Changes In Root Morphology Research Articles

Review: Unravelling the functional relationship between root anatomy and stress tolerance

Salinity is a major environmental constraint limiting the yield of crop plants in many semi-arid and arid regions. A recently developed biophysical model for plant growth in saline environments confirms a critical role for root morphology and hydraulic properties in salinity and soil water deficit tolerance. The identification of genes based on correlations between exposure to salt stress and gene expression in roots and other organs has to date proved to be only marginally successful as a strategy for improving plant salt tolerance. Recently, the identification of genes that function in stress tolerance has advanced considerably by using genetic mutation analysis. However, the power of a genetic approach to understanding the specific mechanisms of root adaptation to saline and osmotic stress environments has not been fully exploited. A review of the available, yet still incomplete, collection of root mutants in Arabidopsis and other species demonstrates the potential usefulness of such mutants as tools in the genetic dissection of root function under osmotic stress. Identification of genes responsible for changes in root morphology that might also be advantageous in the presence of salt stress may open new avenues towards the elucidation of critical mechanisms for plant salt tolerance.

When the going gets tough: Responses ofSorghum bicolorL. (cv. Tegemeo) seedlings to soil penetration resistance

Summary Sorghum bicolor L. (cv. Tegemeo) seedlings were grown for nine days in soil at field capacity packed to give a uniform penetration resistance (PR) of either 0.25, 1.00 or 1.75 MPa. Root biomass was not significantly affected by soil PR treatment. However, as PR increased to 1.75 MPa, the diameter of the seminal root axis increased by 52% whilst its length decreased by 30%. Shoot growth, in terms of oven dry (OD) weight and photo-synthetic area, was reduced in both the 0.25 MPa and 1.75 MPa treatments compared to the 1.00 MPa treatment. A reduced nutrient, water or oxygen supply to the roots were discounted as possible causes of the root and shoot responses to soil PR. It is suggested that the changes in root morphology between treatments were a direct result of the changes in soil PR. For shoot growth, in the 0.25 MPa treatment it is suggested that shoot growth was reduced as a result of an increase in the carbon sink strength of the roots.

Root apical meristems ofAllium porrum L. as affected by arbuscular mycorrhizae and phosphorus

Arbuscular mycorrhizal (AM) fungi significantly improve plant growth in soils with low phosphorus availability and cause many changes in root morphology, similar to those produced by increased P nutrition, mainly depending on root apex size and activity. The aim of this work was to discriminate between the morphogenetic role of AM fungi and P in leek (Allium porrum L.) by feeding mycorrhizal and nonmycorrhizal plants with two nutrient solutions containing 3.2 or 96 μM P and examining specific parameters related to adventitious root apices (apex size, mitotic cycle, and RNA synthesis). The results showed that AM fungi blocked meristem activity as indicated by the higher percentages of inactive apices and metaphases in the apical meristem of mycorrhizal plants, whereas the high P supply lengthened the mitotic cycle without blocking the apices, resulting in steady, slow root growth. The possible involvement of abscisic acid in the regulation of root apex activity is discussed.

Rhizobia Inoculation Improves Nutrient Uptake and Growth of Lowland Rice

Growth‐promoting diazotrophs can enhance the growth and development of associated crops by transferring fixed N or by improving nutrient uptake through modulation of hormone‐linked phenomena in inoculated plants. Six rhizobial diazotrophs isolated from a wide range of legume hosts were investigated to determine their growth‐promoting activities in lowland rice (Oryza sativa L.) during 1997. Seeds and seedlings of rice Pankaj were inoculated with different rhizobia and grown in potted soil supplemented with varied amounts of mineral N. Inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium sp. IRBG74, and Bradyrhizobium sp. IRBG271 increased rice grain and straw yields by 8 to 22 and 4 to 19%, respectively, at different N rates. Nitrogen, P, and K uptake were increased by 10 to 28% due to rhizobial inoculation. Nitrogen‐15‐based studies indicated that the increased N uptake was not due to biological N2 fixation (BNF). Inoculation also increased Fe uptake in rice by 15 to 64%. Indole‐3‐acetic acid (IAA) accumulated in the external root environment of rice plants when grown gnotobiotically with rhizobia. The results indicate that certain strains of rhizobia can promote rice growth and yield, most likely through mechanisms that involve changes in growth physiology or root morphology rather than BNF.

