Showing papers in "Plant and Soil in 1999"

Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Abstract: Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid.

Phytostimulatory effect of Azospirillum brasilense wild type and mutant strains altered in IAA production on wheat

Abstract: Auxin production by Azospirillum is believed to play a major role in the observed plant growth promoting effect. By using different genetically modified strains, the contribution of auxin biosynthesis by A. brasilense in altering root morphology was evaluated in a plate assay. Inoculation with the wild type strains A. brasilense Sp245 and Sp7 resulted in a strong decrease in root length and increase in root hair formation. This effect was abolished when inoculating with an ipdC mutant of A. brasilense. The ipdC gene encodes a key enzyme in the IPyA pathway of IAA synthesis by A. brasilense. On the other hand, the observed auxin effect was further enhanced by adding tryptophan, a precursor of IAA, to the plates and could be mimicked by replacing the Azospirillum cells by a particular concentration of IAA. Furthermore, particular mutants (rpoN, scrp) and transconjugants (extra copy of ipdC) of A. brasilense were tested in the plate assay. Together, these results confirm the important role of IAA produced by Azospirillum in altering root morphology and illustrate the power of combining genetic tools and bioassays to elucidate the mechanism of a beneficial Azospirillum-plant interaction.

Effects of silicon on enzyme activity and sodium, potassium and calcium concentration in barley under salt stress

Abstract: Two contrasting barley (Hordeum vulgare L) cultivars: Kepin No7 (salt sensitive), and Jian 4 (salt tolerant) were grown in a hydroponics system containing 120 mol m-3 NaCl only and 120 mol m-3 NaCl with 10 mol m-3 Si (as potassium silicate) Compared with the plants treated with salt alone, superoxide dismutase (SOD) activity in plant leaves and H+-ATPase activity in plant roots increased, and malondialdehyde (MDA) concentration in plant leaves decreased significantly for both cultivars when treated with salt and Si The addition of Si was also found to reduce sodium but increase potassium concentrations in shoots and roots of salt-stressed barley Sodium uptake and transport into shoots from roots was greatly inhibited by added Si under salt stress conditions However, Si addition exhibited little effect on calcium concentrations in shoots of salt-stressed barley Thus, Si-enhanced salt tolerance is attributed to selective uptake and transport of potassium and sodium by plants The results of the present study suggest that Si is involved in the metabolic or physiological changes in plants

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

Abstract: The concept of critical nitrogen concentration(%N c) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%N c declines as a function of aerial biomassaccumulation (W) and the %N c –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as: If W < 1 t ha-1%N c = 3.40 If 1 t ha-1≤ W ≤ 22 t ha-1%N c = 3.40(W)−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %N c wasallowed. A critical N uptake model (Nuc, kg Nha-1) is proposed as Nuc = 34 (W)0.63 A comparison between Nuc and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops.

Interspecific complementary and competitive interactions between intercropped maize and faba bean

Abstract: Interspecific complementary and competitive interactions between maize (Zea mays L. cv. Zhongdan No. 2) and faba bean (Vicia faba L. cv. Linxia Dacaidou) in maize/faba bean intercropping systems were assessed in two field experiments in Gansu province, northwestern China, plus a microplot experiment in one treatment of one of the field experiments in which root system partitions were used to determine interspecific root interactions. Intercropping effects were detected, with land equivalent ratio values of 1.21–1.23 based on total (grain+straw) yield and 1.13–1.34 based on grain yield. When two rows of maize were intercropped with two rows of faba bean, both total yield and grain yield of both crop species were significantly higher than those of sole maize and faba bean on an equivalent area basis. When two rows of pea (Pisum sativum L. cv. Beijing No. 5) were intercropped with two rows of faba bean, neither total yield nor grain yield of faba bean was higher than of sole faba bean on an equivalent area basis. Interspecific competition between maize and faba bean was relatively weak, with mean relative crowding coefficients of 0.99–1.02 for maize and 1.55–1.59 for faba bean. The microplot experiment in which partitions were placed between root systems showed a significant positive yield effect on maize when the root systems intermingled freely (no partition) or partly (400 mesh nylon net partition) compared with no interspecific root interaction (plastic sheet partition).

The Maturity Index,the evolution of nematode life history traits, adaptive radiation and cp-scaling

Abstract: Nematodes are increasingly being used in environmental studies. One of the potential parameters to measure the impact of disturbances and to monitor changes in structure and functioning of the below-ground ecosystem is the nematode Maturity Index; an index based on the proportion of colonizers (r-strategists s.l.) and persisters (K-strategists s.l.) in samples. In this paper the original allocation of nematode taxa on the colonizer-persister scale, and the tolerance and sensitivity of colonizers and persisters are discussed from an evolutionary viewpoint. The phenomenon that neither relative egg size nor body length is an unequivocal character to scale nematodes suggests that the main selection for life history traits occurred independently in the major evolutionary branches.

