Effect of nitrogen supply on carbon and nitrogen partitioning after flowering in maize

Low nitrogen (N) supply may change assimilate partitioning between plant organs. We measured the effect of N supply on partitioning of recently assimilated ¹³C and recently absorbed ¹⁵N between generative and vegetative plant organs of two maize genotypes (Zea mays L.) 14 d after silking, i.e., during the lag phase of kernel growth. Furthermore, net partitioning of dry matter and N were assessed during grain filling. Plants were grown in a greenhouse in large containers. Our hypothesis was that N deficiency reduces grain set due to low partitioning of carbon (C) and N to the grains during the lag phase and reduces grain yield also because of excessive remobilization of N from the leaves during grain filling. During the lag phase, low N supply increased partitioning of recently assimilated photosynthates towards stem and roots at the expense of partitioning towards reproductive organs. However, despite of diminished sink strength of the reproductive organs for photosynthates, sugar concentrations in the grains of N‐deficient plants were increased, indicating that kernel set and potential kernel weight were not limited by low C supply at the end of the lag phase. In contrast to C, partitioning of recently absorbed N towards the reproductive organs was increased at low N supply at the expense of partitioning towards the roots. This indicates different mechanisms for the regulation of C and N distribution within the plant. During grain filling, biomass partitioning between plant organs was more affected by genotype than by rate of N supply. Nitrogen accumulation in the grains substantially exceeded total N uptake in the plant after flowering. Excess N accumulation in the grains was covered mainly by depletion of stem N at high N supply and by depletion of leaf N at low N supply. However, high concentrations of nonstructural carbohydrates in the stem at maturity indicated that grain yield of N‐deficient plants was not limited by low source strength of N‐depleted leaves.     

不同水分状况下施锌对玉米生长和锌吸收的影响

选择潮土(砂壤)和土(粘壤)两种质地不同的土壤,进行盆栽试验,研究不同土壤水分条件下施锌对玉米生长和锌吸收的影响。结果表明,施锌显著增加了玉米植株根、茎、叶以及整株干物质重;缺锌条件下玉米植株根冠比、根叶比和根茎比趋向增大。施锌显著提高了玉米植株各器官中锌的浓度和吸收量,并明显促进锌向地上部运移。干旱胁迫抑制了玉米植株生长,根冠比、根茎比、根叶比增大;随着土壤水分供应增加,植株生长加快,各器官生物量以茎和叶增加大于根。水分胁迫下,在潮土上玉米叶片中锌浓度上升;在土上叶片中锌浓度下降。但增施锌后,根和茎锌浓度增加幅度较大,叶片增加幅度较小;施锌和水分胁迫对根和茎锌浓度的交互作用极显著。水分胁迫下,玉米植株对锌的吸收总量减少。水分胁迫和锌肥施用对玉米叶片、茎锌吸收量的交互作用十分显著,但对根锌吸收量的交互影响不显著。     

Root‐growth characteristics contributing to genotypic variation in nitrogen efficiency of oilseed rape

A 3‐year field experiment was carried out to determine the significance of root‐growth characteristics contributing to N‐uptake efficiency of two oilseed rape (Brassica napus L.) cultivars differing in N efficiency. Two N treatments were applied, and the core and minirhizotron techniques were used to study root‐length density and number of living roots, respectively. Fertilizer‐N supply increased shoot dry matter, grain yield, total N uptake, and total soil N_min contents particularly in the top soil. Although significant differences occurred in all parameters between years, the interactions between years and cultivars were mostly not significant. Compared to cv. Capitol, the N‐efficient cv. Apex was characterized by a higher grain yield at N0 and a higher N uptake during reproductive growth. This genotype also had a higher root‐length density and more living fine roots particularly in the topsoil layer. Root growth of this genotype was especially high from beginning of shooting to beginning of flowering, while shoot growth and N uptake during vegetative growth were comparatively low. Our results suggest that N‐efficient cultivars can be characterized by a high investment in root growth during the vegetative stage with a comparatively slow shoot growth and N‐uptake rate until beginning of flowering, which, however, continues during reproductive growth. High root production only during reproductive growth seems to be less effective to achieve high N efficiency, because this may lead to a shortage of assimilates for seed filling. High root‐length density at vegetative stages may thus be advantageous for N uptake and reproductive growth and could be a useful morphological character for the selection and breeding of N‐efficient cultivars.     

