Focus Issue on Roots: Root Nutrient Foraging

During a plant''s lifecycle, the availability of nutrients in the soil is mostly heterogeneous in space and time. Plants are able to adapt to nutrient shortage or localized nutrient availability by altering their root system architecture to efficiently explore soil zones containing the limited nutrient. It has been shown that the deficiency of different nutrients induces root architectural and morphological changes that are, at least to some extent, nutrient specific. Here, we highlight what is known about the importance of individual root system components for nutrient acquisition and how developmental and physiological responses can be coupled to increase nutrient foraging by roots. In addition, we review prominent molecular mechanisms involved in altering the root system in response to local nutrient availability or to the plant''s nutritional status.In natural and agricultural soils, the ability of plants to quickly and efficiently acquire nutrients may determine their competitive success and productivity. Because mineral elements interact differently with themselves and other soil constituents or are carried by water out of the rooted soil volume, their availability to plants may decrease and lead to nutrient deficiency. Under these conditions, plants activate foraging responses that include morphological changes, such as the modulation of root system architecture (RSA) or root hair formation, and physiological changes, such as the release of nutrient-mobilizing root exudates or the expression of nutrient transporters (Gojon et al., 2009; Hinsinger et al., 2009; Gruber et al., 2013). These responses are often spatially coupled to increase the root-soil interaction zone and improve the ability of the plant to intercept immobile nutrients. Noteworthy, although not discussed herein, symbiosis or associative rhizosphere microorganisms can also alter the RSA and enhance the foraging capacity of the plant (Gutjahr and Paszkowski, 2013). Here, we provide an update on the morphological responses induced by plants to forage sparingly available nutrients and some of the underlying molecular mechanisms known to date to be involved in RSA adaptations to nutrient availabilities.     

Weak Competition Among Tropical Tree Seedlings: Implications for Species Coexistence

The intensity of competition among forest tree seedlings is poorly understood, but has important ramifications for their recruitment and for the maintenance of species diversity. Intense competition among seedlings could allow competitively dominant species to exclude subordinate species. Alternatively, the low density and small stature of forest tree seedlings could preclude intense interseedling competition. In this case, other processes, such as size-asymmetric competition with adults, interactions with consumers, or neutral dynamics would prevail as those structuring the forest understory. We tested the intensity of, and potential for, intraspecific competition among tree seedlings of three species ( Brosimum alicastrum , Matisia cordata , and Pouteria reticulata ) in two Neotropical rain forests. We reduced stem densities by up to 90 percent and monitored individual growth and survival rates for up to 24 mo. Individual growth and survival rates were generally unrelated to stem density. Contrary to the predicted behavior of intensely competing plant populations, the distribution of individual heights did not become more left-skewed with time for any species, regardless of plot density; i.e. , excesses of short, suppressed individuals did not accumulate in high-density plots. We further measured the overlap of zones of influence (ZOIs) to assess the potential for resource competition. Seedling ZOIs overlapped only slightly in extremely dense monodominant plots, and even less in ambient-density plots of mixed composition. Our results thus suggest that interseedling competition was weak. Given the low density of tree seedlings in Neotropical forests, we infer that resource competition among seedlings may be irrelevant to their recruitment.     

外源NO对铜胁迫下番茄幼苗根系构型及其超微结构的影响

采用营养液水培的方法,以“改良毛粉802F1”番茄为材料,硝普钠（sodiumnitroprusside,sNP）为一氧化氮（N0）供体,研究外源N0对铜胁迫下番茄幼苗根系构型及其超微结构的影响。结果表明,50μmol·L-1的铜胁迫下,外施100μmol·L-1 SNP能够显著增加番茄幼苗植株的生物量、株高和茎粗,提高根系活力,改善根系构型中的根长度、根平均直径、根表面积和根体积,缓解番茄幼苗亚细胞结构（细胞核、线粒体、叶绿体、液泡、核膜）的改变,维持番茄幼苗组织结构的稳定,减缓铜胁迫对植株生长的抑制作用,添加NO清除剂牛血红蛋白后,能显著消除NO的缓解效果。     