Predominance of ecophysiological controls on soil CO2 flux in a Minnesota grassland

Abstract: Ecosystem studies often study soil CO2 flux as a function of environmental factors, such as temperature, that affect respiration rates by changing the rate of utilization of carbon substrates. These studies tend not to include factors, such as photosynthesis, that affect the supply of carbon substrates to roots and root-associated processes. We examined the role of decreased carbohydrate source on soil CO2 flux and root respiration in an annually-burned grassland through manipulations of light intensity and removal of above ground biomass. We also quantified the contribution of root respiration to soil CO2 flux by measuring the respiration rates of excised roots. Two days of shading caused a 40% reduction in soil CO2 flux, while clipping was associated with a 19% reduction in soil CO2 flux. Both reductions were independent of soil and air temperature at the time of measurement. The relative decrease in soil CO2 flux observed in the clipping experiment was similar in magnitude to an observed decrease in root respiration per gram of root, linking decreased root activity and soil CO2 flux. From these experiments, we conclude that variation in factors that affect carbon availability to roots can be important determinants of soil CO2 flux and should be included explicitly in studies that measure or model soil CO2 flux.

EDTA and Pb-EDTA accumulation in Brassica juncea grown in Pb-amended soil

Abstract: Previous studies have shown that EDTA is necessary to solubilize soil Pb and facilitate its transport from the soil to the above ground plant tissues These studies have also suggested that Pb is accumulated in the plant tissue with transpiration as the driving force We conducted further studies to evaluate the relationship between EDTA soil treatment, plant transpiration, and plant accumulation of Pb and EDTA Indian mustard (Brassica juncea) plants were grown in soils containing Pb at three different concentrations (15, 30 and 48 mmol/kg) for 5 weeks before being treated with EDTA concentrations ranging from 0 to 10 mmol/kg Plant shoots and xylem sap were collected and analyzed for Pb and EDTA content using ICP and HPLC, respectively Water loss was measured for 7 days following EDTA application Transpiration was not affected at <5 mmol/kg EDTA but, at 10 mmol/kg EDTA transpiration decreased by 80%, whereas accumulation of Pb and EDTA increased In the Sassafras soil, Pb and EDTA accumulation in the plant shoots continued to increase as the applied EDTA concentration increased, except at the highest level (10 mmol/kg) In soil amended with 48 mmol/kg Pb and 10 mmol/kg EDTA, the concentrations of EDTA and Pb in shoots decreased and visible signs of phytotoxicity were observed The results presented herein support recent studies in hydroponic systems showing that EDTA and Pb are taken up by the plant and suggest that Pb is translocated in the plant as the Pb-EDTA complex The results also show that the maximum Pb accumulation by plants occurs by maximizing the concentration of the Pb-EDTA complex based on the EDTA extractable soil Pb

Plant availability of selenite and selenate as influenced by the competing ions phosphate and sulfate

Abstract: The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L cv Evening Shade) and strawberry clover (Trifolium fragiferrum L cv O'Conner) at basal concentrations of SO4 (05 mM) and PO4 (2 μM) similar to those found in soil solution Concentrations up to 5 mM SO4 and 200 μM PO4 allowed quantitative comparison of the efficacy of the competing ions In both species, selenite was more phytotoxic than selenate, especially for shoot growth Selenate was less toxic, and tended to preferentially inhibit root growth Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%) A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se