Hormones induce an Fe-deficiency-like root epidermal cell pattern in the Fe-inefficient tomato mutantfer

The tomato mutantfer (Lycopersion esculentum L. T3238fer) displayed a chlorotic phenotype at normal external Fe levels. Root cells of the mutant are incompetent to take up iron in adequate amounts and are incapable to induce any of the known responses to Fe deficiency stress. We report here that the ethylene precursor 1-aminocyclopropane-l-carboxylic acid and the auxin analog 2,4-dichlorophenoxyacetic acid induce the formation of extra root hairs and transfer cells in the epidermis, thus mimicking the root-morphological Fe stress responses. In contrast, the physiological reactions involved in iron acquisition are not affected by the hormone treatment. These results indicate that ethylene is essential for transducing environmental signals into adaptive changes in root morphology. The data further suggest that the mutation does not affect necessary steps in the differentiation processes of epidermal cells. TheFER gene appears to control sensing of iron levels and/or the regulation of mechanisms involved in iron uptake.

Changes in root morphology of white lupin ( Lupinus albus L.) and its adaptation to soils with heterogeneous alkaline/acid profiles

The ability of Lupinus albus L. to adapt to a heterogeneous soil profile containing acid subsoil below limed topsoil of the same type, and to utilize nutrients by significantly altering its root system structure, was investigated using specially constructed soil profile tubes. Plants grown in homogeneous acid profiles had the fastest growth while those grown in homogeneous limed-soil profiles showed the slowest growth and exhibited some chlorosis after 19 days. Limed topsoil combined with an acid subsoil profile initially retarded plant growth similar to that in a homogeneous limed soil. However, after 68 days significantly greater growth had occurred in the limed/acid soil treatment relative to the homogeneous limed soil, indicating plants had benefited from the acid subsoil stratum. Plants in the homogeneous limed soil profile had lower concentrations of P, Fe and Mn in shoots compared with those in heterogeneous soils. In contrast, the concentration of Ca increased by 74%, due mainly to an increase in the water-soluble Ca fraction. When grown in a heterogeneous limed/acid soil profile, concentrations of P, Ca, K, Mg, Fe, Mn and Zn in shoots were comparable to those grown in a soil with a homogeneous acid profile. Although total root production was lower in the homogeneous limed-soil profile compared to the acid-soil containing profiles, cluster root mass was maintained at a level comparable with that in acid soil. The roots in heterogeneous soil profiles exhibited extensive plasticity, demonstrating a root-type specific, morphological response to the soil conditions. Within the acid subsoil of a heterogeneous profile, there was a large increase in cluster root mass compared with non-cluster roots. The proliferation of cluster roots in acid soil below limed topsoil may enhance the plant's ability to exploit this soil and facilitate the cultivation of L. albus on limed soil.

Induction of in vivo root ferric chelate reductase activity in fruit tree rootstock

A method has been developed to consistently induce increases in root ferric chelate reductase activity in the fruit tree rootstock GF 677 (Prunus amygdalopersica) grown under iron (Fe) deficiency. Clonal GF 677 plants were grown hydroponically in a growth chamber with 0 or 90 μM Fe(III)‐EDTA. Root ferric chelate reductase activity was measured in vivo using BPDS. Plants grown without Fe developed visible symptoms of chlorosis and had lower root ferric chelate reductase activities than those grown with Fe. Root ferric chelate reductase activities were 0.1–1.9 and 0.6–5.3 nmol of Fe reduced per gram of fresh mass and minute, respectively, in Fe‐deficient and sufficient plants. However, when plants grown without Fe for several days were resupplied with 180 μM of Fe(III)‐EDTA, FC‐R activities increased within 1 day. The FC‐R values after Fe resupply were 20‐fold higher than those found in Fe‐deficient plants and 5‐fold higher than those found in the Fe‐sufficient controls. After three days of the Fe treatments the FC‐R activities had decreased again to the control values. The reduction of Fe was localized at the subapical root zone. In the conditions used we have found no decreases of the nutrient solution pH values, indicating that this type of response is not strong enough to be detected in peach tree rootstocks. Also, no major changes in root morphology have been found in response to Fe deficiency. This ferric chelate reductase induction protocol may be used in screening assays to select rootstock genotypes tolerant to Fe chlorosis.