Der Einfluß von Bodenart,Durchlüftung des Bodens,N-Ernährung und Rhizosphärenflora auf die Morphologie des seminalen Wurzelsystems von Mais

Influence of soil type, soil aeration, nitrogen supply and rhizosphere flora on the morphology of the seminal root system of maize The influence of the soil type (quartz sand – humous loamy sandy soil), soil aeration, nitrogen supply and rhizosphere flora on the morphology of the seminal root system of maize plants grown in pot culture was investigated. The morphological parameters of number, length, diameter and root hair formation (both length and density) of the main and the lateral roots were determined in addition to the total root length and number and the lateral root density. 1. The biomass production of the shoot and root system was nearly identical in both soils. The total root length growth, however, was enhanced in the sandy soil due to the stimulated formation of first order lateral roots. This increase was correlated with a decrease in the mean diameter and root hair length of the main and lateral roots. 2. A decreased O₂-supply to the soil resulted in a drastic reduction of root biomass, which was correlated, however, with a (relative) increase in total root length (due to the stimulation of the length growth of the first order lateral roots). The root hair length, on the other hand, was reduced under O₂-deficiency. 3. Reduced N-supply resulted in a decrease of the shoot/root-ratio with both substrates which could be ascribed to the enhanced formation and length of the first order lateral roots. 4. The presence of soil microorganisms in quartz sand culture resulted in a reduction of shoot biomass. In comparison with the sterile control culture the total length of the main roots was retarded, the main and lateral roots were more slender and root hair formation was reduced. 5. The experimental results show that the lateral root system demonstrates a significantly greater plasticity than does the main root system.     

欧洲缬草不定根悬浮培养及其有效成分的测定

为高效生产欧洲缬草有效成分,通过分析培养基pH值、碳源、糖浓度、盐浓度、总氮(N)浓度、总磷(P)浓度等因素对其不定根悬浮培养生长的影响,确定欧洲缬草不定根悬浮培养的适宜条件,并测定在该培养条件下不定根的生长状况,同时比较正常不定根、褐化不定根、无菌苗根中缬草三酯、乙酰缬草三酯和缬草烯酸3种有效成分的含量。结果表明,欧洲缬草不定根的最佳培养条件为2倍大量元素浓度、48.46 mmol·L^-1总N浓度、1.5 mmol·L^-1总P浓度的RCM培养基,添加40 g·L^-1蔗糖、0.5 mg·L^-1萘乙酸(NAA),pH值5.7,温度25±2℃,转速100 r·min^-1暗培养,28 d后收获。最佳培养条件下的欧洲缬草正常不定根中缬草三酯、乙酰缬草三酯、缬草烯酸的含量均最高,分别为7.222、0.355、4.016 mg·g^-1,褐化不定根中成分含量显著降低,无菌苗根中成分含量最低。本研究结果可为欧洲缬草不定根及其有效成分的规模化生产提供参考。     

Uptake and translocation of nitrogen in patumma (Curcuma alismatifolia) by leaves or root

An adequate supply of nitrogen (N) is important for patumma growth and flower quality. This study aimed to compare the uptake and translocation of N by foliar and root application. Fertilization with ¹⁵ nitrate (NO₃)-N via roots or leaves was carried out at four stages, at the 1st to 4th fully expanded leaf (FEL) stages, and the plants were sampled at each successive stage. The uptake and translocation of ¹⁵N from foliar or root applications showed relatively similar patterns at all stages. Although the N fertilizer utilization rate by roots was higher than that via leaves, the foliar application stimulated reproductive growth by earlier flowering. The N supplied at the 1st FEL and the 2nd FEL was utilized mainly in leaves, whereas supplying N at the 3rd and 4th FEL promoted flower quality. Fertilizer application method and stage of application influence the utilization rate and translocation of N to the sink organs.     

