Effect of selected soil factors on chlorosis of peanut plants grown in calcareous soils in Cyprus

In 1986 and 1987 surveys were conducted of 34 (1986) and 35 (1987) peanut (Arachis hypogaea L.) fields in which the plants showed various degrees of chlorosis. In the areas concerned, plant appearance was classified according to a chlorotic index and corresponding soil samples were taken and analysed for CaCO₃, pH, NO₃–N and DTPA-extractable Fe in 1986 and for CaCO₃, NO₃–N and active lime in 1987.Regression analyses showed that CaCO₃, active lime and NO₃–N were positively correlated, while DTPA-extractable Fe was negatively correlated, with the chlorosis problem. The critical levels above which plants were chlorotic were 20 to 25% CaCO₃ and 10% active lime. Plants began to be chlorotic when DTPA-extractable Fe was below 2.5 mg·kg^–1. The soil factors examined explained about 60% of the variability in plant chlorosis.     

Effects of nitric oxide and Fe supply on recovery of Fe deficiency induced chlorosis in peanut plants

The effects of nitric oxide (NO) and/or iron (Fe) supplied to Fe deficient plants have been investigated in peanut (Arachis hypogaea L.) grown in Hoagland nutrient solution with or without Fe. Two weeks after Fe deprivation, recovery was induced by addition of 250 μM sodium nitroprusside (SNP, a NO donor) and/or 50 μM Fe (Fe-EDTA) to the Fe deprived (-Fe) nutrient solution. Activities of antioxidant enzymes, leaf chlorophyll (Chl), and active Fe content decreased, whereas activities of H⁺-ATPase, ferric-chelate reductase (FCR), nitrate reductase, and nitric oxide synthase and NO production increased in Fe deficient plants, consequently an Fe chlorosis symptom appeared obviously. In contrast, these symptoms disappeared gradually after two weeks with NO and/or Fe supply, which caused an increases in leaf Chl and active Fe content, especially following by co-treatment with NO and Fe to values found in Fe sufficient plants. Increased activities of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) and decreased accumulation of reactive oxygen species (H₂O₂, O ₂ ^?? ) and malondialdehyde enhanced the ability of resistance to oxidative stress. Supplied NO alone had the obvious effect on increased NO production and on activity of H⁺-ATPase and FCR, whereas root length and root/shoot ratio were most effectively increased by Fe supplied alone. Co-treatment with NO and Fe did the best effects on recovery peanut chlorosis symptoms by significantly increased Chl and available Fe content and adjusted distribution of Fe and other mineral elements (Ca, Mg, and Zn) in both leaves and roots.     

Growth, water use efficiency, and proline content of hydroponically grown tomato plants as affected by nitrogen source and nutrient concentration

Tomato plants (Lycopersicon esculentum Mill. cv. Counter) were grown in 12-L polyethylene containers in aerated and CaCO₃-buffered nutrient solutions containing different concentrations of complete stock solutions with either nitrate (stock solution N) or ammonium (stock solution A) as the only nitrogen source (X1 = standard concentration with 5 mM NO₃ ^–-N or NH₄ ⁺-N, and X3, X5.5, X8 and X11 = 3, 5.5, 8, 11 times the standard concentration), or a mixture of both stock solutions (N:A ratio = 100:0, 75:25, 50:50, 25:75, 0:100) at moderate nutrient concentration (X3). Total dry matter production and fruit dry weight were only slightly affected by increasing nutrient concentration if nitrate was supplied as the sole nitrogen source. Compared to nitrate, ammonium nitrogen caused a decrease in total dry weight (32–86% between X1 and X11), but led to an increase in both total dry weight and fruit dry weight (11% and 30%) at low concentration if supplied in addition to nitrate nitrogen (N:A ratio = 75:25). Dry matter partitioning in plants was affected by the strength of the nutrient solution, but even more by ammonium nitrogen. Fruits accumulated relatively less dry matter than did the vegetative parts of tomato plants when supplied with nutrient solutions containing ammonium as the only nitrogen source (fruit dry weight to total dry weight ratio 0.37 and 0.15 at low and high nutrient concentration), while nitrate nitrogen rather supported an increase in dry matter accumulation in the reproductive organs (fruit dry weight to total dry weight ratio 0.39–0.46). The water use efficiency (WUE) was only slightly affected by the strength of the nutrient solutions containing nitrate nitrogen (2.9–3.4 g DW (kg H₂O)^–1), while ammonium nitrogen led to a decrease in WUE from 2.4 to 1.3 g DW (kg H₂O)^–1at low (X1) and high (X11) nutrient concentration, respectively. The proline content of leaves fluctuated (0.1–5.0 mol (g fresh weight)^–1) according to nutrient concentration and global radiation, and reflected enhanced sensitivity of plants to these potential stress factors if ammonium was the predominant N source supplied. It was concluded, that proline is a reliable indicator of the environmental stress imposed on hydroponically grown tomato plants.     