Flooding effects in seedlings of Cytharexyllum myrianthum Cham. and Genipa americana L.: responses of two neotropical lowland tree species

Six-month-old seedlings of Cytharexyllum myrianthum and Genipa americana, two common tree species in different flood-prone areas in Brazil, were flooded for up to 90 days to compare their survival and growth responses under these conditions. Seedlings of both species were found to be relatively tolerant to flooding but growth responses changed according to treatment and plant species. Growth of G. americana was reduced by flooding, showing a decrease in root and leaf dry mass, root/shoot ratio and height, without showing any adaptive morphological changes. On the other hand, growth of C. myrianthum seedlings was stimulated under flooding conditions, showing an increase in root dry mass, root/shoot ratio, height, stem diameter and some morphological changes in roots and stems, i.e., development of new roots and stem base hypertrophy. These results could be regarded as an experimental corroboration of the field observations, showing that these species could be indicated for restoration programs of some Neotropical wetlands.

Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the formation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to examine the role of auxin, cytokinin, ethylene and nutrients in the regulation of root architecture. Surprisingly, extensive dichotomous and coralloid branching of lateral roots occurred spontaneously in Pinus taeda, P. halepensis and P. muricata. In P. sylvestris, P. ponderosa and P. nigra, treatment with auxin transport inhibitors (ATIs), the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid (ACC) or the ethylene‐releasing compound 2‐chloroethylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branching and coralloid organ formation. Formation of both spontaneous and ATI‐induced coralloid structures was blocked by treatment with an ethylene synthesis inhibitor L‐α‐(2‐aminoethoxyvinyl)glycine; this inhibition was reversed by either ACC or CEPA. In addition, the induction of this unique morphogenetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomous and coralloid structures, as well as the regulation of their formation by nutrient levels, show a striking similarity to those of ectomycorrhizas.

Ectomycorrhizas and cadmium toxicity in Norway spruce seedlings.

We studied the effects of ectomycorrhizal colonization by Laccaria bicolor (Maire) Orton S238 and Paxillus involutus (Batsch) Fr. 533 on cadmium (Cd) toxicity in Norway spruce seedlings (Picea abies (L.) Karst.). Both mycorrhizal and nonmycorrhizal seedlings were exposed to 0 (control), 0.5 or 5 &mgr;M CdSO(4) for 9 weeks in a sand culture system with frequent addition of nutrient solutions. In pure culture, P. involutus and L. bicolor showed similar Cd tolerance. However, in symbiosis, the Cd treatments decreased colonization by L. bicolor but not by P. involutus. Paxillus involutus ameliorated the negative effects of 0.5 &mgr;M Cd on shoot and root growth and chlorophyll content of old needles, whereas L. bicolor did not. Mycorrhizal colonization did not affect Cd concentrations of old needles and roots of seedlings. Despite differences between the ectomycorrhizal fungi in colonization and ability to alleviate Cd toxicity of seedlings, both species reduced Cd concentrations of young needles to a similar degree compared with nonmycorrhizal seedlings. However, in the 0.5 &mgr;M Cd treatment, the Cd content of needles of seedlings colonized by P. involutus was increased, whereas the Cd content of needles of seedlings colonized by L. bicolor was similar to that of needles of nonmycorrhizal seedings. When the amount of Cd translocated to needles was expressed on a root length basis to account for differences in the size of the root systems, the amount of Cd translocated to the needles was similar in seedlings mycorrhizal with P. involutus and in nonmycorrhizal seedlings. All mycorrhizal seedlings were similarly affected by 5 &mgr;M Cd, indicating that the amelioration efficiency of ectomycorrhizal fungi is dependent on the metal concentration to which the roots are exposed. Concentrations of P, K, Ca, Mg and Mn were decreased by 5 &mgr;M Cd to a similar extent in both nonmycorrhizal and mycorrhizal seedlings. In contrast to L. bicolor, P. involutus increased P uptake and altered patterns of root branching. We conclude that mycorrhizas alleviate Cd-induced reductions in growth of Picea abies seedlings. Although the two mycorrhizal fungi examined differed in their ability to alleviate Cd toxicity, these differences were not related to differences in Cd uptake or translocation to the shoot of the mycorrhizal seedlings. We suggest that amelioration of Cd toxicity by P. involutus may be a result of improved P nutrition or changes in root morphology, or both.