Ecological succession and the rehabilitation of disturbed terrestrial ecosystems

Abstract: Soil and vegetation development on surface-mined coal sites in a mixed grass prairie region were studied as (a) naturally revegetated chronosequences of 1, 7, 17, 30 and 45-year old sites, and (b) on sites that were regraded, received topsoil, and were fertilized and seeded. In both cases, vegetation and soils of adjacent, relatively undisturbed areas were also studied for comparison. Species richness was lowest (26 species) at the 1-year old site and highest (114 species) at unmined sites; species richness increased youngest to the oldest sites. Stand ordinations revealed that the site age was the most important factor influencing species richness and abundance. On the younger sites, the environment was characterized by unleached parent materials high in electrical conductivity, SO4, Mg, Ca, and Na and plants growing there were non-native, annual pioneers. At about 30 years, the edaphic conditions had moderated somewhat, and the species complex had a greater number of native species. However, even after 45 years, there were clear differences among mined and unmined sites both in species composition and edaphic conditions. Rates of nutrient accumulations over the 45-year time gradient were as follows: organic C 131 kg ha−1 yr−1, N 25 kg ha−1 yr−1, P 0.1 kg ha−1 yr−1, and K 4.9 kg ha−1 yr−1. C/N ratios showed the widest range for 1-year old sites (5–40), but 70% of the 45-year old sites showed values below 15, comparable to unmined sites. Distributional diagrams of plant species revealed their topographic and habitat preferences which may be helpful in formulating species prescriptions for revegetation. Stand-environmental complex ordinations with 53 variables in the matrix showed topographic variables to be the most important, followed by site age. This study attempted to evaluate simultaneously two major environmental factors—time and topography, as these influence the development of soils and vegetation concomitantly. The use of canonical correspondence analysis facilitated the resolution of these two major environmental factors in illustrating the relationships between the species and the environments in which they grow. Simulations with the CENTURY model of C and N dynamics on north- and south-facing slopes provided useful insights into the recovery time of mined systems. All carbon pools (except passive) were larger on the north-facing slopes than on the south, as was the accumulation of organic N. Rehabilitation of mined areas is now required by law and regulations applicable in the region under discussion mandate the following steps: regrading of ridges to blend in with the surrounding topography, spreading topsoil, fertilizing and seeding. The course of vegetation and soil development of such sites was studied intensively for four years and revealed that the first year vegetation, irrespective of desirable species seeded, was dominated by the pioneer forb, Kochia scoparia. Its replacement by seeded grasses was quick. The rapid replacement of Kochia by grasses appears to be related to the production of allelochemics by Kochia and its acting as a nurse crop in the first year. Although allelochemics may be produced by Kochia as a defense mechanism for herbivory, the compounds seem to produce ionic imbalances (P, Mn, Zn) which hasten its own demise through autotoxicity. Like the abandoned mined sites, dispersal of seeds in space (immigration) seemed to be more important than from seeds in the topsoil (seed banks- dispersal in time). But in contrast to abandoned mined sites, the managed sites showed (a) a very rapid replacement of pioneers, (b) three to five times faster rates of leaching of ions capable of diminishing plant growth, (c) 5–8 times faster rates of mineralization of ions that promote growth, and (d) successional trends followed the facilitation and auto-inhibition pathways. A comparison of abandoned and managed systems clearly shows that by following the legal mandates, the rehabilitation process can be achieved many decades sooner with human assistance.

The influence of nematodes on below-ground processes in grassland ecosystems

Abstract: This review summarises recent information on beneficial roles that soil nematodes play in the cycling of carbon and other plant nutrients in grassland ecosystems. In particular, we focus on the role of the two dominant functional groups of nematodes, namely the microbial- and root-feeders, and how their activities may enhance soil ecosystem-level processes of nutrient cycling and, ultimately, plant productivity in managed and unmanaged grassland ecosystems. We report recent experiments which show that low amounts of root herbivory by nematodes can increase the allocation of photoassimilate carbon to roots, leading to increased root exudation and microbial activity in the rhizosphere. The effects of these interactions on soil nutrient cycling and plant productivity are discussed. Evidence is presented to show that the feeding activities of microbial-feeding nematodes can enhance nutrient mineralization and plant nutrient uptake in grasslands, but that these responses are highly species-specific and appear to be strongly regulated by higher trophic groups of fauna (top-down regulation). We recommend that future studies of the roles of nematodes in grasslands ecosystems should consider these more complex trophic interactions and also the effects of species diversity of nematodes on soil ecosystem-level processes.

Utility of nematode community analysis as an integrated measure of the functional state of soils: perspectives and challenges

Abstract: Soil nematode communities have the potential to provide unique insights into many aspects of soil processes Since most nematodes are active in soil throughout the year, they can potentially provide a holistic measure of the biotic and functional status of soils In contrast to other soil microbial groups, representative samples of soil nematode communities are relatively easy to obtain However, most current nematode ecological information has been survey-based or purely observational in nature, with a persistent focus on detailed taxonomic analysis of nematode communities The development of a Maturity Index, MI, represents a significant advance in classifying communities and it continues to be refined and developed But, to develop a wide capacity to use soil nematode information for diagnostic and predictive purposes, particularly for agricultural soils, we need a new, more robust approach, which does not require extensive taxonomic skill and includes more functional criteria One of the key attributes of nematodes is the relationship between structural form (principally oesophagal feeding apparatus) and function (ie trophic group) Nematode form is readily determinable by direct observation of extracted nematodes and high-level taxonomic skills are not needed to assign the major community components to their different trophic and ecological groups Consequently, the trophic structure of nematode communities is relatively easy to determine and can provide an integrated measure of the status of the other groups on which they feed Similarly, population numbers and proportions of juveniles and adults can be readily determined, permitting calculation of relative biomass and dynamics of population growth The size distribution of individuals within the community is likely also to be an indicator of the structural status of soils from a biotic standpoint However, fundamental gaps remain in our understanding which limit our ability to relate differences in nematode communities to functional differences There needs to be a greater emphasis on the development and experimental testing of hypotheses, a greater integration of nematology into soil-process related studies, and the development of a specific, soil-nematode related theoretical framework for understanding epidemiological and soil colonisation processes

Significance and limits in the use of predawn leaf water potential for tree irrigation

Abstract: Research in estimating the water status of crops is increasingly based on plant responses to water stress. Several indicators can now be used to estimate this response, the most widely available of which is leaf water potential (ΨLWP) as measured with a pressure chamber. For many annual crops, the predawn leaf water potential (ΨPLWP), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relative transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common when soil water content is markedly heterogeneous.