Photosynthetic and leaf morphological characteristics in Leucaena leucocephala as affected by growth under different neutral shade levels

Morphological and physiological measurements on individual leaves of Leucaena leucocephala seedlings were used to study acclimation to neutral shading. The light-saturated photosynthetic rate (P_{n max}) ranged from 19.6 to 6.5 mol CO₂ m^–2 s^–1 as photosynthetic photon flux density (PPFD) during growth decreased from 27 to 1.6 mol m^–2 s^–1. Stomatal density varied from 144 mm^–2 in plants grown in high PPFD to 84 mm^–2 in plants grown in low PPFD. Average maximal stomatal conductance for H₂O was 1.1 in plants grown in high PPFD and 0.3 for plants grown in low PPFD. Plants grown in low PPFD had a greater total chlorophyll content than plants grown in high PPFD (7.2 vs 2.9 mg g^–1 on a unit fresh weight basis, and 4.3 vs 3.7 mg dm^–2 on a unit leaf area basis). Leaf area was largest when plants were grown under the intermediate PPFDs. Leaf density thickness was largest when plants were grown under the largest PPFDs. It is concluded that L. leucocephala shows extensive ability to acclimate to neutral shade, and could be considered a facultative shade plant.Abbreviations the initial slope of the photosynthesis vs PPFD curve - P_{n max} the light-saturated photosynthetic rate - PPFD photosynthetic photon flux density     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Fe chelates from compost microorganisms improve Fe nutrition of soybean and oat

Iron availability to plants is often limited when soil pH is 7 or higher. In C rich, but Fe limiting environments, microorganisms may produce organic chelators that complex Fe and increase its availability to plants. Seedlings of soybean (Glycine max L.) and oat (Avena sativa L.) plants, with Fe-efficient or inefficient uptake mechanisms, were grown in an Fe free nutrient solution at pH 7.5. Experiments (using a complete factorial design) were conducted in which these seedlings were transferred to a fresh nutrient solution and treated with Fe sources (FeCl₃, FeEDDHA, and Fe complexed with chelators produced by compost microorganisms (CCMs) after their enrichment in an Fe free, glucose medium), Fe concentrations (0 and 6.7 M) and antibiotic (0 and 100 mg streptomycin L^-1). Dry weight of soybean plants and Fe uptake were significantly (P 0.05) higher when Fe was supplied as ⁵⁹FeCCM than as⁵⁹ FeCl₃ and similar to when Fe was supplied as⁵⁹ FeEDDHA. Dry weight of the Fe-inefficient Tam 0-312 oat cultivar was also significantly higher when Fe was supplied as FeCCM. Fe uptake by oat, when supplied as ⁵⁹FeCCM, was twice that for⁵⁹ FeEDDHA and ⁵⁹FeCl₃. Chlorophyll concentration in plants supplied with FeCCM and FeEDDHA was significantly greater (P 0.05) than in minus Fe control plants and in FeCl₃ supplied plants. Activities of catalase and peroxidase, measured as indicators of Fe nutrition in soybean and oats, were generally increased when Fe was supplied with FeCCM as compared to the other Fe sources. The experimental conditions in which the CCMs were produced are similar to those in soil after amendment with manures or other readily available organic materials. These CCMs can bind with Fe, even under slightly alkaline conditions, and effectively improve Fe nutrition of soybean and oat.     

Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation: A Study with Barley,Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase

The rates of respiration in light and darkness, C _i/C _a and carbon isotope fractionation were investigated in glycine decarboxylase-deficient plants of barley, potato and Arabidopsis thaliana grown in climate chambers with controlled light intensity, temperature, humidity, irradiation and different CO₂ concentrations (360, 700 and 1400 µl l^–1) and compared to the wild-type plants. All photorespiration-impaired plants exhibited higher C _i/C _a and corresponding lower apparent water-use efficiencies, which were more expressed under high irradiance and elevated temperature. The mutants were depleted in ¹³C as compared to the wild-type plants, with a difference of up to 6 following growth in 360 µl l^–1 CO₂. We determined the carbon isotope content at different CO₂ concentrations to calculate the contribution of both C _i/C _a and photorespiration for ¹³C/¹²C fractionation. The direct effect of photorespiration was in the range of 0.7–1.0, from which we calculated the value of fractionation at the site of glycine decarboxylation as being 10–13, which is in agreement with the previously reported carbon isotope discrimination exerted by the glycine decarboxylase. Respiratory rates, particularly in the light, were increased in the glycine decarboxylase mutants. The necessity of the maintenance of a high CO₂ concentration near the site of carboxylation in chloroplasts in plants deficient in photorespiratory enzymes, requires an increased opening of the stomata with a corresponding decrease in water-use efficiency. It is concluded that photorespiration participates in the regulation of C _i/C _a and contributes to carbon isotope fractionation, both via effects on stomata and via discrimination of ¹³C in the glycine decarboxylase reaction.This revised version was published online in October 2005 with corrections to the Cover Date.     

Peat and solution chemistry responses to CaCO3 application in wetlands next to Woods Lake,New York

We studied the effect of a calcite (CaCO₃) treatment on peat and pore water chemistry in poor fen and conifer swamp wetlands next to Woods Lake and its tributaries to evaluate the role of wetlands in an Experimental Watershed Liming Study (EWLS). Peat was characteristically organic rich and nutrient poor, with exchangeable Ca concentrations of < 13 cmol_ckg^–1. We estimated that between 0.4 to 4 Mg (CaCO₃) ha^–1 fell directly on three study sites; however, one year after the treatment the increase in Ca concentration (0–8 cm depth) was equivalent to a (CaCO₃) dosage of 3 Mg ha^–1 with an additional 2–4 Mg ha^–1 of undissolved (CaCO₃) still present, suggesting the peat retained Ca supplied from uplands. Most aspects of peat chemistry including microbial respiration and SO₄ reduction did not respond to the treatment.Peat pore water (5 and 20 cm depths) had a mean pH of 4.82 before treatment with high concentrations of dissolved organic carbon (DOC mean of 790 mol C/l) and low Ca²⁺ concentration (mean of 32 mol/l). The (CaCO₃) treatment increased concentrations of Ca²⁺ to a mean of 87 mol/l and dissolved inorganic carbon (DIC) from 205 to a mean of 411 mol/l, whereas it decreased monomeric Al concentration from 19 to 10 mol/l. Otherwise, pore water chemistry showed little response to the treatment, at least within natural spatial and temporal variation of solute concentrations. The results suggest that liming watersheds with the relatively low (CaCO₃) dosage applied in this study can benefit acidic waters downstream by exporting more Ca and DIC and less monomeric Al, with otherwise little effect on the peat itself.     

Whole -root iron(III)-reductase activity throughout the life cycle of iron-grown Pisum sativum L. (Fabaceae): relevance to the iron nutrition of developing seeds

To understand the whole-plant processes which influence the Fe nutrition of developing seeds, we have characterized root Fe(III)-reductase activity and quantified whole-plant Fe balance throughout the complete 10-week (10-wk) life cycle of pea (Pisum sativum L., cv. Sparkle). Plants were grown hydroponically in complete nutrient solution with a continuous supply of chelated Fe; all side shoots were removed at first appearance to yield plants with one main shoot. Root Fe(III)-reductase activity was assayed with Fe(III)-EDTA. Flowering of the experimental plants began on wk 4 and continued until wk 6; seed growth and active seed import occurred during wks 5–10. Vegetative growth terminated at wk 6. Iron(III) reduction in whole-root systems was found to be dynamically modulated throughout the plant's life cycle, even though the plants were maintained on an Fe source. Iron(III)-reductase activity ranged from 1–3 mol Fe reduced · g ^–1 DW · h^–1 at early and late stages of the life cycle to 9.5 mol Fe reduced · ^g–1 DW · h^–1 at wk 6. Visual assays demonstrated that Fe(III)-reductase activity was localized to extensive regions of secondary and tertiary lateral roots during this peak activity. At midstages of growth (wks 6–7), root Fe(III)-reductase activity could be altered by changes in internal shoot Fe demand or external root Fe supply: removal of all pods or interruption of phloem transport from the reproductive portion of the shoot (to the roots) resulted in lowered root Fe(III)-reductase activity, while removal of Fe from the nutrient solution resulted in a stimulation of this activity. Total shoot Fe content increased throughout the 10-wk growth period, with Fe content in the non-seed tissues of the shoot declining by 50% of their maximal level and accounting for 35% of final seed Fe content. At maturity, total seed Fe represented 74% of total shoot Fe; total Fe in the roots (apoplasmic and symplasmic Fe combined) was minimal. These studies demonstrate that the root Fe(III)-reductase system responds to Fe status and/or Fe requirements of the shoot, apparently through shoot-to-root communication involving a phloem-mobile signal. During active seed-fill, enhanced root Fe(III)-reductase activity is necessary to generate sufficient Fe²⁺ for continued root Fe acquisition. This continuing Fe supply to the shoot is essential for the developing seeds to attain their Fe-content potential. Increased rates of root Fe(III) reduction would be necessary for seed Fe content to be enhanced in Pisum sativum.Abbreviations BPDS bathophenanthrolinedisulfonic acid - DAF days after flowering - DW dry weight - EDDHA N,N-ethylenebis[2-(2-hydroxyphenyl)-glycine] - wk week This project has been funded in part with federal funds from the U.S. Department of Agriculture, Agricultural Research Service under Cooperative Agreement number 58-6250-1-003. The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government. The author wishes to acknowledge S. Pezeshgi and K. Koch for their excellent technical assistance, L. Loddeke for editorial comments, and A. Gillum for assistance with the figures.     

