Effects of vertical distribution of soil inorganic nitrogen on root growth and subsequent nitrogen uptake by field vegetable crops

Information is needed about root growth and N uptake of crops under different soil conditions to increase nitrogen use efficiency in horticultural production. The purpose of this study was to investigate if differences in vertical distribution of soil nitrogen (N_inorg) affected root growth and N uptake of a variety of horticultural crops. Two field experiments were performed each over 2 years with shallow or deep placement of soil N_inorg obtained by management of cover crops. Vegetable crops of leek, potato, Chinese cabbage, beetroot, summer squash and white cabbage reached root depths of 0.5, 0.7, 1.3, 1.9, 1.9 and more than 2.4 m, respectively, at harvest, and showed rates of root depth penetration from 0.2 to 1.5 mm day^?1 °C^?1. Shallow placement of soil N_inorg resulted in greater N uptake in the shallow‐rooted leek and potato. Deep placement of soil N_inorg resulted in greater rates of root depth penetration in the deep‐rooted Chinese cabbage, summer squash and white cabbage, which increased their depth by 0.2–0.4 m. The root frequency was decreased in shallow soil layers (white cabbage) and increased in deep soil layers (Chinese cabbage, summer squash and white cabbage). The influence of vertical distribution of soil N_inorg on root distribution and capacity for depletion of soil N_inorg was much less than the effect of inherent differences between species. Thus, knowledge about differences in root growth between species should be used when designing crop rotations with high N use efficiency.     

Root Morphology,Proton Release,and Carboxylate Exudation in Lupin in Response to Phosphorus Deficiency

Narrow-leafed lupin (Lupinus angustifolius L.) is widely planted in infertile acidic soils where phosphorus (P) deficiency is one of the major limiting factors for plant growth. A hydroponic experiment was conducted to examine the morphological and physiological responses of roots of narrow-leafed lupin in response to altered P supply at 0, 1, 10, 25 or 75 μ M P as monopotassium phosphate (KH₂PO₄). Low P (P₀ and P₁) significantly decreased the plant biomass, but the supply of 10 μ M P was sufficient to produce similar plant biomass as the maximal P supply (P₇₅), indicating an efficient P acquisition by narrow-leafed lupin. Phosphorus deficiency did not enhance rates of carboxylate exudation and proton release by plant roots, indicating that carboxylate exudation and proton release are not the mechanisms for efficient P acquisition. In contrast, low P supply evidently modified the root morphology by increasing the primary root elongation, and developing a large number of cluster-like first-order lateral roots with dense root hairs, thus allowing efficient P acquisition by narrow-leafed lupin under low P supply.     

Phosphorus efficiency of ornamental plants in peat substrates

A pot experiment was conducted to investigate factors contributing to phosphorous (P) efficiency of ornamental plants. Marigold (Tagetes patula) and poinsettia (Euphorbia pulcherima) were cultivated in a peat substrate (black peat 80% + mineral component 20% on a volume basis), treated with P rates of 0, 10, 35, 100, and 170 mg (L substrate)^–1. During the cultivation period, plants were fertigated with a complete nutrient solution (including 18 mg P L^–1) every 2 d. Both poinsettia and marigold attained their optimum yield at the rate of 35 mg P (L substrate)^–1 and the critical level of P in shoot dry matter of both crops was 5–6 mg g^–1. After planting, plant‐available P increased at lower P rates to a higher level for poinsettia than for marigold, but no significant change was observed at higher P rates. Balance sheet calculations indicated that at lower P rates more P was fertigated than was taken up by the plants. Root‐length density, root‐to‐shoot ratio, and root‐hair length of marigold were doubled compared to that of poinsettia. Root‐length density increased with crop growth, and 10 d after planting the mean half distance between roots exceeded the P‐depletion zone around roots by a factor of 3 and 1.5 for poinsettia and marigold, respectively. Thus, at this early stage poinsettia exploited only 10% of the substrate volume whereas marigold utilized 43%. Later in the cultivation period, the depletion zones around roots overlapped for both crops. Taking into account P uptake via root hairs, the simulation revealed that this was more important for marigold compared to poinsettia especially at low P‐supply levels. However, increase of P uptake due to root hairs was only 10%–20% at optimum P supply. For the two lower P levels, the P‐depletion profile around roots calculated for 10 d after planting showed that after 2 d of depletion the concentration at the root surface was below the assumed K_m value (5 μM) and the concentration gradient was insufficient to fit the demand. A higher content of plant‐available P in the substrate was observed for poinsettia compared to marigold in the treatment with P application adequate for optimum growth, because more fertigated P was accumulated during early stages of cultivation due to lower root‐length density of poinsettia. The observed difference of root morphological parameters did not contribute significantly to P‐uptake efficiency, since P mobility in the peat substrate was high.     