Root distribution and water uptake patterns of corn under surface and subsurface drip irrigation

Abstract: Information on root distribution and uptake patterns is useful to better understand crop responses to irrigation and fertigation, especially with the limited wetted soil volumes which develop under drip irrigation. Plant water uptake patterns play an important role in the success of drip irrigation system design and management. Here the root systems of corn were characterized by their length density (RLD) and root water uptake (RWU). Comparisons were made between the spatial patterns of corn RWU and RLD under surface and subsurface drip irrigation in a silt loam soil, considering a drip line on a crop row and between crop rows. Water uptake distribution was measured with an array of TDR probes at high spatial and temporal resolution. Root length density was measured by sampling soil cores on a grid centered on crop row. Roots were separated and an estimation of root geometrical attributes was made using two different image analysis programs. Comparisons of these programs yielded nearly identical estimates of RLD. The spatial patterns of RWU and RLD distributions, respectively normalized to the total uptake and root length, were generally similar only for drip line on a crop row, but with some local variations between the two measures. Both RLD and RWU were adequately fitted with parametric models based on semi-lognormal and normal Gaussian bivariate density functions (Coelho and Or, 1996; Soil Sci. Soc. Am. J. 60, 1039–1049).

Post-drainage changes in vegetation composition and carbon balance in Lakkasuo mire, Central Finland

Abstract: The post-drainage changes in vegetation composition and carbon balance were studied on four site types (from minero- to ombrotrophic conditions) in Lakkasuo mire, central Finland, by directly comparing undrained and drained parts (30 years ago) of the mire. Drainage had drastically changed the species composition of the sites, especially at the minerotrophic sites, where almost all Sphagna had been replaced by forest mosses. On the ombrotrophic sites much of the mire vegetation still remained 30 years after drainage. Drainage had decreased the C stores in ground vegetation on the minerotrophic sites but increased them on the ombrotrophic sites. The changes were, however, very small compared to the changes in the tree stand, where the C stores had increased at all sites (increasing with nutrient level). The change in peat C balance over the 30-year post-drainage period was negative on the most nutrient-rich site, and positive on the others, increasing with lower nutrient levels. The decrease in the peat C balance on the most nutrient-rich site was compensated by the greater increase in the tree stand C stores and the changes in the total C balance (peat+tree stand+ground vegetation) remained positive on all sites.

Estimating C inputs retained as soil organic matter from corn (Zea Mays L.)

Abstract: In agroecosystems, the annual C inputs to soil are a major factor controlling soil organic matter (SOM) dynamics. However, the ability to predict soil C balance for agroecosystems is limited because of difficulties in estimating C inputs and in particular from the below-ground part. The objective of this paper was to estimate the proportion of corn residue retained as SOM. For that purpose, the results of a 13 C long-term (15 yr) field study conducted on continuous silage corn and two silage corn rotations along with data from the existing literature were analyzed. The total amount of corn-derived C (0‐30 cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445 g m 2 ), compared to the two rotational treatments (175 and 133 g m 2 for the corn-barley-barley-wheat and corn-underseeded barley hay-hay rotations, respectively). Assuming that the C inputs to the soil from silage-corn was mainly roots and would have been similar across treatments on an annual basis, the total amount of cornderived C for the two rotational treatments was approximately proportional to the number of years the silage-corn was present in the rotation (4 yr). The results from the current study indicate that about 17% of root-derived C is retained as SOM. This value is higher than those reported in the literature for long-term studies on shoot-derived C (range of 7.7 to 20%, average of 12.2%), which is in agreement with previous studies showing that more C is retained as SOM from roots than from shoots.