Effects of supplied nitrogen form on growth and water uptake of French bean (Phaseolus vulgaris L.) plants

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO^– ₃) supply (NN), sole ammonium (NH⁺ ₄) supply (AA) and spatially separated supply of NO^– ₃ and NH⁺ ₄ (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO^– ₃ revealed a 170% higher DM than the root half exposed to NH⁺ ₄. N form affected stable carbon-isotope discrimination () of leaf tissue. In leaves of plants which were supplied with NH⁺ ₄ (AA; NA) was significantly more negative (–29.4, –29.6) than in NN treatment (–28.2). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative of root tissue under NH⁺ ₄ supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO^– ₃ was 104% higher than in those receiving NH⁺ ₄. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

Effects of salt stress on the growth,ion content,stomatal behaviour and photosynthetic capacity of a salt-sensitive species,Phaseolus vulgaris L.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0–150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl^- concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K⁺ concentration, although to a lesser degree. Increases in leaf Na⁺ concentration occurred only at the higher external NaCl concentrations (100 mM). Increases in leaf Cl^- were primarily balanced by increases in K⁺ and Na⁺. X-ray microanalysis of leaf cells from salinized plants showed that Cl^- concentration was high in both the cell vacuole and chloroplast-cytoplasm (250–300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO₂ concentration (C _i) was reduced by up to 30%. Salinization of plants was found to alter the ¹³C value of leaves of Phaseolus by up to 5 and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO₂ concentration. Salt stress also brought about a reduction in photosynthetic CO₂ fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C _i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl^- concentrations. The quantum yield for net CO₂ uptake was also reduced by salt stress.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - RuBP ribulose-1,5-bisphosphate - ¹³C ratio of ¹³C to ¹²C relative to standard limestone     

Interactions between nitrate uptake and N2 fixation in white clover

Summary White clover (Trifolium repens L.) plants grown in pots and supplied with the same concentration x days of¹⁵N labelled nitrate, but in contrasting patterns and doses had similar N concentrations but differed in the proportions devived from N₂ fixation and nitrate. N₂-fixation and nodule dry weight responded rapidly (2–3 days) to changes in nitrate availability. Plants exposed frequently to small doses of nitrate took up more nitrate (and hence relied less on N₂-fixation) and had greater dry weights and shoot: root ratios than those exposed to larger doses less often. In mixed ryegrass (Lolium perenne L.)/clover communities clover's ability to either successfully compete for nitrate or fix N₂ gave it consistently higher N concentrations than grass whether they were given high or low nitrate nutrient. This higher N concentration was accompanied by greater dry weights than grass in the low nitrate swards but not where high levels of nitrate were applied.     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

Carbon translocation to the rhizosphere of maize and wheat and influence on the turnover of native soil organic matter at different soil nitrogen levels

Wheat and maize were grown in a growth chamber with the atmospheric CO₂ continuously labelled with ¹⁴C to study the translocation of assimilated carbon to the rhizosphere. Two different N levels in soil were applied. In maize 26–34% of the net assimilated ¹⁴C was translocated below ground, while in wheat higher values (40–58%) were found. However, due to the much higher shoot production in maize the total amount of carbon translocated below ground was similar to that of wheat. At high N relatively more of the C that was translocated to the root, was released into the soil due to increased root respiration and/or root exudation and subsequent microbial utilization and respiration. The evolution rate of unlabelled CO₂ from the native soil organic matter decreased after about 25 days when wheat was grown at high N as compared to low N. This negative effect of high N in soil was not observed with maize.     

