Hydraulic lift: Substantial nocturnal water transport between soil layers by Artemisia tridentata roots

Summary Diel soil water potential fluctuations reflected daytime depletion and nocturnal resupply of water in upper soil layers. Transpiration suppression experiments demonstrated that water absorption by roots caused the daytime depletion. The soil water potential data and experimental results suggest that at night water absorbed from moist soil by deeper roots is transported to and lost from roots into drier upper soil layers. The deeper roots appear to absorb and transport water both day and night. Implications for the efficiency of deep roots and water storage, nutrient uptake and water parasitism in upper soil layers are discussed.     

Osmotic relations of the drought-tolerant shrub Artemisia tridentata in response to water stress

Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above ?2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to ?5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.     

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl(2). Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature 'transition' zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83-95%) in response to HgCl(2) was observed in cortical cells. Water flow through roots and plants was reduced less (40-74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue.     

Relative Abundance of and Composition within Fungal Orders Differ between Cheatgrass (Bromus tectorum) and Sagebrush (Artemisia tridentata)-Associated Soils

Nonnative Bromus tectorum (cheatgrass) is decimating sagebrush steppe, one of the largest ecosystems in the Western United States, and is causing regional-scale shifts in the predominant plant-fungal interactions. Sagebrush, a native perennial, hosts arbuscular mycorrhizal fungi (AMF), whereas cheatgrass, a winter annual, is a relatively poor host of AMF. This shift is likely intertwined with decreased carbon (C)-sequestration in cheatgrass-invaded soils and alterations in overall soil fungal community composition and structure, but the latter remain unresolved. We examined soil fungal communities using high throughput amplicon sequencing (ribosomal large subunit gene) in the 0–4 cm and 4–8 cm depth intervals of six cores from cheatgrass- and six cores from sagebrush-dominated soils. Sagebrush core surfaces (0–4 cm) contained higher nitrogen and total C than cheatgrass core surfaces; these differences mirrored the presence of glomalin related soil proteins (GRSP), which has been associated with AMF activity and increased C-sequestration. Fungal richness was not significantly affected by vegetation type, depth or an interaction of the two factors. However, the relative abundance of seven taxonomic orders was significantly affected by vegetation type or the interaction between vegetation type and depth. Teloschistales, Spizellomycetales, Pezizales and Cantharellales were more abundant in sagebrush libraries and contain mycorrhizal, lichenized and basal lineages of fungi. Only two orders (Coniochaetales and Sordariales), which contain numerous economically important pathogens and opportunistic saprotrophs, were more abundant in cheatgrass libraries. Pleosporales, Agaricales, Helotiales and Hypocreales were most abundant across all libraries, but the number of genera detected within these orders was as much as 29 times lower in cheatgrass relative to sagebrush libraries. These compositional differences between fungal communities associated with cheatgrass- and sagebrush-dominated soils warrant future research to examine soil fungal community composition across more sites and time points as well as in association with native grass species that also occupy cheatgrass- invaded ecosystems.     

Temporal variation in nutrient uptake capacity by intact roots of mature loblolly pine

Nutrient uptake is generally thought to exhibit a simple seasonal pattern, but few studies have measured temporal variation of nutrient uptake capacity in mature trees. We measured net uptake capacity of K, NH⁺₄, NO₃^–, Mg and Ca across a range of solution concentrations by roots of mature loblolly pine at Calhoun Experimental Forest in October 2001, July 2001, and April 2002. Uptake capacity was generally lowest in July; rates in October were similar to those in April. Across a range of concentrations, antecedent nutrient solution concentrations affected the temporal patterns in uptake in July but not in October or April. In July, uptake of NH⁺₄, Mg and Ca was positively correlated with concentration when roots were exposed to successively lower concentrations, but negatively correlated with concentration when exposed to successively higher concentrations. In contrast, uptake in October was constant across the range of concentrations, while uptake increased with concentration in April. As in studies of other species, we found greater uptake of NH⁺₄ than NO₃^–. Temporal patterns of uptake capacity are difficult to predict, and our results indicate that experimental conditions, such as experiment duration, antecedent root conditions and nutrient solution concentration, affect measured rates of nutrient uptake.     