Impact of chemical composition of legume residues and initial soil pH on pH change of a soil after residue incorporation

Abstract: Reports on the effect of organic matter addition on soil pH have been contradictory. This study examined the effect of applying legume residues differing in concentrations of N (4.3-45.5 mg g-1) and excess cations/organic anions (0.22–1.56 mmol g-1) on pH change of five soils differing in initial pH (3.60–5.58 in 0.01 M CaCl2) under sterile and non-sterile conditions. Addition of the legume residues at a level of 1% soil weight increased the pH of all soils by up to 2 units after incubation for 35 and 100 d under non-sterile conditions. Exceptions were the Lancelin (initial pH 5.06) and Kellerberin (pH 5.58) soils with addition of clover roots (excess cations 22 cmol/kg) for 100 d where soil pH decreased by 0.13–0.15 units as compared to the control. The amounts of alkalinity produced in soil correlated positively with concentrations of excess cations and total nitrogen of the added legume residues, and negatively with the initial pH of the soil. When soil was fumigated with chloroform during incubation, similar trends of soil pH changes and alkalinity production, due to legume residues addition, were displayed but the effects of the residue on alkalinity production in the Wodjil and Lancelin soils were much less than under non-sterile conditions. Direct shaking of soil with the residues under sterile conditions increased the amount of alkalinity in the soils with initial pH of 3.60–4.54, but not in the soils with initial pH of 5.06 and 5.58. The maximal alkalinity production was less than one third of that produced in the soil after 100 d of incubation under non-sterile conditions. The results suggest that the direction and the magnitude of pH change depend largely on the concentration of organic anions in the residues, initial soil pH and the degree of residue decomposition. The incorporation of crop residues, especially those with high concentrations of excess cations, is recommended in minimizing soil acidification in farming systems.

Effect of ammonium or nitrate nutrition on net photosynthesis, growth, and activity of the enzymes nitrate reductase and glutamine synthetase in blueberry, raspberry and strawberry

Abstract: Blueberry, raspberry and strawberry may have evolved strategies for survival due to the different soil conditions available in their natural environment. Since this might be reflected in their response to rhizosphere pH and N form supplied, investigations were carried out in order to compare effects of nitrate and ammonium nutrition (the latter at two different pH regimes) on growth, CO2 gas exchange, and on the activity of key enzymes of the nitrogen metabolism of these plant species. Highbush blueberry (Vaccinium corymbosum L. cv. 13–16–A), raspberry (Rubus idaeus L. cv. Zeva II) and strawberry (Fragaria × ananassa Duch. cv. Senga Sengana) were grown in 10 L black polyethylene pots in quartz sand with and without 1% CaCO3 (w: v), respectively. Nutrient solutions supplied contained nitrate (6 mM) or ammonium (6 mM) as the sole nitrogen source. Compared with strawberries fed with nitrate nitrogen, supply of ammonium nitrogen caused a decrease in net photosynthesis and dry matter production when plants were grown in quartz sand without added CaCO3. In contrast, net photosynthesis and dry matter production increased in blueberries fed with ammonium nitrogen, while dry matter production of raspberries was not affected by the N form supplied. In quartz sand with CaCO3, ammonium nutrition caused less deleterious effects on strawberries, and net photosynthesis in raspberries increased as compared to plants grown in quartz sand without CaCO3 addition. Activity of nitrate reductase (NR) was low in blueberries and could only be detected in the roots of plants supplied with nitrate nitrogen. In contrast, NR activity was high in leaves, but low in roots of raspberry and strawberry plants. Ammonium nutrition caused a decrease in NR level in leaves. Activity of glutamine synthetase (GS) was high in leaves but lower in roots of blueberry, raspberry and strawberry plants. The GS level was not significantly affected by the nitrogen source supplied. The effects of nitrate or ammonium nitrogen on net photosynthesis, growth, and activity of enzymes in blueberry, raspberry and strawberry cultivars appear to reflect their different adaptability to soil pH and N form due to the conditions of their natural environment.

Simulated climate change affecting microorganisms, nematode density and biodiversity in subarctic soils

Abstract: Arctic terrestrial ecosystems are strongly dominated by temperature, and global warming is expected to have a particularly strong impact in high latitudes. The Arctic will therefore be an important region for early detection of global change. In the present study the effects of environmental manipulations simulating climate change on soil microorganisms and nematode populations were investigated. Study sites were a dwarf shrub dominated tree-line heath (450 m a.s.l.) and a high altitude fellfield (1150 m a.s.l.) at Abisko, Swedish Lapland. Soil temperature was enhanced by using passive greenhouses and the impact on soil organisms with and without NPK fertilizer addition was assessed. The nematode community was strongly affected by warming and nutrient application. Population density was twice as high for all treatments at the fellfield as compared to controls. At the heath temperature enhancement with or without fertilizer application also led to a doubling of the population density, whereas fertilization alone caused an increase of about one third. The environmental manipulations resulted in a greater microbial biomass C and active fungal biomass in the heath soil. Increased density was also recorded for bacterial and fungal feeding nematodes at both sites. The results suggest that nematodes have an important impact on microbial biomass and turnover rates in the two subarctic systems. Elevated soil temperature apparently will lead to increased grazing on microorganisms, contributing to enhanced net N and P mineralization rates and plant nutrient availability. However, biodiversity was generally affected negatively by the environmental manipulations. The effects were more severe at the high altitude fellfield indicating that the influence of elevated temperature will be more pronounced in systems already stressed by extreme climatic conditions.