Root System Development of Oak Seedlings Analysed using an Architectural Model. Effects of Competition with Grass

A dynamic 3D model of root system development was adapted to young sessile oak seedlings, in order to evaluate the effects of grass competition on seedling root system development. The model is based on a root typology and the implementation of a series of developmental processes (axial and radial growth, branching, reiteration, decay and abscission). Parameters describing the different processes are estimated for each root type. Young oak seedlings were grown for 4 years in bare soil or with grass competition and were periodically excavated for root system observation and measurements (topology of the root system, length and diameter of all roots with a diameter greater than 0.3 mm). In the fourth year, 40 cm×20 cm×20 cm soil monoliths were excavated for fine root measurement (root density and root length). Root spatial development was analysed on a sub-sample of roots selected on four seedlings. The model was a guideline that provided a complete and consistent set of parameters to represent root system development. It gave a comprehensive view of the root systems and made it possible to quantify the effects of competition on the different root growth processes. The same root typology was used to describe the seedlings in bare soil and in grass. Five root types were defined, from large tap roots to fine roots. Root system size was considerably reduced by grass competition. Branching density was not affected but the branch roots were always smaller for the seedlings grown in competition. Reiteration capacity was also reduced by competition. Cross sectional areas before and after branching were linearly related with a scaling coefficient close to 1, as predicted by the pipe model theory. This relationship was not affected by grass competition.     

Effects of Waterlogging on Water Relations of Actively-growing and Dormant Sitka Spruce Seedlings

Two-year-old Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings,either actively growing or dormant, were waterlogged in a growthroom at 15 °C. Shoot and root growth, transpiration andleaf water potential were observed. In actively-growing plants shoot extension continued after waterlogging,though at a reduced rate, and shoots of dormant plants brokebud and extended during the waterlogging period. Root growthwas suppressed by waterlogging in both types of plant. The 22day waterlogging treatment eventually killed the actively-growingplants but plants which were dormant at the time of waterloggingwere more tolerant. Changes in plant water relations after waterloggingwere entirely different depending on the condition of the plantswhen the soil was flooded. Dormant plants showed a gradual reductionin transpiration and increased water stress over the waterloggingperiod; after the soil was drained leaf water potential increasedto equal the value of control plants which had been maintainedin a freely drained condition, but transpiration did not increaseuntil root growth began. Actively-growing plants exhibited amore complex behaviour, characterized by a very rapid reductionin transpiration after waterlogging, accompanied by a briefperiod of water stress, followed by a period of increasing transpirationrate in the absence of water stress. Finally a second reductionin transpiration occurred and water stress increased as theseedlings died. The importance of the stage of activity of theroot system to the response of plants to waterlogging is discussed. Picea sitchensis (Bong.) Carr., Sitka spruce, waterlogging, water relations, dormancy, transpiration, water potential     

氮沉降及菌根真菌对长白落叶松苗木根系构型及根际酶活性的影响

为探究氮沉降和接种菌根真菌对长白落叶松苗木根系构型和根际土壤酶活性的影响,以1年生长白落叶松(Larix olgensis)的盆栽菌根苗(简称+M,混合接种8种外生菌根真菌)和非菌根苗(简称-M,未接种处理)为研究对象,设置4个氮沉降处理(不施氮(0N,0 kg·N·hm^-1·yr^-1)、低氮(LN,15 kg·N·hm^-1·yr^-1)、中氮(MN,30 kg·N·hm^-1·a^-1)和高氮(HN,60 kg·N·hm^-1·a^-1)),测定直径0~0.5 mm根系的总根长、总表面积、总体积和根尖数等根系形态指标,对比分析氮沉降和接种菌根真菌处理对苗木根际土壤酶(β-1,4葡萄糖苷酶(BG)、亮氨酸氨基肽酶(LAP)、β-1,4-N-乙酰-氨基葡糖氨糖苷酶(NAG)、酸性磷酸酶(ACP)和碱性磷酸酶(ALP))活性的影响。结果表明:①长白落叶松苗木直径0~0.5 mm根系的总根长、总表面积、总体积和根尖数均随氮浓度的递增呈下降的趋势;在0N、LN和MN处理下,-M处理的根系形态指标均高于+M处理。②随氮浓度增加,+M和-M处理苗木根际土壤中BG、LAP、ACP和ALP活性均呈先增加后下降的趋势,而NAG活性呈下降的趋势。③+M和-M处理下,长白落叶松直径0~0.5 mm根系的形态指标与根际土壤BG活性均呈显著负相关关系(P<0.05);除根尖数外,其它根系形态指标与NAG活性相关性均为正相关(P<0.05)。综上所述,苗木菌根化处理削弱了氮沉降对落叶松苗木根系构型的影响;而低氮处理下,+M处理对苗木根际土壤酶的活化程度高于-M处理。     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition. Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattem.     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition.Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattern.     