The maize root system in situ: Evaluation of structure and capability of utilization of phytate and inorganic soil phosphates

• 1 The dependence of the morphology of the maize (Zea mays L.) seminal root system on physical, chemical and biotic parameters was investigated with pot cultures in quartz sand and in a natural loamy sand soil. Low O₂-supply to the soil resulted in a substantially smaller root biomass despite a relative increase in total root length. Reduced N-supply also stimulated root length growth, but also enhanced the formation of laterals. The presence of soil microorganisms, in comparison to sterile cultures, resulted in a reduced length of the main roots, and the production of slender laterals with a decreased root hair density. Generally, the structural variability of laterals in response to different growth conditions was much more pronounced than that of the main roots.
• 2 A major part of the work reported here was dedicated to a detailed study of phosphate (P) acquisition by the maize root system under field conditions. Radioactive labelling of the roots and radioautography of soil cores revealed the in situ distribution pattern of the maize root system. Controlled labelling of the soil with radioactive phosphate allowed the documentation of the development and replenishment of the phosphate depletion zone around roots. Finally, the longevity and phosphate uptake activity of the different parts and tissues of the primary root system of maize was examined by electron microscopy and tracer studies including pulse chase experiments. From these studies the phosphate-acquiring strategy of the maize root system appears as follows: The capability of P uptake decreases in the order: root hairs, 1^st order laterals, 2^nd order laterals, main root. The life-spans of the components of the maize root system increase by the sequence: root hairs, laterals, main root. Inorganic P uptake, therefore, mainly occurs during the first weeks of root development. Dying back of the root occurs in an ordered manner resulting in a relocation of stored P predominantly into the main root cortex. Furthermore, it could be shown that competition for P between roots of the same or of adjacent maize and/or lupin plants virtually does not occur in situ.
• 3 The utilization of phytate-P was studied with ¹⁴C/³²P-labelled Camyo-inositol-hexaphosphate supplied to maize plants grown in sterile quartz sand or in hydroponic cultures. The ratio of P- and C-uptake as well as the incidence of phytate hydrolysis products in the rooting medium indicated the capability of maize roots to acquire P from phytate by enzymatic hydrolysis. This was confirmed by enzyme studies of the root tissues. A specific hydrolyzing enzyme (phytase; molecular weight 51 kD) could be detected in the cell wall of the root, especially in the root tip, which initiates phytate dephosphorylation. Further breakdown is presumably accomplished by monophosphoric phosphohydrolases.

     

田间玉米和蚕豆对低磷胁迫响应的差异比较

【目的】植物在长期进化过程中形成了一系列适应机制,以应对低磷胁迫。本文提出玉米主要通过根系形态变化适应低磷胁迫的假设,并通过与蚕豆植株在根系形态与生理方面对低磷胁迫反应的比较试验加以验证。【方法】在中国农业大学上庄长期定位试验田进行两年田间实验,玉米和蚕豆分别单作,重复3次。在玉米抽雄前的拔节至大喇叭口期和蚕豆的初花至盛花期两次取样(两年的两次取样时间间隔10~12天),比较研究了不供磷和供磷100 kg/hm2下玉米和蚕豆生长和磷素吸收、根系在0—40 cm土层中分布、以及根际p H值和酸性磷酸酶活性的差异。【结果】1)玉米植株的生物量和含磷量远远高于蚕豆;第一次取样时蚕豆的根冠比高于玉米,而且两种植物低磷下的根冠比高于供磷充足处理。两次取样时玉米的总根长大于蚕豆,两种植物的大部分根系分布在0—20 cm表层土壤,玉米根系在0—10 cm土层的分布更多。2)蚕豆根系的比根长明显大于玉米,但单位根长吸磷量低于玉米,两种植物间的上述差异不受取样时间和供磷水平的影响。3)两次取样时,蚕豆根表的酸性磷酸酶活性均明显高于玉米。玉米根表的酸性磷酸酶活性在两个供磷水平下没有差异。第一次取样时,缺磷蚕豆根表的酸性磷酸酶活性高于供磷充足的蚕豆植株。4)缺磷蚕豆的根际土壤p H值明显低于供磷充足蚕豆;但玉米根际土壤p H值在缺磷和供磷充足条件下无显著差异。【结论】低磷条件下两种植物的根冠比均明显增加。玉米根系单位根长的吸磷量高于蚕豆,并且在含磷量丰富的表层土壤分布有更多根系,但缺磷条件下玉米没有增加根系的质子和酸性磷酸酶的分泌,主要以根系形态变化来适应低磷胁迫。结果支持本文提出的玉米主要通过根系形态变化适应低磷胁迫的假设。但蚕豆在低磷条件下除了增加根系生长外,还具有通过增加质子分泌和根表酸性磷酸酶活性提高根际土壤有效磷浓度的潜力。     

