Root Development and Nutrient Uptake

Root system formation proceeds in close coordination with shoot growth. Accordingly, root growth and its functions are regulated tightly by the shoot through materials cycling between roots and shoots. A plant root system consists of different kinds of roots that differ in morphology and functions. The spatial configuration and distribution of these roots determine root system architecture in the soil, which in turn primarily regulates the acquisition of soil resources like nutrients and water. Morphological and physiological properties of each root and the concomitant tissues further affect nutrient uptake and transport, while the root traits that are related to such acquisition also depend on the kinds of nutrients and their mobility in the soil. In addition, mechanisms involved in the uptake and transport of mineral nutrients recently have been elucidated at the molecular level. A number of genes for acquisition and transport of various mineral nutrients have been identified in model plant systems such as Arabidopsis thaliana, and rice, and in other plant species. An integration of studies on nutrient behavior in soils and the morphological and physiological functions of root systems will further elucidate the mechanism of plant nutrient uptake and transport by roots, and offer a real possibility of genetically improving crop productivity in problem soils.

     

Effect of soil water content on the gravitropic behavior of nodal roots in maize

The direction of root growth is an important factor that determines the spatial distribution of roots in the soil. The influence of soil water content on the direction of growth of maize nodal roots was studied both in the field and in the greenhouse. In the field experiment, the one plot was regularly irrigated (I-plot) and the other non-irrigated (N-plot). In the greenhouse experiment, three water treatments were conducted on plants grown in pots: continuously wet (CW), early drying (ED), and late drying (LD). The direction of root growth was quantified by the angle from the vertical, measured at 1 cm intervals for 10 cm from the first five internodes. Nodal roots grew more vertically in the N-plot and ED treatment than those in the I-plot and CW treatment. This was due to the decrease of the initial angle and/or the liminal angle. It is therefore thought that two events regulate the growth direction of nodal roots under dry soil conditions: gravitropic bending at root emergence from the stem and the later establishment of the angle of growth. Nodal roots appearing after rewatering in the ED treatment grew in a similar direction as those in the CW treatment. It follows from this that the water content of the surrounding soil has a direct effect on the direction of growth. Nodal roots that emerged in rapidly drying soil in the LD treatment ceased growing after showing negative gravitropism. The possible mechanisms determining the growth direction of nodal roots in drier soils are discussed.     

Rice cultivar evaluation for phosphorus use efficiency

Phosphorus deficiency is one of the most growth-limiting factors in acid soils in various parts of the world. The objective of this study was to screen 25 rice cultivars (Oryza sativa L.) at low, medium, and high levels of soil P. Number of tillers, root length, plant height, root dry weight and shoot dry weight were related to tissue P concentrations, P uptake and P-use efficiency. Shoot weight was found to be the plant parameter most sensitive to P deficiency. Significant cultivar differences in P use efficiency were found. Phosphorus use efficiency was higher in roots than shoots and decreased with increasing levels of soil P. Positive correlations were found among growth parameters such as plant height, tillers, root and shoot weight, and P content of roots and shoots. These results indicate selection of rice cultivars for satisfactory performance under low P availability can be carried out using shoot and root dry weight as criteria.     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

根区交替控制灌溉条件下玉米根系吸水规律

为了阐明根区交替控制灌溉(CRDAI)条件下玉米根系吸水规律,通过田间试验,在沟灌垄植模式下采用根区交替控制灌溉研究玉米根区不同点位(沟位、坡位和垄位)的根长密度(RLD)及根系吸水动态。研究表明,根区土壤水分的干湿交替引起玉米RLD的空间动态变化,在垄位两侧不对称分布,并存在层间差异;土壤水分和RLD是根区交替控制灌溉下根系吸水速率的主要限制因素。在同一土层,根系吸水贡献率以垄位最大,沟位最低;玉米营养生长阶段,10—30 cm土层的根系吸水速率最大;玉米生殖生长阶段,20—70 cm为根系吸水速率最大的土层,根系吸水贡献率为43.21%—55.48%。研究阐明了交替控制灌溉下根系吸水与土壤水分、RLD间相互作用的动态规律,对控制灌溉下水分调控机理研究具有理论意义。     

Comparative effects of NaCl and polyethylene glycol on root distribution,growth, and stomatal conductance of sour orange seedlings