Root Architecture and Root:Shoot Allocation of Shrubs and Saplings in Two Lowland Tropical Forests: Implications for Life-Form Composition1

Entire root systems of saplings of five canopy species and of six shrub and treelet species growing in lowland mixed dipterocarp forest at Andulau, Brunei were excavated and measured. Referring to a prior study at Gigante, Panama, two-way, fixed-factor ANOVAs were used to compare life–forms and sites. Rooting depth and the proportion of root surface area in the upper 20 cm of soil did not differ significantly between life-forms because some treelets/shrubs at Andulau were deep-rooted; all saplings studied were deep-rooted. The root:leaf area ratios of both saplings and treelets/ shrubs at Andulau were significantly higher than those at Gigante. We attribute this strong difference to the lower soil content of available nutrients at Andulau where rainfall shortage is less severe and regular than at Gigante. Available data on life-form composition and mortality rates in large plots are consistent with our proposal that shallow-rooted shrubs and treelets are more vulnerable to drought than deep-rooted life-forms. We suggest that future studies of water use partitioning, wood anatomy, leaf morphology, and associations with neighboring plants would benefit from an explicit examination of their relation to rooting depth.     

Nutrient and Carbon Additions to the Microbial Soil Community and its Impact on Tree Seedlings in a Boreal Spruce Forest

We have added glucose and nutrients to manipulate soil microbial activity and nutrient availability in a boreal spruce forest to study the performance of birch and spruce seedlings in relation to the soil microbial community. The proportion of aboveground biomass in the seedlings was largest in plots amended with extra nutrients, while ectomycorrhizal (ECM) colonisation was low in these plots. ECM appeared beneficial for growth of both species, but only at low levels of colonisation (<25% ECM colonised root-tips). The soil microbial biomass, as determined by total PLFA, was largest in plots treated with glucose and there was a significant negative relationship between birch seedling size and levels of total PLFA in soil. This could be taken to suggest that poor seedling growth was due to nutrient limitation caused by microbial assimilation. However, the treatment response of the birch seedlings was generally weak, and spruce often showed no response at all to the addition of nutrients and glucose. The most consistent parameter for the variation in plant performance, as well as for the microbial soil community, was the block-effect. This suggests a strong spatial structure in the soil microbial community, and that this structure was robust with respect to our treatments even though they continued over a 3-year period.     

Sexual Segregation in Juvenile New Zealand Sea Lion Foraging Ranges: Implications for Intraspecific Competition,Population Dynamics and Conservation

Sexual segregation (sex differences in spatial organisation and resource use) is observed in a large range of taxa. Investigating causes for sexual segregation is vital for understanding population dynamics and has important conservation implications, as sex differences in foraging ecology may affect vulnerability to area-specific human activities. Although behavioural ecologists have proposed numerous hypotheses for this phenomenon, the underlying causes of sexual segregation are poorly understood. We examined the size-dimorphism and niche divergence hypotheses as potential explanations for sexual segregation in the New Zealand (NZ) sea lion (Phocarctos hookeri), a nationally critical, declining species impacted by trawl fisheries. We used satellite telemetry and linear mixed effects models to investigate sex differences in the foraging ranges of juvenile NZ sea lions. Male trip distances and durations were almost twice as long as female trips, with males foraging over the Auckland Island shelf and in further locations than females. Sex was the most important variable in trip distance, maximum distance travelled from study site, foraging cycle duration and percent time at sea whereas mass and age had small effects on these characteristics. Our findings support the predictions of the niche divergence hypothesis, which suggests that sexual segregation acts to decrease intraspecific resource competition. As a consequence of sexual segregation in foraging ranges, female foraging grounds had proportionally double the overlap with fisheries operations than males. This distribution exposes female juvenile NZ sea lions to a greater risk of resource competition and bycatch from fisheries than males, which can result in higher female mortality. Such sex-biased mortality could impact population dynamics, because female population decline can lead to decreased population fecundity. Thus, effective conservation and management strategies must take into account sex differences in foraging behaviour, as well as differential threat-risk to external impacts such as fisheries bycatch.     