Root architectural responses of wheat cultivars to localised phosphorus application are phenotypically similar

Differences in nutrient recovery from fertiliser bands may improve cereal variety selection. The objective of this study was to identify the variation in root plasticity across commonly grown Australian wheat (Triticum aestivum L.) cultivars in response to a phosphorus (P)‐enriched band. Ten wheat cultivars were screened for root proliferation within a 150 mg P kg⁻¹ band in P‐responsive soil. Plants were destructively harvested at the four‐leaf phenological stage and various growth parameters, including root length density (RLD), were measured on banded and uniformly adequate P treatments. All wheat cultivars increased RLD between three and nine times in the P band. However, there was no significant difference in root plasticity among the cultivars tested. Although all cultivars produced longer, though ≈ 9% thinner roots when responding to the P band, the phenotypic response was unable to compensate fully for the lower P status encountered in the soil. Despite 23% longer root lengths in the P‐band treatments, P uptake per unit root length was 78% lower than in uniformly adequate P treatments. Our results indicate that root plasticity of wheat cultivars in a P‐enriched band was phenotypically similar. Further research is necessary before selecting for wheat cultivars that respond to localised nutrient patches with increased RLD.     

Cluster Root Formation by Lupinus Albus is Modified by Stratified Application of Phosphorus in a Split-Root System

ABSTRACT

Cluster root formation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to stratified application of hydroxyapatite was examined in a split-root system. The system consisted of two vertical compartments, each divided horizontally into five 60-mm layers. Hydroxyapatite was applied to different layers at 150 mg phosphorus(P) kg^?1 soil. The proportion of dry biomass of cluster roots in the whole root system was significantly reduced when P concentration was high in shoots due to P application, suggesting that cluster root formation was regulated by the shoot P status. However, the cluster root percentage increased in the soil layer supplemented with P, and decreased in other layers, especially when P was applied in a deep layer. The formation of cluster roots is regulated by internal plant P status, but is also greatly affected by localized P supply. Heterogeneous P supply can modify the distribution of cluster roots.     

Aluminium-Toleranz von Ackerbohne (Vicia faba), Lupine (Lupinus luteus), Gerste (Hordeum vulgare) und Roggen (Secale cereale). II. Mineralstoffgehalte in Sproß und Wurzeln in Abhängigkeit vom Aluminium-Angebot

Al tolerance of horse bean, yellow lupin, barley and rye. II. Mineral element concentrations in shoots and roots as affected by Al supply Inhibition of seminal root elongation by Al in solution culture gave the following ranking for Al tolerance: yellow lupin (Lupinus luteus ?Schwako”?) ? rye (Secale cereale ?Kustro)”? « horse bean (Vicia faba ?Herz Freya”?) > barley (Hordeum vulgare ?Roland”?). Exclusion from uptake by inactivation of Al outside the root was not responsible for the higher Al tolerance of lupin and rye, because comparable inhibition of root elongation occured at much higher Al concentration of the root and the root tips (5 mm) compared to barley and horse bean. The plant species differed considerable in nutrient concentrations of the roots: higher Ca concentrations in horse bean and rye, higher Mg concentrations in rye and lupin and higher P concentration in lupin. Al supply reduced Ca and Mg concentrations (Ca > Mg) in shoots and roots of all species. P concentrations were hardly affected. The nutrient concentrations in the root tips did not indicate that induction of nutrient deficiency was responsible for the effect of Al on root elongation and Al sensitivity of barley and horse bean. The considerable differences in Ca, Mg and P concentrations of the roots between the Al-tolerant plant species rye and lupin do not suggest a common physiological mechanism responsible for Al tolerance.     