An experiment was conducted to study sour orange (Citrus aurantium L.) seedling root density, distribution, and morphological development under NaCl and polyethylene glycol (PEG) stresses in relation to shoot growth and stomatal conductance. Plants were treated with 2 stress levels (– 0.12 and – 0.24 MPa) of NaCl and PEG 4000 for 7 months. Root observation chambers were used to monitor root growth and distribution under stressed and non-stressed conditions. Seedlings receiving NaCl or PEG treatments produced fewer roots and shallower root systems with 46 to 65% of the roots occurring in the top portion of the soil. Fibrous root weight per unit length was increased by 24 to 30% by PEG but was not significantly increased by NaCl.Root growth rate usually alternated with shoot growth in a 2-month cycle. This alternating pattern was not shifted by NaCl and PEG stresses. In all NaCl and PEG treatments, growth was depressed and stomatal conductance was reduced. Compared to controls, plants that received NaCl or PEG had smaller shoot and root dry weights, fewer leaves, shorter height, and fewer roots. Sodium chloride usually caused less damage than PEG to sour orange seedlings suggesting that NaCl and PEG acted through different mechanisms.Florida Agricultural Experiment Station Journal Series No. 9941.     

Number,position, diameter and initial direction of growth of primary roots in Musa

To understand soil colonization by a root system, information is needed on the architecture of the root system. In monocotyledons, soil exploration is mainly due to the growth of adventitious primary roots. Primary root emergence in banana was quantified in relation to shoot and corm development. Root emergence kinetics were closely related to the development of aerial organs. Root position at emergence on the corm followed an asymptotic function of corm dry weight, so that the age of each root at a given time could be deduced from its position. Root diameter at emergence was related to the position of the roots on the corm, with younger roots being thicker than older ones. However, root diameters were not constant along a given root, but instead decreased with the distance to the base; roots appear to be conical in their basal and apical parts. Root growth directions at emergence were variable, but a high proportion of the primary roots emerged with a low angle to the horizontal. Further research is needed to evaluate whether these initial trajectories are conserved during root development. Results presented in this study are in good agreement with those reported for other monocotyledons such as maize and rice. They give quantitative information that will facilitate the development of models of root system architecture in banana.     

Plagiogravitropism of maize roots

The direction of root growth can be studied by analyzing the trajectories of roots growing in soil. Both the primary seminal root and nodal roots of maize attain a preferred, or liminal, angle of growth that deviates from the vertical. These roots are said to be plagiogravitropic. Experiments using plants grown in soil-filled boxes revealed that the primary seminal root is truly plagiogravitropic. It shows both positive and negative gravitropism in response to gravity stimuli and tends to maintain its direction even after growing around obstacles. These are experimental results suggesting that plagiogravitropic growth is controlled by internal factors. The orientation of the grain affects the establishment of the liminal angle of the primary seminal root, and both the position of their node of origin and the root diameter are closely related to the plagiogravitropic behaviour of nodal roots. Several external factors are also known to influence plagiogravitropism. Low soil water content causes a decrease in the angle of growth and soil mechanical resistance suppresses the gravitropic curvature. Plagiogravitropic behaviour of both seminal and nodal roots plays a significant role in shaping the root system.     

Ara-Rhizotron: An Effective Culture System to Study Simultaneously Root and Shoot Development of Arabidopsis

Studying Arabidopsis thaliana (L.) Heynh. root development in situ at the whole plant level without affecting shoot development has always been a challenge. Such studies are usually carried out on individual plants, neglecting competition of a plant population, using hydroponic systems or Agar-filled Petri dishes. Those both systems, however, present some limitations, such as difficulty to study precisely root morphogenesis or time-limited culture period, respectively. In this paper, we present a method of Arabidopsis thaliana (L.) Heynh. cultivation in soil medium, named “Ara-rhizotron”. It allows the non-destructive study of shoot and root development simultaneously during the entire period of vegetative growth. In this system, roots are grown in 2D conditions, comparable to other soil cultures. Moreover, grouping several Ara-rhizotrons in a box enables the establishment of 3D shoot competition as for plants grown in a population. In comparison to a control culture grown in pots in the same environmental conditions, the Ara-rhizotron resulted in comparable shoot development in terms of dry mass, leaf area, number of leaves and nitrogen content. We used this new culture system to study the effect of irrigation modalities on plant development. We found that irrigation frequency only affected root partitioning in the soil and shoot nitrogen content, but not shoot or root growth. These effects appeared at the end of the vegetative growth period. This experiment highlights the opportunity offered by the Ara-rhizotron to point out tardy effects, affecting simultaneously shoot development and root architecture of plants grown in a population. We discuss its advantages in relation to root development and physiology, as well as its possible applications.     