The development of symmetry, rigidity and anchorage in the structural root system of conifers

Abstract: The stability of shallowly rooted trees can be strongly influenced by the symmetry of the ‘structural’ system of woody roots. Root systems of forest trees are often markedly asymmetric, and many of the factors affecting symmetry, including root initiation and the growth of primary and woody roots, are poorly understood. The internal and environmental factors that control the development, with respect to symmetry and rigidity, of shallow structural root systems are reviewed and discussed with particular reference to Sitka spruce (Picea sitchensis Bong. Carr.). Areas where there is insufficient knowledge are highlighted. A scheme is proposed that represents the root system as a set of spokes that are variable in number, size and radial distribution. Rigidity can vary between and along each of the spokes. The root system is presented as a zone of competition for assimilates, where allocation to individual roots depends upon their position and local variations in conditions. Factors considered include the production of root primordia of different sizes, effects of soil conditions such as the supply of mineral nutrients and water on growth of primary and woody roots, and the effect of forces caused by wind action on growth of the cambium, giving rise to roots which, in cross section, resemble I- or T-beams, and efficiently resist bending.

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

Abstract: The mechanism of aluminium-induced inhibition of root elongation is still not well understood It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm The most Al-sensitive apical root zone appeared to be the 1–2 mm segment Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum

Modeling soil water movement with water uptake by roots

Abstract: Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils.

Causes of increased nutrient concentrations in post-fire regrowth in an East African savanna

Abstract: The aim of the present study was to investigate the causes of increased macronutrient concentrations in above-ground post-fire regrowth in an East African savanna (Northern Tanzania). Experiments were set up to discriminate between the following possible causes: (1) increased soil nutrient supply after fire, (2) relocation of nutrients from the roots to the new shoots, (3) rejuvenation and related changes in plant tissue composition and (4) changes in nutrient uptake in relation to above-ground carbon gains. N, P, K, Ca and Mg concentrations in post-burn graminoid vegetation were compared with clipped and with unburned, control vegetation during the post-burn growth season. One month after burning and clipping, nutrient concentrations in live grass shoots in the burned and clipped treatments were significantly higher than in the control. This effect, however, declined in the course of the season and, except for Ca, disappeared three months after onset of the treatments. There were no significant differences in live grass shoot nutrient concentrations between burned and clipped treatments which suggests that the increased nutrient concentration in post-fire regrowth is not due to increased soil nutrient supply via ash deposition. The relatively low input of nutrients through ash deposition, compared to the amount of nutrients released through mineralisation during the first month after burning and to the total nutrient pools, supports this suggestion. There was no difference between burned and unburned vegetation in total root biomass and root nutrient concentrations. Relocation of nutrients from the roots to the new shoots did not, therefore, appear to be a cause of higher post-fire shoot nutrient concentrations. The present study shows that in this relatively nutrient-rich savanna, the increased nutrient concentration in above-ground post-fire regrowth is primarily due to increased leaf:stem ratios, rejuvenation of plant material and the distribution of a similar amount of nutrients over less above-ground biomass.

The root zone dynamics of water uptake by a mature apple tree

Abstract: We report the results from a field experiment in which we examined the spatial and temporal patterns of water uptake by a mature apple tree (Malus domestica Borkh., ‘Splendour’) in an orchard. Time Domain Reflectometry (TDR) was used to measure changes in the soil's volumetric water content, and heat-pulse was used to monitor locally the rates of sap flow in the trunk and roots of the tree. We also measured the tree's distribution of root-length density and obtained supporting data to characterize the soil's hydraulic properties. The experimental data were used to examine the output of the WAVE-model (Vanclooster et al, 1995; Ecol. Model. 81, 183–185) in which soil water transport is predicted using Richards' equation, and where root uptake is represented by a distributed macroscopic sink term. When the surface soil layers were uniformly wet, 70% of the trees water uptake occurred in the top 0.4 m of the root zone, in which approximately 70% of the tree's fine roots were located. When a partial irrigation was applied to just one side of the root zone, the apple tree quickly shifted its pattern of water uptake with an almost two-fold increase in uptake from the wetter soil parts and a corresponding reduction in uptake from the drier parts. The response of root-sap flow to irrigation was almost immediate (i.e. root flow increased within hours of the irrigation). Following subsequent irrigations over the whole soil surface, TDR measurements revealed a surface-ward shift in the pattern of water extraction, and root flow measurements revealed a recovery in the uptake function of seemingly inactive roots located in the previously-dry soil. Via our root sap flow measurements, we observed two roots on the same tree locally responding quite differently to similar events of soil wetting. This observation suggests that there may be considerable functional variability across the apple root system. Our measurement-model calculations yielded similar results and stress the prime role played by the plant in modifying the root zone balance of water. Following an irrigation or rainfall event, root uptake by apple appears to be more dependent upon the near-surface availability of water than it is related to the distribution of fine roots.