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.     

Root growth and nitrogen uptake of carrot, early cabbage, onion and lettuce following a range of green manures

An experiment was performed to study the significance of rooting depth of four vegetable crops on their utilization of green manure nitrogen (N). Rates of rooting depth development were estimated as approximately 0.2, 0.7, 1.2 and 1.2 mm day °C^?1 for onion, carrot, lettuce and cabbage, respectively. At harvest, onion and lettuce were found to be shallow‐rooted with final rooting depths of only 0.3 and 0.6 m, respectively, whereas carrot and cabbage reached rooting depths of at least 1.1 m. The two deep‐rooted vegetables increased their N uptake by 46, 24 and 7 kg N ha^?1 when following winter‐hardy legumes, non‐hardy legumes and rye, respectively; the equivalent responses by the two shallow‐rooted crops were 23, 9 and 15 kg N ha^?1, respectively. Thus the deep‐rooted crops used the legume N more efficiently but the shallow‐rooted crops made better use of N left by the non‐legume rye crop. These interactions between green manure type and vegetable crop N response are the result of the dual effects of the green manures: biological N fixation by the legumes, and the variable ability of the green manure crops to concentrate available N in the topsoil. Before shallow‐rooted crops, the ability of rye to concentrate N in the topsoil may be as important as the N fixing ability of legumes.     

Growth and nutrient uptake of temperate perennial pastures are influenced by grass species and fertilisation with a microbial consortium inoculant

Background: The low fertility of sandy soils in South‐Western Australia is challenging for the establishment of temperate perennial pastures. Aims: To assess whether microbial consortium inoculant may improve plant growth by increasing nutrient supply, root biomass and nutrient uptake capacity. Methods: Five temperate perennial pasture grasses–cocksfoot (Dactylis glomerata L. cv. Howlong), phalaris (Phalaris aquatica L. cv. Atlas PG), tall fescue (Festuca arundinacea L. cv. Prosper), tall wheatgrass (Thinopyrum ponticum L. cv. Dundas), and veldt grass (Ehrharta calycina Sm. cv. Mission) were tested in a controlled environment on the growth and nutrition with the microbial consortium inoculant and rock mineral fertiliser. Results: Veldt grass produced the highest shoot and root growth, while tall fescue yielded the lowest. Rock mineral fertiliser with or without microbial consortium inoculant significantly increased root and shoot biomass production across the grass species. The benefit of microbial consortium inoculation applied in conjunction with rock mineral fertiliser was significant regarding shoot N content in tall wheatgrass, cocksfoot and tall fescue. Shoot P and K concentrations also increased in the five grass species by microbial consortium inoculation combined with rock mineral fertiliser in comparison with the control treatment. Arbuscular mycorrhizal (AM) colonisation decreased with rock mineral fertilisation with or without microbial consortium inoculant except in cocksfoot. Conclusions: The response to microbial consortium inoculation, either alone or in combination with rock mineral fertiliser, was plant species‐dependent, indicating its potential use in pasture production.     

Root Growth,Nutrient Uptake,and Nutrient-Use Efficiency by Roots of Tropical Legume Cover Crops as Influenced by Phosphorus Fertilization

Roots are important organs that supply water and nutrients to growing plants. Data related to root growth and nutrient uptake by tropical legume cover crops are limited. The objective of this study was to evaluate root growth of tropical legume cover crops and nutrient uptake and use efficiency under different phosphorus (P) levels. The P levels used were 0 (low), 100 (medium), and 200 (high) mg kg^?1 of soil, and five cover crops were evaluated. Root dry weight, maximum root length, and specific root length were significantly influenced by P and cover crop treatments. Maximum values of these root growth parameters were achieved with the addition of 100 mg P kg^?1 soil. The P?×?cover crops interactions for all the macro- and micronutrients, except manganese (Mn), were significant, indicating variation in uptake pattern of these nutrients by cover crops with the variation in P rates. Overall, uptake pattern of macronutrients was in the order of nitrogen (N) > calcium (Ca) > potassium (K) > magnesium (Mg) > P and micronutrient uptake pattern was in the order of iron (Fe) > Mn > zinc (Zn) > copper (Cu). Cover crops which produced maximum root dry weight also accumulated greater amount of nutrients, including N, compared to cover crops, which produced lower root dry weight. Greater uptake of N compared to other nutrients by cover crops indicated that use of cover crops in the cropping systems could reduce loss of nitrate (NO₃ ^?) from soil–plant systems. Increase in root length and root dry weight with the addition of P can improve nutrient uptake from the soil and lessen loss of macro- and micronutrients from the soil–plant systems.     