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.): Phosphorus uptake

Under anaerobic conditions, ferric hydroxide deposits on the surface of rice roots have been shown to affect the uptake of some nutrients. In the present experiment, different amount of this iron plaque were induced on the roots of rice (Oryza sativa L. cv. TZ88-145) by supplying different Fe(OH)₃ concentrations in nutrient solutions, and the effect of the iron plaque on phosphorus uptake was investigated. Results showed that 1) iron plaque adsorbed phosphorus from the growth medium, and that the amount of phosphorus adsorbed by the plaque was correlated with the amount of plaque; 2) the phosphorus concentration in the shoot increased by up to 72% after 72 h at concentration of Fe(OH)₃ in the nutrient solution from 0 to 30 mg Fe/L, corresponding with amounts of iron plaque from 0.2 to 24.5 mg g^-1 (root d. wt); 3) the phosphorus concentration in the shoots of rice with iron plaque was higher than that without iron plaque though the concentration in the shoot decreased when Fe(OH)₃ was added at 50 mg Fe/L producing 28.3 mg g^-1 (root d. wt) of plaque; and 4) the phosphorus concentrations in Fe-deficient and Fe-sufficient rice plants with iron plaque were the same, although phytosiderophores were released from the Fe-deficient roots. The phytosiderophores evidently did not mobilise phosphorus adsorbed on plaque. The results suggest that iron plaque on rice plant roots might be considered a phosphorus reservoir. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate uptake by bean (Phaseolus vulgaris L.) roots under phosphate deficiency

Bean (Phaseolus vulgaris L.) plants were cultured for 19 d on complete or on phosphate deficient culture media. Low inorganic phosphate concentration in the roots decreased ATP level and nitrate uptake rate. The mechanisms which may control nitrate uptake rate during phosphate deficiency were examined. Plasma membrane enriched fractions from phosphate sufficient and phosphate deficient plants were isolated and compared. The decrease in total phospholipid content was observed in plasma membranes from phosphate deficient roots, but phospholipid composition was similar. No changes in ATPase and proton pumping activities measured in isolated plasma membrane of phosphate sufficient and phosphate deficient bean roots were noted. The electron microscope observations carried out on cortical meristematic cells of the roots showed that active ATPases were found in plasma membrane of both phosphate sufficient and phosphate deficient plants. The decrease in inorganic phosphate concentration in roots led to increased nitrate accumulation in roots, accompanied by a corresponding alterations in NO₃ distribution between shoots and roots. Nitrate reductase activity in roots of phosphate deficient plants estimated in vivo and in vitro was reduced to 50–60% of the control. The increased NO₃ concentration in root tissue may be explained by decreased NR activity and lower transport of nitrate from roots to shoots. Therefore, the reduction of nitrate uptake during phosphate starvation is mainly a consequence of nitrate accumulation in the roots.     

Seed provenance determines germination responses of Rumex crispus (L.) under water stress and nutrient availability

Aims Seeds ofRumex crispusfrom six provenances were studied in relation to their germination under drought and presence of nitrogen in the germination and emergence media. We also investigated whether adaptation to soil increases the ability of the species to colonize and establish in contrasting environments along a longitudinal gradient in western Spain by means of a reciprocal transplantation experiment.     

外源褪黑素对干旱胁迫下黑麦草和苜蓿抗氧化能力及养分吸收的影响

以黑麦草和苜蓿为对象,分别叶面喷施和根施100 μmol·L^-1的褪黑素溶液,在干旱胁迫下测定了生物量、丙二醛(MDA)含量、相对电导率、超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT)活性、养分含量(有机碳C、全氮N、全磷P)等指标,研究外源褪黑素对干旱胁迫下植物抗氧化能力及养分吸收的影响。结果表明: 干旱胁迫下,黑麦草和苜蓿的地上、地下生物量显著降低,外施褪黑素能够有效缓解干旱胁迫对黑麦草和苜蓿生长的抑制作用,叶面喷施和根施褪黑素使干旱胁迫下黑麦草的生物量分别增加14.5%和29.6%,苜蓿的生物量分别增加36.6%和49.1%。干旱胁迫下,黑麦草的SOD、POD活性和苜蓿的SOD活性显著降低,外施褪黑素显著提高黑麦草和苜蓿的SOD、POD、CAT活性,减少叶片中MDA的积累,使叶片相对电导率显著下降,抗氧化能力显著提高。干旱和外施褪黑素对黑麦草和苜蓿有机碳含量无显著影响。干旱胁迫下,黑麦草叶片和根中的N、P含量以及苜蓿根中的N含量降低,外施褪黑素提高黑麦草和苜蓿根和叶片中的N、P含量,这表明褪黑素对干旱胁迫下黑麦草和苜蓿的养分吸收有一定的调节作用。施用褪黑素不仅能改善植物的抗氧化能力,还能调节养分吸收以增强植物对干旱胁迫的适应性,而且叶面喷施褪黑素效果好于根施。     