Root morphological and physiological plasticity of perennial grass species and the exploitation of spatial and temporal heterogeneous nutrient patches

Root morphological and physiological characteristics of four perennial grass species were investigated in response to spatial and temporal heterogeneous nutrient patches. Two species from nutrient-rich habitats (i.e. Holcus lanatus and Lolium perenne) and two species from nutrient-poor habitats (i.e. Festuca rubra and Anthoxanthum odoratum) were included in the study. Patches were created by injecting equal amounts of nutrient solution into the soil either on one location (i.e. spatial heterogeneity) or on several, alternating locations (i.e. temporal heterogeneity) within the pot. The consequences of changes in root morphology and the implications for the exploitation of the nutrient patches by individual plants were quantified by the amount of 15N captured from the enriched patches. The effects of nutrient heterogeneity on the acquisition of nutrients by species were determined by comparing the total nitrogen and phosphorus acquisition of the species in the two heterogeneous habitats with the total nitrogen and phosphorus acquisition in a homogeneous treatment. In this homogeneous treatment the same amount of nutrient solution was supplied homogeneously over the soil surface. The experiment lasted for 27 days and comprised one harvest.

Aluminum induces changes in the orientation of microtubules and the division plane in root meristem of Zea mays

Aluminum (Al) induces agricultural problems limiting crop productivity in acid soils. Since Al causes morphological changes in roots, and because microtubules (MTs) play important roles in determination of tissue morphology, we investigated whether Al affects the arrangement of MTs in maize root meristem using immunolocalization techniques. When seedling roots were treated with 50 μM Al, the orientations of MTs were dramatically altered in a population of cells located in the protoderm and the two outer layers of cortex: interphase cortical MT arrays lost their normal transverse organization and became random or longitudinal; the preprophase band of MTs, mitotic spindle, and phragmoplast developed at planes 90° rotated compared to their counterparts in controls. These changes in MT orientation resulted in the change of the division plane from transverse to longitudinal, producing daughter cells positioned side by side instead of above and below. The rotation of the otherwise normal MT arrays and the division plane in Al-treated roots indicates that Al interferes with the normal polarity sensing mechanism, which may contribute to the reduced axial growth of the Al-treated roots.

Length to weight ratio of chickpea roots under progressively receding soil moisture conditions in a Vertisol

Successful simulation of root growth relies on the conversion of root dry mass into root length, and the pattern of changes in length-to-weight ratio (LWR) of roots with soil depth and crop age. Experiments were conducted in a Vertisol under receding soil moisture conditions with five chickpea ( Cicer arietinum L.) genotypes during the 1992–1993 and 1993–1994 post-rainy seasons. Roots were sampled at 14-day intervals using a monolith method. Changes in LWR across genotypes, in general, were not significant as the genotypes had similar growth duration. With increasing age, root LWR declined gradually in the 0–10 cm depth and remained nearly unchanged in the 10–30 cm soil layer. It showed an apparent increasing trend at 30–135 cm soil depths, contrary to the observations made in other crops. Between the two seasons, root LWR was the lowest at all times and depths during the relatively wet 1992–1993 season. The changes in LWR with increasing soil depth exhibited lowest values at the soil surface with steep increases in the middle layers and a constant or declining pattern in deeper layers. There was a clear negative relationship between available soil moisture and LWR in the lower half of the rooting zone at all stages of crop growth in both years, a possible effect of oxygen deficiency caused by excessive soil water status. Changes in root morphology over depth indicated that poor soil aeration decreased LWR in Vertisols.

Changes in root hydraulic conductance (KR) of Olea oleaster seedlings following drought stress and irrigation

Quasi‐steady‐state measurements of root hydraulic conductance (KR) of Olea oleaster Hoffmgg. et Link potted seedlings were performed using a pressure chamber with the aim of: (a) measuring the impact of different water‐stress levels on a KR; (b) measuring the kinetics of KR recovery several days after soil rewetting; (c) relating changes in KR to changes in root anatomy and morphology. Increasing water‐stress was applied in terms of ratio of leaf water potential (ΨL) measured at midday to that at zero turgor (ΨTLP), i.e. ΨL/ΨTLP=0·5, 1·0, 1·2, 1·6; KR was measured initially and at 24, 48, 72, 96 h after irrigation.Values of KR in seedlings stressed to ΨL/ΨTLP=1·2 increased for 48 h after irrigation from 0·23 to 0·97×10−5 kg s−1 m−2 MPa−1 i.e. from 16% to 66% of that measured in unstressed seedlings. A marked shift of the x‐axis intercept of the straight line relating flow to pressure (zero flow at non‐zero pressure) was recorded initially after irrigation and persisted up to 48 h. Recovery of KR occurred within 24 h after irrigation in seedlings at ΨL/ΨTLP=0·5 and 48 h later in those at ΨL/ΨTLP=1·0.Severe drought stress (ΨL/ΨTLP=1·6) caused anatomical changes to roots which formed a two‐layered exodermis with thicker suberized walls and a three‐ to four‐layered endodermis with completely suberized tangential walls. Recovery of KR in these roots required resumed growth of root tips and emergence of new lateral roots.