Cluster root allocation of white lupin (Lupinus albus L.) in soil with heterogeneous phosphorus and water distribution

Cluster roots are structures formed by many plants adapted to phosphorus (P)-deficient soils. We investigated the combined influence of spatial heterogeneity in soil water and P distribution on the allocation of cluster root formation in white lupin (Lupinus albus L.). In this study, single plants were grown at a low or a high rate of water supply in containers filled with a P-poor sand to which either no P was added or which was fertilized homogeneously or heterogeneously. Furthermore, heterogeneous soil water distribution was established in half of the containers by using a finer instead of a coarser sand in a lateral third of the containers. Plant growth increased with water supply rate, but P fertilization had no influence on shoot biomass production. Although overall cluster root production decreased with increasing homogeneous P supply, localized P fertilization had no effect on cluster root allocation. However, cluster roots were preferentially allocated in the soil sections with lower water availability when overall water supply rate was low. The results suggest that overall cluster root production was a systemic response to initial plant P status, while cluster root growth was stimulated locally in drier patches when overall water supply was limiting plant growth.     

Effects of soil bulk density on seminal and lateral roots of young maize plants (Zea mays L.)

It is well established that increasing soil bulk density (SBD) above some threshold value reduces plant root growth and thus may reduce water and nutrient acquisition. However, formation and elongation of maize seminal roots and first order lateral (FOL) roots in various soil layers under the influence of SBD has not been documented. Two studies were conducted on a loamy sand soil at SBD ranging from 1.25 g cm^–3 to 1.66 g cm^–3. Rhizotrons with a soil layer 7 mm thick were used and pre‐germinated plants were grown for 15 days. Over the range of SBD tested, the shoot growth was not influenced whereas total root length was reduced by 30 % with increasing SBD. Absolute growth rate of seminal roots was highest in the top soil layer and decreased with increasing distance from the surface. Increasing SBD amplified this effect by 20 % and 50 % for the top soil layer and lower soil layers, respectively. At the end of the experiment, total seminal roots attributed to approximately 15 % of the total plant root length. Increasing SBD reduced seminal root growth in the lowest soil layer only, whereas FOL root length decreased with SBD in all but the uppermost soil layer. For FOL, there was a positive interaction of SBD with distance from the soil surface. Both, increasing SBD and soil depth reduced root length by a reduction of number of FOL roots formed while the length of individual FOL roots was not influenced. Hence, increasing SBD may reduce spatial access to nutrients and water by (i) reducing seminal root development in deeper soil layers, aggravated by (ii) the reduction of the number of FOL roots that originate from these seminal roots.     

Effects of mes [2(n‐morpholino)‐ethanesulfonic acid] and ph on mineral nutrient uptake by mycor‐rhizal and nonmycorrhizal maize

Mycorrhizal (+VAM) and nonmycorrhizal (‐VAM) maize (Zea mays L.) plants were grown in sand culture in a greenhouse to determine effects of MES [2(N‐morpholino)‐ethanesulfonic acid] (2.0 mM) and pH (4.0, 5.0, 6.0, and 7.0) on mineral nutrient uptake. Plants were inoculated with the vesicular‐arbuscular mycorrhizal (VAM) isolate Glomus intraradices UT143. Shoot and root dry matter yields were lower in plants grown with MES (+MES) than without MES (‐MES), and decreased as pH increased. Shoot concentrations of N, Ca, Mg, Mn, and Zn were generally higher in +MES than in ‐MES plants, and nutrient contents of most nutrients were generally higher in + MES than in ‐MES plants. Concentrations of N, Ca, Mg, and Mn increased and P, S, and Fe decreased, while contents of all measured nutrients except Mn and Zn decreased as pH increased. Concentrations of Mn, Fe, Zn, and Cu were higher in +VAM than in ‐VAM plants, and contents of P and Ca were higher in ‐VAM than in +VAM plants and Zn content was higher in +VAM than in ‐VAM plants. MES had marked effects on mineral nutrient uptake which should be considered when MES is used to control pH of nutrient solutions for growth of maize.     