非均匀水势下作物根系吸水模型分析

根据土壤-根系统中水分守恒和水势对水分运输作用的原理, 建立了土壤中非均匀水势作物根系吸水模型。在该模型中, 分别对一次函数和指数函数两种不同的非均匀土壤水势的表达形式建立模型, 并对非均匀水势和均匀水势下模型的解析解之间的关系进行了探讨; 利用该模型讨论根系的吸收阻力和木质部传导阻力的比率对根吸水的影响; 运用阻力比率的合理生理范围确定根生长的优化长度。结果表明: 在特定情况下, 非均匀水势下的根系吸水模型可以用于均匀水势, 对Poiseuille公式进行修正后得到的根的优化长度接近实际值。     

Estimating nutrient uptake by mature tree roots under field conditions: challenges and opportunities

Nutrient uptake by roots of mature trees is difficult to measure accurately under field conditions using existing methods. In this review, we discuss current techniques for measuring uptake at the root surface including excised roots, isotopic tracers, autoradiography, depletion, and lysimeters. Although these methods have provided many insights, each has drawbacks. Estimates of uptake are affected by the sampling scheme, experimental conditions, whether roots are excised or not, concentrations of ions, and the rate of efflux of ions. Microbes and mycorrhizas can also affect estimates of uptake. A greater focus on methods development is critical to advancing our understanding of nutrient uptake of mature trees under conditions representative of those in the field.     

野牛草叶片活性氧及其清除系统对水分胁迫的响应

逆境条件下活性氧产生及其清除效率是衡量植物抗性的重要指标,而活性氧对植物组织的氧化伤害和抗氧化酶活性常因材料的遗传或生理差异性而发生变化。以同一基因型的野牛草(Buchloe dactyloides(Nutt.)Engelm'texoka')克隆分株叶片为材料,采用Hoagland营养液培养,研究了10%、20%和30%PEG-6000模拟干旱胁迫下,野牛草叶片活性氧的产生、脂质过氧化和抗氧化酶活性变化规律。结果表明:随着PEG-6000浓度的增加及胁迫时间的延长,超氧阴离子(O2-.)的产生速率、过氧化氢(H2O2)和丙二醛(MDA)含量均显著增加;超氧化物歧化酶(SOD EC1.15.1.1)、愈创木酚过氧化物酶(G-POD EC1.11.1.7)和过氧化氢酶(CATEC1.11.1.6)活性呈先上升后下降的变化趋势;SOD、G-POD与CAT活性达到峰值的时间随PEG-6000浓度的增加而提前。认为水分胁迫下,抗氧化酶可有效清除活性氧自由基,但随胁迫时间的延长,抗氧化酶活性受到抑制。     

Regulation of root water uptake under abiotic stress conditions

A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.     

5-Aminolevulinic acid improves photosynthetic gas exchange capacity and ion uptake under salinity stress in oilseed rape (Brassica napus L.)