Effect of growth regulators on transformed root cultures of Hyoscyamus muticus

Summary The transformed root cultures of Hyoscyamus muticus are usually cultivated in hormone-free medium in which the production of hyoscyamine is high. We studied whether the addition of exogenous growth regulators could enhance tropane alkaloid accumulation without affecting the growth and root phenotype. In order to rapidly screen a large number of hairy root samples a monoclonal antibody based enzyme immunological assay was developed for scopolamine analysis. The hairy root cultures tolerated exogenous auxins in the concentration range 0.01–5μmol/L well. The growth of the roots was not significantly affected by IAA and NAA, but the alkaloid accumulation was doubled compared with that of the roots growing in the absence of auxin. At optimum IAA concentration (0.1–1.0 μmol/L) the maximum hyoscyamine content was 117 mg L −1 and scopolamine content 0.2mgL −1 . In general, the scopolamine contents were by a factor of 300–500 lower than the hyoscyamine ones. The phenotype, growth rate and alkaloid accumulation of the roots did not change despite cytokinin addition. Both ABA and GA 3 strongly inhibited hyoscyamine accumulation. However, the inhibition was not associated with the changes in root morphology. We suggest that endogenous auxin production in the roots is not sufficient for maximum tropane alkaloid accumulation. To our knowledge this is the first time that alkaloid accumulation has been significantly increased by exogenous auxins without influencing growth.

Mechanisms for the increase in phosphorus uptake of waterlogged plants: soil phosphorus availability, root morphology and uptake kinetics.

Waterlogging frequently reduces plant biomass allocation to roots. This response may result in a variety of alterations in mineral nutrition, which range from a proportional lowering of whole-plant nutrient concentration as a result of unchanged uptake per unit of root biomass, to a maintenance of nutrient concentration by means of an increase in uptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experiment, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was affected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root-shoot ratios, but did not reduce P concentration. Thus, uptake of P per unit of root biomass increased under waterlogging. Our second objective was to test three non-exclusive hypotheses about potential mechanisms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root morphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) changes in P availability induced by waterlogging; (2) specific root length of waterlogged and control plants, and (3) P uptake kinetics in excised roots from waterlogged and control plants. The results supported the three hypotheses. Soil P avail-ability was higher during waterlogging periods, roots of waterlogged plants showed a morphology more favorable to nutrient uptake (finer roots) and these roots showed a higher physiological capacity to absorb P. The results suggest that both soil and plant mechanisms contributed to compensate, in terms of P nutrition, for the reduction in allocation to root growth. The rapid transformation of the P uptake system is likely an advantage for plants inhabiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the increased relative abundance of P. dilatatum in the community after waterlogging periods.

Growth and physiological responses of neotropical mangrove seedlings to root zone hypoxia.

Seedlings of Rhizophora mangle L., Avicennia germinans (L.) Stearn., and Laguncularia racemosa (L.) Gaertn. f. were cultured in aerated or N(2)-purged solution for 12 weeks to assess their relative responses to low oxygen tensions. All three species responded to low oxygen treatment by modifying physiological and morphological patterns to decrease carbon loss by root respiration. However, the extent to which seedling physiology and morphology were altered by low oxygen treatment differed among species. Maintenance of root oxygen concentrations, root respiration rates and root extension rates by R. mangle demonstrated an ability to avoid low oxygen stress with minimal changes in root morphology and physiology. In contrast, oxygen concentrations in A. germinans and L. racemosa roots declined from 16 to 5% or lower within 6 h of treatment. Root hypoxia led to significant decreases in respiration rates of intact root systems (31 and 53% below controls) and root extension rates (38 and 76% below controls) by A. germinans and L. racemosa, respectively, indicating a greater vulnerability of these species to low oxygen tensions in the root zone compared with R. mangle. I conclude that the relative performance of mangrove seedlings growing in anaerobic soils is influenced by interspecific differences in root aeration and concomitant effects on root morphology and physiology.