Physiological studies on salinity and nitrogen interaction in alfalfa. II. Photosynthesis and transpiration

The interaction between salinity and nitrogen (N) forms and concentration was studied with alfalfa (Medicago sativa L.) grown in pots with fine sand under greenhouse conditions. Salinity (0–100 mM NaCl) caused a substantial reduction in carbon assimilation rate, stomatal conductance, water use efficiency, and leaf area, while transpiration rate was least affected. Salinity effects were considerably moderated by additional N supply, varied with form, concentration, and stage of plant growth. The photosynthesis was reduced more in ammonium‐ than in nitrate‐fed plants, while the transpiration rate was relatively lower in nitrate‐fed plants grown either with or without NaCl. The plants also responded differently to salinity and N levels at two harvests. This indicated a change in plant behaviour with age. The promotive effect of N on photosynthesis and other parameters in saline as well as in non‐saline conditions may be attributed to the enhanced synthesis and availability of carbon assimilatory enzymes and cofactors required for optimal photosynthesis.     

Interactions of metolachlor and excess hydrogen (H+) influences on sorghum (sorghum bicolor) cultivar roots

Sorghum [Sorghum bicolor (L.) Moench] cultivars were planted in 8 cm × 8 cm × 8 cm pots filled with ‘white quartz flintshot’ sand containing 0, 0.25, 0.50, 1.0, or 2.0 mg/kg metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2 methoxy‐1‐methylethyl)acetamide] and the pots were watered on alternate days with 100 mL 0.1 M sodium acetate at pH 6.0, 5.5, 5.0, 4.5, or 4.0 to determine the influence of excess H⁺ and metolachlor concentrations on sorghum root growth. Cultivars utilized were Funk G522DR, SC574, SC283, GP‐10, 58M, and 38M. At pH 4.5 and 4.0 (0 metolachlor), root lengths of Funk G522DR and SC574 were significantly decreased compared to roots from plants grown at pH 6.0. The other four cultivars had decreased root growth at pH 4.0 (0 metolachlor). Metolachlor influence on sorghum cultivar root growth was dependent on pH, cultivar, and metolachlor concentration. None of the cultivars showed increased metolachlor activity which was influenced by pH. Metolachlor (0.25 mg/kg) reversed the influence of excess H⁺ concentration (pH 4.0) in SC574. Metolachlor (0.5, 1.0, and 2.0 ppmw) reversed the excess H⁺ concentration inhibition of root growth at pH 4.0 in Funk G522DR.     

Mapping water,oxygen, and pH dynamics in the rhizosphere of young maize roots

Rhizosphere processes are highly dynamic in time and space and strongly depend on each other. Key factors influencing pH changes in the rhizosphere are root exudation, respiration, and nutrient supply, which are influenced by soil water content levels. In this study, we measured the real‐time distribution of soil water, pH changes, and oxygen distribution in the rhizosphere of young maize plants using a recently developed imaging approach. Neutron radiography was used to capture the root system and soil water distribution, while fluorescence imaging was employed to map soil pH and soil oxygen changes. Germinated seeds of maize (Zea mays L.) were planted in glass rhizotrons equipped with pH and oxygen‐sensitive sensor foils. After 20 d, the rhizotrons were wetted from the bottom and time‐lapsed images via fluorescence and neutron imaging were taken during the subsequent day and night cycles for 5 d. We found higher water content and stronger acidification in the first 0.5 mm from the root surface compared to the bulk soil, which could be a consequence of root exudation. While lateral roots only slightly acidified their rhizosphere, crown roots induced stronger acidification of up to 1 pH unit. We observed changing oxygen patterns at different soil moisture conditions and increasing towards lateral as well as crown roots while extending laterally with ongoing water logging. Our work indicates that plants alter the rhizosphere pH and oxygen also depending on root type, which may indirectly arise also from differences in age and water content changes. The results presented here were possible only by combining different imaging techniques to examine profiles at the root‐soil interface in a comprehensive way during wetting and drying.     