Salinity is one of the major constraints in oilseed rape (Brassica napus L.) production. One of the means to overcome this constraint is the use of plant growth regulators to induce plant tolerance. To study the plant response to salinity in combination with a growth regulator, 5-aminolevulinic acid (ALA), oilseed rape plants were grown hydroponically in greenhouse conditions under three levels of salinity (0, 100, and 200 mM NaCl) and foliar application of ALA (30 mg/l). Salinity depressed the growth of shoots and roots, and decreased leaf water potential and chlorophyll concentration. Addition of ALA partially improved the growth of shoots and roots, and increased the leaf chlorophyll concentrations of stressed plants. Foliar application of ALA also maintained leaf water potential of plants growing in 100 mM salinity at the same level as that of the control plants, and there was also an improvement in the water relations of ALA-treated plants growing in 200 mM. Net photosynthetic rate and gas exchange parameters were also reduced significantly with increasing salinity; these effects were partially reversed upon foliar application with ALA. Sodium accumulation increased with increasing NaCl concentration which induced a complex response in the macro-and micronutrients uptake and accumulation in both roots and leaves. Generally, analyses of macro- (N, P, K, S, Ca, and Mg) and micronutrients (Mn, Zn, Fe, and Cu) showed no increased accumulation of these ions in the leaves and roots (on dry weight basis) under increasing salinity except for zinc (Zn). Foliar application of ALA enhanced the concentrations of all nutrients other than Mn and Cu. These results suggest that under short-term salinity-induced stress (10 days), exogenous application of ALA helped the plants improve growth, photosynthetic gas exchange capacity, water potential, chlorophyll content, and mineral nutrition by manipulating the uptake of Na⁺.     

Drought-avoidant soybean germplasm maintains nitrogen-fixation capacity under water stress

Inoculated soybeans (Glycine max L. (Merrill)) were grown in controlled environments to evaluate the relationship between genotype and plant water status on nodule function, nitrogen assimilation, growth rates, and seed yield. Plants were grown under well-watered (WW) and water-stressed (WS) conditions during the linear pod-filling growth stage in sand culture using N-free nutrient solution. Dry matter and N accumulation were greater for the drought-adapted Plant Introduction 416937 (PI) than for Forrest, a commercially adapted genotype of similar phenology. These differences are attributed to: (i) more favorable internal water balance throughout the pod-filling period (higher total leaf water potential), (ii) higher photosynthetic function (more total leaf area and higher net carbon exchange rates), and (iii) stronger nodule function (larger nodule mass, greater specific and total nodule activity, and thus more nitrogen assimilation) for the PI than for Forrest. While Forrest out yielded the PI under WW conditions, the percentage reduction in seed mass per plant was less for the PI than for Forrest when both genotypes were exposed to desiccating conditions. The inference is that soybean germplasm with the capacity to maintain tissue turgidity, and thus leaf and nodule function, during reproductively-imposed desiccation may reduce the extent to which yield is compromised during drought. These findings have implications for the role of symbiotic nitrogen fixation in conserving yield under dry weather conditions.Abbreviations DAE Days After Emergence - NCE Net CO₂ Exchange - PI PI 416937 - SNA Specific Nodule Activity - TNA Total Nodule Activity - WS Water Stressed - WW Well Watered     

三角叶滨藜根吸水特点与其抗盐性的关系

采用压力室和冰点渗透压计测定了三角叶滨藜在不同浓度NaCl的根系环境溶液中根木质部的压力势和伤流液的渗透势,并利用原子吸收分光光度计测定了植株和伤流液以及环境溶液中Na 含量。结果表明:随着根环境溶液NaCl浓度的增加,三角叶滨藜植株和木质部伤流液中Na 含量虽呈上升趋势,但根系的过滤系数和体内Na 相对累积量逐渐降低,说明三角叶滨藜根细胞对盐分有很强的过滤作用;木质部伤流液的渗透势随着环境溶液渗透势的降低而降低,但根木质部溶液的水势则逐渐高出根外环境溶液的渗透势;表明三角叶滨藜能够利用较低的木质部负压来抵抗根外溶液的低渗透势而反渗透吸水,并利用根细胞对盐分的过滤作用来避免从环境摄取过量的盐分。     

Genetic studies on the acidification capacity of sunflower roots induced under iron stress

Under stress of iron deficiency roots of sunflower (Helianthus annuus L.) increase proton efflux which acidifies the root medium, increase the ferric reducing capacity and the exudation of phenolic compounds. Differences have been found previously among sunflower inbred lines in the capacity of their roots to lower pH and it was also found that this character is under genetic control.This work presents the results of an inheritance study made by crossing two genotypes, one (CMS HA 89) without acidification capacity and another (RHA 271) with it. Plants were grown individually in 75 mL vessels with an aerated solution low in iron. After 4 days, solutions were changed to one without iron and the pH of the medium was measured during the following days. Results from F₁, F₂, and backcross generations can be explained with two pairs of alleles controlling the character, being the relation between alleles of complete dominance at both gene pairs, but either gene, when dominant is epistatic to the other.