissions, leading to a decline in SOC storage. An explicit representation of soil microbial seasonality represents a critical improvement for projecting soil C decomposition and reducing the uncertainties in global C cycle projection under the changing climate.

Linking root respiration to chemistry and morphology across species.

Root respiration is a critical physiological trait involved in root resource acquisition strategies, yet it is less represented in root trait syndrome. Here we compiled a large dataset of root respiration associated with root chemical and morphological traits from 245 plant species. Our results demonstrated that root respiration correlated positively with root nitrogen concentration (RNC) and negatively with root tissue density (RTD) across and within woody and non-woody species. However, the relationships between root respiration and specific root length (SRL) and root diameter (RD) were weak or even insignificant. Such root respiration-traits relationships were not completely in line with predictions by the root economics spectrum (RES). Furthermore, the principal component analysis showed that root trait syndrome was multidimensional. Root respiration was associated more strongly with the RNC-RTD axis (the classical RES) than with the orthogonal SRL-RD axis for woody species, but not for non-woody species. Collectively, the linkages of root physiological, chemical, and morphological traits provide a better understanding of root trait covariation and root resource acquisition strategies.

Patterns and dynamics of canopy-root coupling in tropical tree saplings vary with light intensity but not with root depth.

Carbon (C) dynamics in canopy and roots influence whole-tree carbon fluxes, but little is known about canopy regulation of tree-root activity. Here, the patterns and dynamics of canopy-root C coupling are assessed in tropical trees. Large aeroponics facility was used to study the root systems of Ceiba pentandra and Khaya anthotheca saplings directly at different light intensities. In Ceiba, root respiration (Rr ) co-varied with photosynthesis (An ) in large saplings (3-to-7-m canopy-root axis) at high-light, but showed no consistent pattern at low-light. At medium-light and in small saplings (c. 1-m axis), Rr tended to decrease transiently towards midday. Proximal roots had higher Rr and nonstructural carbohydrate concentrations than distal roots, but canopy-root coupling was unaffected by root location. In medium-sized Khaya, no Rr pattern was observed, and in both species, Rr was unrelated to temperature. The early-afternoon increase in Rr suggests that canopy-root coupling is based on mass flow of newly fixed C in the phloem, whereas the early-morning rise in Rr with An indicates an additional coupling signal that travels faster than the phloem sap. In large saplings and potentially also in higher trees, light and possibly additional environmental factors control the diurnal patterns of canopy-root coupling, irrespective of root location.

The temperature optima for tree seedling photosynthesis and growth depend on water inputs.

Understanding how tree growth is affected by rising temperature is a key to predicting the fate of forests in future warmer climates. Increasing temperature has direct effects on plant physiology, but there are also indirect effects of increased water limitation because evaporative demand increases with temperature in many systems. In this study, we experimentally resolved the direct and indirect effects of temperature on the response of growth and photosynthesis of the widely distributed species Eucalyptus tereticornis. We grew E. tereticornis in an array of six growth temperatures from 18 to 35.5°C, spanning the climatic distribution of the species, with two watering treatments: (a) water inputs increasing with temperature to match plant demand at all temperatures (Wincr ), isolating the direct effect of temperature; and (b) water inputs constant for all temperatures, matching demand for coolest grown plants (Wconst ), such that water limitation increased with growth temperature. We found that constant water inputs resulted in a reduction of temperature optima for both photosynthesis and growth by ~3°C compared to increasing water inputs. Water limitation particularly reduced the total amount of leaf area displayed at Topt and intermediate growth temperatures. The reduction in photosynthesis could be attributed to lower leaf water potential and consequent stomatal closure. The reduction in growth was a result of decreased photosynthesis, reduced total leaf area display and a reduction in specific leaf area. Water availability had no effect on the response of stem and root respiration to warming, but we observed lower leaf respiration rates under constant water inputs compared to increasing water inputs at higher growth temperatures. Overall, this study demonstrates that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures.

Rice acclimation to soil flooding: Low concentrations of organic acids can trigger a barrier to radial oxygen loss in roots.

Waterlogged soils contain monocarboxylic acids produced by anaerobic microorganisms. These "organic acids" can accumulate to phytotoxic levels and promote development of a barrier to radial O2 loss (ROL) in roots of some wetland species. Environmental cues triggering root ROL barrier induction, a feature that together with tissue gas-filled porosity facilitates internal aeration, are important to elucidate for knowledge of plant stress physiology. We tested the hypothesis that comparatively low, non-toxic, concentrations of acetic, propionic, butyric, and/or hexanoic acids might induce root ROL barrier formation in rice. Each organic acid, individually, triggered the ROL barrier in roots but with no effect (acetic or butyric acids) or with only slight effects (propionic or hexanoic acids) on root extension. Transcripts of four genes related to suberin biosynthesis were increased by some of the organic acid treatments. Respiration in root tissues was not, or moderately, inhibited. Beyond a narrow concentration range, however, respiration declined exponentially and the order (least to greatest) for EC50 (effective concentration for 50% inhibition) was butyric, propionic, acetic, then hexanoic acid. An understanding of the environmental cue for root ROL barrier induction should enhance future work to elucidate the molecular regulation of this root trait contributing to plant flooding tolerance.