在富营养土壤斑块中根增值对玉米养分吸收和生长的贡献

Root proliferation can be stimulated in a heterogeneous nutrient patch; however, the functions of the root proliferation in the nutrient-rich soil patches are not fully understood. In the present study, a two-year field experiment was conducted to examine the comparative effects of localized application of ammonium and phosphorus (P) at early or late stages on root growth, nutrient uptake, and biomass of maize (Zea mays L.) on a calcareous soil in an intensive farming system. Localized supply of ammonium and P had a more evident effect on shoot and root growth, and especially stimulated fine root development at the early seedling stage, with most of the maize roots being allocated to the nutrient-rich patch in the topsoil. Although localized ammonium and P supply at the late stage also enhanced the fine root growth, the plant roots in the patch accounted for a low proportion of the whole maize roots in the topsoil at the flowering stage. Compared with the early stage, fine root length in the short-lived nutrient patch decreased by 44%-62% and the shoot dry weight was not different between heterogeneous and homogeneous nutrient supply at the late growth stage. Localized supply of ammonium and P significantly increased N and P accumulation by maize at 35 and 47 days after sowing (DAS); however, no significant difference was found among the treatments at 82 DAS and the later growth stages. The increased nutrient uptake and plant growth was related to the higher proportion of root length in the localized nutrient-enriched patch. The results indicated that root proliferation in nutrient patches contributed more to maize growth and nutrient uptake at the early than late stages.     

Transfer of rubidium‐86 (potassium) from deeply‐rooted alfalfa (Medicago sativa L.) to associated,shallow‐rooted maize (Zea mays L.) or grain sorghum (Sorghum vulgare pers.).

Circumstantial evidence exists for non‐N‐mineral element transfer in legume‐grass associations. Three experiments were conducted in an effort to directly demonstrate transfer of a non‐N‐mineral element in alfalfa (Medicago sativa L.)‐maize (Zea mays L.) and alfalfa‐grain sorghum (Sorghum vulgare Pers.) associations in two rooting media. Associations were established in double‐tube apparatus so that a single alfalfa plant was rooted in media of top‐ and bottom‐tubes, while an associated grass plant was rooted exclusively in the top‐tube (Intact treatment). Severed treatments (the control) were identical to the Intact treatments except the alfalfa roots in an air gap between the top‐and bottom‐tubes were excised.

⁸⁶Rb was dispensed onto the medium of bottom tubes with movement of the radioisotope determined by analyzing the legume and grass tissues over time. ⁸⁶Rb was detected in: i) soil‐grown maize associated with alfalfa within a 40‐day treatment period; ii) sand‐grown maize associated with alfalfa within 20 days after treatment and iii) sand‐grown sorghum associated with alfalfa within 10 days. Detection of ⁸⁶Rb in grass plants associated with alfalfa demonstrated that transfer of this potassium analog can occur via the root systems of legume‐grass associations.     

Accumulation and partitioning of biomass and soluble carbohydrates in maize seedlings as affected by source of nitrogen,nitrogen concentration,and cultivar 1

Abstract

Seedlings of four maize hybrids were grown hydroponically to investigate the impact of different N sources (Ca(NO₃)₂, (NH₄)₂SO₄ and a 1:1 mixture of both) on (i) production and partitioning of root and shoot dry matter, (ii) concentration of soluble carbohydrates in roots and shoots and their partitioning to these plant parts, (iii) concentration of starch in the shoot, and (iv) N uptake. During the main phase of the experiments (duration 14d), the plants were grown in a greenhouse at 25/22°C day/night temperatures and a photoperiod of 16h. Nitrogen was supplied at three concentrations (2.8, 28, and 280 ppm). The root‐zone pH was 6.5. Under the lowest N supply, the N sources produced similar root and shoot dry matters. At the highest N level (280 ppm), NO₃‐fed plants were superior. In contrast, the mixture of NH₄ and NO₃ ^? was optimum at 28 ppm. More or less pronounced N form by N concentration interactions were also found in the concentration and distribution of soluble carbohydrates and in all remaing traits. There were almost statistically significant cultivar by N form interactions in shoot dry matter (P = 0.07) and total dry matter (P = 0.06), indicating the existence of considerable genotypic variation in sensivity to NH₄‐N.     