Regulation of shoot branching patterns by the basal root system: towards a predictive model

This study aimed to underpin the development of a generic predictivemodel of the regulation of shoot branching by roots in nodallyrooting perennial prostrate-stemmed species using knowledgegained from physiological studies of Trifolium repens. Experiment1 demonstrated that the net stimulatory influence from the basalrooted region of the plant on growth of newly emerging axillarybuds on the primary stem decreased as their phytomeric distancefrom the basal root system increased. Experiment 2 found thatat any one time the distribution of net root stimulus (NRS)to the apical bud on the primary stem and all lateral brancheswas fairly uniform within a single plant. Thus, although NRSavailability was uniform throughout the shoot system at anypoint in time, it progressively decreased as shoot apical budsgrew away from the basal root system. Based on these findings,a preliminary predictive model of the physiological regulationof branching pattern was developed. This model can explain thedecline in growth rate of buds on a primary stem as it growsaway from its basal root system but not the rapid progressivedecline in secondary branch development on successive lateralbranches. Thus knowledge of NRS availability to emerging budsis not, by itself, a sufficient basis from which to constructa predictive model. In addition, it seems that the ability ofan emerging bud to become activated in response to its localNRS availability is, at least in part, directly influenced bythe activation level of its parent apical bud. The experimentaltesting of this hypothesis, required for continued developmentof the model, is proceeding. Key words: Axillary bud outgrowth, branch development, bud activation, intra-plant variation, nodal roots, prostrate clonal herbs, root signals, Trifolium repens Received 11 September 2007; Revised 25 November 2007 Accepted 18 January 2008     

Effect of mutual shading on the emergence of nodal roots and the root/shoot ratio of maize

The effect of mutual shading on the root/shoot ratio and on the number of nodal roots of maize was studied. Plants of two varieties (Dea and LG2281) were grown in individual pots of 9 L, at three plant densities: 7.5, 11 and 15 plants m^–2. A control experiment was carried out in order to study if root growth was affected by the small size of the pots. Maize plants (cv Dea) were grown at a low plant density (7.5 plants m^–2) in pots of two different volumes (9 and 25 L respectively). In both experiments plants were watered every two hours with a nutrient solution. Some plants were sampled at five dates in the main experiment and the following data were recorded: foliar stage; root, stem and leaf dry weight; number of root primordia and number of emerged roots per phytomer. The final sampling date occurred at silking.Results of the control experiment showed that the root biomass was lower in small pots but the number of nodal roots per phytomer was not affected.Results of the main experiment showed that the total plant biomass and the root/shoot ratio were lower at high plant density. The number of emerged roots was strongly reduced on the upper phytomer (P₈). This reduction was mainly due to a lower percentage of root primordia which elongated. A proposed interpretation is that the number of roots which emerge on upper phytomers is controlled by carbohydrate availability.     

Seedling establishment,underground kinetics,and clonal reiteration: How do Potentilla inclinata and Inula ensifolia get their multifunctional subterranean systems?

The development of a seedling into an adult plant comprises various underground processes. Time-lapse photography (TLP) makes them visible. This is documented for Potentilla inclinata (Rosaceae) and Inula ensifolia (Asteraceae).After germination, P. inclinata develops a taproot system. Contraction phenomena pull the basal part of the shoot at least 10 mm into the soil. Later, several adventitious roots are generated, and thus the root system changes to a fibrous one. This is followed by cloning without separation of the ramets.Seedlings of I. ensifolia develop a weak primary root. At an early stage, adventitious roots are formed at the leaf rosette. This fibrous root system exerts a strong pulling effect on the shoot. After one vegetation period the basis of the rosette is approx. 30 mm under the soil surface. Cloning includes the formation of many new horizontal shoots, which conquer new sites.These two examples show three functional steps common in the developmental progress of subterranean systems: (I) establishment of the seedling, (II) innovation and survival of the young plant, and (III) reiteration (cloning and dispersal). However, to accomplish these basic development steps the diversity of subterranean systems is enhanced by different organographical components.Furthermore, the development of subterranean systems is a dynamic process consisting of two kinetic processes: the vertical movement during seedling establishment, which brings the innovation buds to a safe soil position, and the horizontal movement during dispersal, which conquers new sites.     