Yearly variation of root distribution with depth in relation to nutrient uptake and corn yield

Abstract

Root length and root distribution in the soil profile is important in determining the amount of nutrients and water taken up by the plant. Data about year to year variation of corn (Zea mays L.) root growth and its relation to nutrient uptake are limited. An evaluation of the importance of root system size and distribution on P and K uptake and corn yield was made from samples taken annually from a long‐term fertility experiment on Raub silt loam, fine silty, mixed, mesic Aquic Argiudolls. Root density varied with soil depth among years, whereas P and K fertilizer treatment had no measureable influence on total root length. Ear leaf P concentration was highly correlated with the amount of roots in the 0 to 15 cm layer which contained most of the available P. Since P was not appreciably limiting corn yield, no significant relation was found between yield and P content of the ear leaf. Yields on K deficient plots were positively correlated with root density in the topsoil. Correlations of root densities in the deeper soil layers with both yield and ear leaf nutrient concentration became increasingly smaller with depth in the soil profile. The results indicate that root length plus root distribution in the soil may influence year to year variation in yield particularily on soils having low available nutrient levels. This variation in root growth may be responsible for differences among years in the response of crops to applied P and/or K.     

Root development of winter wheat in erosion‐affected soils depending on the position in a hummocky ground moraine soil landscape

Agricultural soil landscapes of hummocky ground moraines are characterized by 3D spatial patterns of soil types that result from profile modifications due to the combined effect of water and tillage erosion. We hypothesize that crops reflect such soil landscape patterns by increased or reduced plant and root growth. Root development may depend on the thickness and vertical sequence of soil horizons as well as on the structural development state of these horizons at different landscape positions. The hypotheses were tested using field data of the root density (RD) and the root lengths (RL) of winter wheat using the minirhizotron technique. We compared data from plots at the CarboZALF‐D site (NE Germany) that are representing a non‐eroded reference soil profile (Albic Luvisol) at a plateau position, a strongly eroded profile at steep slope (Calcaric Regosol), and a depositional profile at the footslope (Anocolluvic Regosol). At each of these plots, three Plexiglas access tubes were installed down to approx. 1.5 m soil depth. Root measurements were carried out during the growing season of winter wheat (September 2014–August 2015) on six dates. The root length density (RLD) and the root biomass density were derived from RD values assuming a mean specific root length of 100 m g^?1. Values of RD and RLD were highest for the Anocolluvic Regosol and lowest for the Calcaric Regosol. The maximum root penetration depth was lower in the Anocolluvic Regosol because of a relatively high and fluctuating water table at this landscape position. Results revealed positive relations between below‐ground (root) and above‐ground crop parameters (i.e., leaf area index, plant height, biomass, and yield) for the three soil types. Observed root densities and root lengths in soils at the three landscape positions corroborated the hypothesis that the root system was reflecting erosion‐induced soil profile modifications. Soil landscape position dependent root growth should be considered when attempting to quantify landscape scale water and element balances as well as agricultural productivity.     