Foraging niche separation of social wasps in an invaded area: Implications for their management

Foraging niche separation may be a mechanism to promote coexistence of two competing species by concentrating intraspecific competition relative to interspecific competition. The present study investigated foraging behaviour and microhabitat use of two coexisting species of invasive social wasps, Vespula germanica and Vespula vulgaris, when foraging for two different food resources. Also, we tested the attractiveness of traps baited with a synthetic lure for those two species. We found that V. germanica wasps prefer to forage at ground level regardless of the resource, while V. vulgaris prefers protein resources at the shrubland level given a choice between a protein bait at ground or at shrubland level. However, when baited with the synthetic lure, the species caught was not affected by the height at which traps were placed. That is, in a no choice scenario, the traps were sufficiently attractive to lure both species of wasps to both microhabitats (ground and shrubland levels). Thus, our results support the existence of spatial niche differentiation at least in protein foraging and suggest that the synthetic lure evaluated could be used to trap both species of Vespula wasps present in Argentina. These results could help to improve management strategies of these social wasps in an invaded area.     

褪黑素对玉米幼苗根系发育和抗旱性的影响

褪黑素是一种在生物体内广泛存在的吲哚胺类化合物,参与植物的多种生理和生化过程.近年来研究认为褪黑素可以不同程度地增强植物的抗逆性,但对其作用机理仍知之甚少.通过两种褪黑素的施用方法,详细研究了褪黑素对于玉米根系发育和抗旱性的影响.首先,采用水培根灌褪黑素的方法对玉米幼苗的根系和生长状况进行分析,结果表明施加褪黑素显著提...     

Effects of Intraspecific Competition and Host-Parasitoid Developmental Timing on Foraging Behaviour of a Parasitoid Wasp

In a context where hosts are distributed in patches and susceptible to parasitism for a limited time, female parasitoids foraging for hosts might experience intraspecific competition. We investigated the effects of host and parasitoid developmental stage and intraspecific competition among foraging females on host-searching behaviour in the parasitoid wasp Hyposoter horticola. We found that H. horticola females have a pre-reproductive adult stage during which their eggs are not mature yet and they forage very little for hosts. The wasps foraged for hosts more once they were mature. Behavioural experiments showed that wasps’ foraging activity also increased as host eggs aged and became susceptible to parasitism, and as competition among foraging wasps increased.     

Effects of Root Restriction, Root Pruning and 6-Benzylaminopurine on the Growth of Peach Seedlings

Peach seedlings were grown in aerated nutrient solution in small(15 cm³) or large (500 cm³) containers. Subsequently some plantswere removed from the small to the large containers or werestressed by root pruning. Half the plants received a foliarspray of 6-benzylaminopurine (BAP) every 4 days. Root restriction eventually impaired growth rates and ultimatelyplant size. The effect of root pruning was quickly overcomeby a redistribution of growth in favour of the roots. When restrictedplants were transferred into large containers there was an accompanyingrapid root growth and plants attained a comparable size to unrestrictedplants. BAP applied to the leaves to some extent overcame thereduced growth of restricted plants, however, its major effectwas on the distribution of dry matter. Restriction and BAP application,in general, reduced the levels of mineral nutrients in the leaves. It is argued that the limit set by the roots on top growth involvesan internal regulation by the root, in particular the productionand supply of growth substances. The evidence suggests thatthis is related to root morphology and must be taken into accountwhen studying top:root inter-relationships. The experiment highlights the importance of root restrictionas a technique for manipulating root growth and studying rootfunction and its interaction with the top. Some implicationsof controlled root growth in the field are discussed.     