The effect of Piriformospora indica on the root development of maize (Zea mays L.) and remediation of petroleum contaminated soil

As the depth of soil petroleum contamination can vary substantially under field conditions, a rhizotron experiment was performed to investigate the influence of endophyte, P. indica, on maize growth and degradation of petroleum components in a shallow and a deep-reaching subsurface layer of a soil. For control, a treatment without soil contamination was also included. The degree in contamination and the depth to which it extended had a strong effect on the growth of the plant roots. Contaminated soil layers severely inhibited root growth thus many roots preferred to bypass the shallow contaminated layer and grow in the uncontaminated soil. While the length and branching pattern of these roots were similar to those of uncontaminated treatment. Inoculation of maize with P. indica could improve root distribution and root and shoot growth in all three contamination treatments. This inoculation also enhanced petroleum degradation in soil, especially in the treatment with deep-reaching contamination, consequently the accumulation of petroleum hydrocarbons (PAHs) in the plant tissues were increased.     

Evaluation in field conditions of a three-dimensional architectural model of the maize root system: Comparison of simulated and observed horizontal root maps

Most existing water and nutrient uptake models are based on the assumption that roots are evenly distributed in the soil volume. This assumption is not realistic for field conditions, and significantly alters water or nutrient uptake calculations. Therefore, development of models of root system growth that account for the spatial distribution of roots is necessary.The objective of this work was to test a three dimensional architectural model of the maize root system by comparing simulated horizontal root maps with observed root maps obtained from the field. The model was built using the current knowledge on maize root system morphogenesis and parameters obtained under field conditions. Simulated root maps (0.45 × 0.75 m) of horizontal cross sections at 3 depths and 3 dates were obtained by using the model for a plant population. Actual root maps were obtained in a deep, barrier-free clay-loamy soil by digging pits, preparing selected horizontal planes and recording root contacts on plastic sheets.Results showed that both the number of cross-sections of axile roots, and their spatial distribution characterized with the R-index value of Clark and Evans (1954), were correctly accounted for by the model at all dates and depths. The number of cross-sections of laterals was also correctly predicted. However, laterals were more clustered around axile roots on simulated root maps than on observed root maps. Although slight discrepancies appeared between simulated and observed root maps in this respect, it was concluded that the model correctly accounted for the general colonization pattern of the soil volume by roots under a maize crop.     

Response of millet and sorghum to a varying water supply around the primary and nodal roots

Background and Aims

Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant''s life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.

Methods

Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.

Key Results

When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0·6–1·5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.

Conclusions

Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.     

Genotypic Variation in Root Growth Angle in Rice (Oryza sativa L.) and its Association with Deep Root Development in Upland Fields with Different Water Regimes

Deep root development, which is important for the drought resistance in rice (Oryza sativa L.), is a complex trait combining various root morphologies. The objective of this study was to elucidate genotypic variation in deep root development in relation to morphological indicators such as vertical root distribution and root growth angle. Two experiments were conducted: one on upland fields, and one in pots and fields. In experiment 1, the root systems of six rice cultivars on upland fields were physio-morphologically analyzed under different water regimes (irrigated and intermittent drought conditions during panicle development). In experiment 2, cultivar differences in root growth angles were evaluated with 12 cultivars using the basket method under irrigated conditions. No cultivar × environment interactions were found for total root length or deep root length between irrigated and drought conditions in experiment 1. This suggests that constitutive root growth, which is genetically determined, is important for deep root development under intermittent drought conditions during reproductive stage. Among root traits, the deep root ratio (i.e., deep root weight divided by total root weight) was most closely related to deep root length under both water regimes. This suggested that vertical root distribution constitutively affects deep root length. Significant genotypic variation existed in the nodal root diameter and root growth angle of upland rice in experiment 2. It was considered that genotypes with thick roots allocated more assimilates to deep roots through root growth angles higher to the horizontal plane on upland fields. This is the first report on genotypic variation in the root growth angle of rice on upland fields. It should prove useful for rough estimations of genotypic variation in the vertical root distribution of upland rice because root growth angle is rapidly and easily measured.     