Influence of large termitaria on soil characteristics, soil water regime, and tree leaf shedding pattern in a West African savanna

Abstract: Termitaria are major sites of functional heterogeneity in tropical ecosystems, through their strong influence on soil characteristics, in particular soil physico-chemical properties and water status. These factors have important consequences on nutrient availability for plants, plant spatial distribution, and vegetation dynamics. However, comprehensive information about the influence of termite-rehandled soil on soil water regime is lacking. In a humid shrubby savanna, we characterized the spatial variations in soil texture, soil structure and maximum soil water content available for plants (AWC max) induced by a large termite mound, at three deepths (0–0.10, 0.20–0.30 and 0.50–0.60 m). In addition, during a three month period at the end of the rainy season, soil water potential was surveyed by matrix sensors located on the termite mound and in the surrounding soil at the same depths and for the 80–90 cm layer. Concurrently, the leaf shedding patterns of two coexisting deciduous shrub species exhibiting contrasted soil water uptake patterns were compared for individuals located on termite mounds and in undisturbed control areas. For all the soil layers studied, clay and silt contents were higher for the mound soil. Total soil clods porosity was higher on the mound than in control areas, particularly in the 0.20-0.60 m layer, and mound soil exhibited a high shrinking/swelling capacity. AWCmax of the 0-0.60 m soil layer was substantially higher on the termite mound (112 mm) than in the surroundings (84 mm). Furthermore, during the beginning of the dry season, soil water potential measured in situ for the 0.20-0.90 m soil layer was higher on the mound than in the control soil. In contrast, soil water potential of the 0-0.10 m soil layer was similar on the mound and in the control soil. In the middle of the dry season, the leaf shedding pattern of Crossopteryx febrifuga shrubs (which have limited access to soil layers below 0.60 m) located on mounds was less pronounced than that of individuals located on control soil. In contrast, the leaf shedding pattern of the shrub Cussonia barteri (which has a good access to deep soil layers) was not influenced by the termite mound. We conclude that in this savanna ecosystem, termite mounds appear as peculiar sites which exhibit improved soil water availability for plants in upper soil layers, and significantly influence aspects of plant function. Implications of these results for understanding and modelling savanna function and dynamics, and particularly competitive interactions between plant species, are discussed.

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.): Phosphorus uptake

Abstract: Under anaerobic conditions, ferric hydroxide deposits on the surface of rice roots have been shown to affect the uptake of some nutrients. In the present experiment, different amount of this iron plaque were induced on the roots of rice (Oryza sativa L. cv. TZ88-145) by supplying different Fe(OH)3 concentrations in nutrient solutions, and the effect of the iron plaque on phosphorus uptake was investigated. Results showed that 1) iron plaque adsorbed phosphorus from the growth medium, and that the amount of phosphorus adsorbed by the plaque was correlated with the amount of plaque; 2) the phosphorus concentration in the shoot increased by up to 72% after 72 h at concentration of Fe(OH)3 in the nutrient solution from 0 to 30 mg Fe/L, corresponding with amounts of iron plaque from 0.2 to 24.5 mg g-1 (root d. wt); 3) the phosphorus concentration in the shoots of rice with iron plaque was higher than that without iron plaque though the concentration in the shoot decreased when Fe(OH)3 was added at 50 mg Fe/L producing 28.3 mg g-1 (root d. wt) of plaque; and 4) the phosphorus concentrations in Fe-deficient and Fe-sufficient rice plants with iron plaque were the same, although phytosiderophores were released from the Fe-deficient roots. The phytosiderophores evidently did not mobilise phosphorus adsorbed on plaque. The results suggest that iron plaque on rice plant roots might be considered a phosphorus reservoir.