Partitioning of biomass in water‐ and nitrogen‐stressed cotton during pre‐bloom stage

The partitioning of biomass between aboveground parts and roots, and between vegetative and reproductive plant parts plays a major role in determining the ability of cotton (Gossypium hirsutum L.) to produce a crop in a given environment. We evaluated the single and combined effects of water and N supply on the partitioning of biomass in cotton plants exposed to two N supply levels, 0 and 12 mM of N, and two water regimes, well irrigated and water‐stressed at an early reproductive stage. The N treatments began when the third true leaf was visible, while water deficit treatments were imposed over the N treatments when the plants were transferred into controlled‐environment chambers at a leaf area near 0.05 m². Both water deficits and N deficits inhibited total biomass accumulation and its partitioning in cotton. Water deficit alone and N deficit alone inhibited the growth of leaves, petioles, and branches, but did not inhibit growth of the stem and enhanced the accumulation of biomass in squares. When water deficit was superimposed on N deficit, leaf growth was inhibited, although to a lesser extent than when it was the sole stress factor, and the accumulation of biomass in squares was also inhibited. Yet, the dry weight of squares in plants exposed to water and N deficits was greater than that of non‐stressed plants. Water and N deficits, either alone or in combination, did not inhibit the growth of the tap root. Growth of lateral roots was not inhibited either by water deficit alone or in combination with N deficit, but was enhanced when plants were exposed to N deficit alone. Exposure to water deficit alone or in combination with N deficit decreased the shoot:root ratio through the inhibition of shoot growth. Exposure to N deficit alone decreased the shoot:root ratio through the combination of shoot growth inhibition and root growth enhancement.     

Visualization of root growth in heterogeneously contaminated soil using neutron radiography

We used neutron radiography (NR), a non-invasive and in situ technique, to study living plant roots in soil. Plant roots have a larger water content than their unsaturated surrounding media. As water strongly attenuates a neutron-beam, NR can identify root structures in detail. We investigated the use of NR to visualize the root growth of lupin in quartz sand and in a loamy sand field soil. Further experiments elucidated the root growth of lupin in the loamy sand heterogeneously contaminated with 10 and 20 mg kg⁻¹ boron (B) and 100 mg kg⁻¹ zinc (Zn). We obtained high-quality images of root growth dynamics in both media with a resolution range of 110–270 μm. The images with quartz sand revealed fine structures such as proteoid roots that are difficult to locate in situ by other methods without destruction of the soil. Though quartz sand provided excellent visibility of roots, it proved to be a poor medium for growing plants, probably because of its bulk density (1.8 Mg m⁻³). The images with field soil showed normal root growth with slightly less contrast than the quartz sand. The poorer contrast was due to the greater neutron interaction with soil water and soil organic matter. In the heterogeneously contaminated soil, root growth was significantly reduced in the contaminated part of the soil in all B and Zn treatments. This study shows that NR has potential as a non-invasive method to investigate root growth over time as well as the response of roots to various abiotic stress factors.     

Mycorrhizal colonization and nitrogen uptake by maize: combined effect of tropical earthworms and velvetbean mulch

Earthworms and mulch can have positive or negative effects on mycorrhizae (fungus-roots) and N uptake by plants. In the present experiment, maize plants were grown under greenhouse conditions with or without tropical earthworms (Balanteodrilus pearsei) and mulch of velvetbean (Mucuna pruriens var. utilis). The formation of vesicles and hyphae of arbuscular-mycorrhizal (AM) fungi in roots and N uptake by maize plants was measured at harvest. The addition of earthworms and velvetbean reduced AM root colonization. Earthworms had no effect on plant root or shoot biomass. In the absence of velvetbean, earthworms reduced AM colonization, but when velvetbean was present, this effect disappeared. The addition of velvetbean mulch, on the other hand, had an effect on plant biomass (above- and belowground) and a positive effect on AM fungal colonization of roots in presence of worms, but a negative effect when worms were absent. When both M. pruriens and B. pearsei were added, shoot and root biomass and N concentrations increased. Vesicle formation was related to velvetbean mulch decomposition as well as the higher N concentration in maize roots. Management of mulch–earthworm interactions may be of value, particularly in low-input and organic agricultural systems, and deserves further investigation.