Effect of a genetically modified Rhizobium meliloti inoculant on the development of arbuscular mycorrhizas, root morphology, nutrient uptake and biomass accumulation in Medicago sativa

summaryA soil microcosm system was used to evaluate the impact of a genetically modified (GM) Rhizobium meliloti strain on development and function of arbuscular mycorrhiza in alfalfa plants (Medicago sativa L.). There was no indication that this GM Rhizobium strain, which had an enhanced nodulation competitiveness ability, interfered with mycorrhiza formation by Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe. Indeed, inoculation with the GM Rhizobium strain greatly increased the number of mycorrhizal entry points in the alfalfa root system in comparison to the wild‐type strain. Mycorrhizal development and quality of nodulation increased with time and coincided with increased biomass of and nutrient (N, P) uptake by the host plant. The establishment of the symbiotic interactions also induced changes in root morphology; in particular, the degree of branching increased and the number of lateral roots was greater in plants inoculated with the GM Rhizobium strain. These results demonstrate that the GM Rhizobium strain does affect adversely the performance of arbuscular mycorrhizal symbiosis, a biosafety model system based on a functional rhizosphere.

Effects of varied soil nitrogen supply on Norway spruce (Picea abies [L.] Karst.)

During a seven-month period the effect of different nitrogen (N) availability in soil on growth and nutrient uptake was studied in three-year-old Norway spruce (Picea abies [L.] Karst.) trees. The plants were grown in pots on N-poor forest soil supplied with various amounts and forms (inorganic and organic) of N. Increasing supply of inorganic N (as NH 4 NO 3 ) increased the formation of new shoots and shoot dry weight. The root/shoot dry weight ratio of new growth was drastically decreased from 1.6 in plants without N supply to 0.5 in plants supplied with high levels of NH 4 NO 3 . This decrease in root/shoot dry weight ratio was associated with distinct changes in root morphology in favour of shorter and thicker roots. The addition of keratin as organic N source did neither affect growth nor root morphology of the trees. The amount of N taken up by plants was closely related to the supply of inorganic N, and trees supplied with highest levels of NH 4 NO 3 also had the highest N contents in the dry matter of needles and roots. In contrast, N contents in needles of trees grown without additional N, or with keratin supply, were in the deficiency range. Supply of NH 4 NO 3 decreased the contents of phosphate (P) and potassium (K) and therefore markedly increased N/P and N/K ratios in the needles. On the other hand, the contents of calcium (Ca), magnesium (Mg), and manganese (Mn) in the needles were increased in the plants supplied with inorganic N, suggesting high soil availability and promotion of uptake of these divalent cations by high nitrate uptake. The observed effects on root/shoot dry weight ratio, root morphology, and mineral nutrient composition of the needles indicated that high inorganic N supply may increase above-ground productivity but at the same time decrease the tolerance of trees against soil-borne (e.g. deficiency of other mineral nutrients) stress factors.

Nitrate and ammonium uptake for single-and mixed-species communities grown at elevated CO2.

Sustained increases in plant production in elevated CO2 depend on adequate belowground resources. Mechanisms for acquiring additional soil resources include increased root allocation and changes in root morphology or physiology. CO2 research to date has focused almost exclusively on changes in biomass and allocation. We examined physiological changes in nitrate and ammonium uptake in elevated CO2, hypothesizing that uptake rates would increase with the amount of available CO2. We combined our physiological estimates of nitrogen uptake with measurements of root biomass to assess whole root-system rates of nitrogen uptake. Surprisingly, physiological rates of ammonium uptake were unchanged with CO2, and rates of nitrate uptake actually decreased significantly (P<0.005). Root boomass increased 23% in elevated CO2 (P<0.005), but almost all of this increase came in fertilized replicates. Rates of root-system nitrogen uptake in elevated CO2 increased for ammonium in nutrient-rich soil (P<0.05) and were unchanged for nitrate (P>0.80). Root-system rates of nitrogen uptake were more strongly correlated with physiological uptake rates than with root biomass in unamended soil, but the reverse was true in fertilized replicates. We discuss nitrogen uptake and changes in root biomass in the context of root nutrient concentrations (which were generally unchanged with CO2) and standing pools of belowground plant nitrogen. In research to date, there appears to be a fairly general increase in root biomass with elevated CO2, and little evidence of up-regulation in root physiology.