Rhizodeposition and C-partitioning of Lolium perenne in axenic culture affected by nitrogen supply and defoliation

Abstract: This study investigated the effects of N-supply and partial defoliation on C-partitioning, root morphology and soluble rhizodeposition, for Lolium perenne grown in axenic sand culture systems percolated with nutrient solution Plants were grown for 36 d in nutrient solutions with differing N concentrations (4 mM or 002 mM NH4 +NO3 -), and effects of repeated defoliation to 4 cm were determined The ‘low N’ supply reduced (P < 005) dry matter accumulation, with proportionately increased partitioning to the root systems Root morphology was also altered at ‘low N’, with development of a finer root system, manifest as increased (P < 005) specific root length Concurrent with these effects on growth of L perenne, ‘low N’ increased (P < 005) exudation of C-compounds from roots on a per g root basis Defoliation was found to increase exudation (P < 005) of soluble compounds for periods of 3-5 d following each cut, at both N-supply rates The effects of N-supply and defoliation are of importance in understanding the coupling of plant productivity to nutrient cycling in soils with differing N availabilities and for grassland systems which are subject to grazing

Photosystem II efficiency in low chlorophyll, iron-deficient leaves

Abstract: Iron deficiency (iron chlorosis) is the major nutritional stress affecting fruit tree crops in calcareous soils in the Mediterranean area. This work reviews the changes in PS II efficiency in iron-deficient leaves. The iron deficiency-induced leaf yellowing is due to decreases in the leaf concentrations of photosynthetic pigments, chlorophylls and carotenoids. However, carotenoids, and more specifically lutein and the xanthophylls of the V+A+Z (Violaxanthin+ Antheraxanthin+Zeaxanthin) cycle are less affected than chlorophylls. Therefore, iron-chlorotic leaves grown in either growth chambers or field conditions have increases in the molar ratios lutein/chlorophyll a and (V+A+Z)/chlorophyll a. These pigment changes are associated to changes in leaf absorptance and reflectance. In the chlorotic leaves the amount of light absorbed per unit chlorophyll increases. The low chlorophyll, iron-deficient leaves showed no sustained decreases in PS II efficiency, measured after dark adaptation, except when the deficiency was very severe. This occurred when plants were grown in growth chambers or in field conditions. However, iron-deficient leaves showed decreases in the actual PS II efficiency at steady-state photosynthesis, due to decreases in photochemical quenching and intrinsic PS II efficiency. Iron-chlorotic leaves were protected not only by the decrease in leaf absorptance, but also by down-regulation mechanisms enhancing non-photochemical quenching and thermal dissipation of the light absorbed by PS II within the antenna pigment bed.

Root development of the Zinc-hyperaccumulator plant Thlaspi caerulescens as affected by metal origin, content and localization in soil

Abstract: Penetration into and exploitation of contaminated soils by roots of hyperaccumulator plants is a prerequisite for efficient removal of heavy metals, ie efficacy of phytoextraction This work was undertaken to study the development of roots of the Zn-hyperaccumulator Thlaspi caerulescens under various conditions of soil contamination Rhizoboxes were constructed with a removable plastic front cover, and filled with soils containing different amounts and forms of metals (Zn, Cd and Pb) Treatments were: homogeneous soil profile, superposition of three layers, inclusion of contaminated soil into uncontaminated soil, or inclusion of uncontaminated soil into uniformly contaminated soil Four seedlings were transplanted into each rhizobox, and development of the root system was periodically recorded for 133 days At harvest, the biomass and size of the rosette of aerial parts were determined The aerial biomass/root length fraction as well as the kinetics of root development varied according to the presence and localization of Zn The distribution and morphology of roots at harvest were strongly dependent upon the metal content and form in soil Roots exhibited a high affinity for the Zn-contaminated patches and showed two distinct morphologies according to the concentration of Zn in soil

Fatty acid methyl ester (FAME) profiles as a tool to investigate community structure of two agricultural soils

Abstract: Soil microbiological parameters may be the earliest predictors of soil quality changes. Recently, molecular techniques such as fatty acid methyl ester (FAME) profiles have been used to characterize soil microbial communities. Fatty acid methyl ester (FAME) from whole soil may be derived from live cells, dead cells, humic materials, as well as plant and root exudates. Our objective was to verify differences in FAME profiles from two agricultural soils with different plants. Soil samples were collected from Ritzville and Palouse silt loams for fatty acid analysis. Soil samples from wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), pea (Pisum sativum L.), jointed goatgrass ( Aegilops cylindrica L.) and downy brome (Bromus tectorum L.) rhizospheres were also collected for fatty acid analysis. Principal component analysis (PCA) of the two soils explained 42% of the variance on PC1, which accounted for Palouse soil. Ritzville soil accounted for 19% of the variance on PC2. Factor analysis showed that rhizosphere microbial communities from various plant species may differ depending on the plant species. Presence of Gram-positive bacteria as identified by a15:0, i15:0, a17:0 and i17:0 peaks were similar between rhizosphere and nonrhizosphere soils. Gram-negative bacteria characterized by short chain hydroxy acids (10:03OH and 12:03OH) as well as cyclopropane acids (cy17:0) were higher in rhizosphere soil than nonrhizosphere. This indicates a possible shift in the bacterial community to more Gram-negative bacteria and fewer Gram-positive bacteria in the rhizospheres of the plants species studied.