Phosphorus acquisition and wheat growth are influenced by shoot phosphorus status and soil phosphorus distribution in a split‐root system

Root proliferation and greater uptake per unit of root in the nutrient‐rich zones are often considered to be compensatory responses. This study aimed to examine the influence of plant phosphorus (P) status and P distribution in the root zone on root P acquisition and root and shoot growth of wheat (Triticum aestivum L.) in a split‐root soil culture. One compartment (A) was supplied with either 4 or 14 mg P (kg soil)^–1, whereas the adjoining compartment (B) had 4 mg P kg^–1 with a vertical high‐P strip (44 mg kg^–1) at 90–110 mm from the plant. Three weeks after growing in the split‐root system, plants with 4 mg P kg^–1 (low‐P plants) started to show stimulatory root growth in the high‐P strip. Two weeks later, root dry weight and length density in the high‐P strip were significantly greater for the low‐P plants than for the plants with 14 mg P (kg soil)^–1. However, after 8 weeks of growth in the split‐root system, the two P treatments of compartment A had similar root growth in the high‐P strip of compartment B. The study also showed that shoot P concentrations in the low‐P plants were 0.6–0.8 mg g^–1 compared with 1.7–1.9 mg g^–1 in the 14 mg P kg^–1 plants after 3 and 5 weeks of growth, but were similar (1.1–1.4 mg g^–1) between the two plants by week 8. The low‐P plants had lower root P concentration in both compartments than those with 14 mg P kg^–1 throughout the three harvests. The findings may indicate that root proliferation and P acquisition under heterogeneous conditions are influenced by shoot P status (internal) and soil P distribution (external). There were no differences in the total root and shoot dry weight between the two P treatments at weeks 3 and 5 because enhanced root growth and P uptake in the high‐P strip by the low‐P plants were compensated by reduced root growth elsewhere. In contrast, total plant growth and total root and shoot P contents were greater in the 14 mg P kg^–1 soil than in the low‐P soil at week 8. The two P treatments did not affect the ratio of root to shoot dry weight with time. The results suggest that root proliferation and greater P uptake in the P‐enriched zone may meet the demand for P by P‐deficient plants only for a limited period of time.     

Identification of a maize (Zea mays L.) inbred line adapted to low‐P conditions via analyses of phosphorus utilization,root acidification,and calcium influx

Phosphorus (P) fertilizers are essential for achieving high crop productivity, but declining soil P reserves and cost of fertilizers suggest that improving crop varieties for improved use efficiency of P be important for sustainability. To explore the possibility of selecting crops suitable for low P conditions, two maize (Zea mays L.) inbred lines, i.e., W22 and W23 were compared for growth, root morphology, and electrophysiological parameters, under hydroponic conditions with either insoluble P source (LP) or soluble P source (HP) in a factorial completely randomized design. Relative shoot biomass of W23 was significantly (38%) greater than that of W22 with LP, while relative root biomass of the two inbred lines did not differ. With LP, the P stress factor was the lowest (25%) and P dissolution in hydroponic solution was the greatest for W23. Root electrophysiological analysis revealed that W23 had 89% greater H⁺ efflux and 225% greater Ca²⁺ influx than W22 with LP. The distant elongation zone (DEZ) of W23 root was significantly longer and more shoot‐ward than W22 with LP. Thus, W23, having significantly greater relative shoot biomass, lower P stress factor, greater P dissolution, greater H⁺ efflux and Ca²⁺ influx, longer and more shoot‐ward DEZ, was better adapted to low‐P condition compared to W22. In the future, the W23 inbred line can be used for developing low‐P stress resistant varieties to utilize native insoluble soil P efficiently or to produce commercially acceptable yields using lower rates of soluble P fertilizers.     

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.     

Root cation exchange capacity of cotton cultivars in relation to aluminum toxicity

Root cation exchange capacity (CEC) was analyzed for four cotton cultivars (Pima S‐5, Stoneville 825, Deltapine 41 and Auburn 56) within tvo species (Gossypium barbadense and G. hirsutum) grown in control (O Al) and Al (1.5 mg/l) solution. Pima S‐5, a G. barbadense variety, had significantly (P < 0.10) lower root CEC than G. hirsutum cultivars in control (O Al) solution. Root CEC of Stoneville 825 was numerically but not significantly lower than Auburn 56 and Deltapine Al in control solution. Root CEC was significantly reduced in all cultivars when grown in Al solution. Compared to controls, Pima S‐5 and Stoneville 825 had either numerically or significantly less reduction in root CEC than Auburn 56 or Deltapine 41 in Al solution. Aluminum content of roots after CEC analysis was numerically greater in the former cultivars than the latter.