LeNRT1.1 Improves Nitrate Uptake in Grafted Tomato Plants under High Nitrogen Demand.

Grafting has become a common practice among tomato growers to obtain vigorous plants. These plants present a substantial increase in nitrogen (N) uptake from the root zone. However, the mechanisms involved in this higher uptake capacity have not been investigated. To elucidate whether the increase in N uptake in grafted tomato plants under high N demand conditions is related to the functioning of low- (high capacity) or high-affinity (low capacity) root plasma membrane transporters, a series of experiments were conducted. Plants grafted onto a vigorous rootstock, as well as ungrafted and homograft plants, were exposed to two radiation levels (400 and 800 µmol m-2 s-1). We assessed root plasma membrane nitrate transporters (LeNRT1.1, LeNRT1.2, LeNRT2.1, LeNRT2.2 and LeNRT2.3) expression, Michaelis‒Menten kinetics parameters (Vmax and Km), root and leaf nitrate reductase activity, and root respiration rates. The majority of nitrate uptake is mediated by LeNRT1.1 and LeNRT1.2 in grafted and ungrafted plants. Under high N demand conditions, vigorous rootstocks show similar levels of expression for LeNRT1.1 and LeNRT1.2, whereas ungrafted plants present a higher expression of LeNRT1.2. No differences in the uptake capacity (evaluated as Vmax), root respiration rates, or root nitrate assimilation capacity were found among treatments.

Root structure-function relationships in 74 species: evidence of a root economics spectrum related to carbon economy.

Although fine roots are important components of the global carbon cycle, there is limited understanding of root structure-function relationships among species. We determined whether root respiration rate and decomposability, two key processes driving carbon cycling but always studied separately, varied with root morphological and chemical traits, in a coordinated way that would demonstrate the existence of a root economics spectrum (RES). Twelve traits were measured on fine roots (diameter ≤ 2 mm) of 74 species (31 graminoids and 43 herbaceous and dwarf shrub eudicots) collected in three biomes. The findings of this study support the existence of a RES representing an axis of trait variation in which root respiration was positively correlated to nitrogen concentration and specific root length and negatively correlated to the root dry matter content, lignin : nitrogen ratio and the remaining mass after decomposition. This pattern of traits was highly consistent within graminoids but less consistent within eudicots, as a result of an uncoupling between decomposability and morphology, and of heterogeneity of individual roots of eudicots within the fine-root pool. The positive relationship found between root respiration and decomposability is essential for a better understanding of vegetation-soil feedbacks and for improving terrestrial biosphere models predicting the consequences of plant community changes for carbon cycling.

Enhancing cytokinin synthesis by overexpressing ipt alleviated drought inhibition of root growth through activating ROS-scavenging systems in Agrostis stolonifera.

Drought stress limits root growth and inhibits cytokinin (CK) production. Increases in CK production through overexpression of isopentenyltransferase (ipt) alleviate drought damages to promote root growth. The objective of this study was to investigate whether CK-regulated root growth was involved in the alteration of reactive oxygen species (ROS) production and ROS scavenging capacity under drought stress. Wild-type (WT) creeping bentgrass (Agrostis stolonifera L. 'Penncross') and a transgenic line (S41) overexpressing ipt ligated to a senescence-activated promoter (SAG12) were exposed to drought stress for 21 d in growth chambers. SAG12-ipt transgenic S41 developed a more extensive root system under drought stress compared to the WT. Root physiological analysis (electrolyte leakage and lipid peroxidation) showed that S41 roots exhibited less cellular damage compared to the WT under drought stress. Roots of SAG12-ipt transgenic S41 had significantly higher endogenous CK content than the WT roots under drought stress. ROS (hydrogen peroxide and superoxide) content was significantly lower and content of total and free ascorbate was significantly higher in S41 roots compared to the WT roots under drought stress. Enzymatic assays and transcript abundance analysis showed that superoxide dismutase, catalase, peroxidase, and dehydroascorbate reductase were significantly higher in S41 roots compared to the WT roots under drought stress. S41 roots also maintained significantly higher alternative respiration rates compared to the WT under drought stress. The improved root growth of transgenic creeping bentgrass may be facilitated by CK-enhanced ROS scavenging through antioxidant accumulation and activation of antioxidant enzymes, as well as higher alternative respiration rates when soil water is limited.

Root respiratory costs of ion uptake, root growth, and root maintenance in wetland plants: efficiency and strategy of O2 use for adaptation to hypoxia.