菌根真菌与施肥对杜鹃苗养分吸收的影响

以2年生杜鹃(Rhododendron simsii)苗为研究对象,采用J1(杜鹃花类菌根真菌混合接菌)、J2(杜鹃花类菌根真菌和马尾松外生菌混合接菌)、J3(不接菌,对照)等3种接菌组合与不同氮磷钾施肥组合研究菌根真菌与氮磷钾肥对杜鹃苗生长及养分吸收的影响。结果表明,杜鹃幼苗的生长指标和氮磷钾含量总体表现为接菌处理优于不接菌处理,J2处理下施用尿素1.1 g·株-1、钙镁磷1 g·株-1、氯化钾0.7 g·株-1对杜鹃苗生长及氮磷钾含量积累的促进作用最为明显。数据表明,J2处理促进N向叶积累,促进K向茎叶积累,促进P向根茎积累;J1处理促进N和P向根茎积累,促进K向茎叶积累。     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.     

Effects of KCN and Salicylhydroxamic Acid on the Root Respiration of Pea Seedlings

Polarography, using cylindrical platinum electrodes, proved suitable for measuring changes in the internal apical O₂ concentration of the primary root of pea (Pisum sativum L. cv Meteor) effected by KCN and/or salicylhydroxamic acid (SHAM) in the bathing medium. An electrical rootaeration analog was used to help evaluate some of the results. Concentrations of KCN ≤0.05 millimolar had no significant effect. In response to 0.1 millimolar KCN, the O₂ concentration rose substantially for approximately 2 hours, then declined, and after 10 hours had frequently fallen below the pretreatment level. Such changes suggest an initial inhibition of cytochrome oxidase-mediated O₂ uptake followed by an induction of the alternative, cyanide-resistant respiratory pathway. These treatments proved nonlethal. Changes in O₂ concentration similar to those described for 0.1 millimolar KCN were observed in response to 1 and 10 millimolar KCN but these treatments were lethal and the root apex became soft and often appeared flooded. Roots survived and showed no significant responses when treated with SHAM at concentrations ≤5 millimolar. However, when the alternative pathway had been (apparently) induced by 0.1 millimolar KCN, the addition of 5 millimolar SHAM to the bathing medium caused a substantial and persistent rise in the root apical O₂ concentration, suggesting that this (nonlethal) concentration of SHAM could indeed inhibit O₂ uptake via the cyanide-resistant pathway.

It is concluded that while O₂ uptake normally occurs by the cytochrome pathway in the primary pea root, the alternative, cyanide-resistant pathway can be induced by 0.1 millimolar KCN.

     

接种丛枝菌根真菌对模拟大气氮沉降下灌木铁线莲根系形态及养分承载的影响

为了解菌根化处理的灌木铁线莲(Clematis fruticosa)苗木根系形态及养分承载对氮沉降的应激响应,以1年生盆栽灌木铁线莲为对象,分别采用单接种和混合接种,即：单接种根内根孢囊霉(Rhizophagus intraradices,以下简称+R),单接种摩西斗管囊霉(Funneliformis mosseae,以下简称+F);混合接菌(上述2菌种菌剂按体积1∶1混合,以下简称+RF)的菌根苗。以非菌根苗(未接菌,以下简称-M)为对照。氮沉降处理设置4个梯度(不施氮(0N,0 g·m^-2·a^-1)、低氮(LN,3 g·m^-2·a^-1)、中氮(MN,6 g·m^-2·a^-1)、高氮(HN,9 g·m^-2·a^-1)),1年后测定各处理细根形态(直径≤0.5 mm的总根长、总表面积、总体积、根尖数量)、菌根侵染率、土壤孢子密度及根、茎、叶各器官的养分(碳、氮、磷)含量等指标。(1)在+R和+RF处理下,LN处理的苗...