The lower root CEC of Pima S‐5 and Stoneville 825 in non‐toxic conditions could provide an initially greater Al tolerance when roots grow into marginally Al toxic soil. Under sustained, Al toxic conditions, root CEC becomes altered and is more of an indirect indicator of root growth as affected by as yet undetermined Al tolerance mechanism(s).

The steady‐state technique to determine root CEC virtually eliminated the inherent problems of CO. effects on pH and titrating to an end point in a specific period of time in a dynamic system.     

Minirhizotron quantification of soybean root growth as affected by reduced A horizon in soil

Shallow soil A horizon (topsoil) caused by soil erosion and soil movement from cultivation is known to reduce soil and crop productivity. The reduction may be related to limitation of root growth. A field study was conducted to investigate the effects of topsoil thickness on distributions of root density and growth. Soybeans [Glycine max (L.) Merr.] were grown on plots of Mexico silt loam (fine, montmorillonitic, mesic Mollic Endoaqualfs) with topsoil thicknesses of 0, 12.5, 25.0, and 37.5 cm above the Bt horizons. Root density was measured 60 and 90 days after planting using a minirhizotron video‐camera system. Root density was significantly reduced as topsoil thickness decreased from 37.5 to 0 cm. Mean density and net change of the density across profile between 30 and 60 days of growth had a linear function of topsoil thickness. The reduction and lower activity induced by shallow topsoil were attributed to detrimental properties in the Bt horizons. Root distribution pattern and rooting depth were not significantly affected by topsoil thickness. The roots appeared to be accumulated on the upper layers of the Bt horizons. Roots growing in thicker topsoil were more active than roots growing without topsoil. High soil moisture content during the growing season may mitigate the detrimental effects of shallow topsoil, inhibit root penetration, and enhance root activity.     

COMPARING FERTILIZER PHOSPHORUS REQUIREMENTS OF CANOLA,LUPIN, AND WHEAT

Increases in yield due to applications of phosphorus (P) (0, 5, 10, 15, 20, and 40 kg P/ha) applied as single (ordinary) superphosphate were measured for canola (Brassica napus), lupin (Lupinus angustifolius) and wheat (Triticum aestivum) in a field experiment on a deep sandy soil near Esperance, south-western Australia (WA). There are no data comparing the P requirements of these species grown at the same time, which was done by determining the amount of P required to produce 90% of the maximum yield for each species. The amount of P required was about 50% less for canola than wheat and about 10% more for lupin than wheat (60% more than canola). For each amount of P applied, the concentration of P in shoots and grain was greater for canola, followed by lupin and then wheat, suggesting that canola and lupin roots were better at accessing soil P than wheat. The critical concentration of P (diagnostic) required for 90% maximum yield of dried shoots measured in September was about 2.3 g/kg P for wheat, 2.8 g/kg P for lupin, and 3 g/kg P for canola. Similar critical values were obtained when P concentration in the shoots was related to grain yield (prognostic).     

Root Architecture Plasticity of Citrus Rootstocks in Response to Nitrate Availability

Under nutrient heterogeneous soil conditions, the ability of plants to compete for resource acquisition was closely related to the plasticity of root architecture, intended as a change of topology and geometry.

Root architecture variations in response to different nitrate supplies of co-generic citrus rootstocks, ‘Rough Lemon’ (Citrus jambhiri Lush), ‘Sweet Orange’ (Citrus sinensis (L) Osbeck), ‘Cleopatra Mandarin’ (Citrus reshni Hort ex Tan.), and ‘Sour Orange’ (Citrus aurantium L.) are reported here. Root architecture was gauged by an image-specific analysis system (WinRHIZO).

Based on the topological index, root topology of citrus rootstocks varied in response to nitrate treatments. In terms of root geometry, external-external and, to a lesser degree, internal-internal links of all rootstocks changed in response to nitrate supply. Among the rootstocks, ‘Cleopatra Mandarin’, both in terms of root topology (high TI) and root geometry (high length of exterior and interior link), could be considered less competitive owing to resource acquisition compared to the other rootstocks.