Oxygen use in roots is an important aspect of wetland plant ecophysiology, and it depends on the respiratory costs of three major processes: ion uptake, root growth, and root maintenance. However, O2 allocation in wetland plants has received little attention. This study aimed to determine the O2 allocation and specific respiratory cost of each process under hypoxic conditions, to better understand the strategy and efficiency of O2 use in wetland plants. The root respiration rate, nitrogen uptake, and root growth in three Carex species with different growth rates were examined under hypoxic conditions using different N sources, and the respiratory costs of ion uptake, root growth, and root maintenance were statistically estimated. All species exhibited low specific costs and low ratios of O2 allocation for root growth (2.0 ± 0.4 mmol O2 g(-1) and 15.2 ± 2.7 %, respectively). The specific cost of ion uptake was 20-30 % lower in fast-growing species than in slow-growing species. As plant growth rate increased, the O2 allocation ratio for ion uptake increased, and that for root maintenance decreased. The cost was higher when NO3 (-) was fed, than when NH4 (+) was fed, although the pattern of O2 allocation ratios for three processes was similar for NO3 (-) and NH4 (+). Our results indicate that wetland plants primarily employ an O2 use strategy of minimising the respiratory costs of root growth, and fast-growing plants specifically use O2 to maximise ion uptake. These findings provide new insights into ecophysiological behaviours of roots in adaptation to hypoxia.

Exaggerated root respiration accounts for growth retardation in a starchless mutant of Arabidopsis thaliana.

The knock-out mutation of plastidial phosphoglucomutase (pgm) causes a starchless phenotype in Arabidopsis thaliana, and results in a severe growth reduction of plants cultivated under diurnal conditions. It has been speculated that high soluble sugar levels accumulating during the light phase in leaf mesophyll might cause a reduction of photosynthetic activity or that shortage of reduced carbon during the night is the reason for the slow biomass gain of pgm. Separate simultaneous measurements of leaf net photosynthesis and root respiration demonstrate that photosynthetic activity per unit fresh weight is not reduced in pgm, whereas root respiration is strongly elevated. Comparison with a mutant defective in the dominating vacuolar invertase (AtßFruct4) revealed that high sucrose concentration in the cytosol, but not in the vacuole, of leaf cells is responsible for elevated assimilate transport to the root. Increased sugar supply to the root, as observed in pgm mutants, forces substantial respiratory losses. Because root respiration accounts for 80% of total plant respiration under long-day conditions, this gives rise to retarded biomass formation. In contrast, reduced vacuolar invertase activity leads to reduced net photosynthesis in the shoot and lowered root respiration, and affords an increased root/shoot ratio. The results demonstrate that roots have very limited capacity for carbon storage but exert rigid control of supply for their maintenance metabolism.

Exposure of barley plants to low Pi leads to rapid changes in root respiration that correlate with specific alterations in amino acid substrates.

The majority of inorganic phosphate (Pi ) stress studies in plants have focused on the response after growth has been retarded. Evidence from transcript analysis, however, shows that a Pi -stress specific response is initiated within minutes of transfer to low Pi and in crop plants precedes the expression of Pi transporters and depletion of vacuolar Pi reserves by days. In order to investigate the physiological and metabolic events during early exposure to low Pi in grain crops, we monitored the response of whole barley plants during the first hours following Pi withdrawal. Lowering the concentration of Pi led to rapid changes in root respiration and leaf gas exchange throughout the early phase of the light course. Combining amino and organic acid analysis with (15) N labelling we show a root-specific effect on nitrogen metabolism linked to specific substrates of respiration as soon as 1 h following Pi withdrawal; this explains the respiratory responses observed and was confirmed by stimulation of respiration by exogenous addition of these respiratory substrates to roots. The rapid adjustment of substrates for respiration in roots during short-term Pi -stress is highlighted and this could help guide roots towards Pi -rich soil patches without compromising biomass accumulation of the plant.

Summer drought alters carbon allocation to roots and root respiration in mountain grassland.

Drought affects the carbon (C) source and sink activities of plant organs, with potential consequences for belowground C allocation, a key process of the terrestrial C cycle. The responses of belowground C allocation dynamics to drought are so far poorly understood. We combined experimental rain exclusion with (13)C pulse labelling in a mountain meadow to analyse the effects of summer drought on the dynamics of belowground allocation of recently assimilated C and how it is partitioned among different carbohydrate pools and root respiration. Severe soil moisture deficit decreased the ecosystem C uptake and the amounts and velocity of C allocated from shoots to roots. However, the proportion of recently assimilated C translocated belowground remained unaffected by drought. Reduced root respiration, reflecting reduced C demand under drought, was increasingly sustained by C reserves, whilst recent assimilates were preferentially allocated to root storage and an enlarged pool of osmotically active compounds. Our results indicate that under drought conditions the usage of recent photosynthates is shifted from metabolic activity to osmotic adjustment and storage compounds.