Tillage and N fertilization effects on maize root growth and root:shoot ratio

Two methods for estimating the size of the maize (Zea mays l.) root system from soil cores taken in the field were compared. The spatially weighed block method of estimation accounted for variation in root density by using 18 samples per plant which varied in distance from plant and soil depth. This method was compared to an estimation which averaged all of the 18 samples together. Both methods gave surprisingly similar estimates for total root growth. Increased root growth in the surface soil layers, due to tillage and N fertilization, did not impact on the estimation of total root growth. Total root length remained unchanged or increased with N fertilization, while root weight remained the same or decreased. Root mass per length decreased with N fertilization. The estimated size of the root system was used to calculate root:shoot weight ratios. The largest root:shoot ratio was found in the vegetative stage and decreased throughout the rest of the season. In this field experiment, the estimated size of the root system at 8 weeks after planting was not significantly different from the size at silking or harvest. Nitrogen fertilization significantly decreased the root:shoot weight ratio. However, tillage did not significantly change the ratio.     

疏叶骆驼刺根系对土壤异质性和种间竞争的响应

近年来, 植物根系对土壤异质性的响应和植物根系之间的相互作用一直是研究的热点。过去的研究主要是针对一年生短命植物进行的, 而且多是在人工控制的温室条件下进行的。而对于多年生植物根系对养分异质性和竞争的综合作用研究很少。该文对塔里木盆地南缘多年生植物疏叶骆驼刺(Alhagi sparsifolia)根系生长对养分异质性和竞争条件的响应途径与适应策略进行了研究, 结果表明: (1)在无竞争的条件下, 疏叶骆驼刺根系优先向空间大的地方生长, 即使另一侧有养分斑块存在, 其根系也向着空间大的一侧生长; (2)在有竞争的条件下, 疏叶骆驼刺根系生长依然是优先占领空间大的一侧, 但是竞争者的存在抑制了疏叶骆驼刺的生长, 导致其枝叶生物量和根系生物量都明显减少(p < 0.01), 而养分斑块的存在促进了疏叶骆驼刺根系的生长; (3)疏叶骆驼刺根系的生长不仅需要养分, 也需要足够的空间, 空间比养分更重要; (4)有竞争者存在的时候, 两株植物的根系都先长向靠近竞争者一侧的空间, 即先占据“共有空间”。研究结果对理解植物根系觅食行为和植物对环境的适应策略有重要意义。     

Inoculation with arbuscular mycorrhizal fungi enhances growth of Litchi chinensis Sonn. trees after propagation by air-layering

This study investigated the patterns of root growth and water uptake of maize (Zea mays L.) and cowpea (Vigna unguiculata (L.) Walp) grown in a mixture under greenhouse conditions. The plants were grown in root boxes for 5 weeks under 2 watering regimes; fully irrigated and water stress conditions, followed by a 5-day drying cycle imposed during the 6th week of growth. Water uptake patterns were analysed during the drying cycle. The two-dimensional distribution of the roots of both plants in the boxes was determined immediately at the end of the drying cycle. Under well-irrigated conditions, the roots of the component plants grew profusely into all sections of the root box and intermingled considerably. Water stress resulted in the decline of root growth of maize and cowpea but the root:shoot ratios of maize and cowpea were not affected, suggesting that there was no significant effect of water stress on root:shoot partitioning. However, water stress affected the biomass distribution between fine and coarse roots in cowpea. About 64% by weight of cowpea roots under water stress were coarse whereas as against 48% under well-irrigated conditions. Furthermore, water stress generally restricted the lateral extent of the roots of both maize and cowpea with a tendency of clumping together of the root systems and a reduced degree of intermingling. Thus, the extent of mixing of the root systems was apparently controlled by the availability of soil water. Water uptake from the well-irrigated soil in the root boxes was initially restricted to the sections directly below the base of each plant. Although roots of both plants were present in almost all sections of the root box, all the sections did not contribute simultaneously to water uptake by each plant. Water uptake was delayed from the middle intermingled zones. In effect, uptake patterns did not relate generally to the root distribution. The tendency was for the component plants to initially `avoid' water uptake from zones of intense intermingling or competition.