Iron toxicity and other chemical soil constraints to rice in highland swamps of Burundi

Iron toxicity is suspected to be a major nutritional disorder in rice cropping systems established on flooded organic soils that contain reductible iron. A pot trial was carried out to assess Fe toxicity to rice in flooded Burundi highland swamp soils with a wide range of organic carbon contents. Soil and leaf analyses were performed and total grain weight was determined. Clear Fe toxicity was diagnosed, based on leaf Fe content at panicle differentiation. Leaf Fe contents higher than 250 g g^–1 dry matter induced lower Mg (and probably Mn) uptake, and a 50% total grain weight reduction. These features were associated with exchangeable Fe equivalent fractions higher than 86%. Besides, several non-Fe toxic soils exhibited an Mg-Mn imbalance.     

Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert

The Central Asian Taklamakan desert is characterized by a hyperarid climate with less than 50 mm annual precipitation but a permanent shallow groundwater table. The perched groundwater (2–16 m) could present a reliable and constant source of nitrogen throughout the growing season and help overcome temporal nitrogen limitations that are common in arid environments. We investigated the importance of groundwater and nitrogen fixation in the nitrogen metabolism of desert plants by assessing the possible forms and availability of soil N and atmospheric N and the seasonal variation in concentration as well as isotopic composition of plant N. Water availability was experimentally modified in the desert foreland through simulated flooding to estimate the contribution of surface water and temporally increased soil moisture for nutrient uptake and plant–water relations. The natural vegetation of the Taklamakan desert is dominated by plants with high foliar nitrogen concentrations (2–3% DM) and leaf nitrate reductase activity (NRA) (0.2–1 mol NO₂^– g^–1 FW h^–1). There is little evidence that nitrogen is a limiting resource as all perennial plants exhibited fast rates of growth. The extremely dry soil conditions preclude all but minor contributions of soil N to total plant N so that groundwater is suggested as the dominant source of N with concentrations of 100 M NO₃^–. Flood irrigation had little beneficial effect on nitrogen metabolism and growth, further confirming the dependence on groundwater. Nitrogen fixation was determined by the ¹⁵N natural abundance method and was a significant component of the N-requirement of the legume Alhagi, the average contribution of biologically fixed nitrogen in Alhagi was 54.8%. But nitrogen fixing plants had little ecological advantage owing to the more or less constant supply of N available from groundwater. From our data we conclude that the perennial species investigated have adapted to the environmental conditions through development of root systems that access groundwater to satisfy demands for both water and nutrients. This is an ecologically favourable strategy since only groundwater is a predictable and stable resource.     

The effects of elevated atmospheric CO2 and soil P placement on cotton root deployment

Root proliferation into nutrient rich zones is an important mechanism in the exploitation of soil nutrients by plants. No studies have examined atmospheric CO₂ effects on cotton (Gossypium hirsutum L.) root distribution as affected by localized phosphorus (P). Cotton plants were grown in a Troup sand (loamy, thermic Grossarenic Kandiudults) using 17.2-l containers placed in open top field chambers (OTC) under ambient (360 mol mol^–1) or enriched (720 mol mol^–1) atmospheric CO₂ concentrations for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25% of the total soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was unresponsive to CO₂ enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil under elevated CO₂ conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both CO₂ levels. Under ambient CO₂, cotton plant response was positive (shoot mass, and total root mass and length) when soil P was confined to relatively small proportions of the total soil volume (6.25 and 12.5%). However, elevated CO₂ grown plants tended to respond to P regardless of its distribution.     

Long-term effectiveness of vivianite in reducing iron chlorosis in olive trees

Iron (Fe) chlorosis is common in olive (Olea europaea L.) trees growing on highly calcareous soils in Southern Spain, where generally causes reduction in yield, size and commercial value of the olives. The objective of this research was to study the effectiveness of synthetic vivianite (Fe₃(PO₄)₂H₂O) to reduce Fe chlorosis in olive. Experiments were established in three orchards with cultivars `Hojiblanco', `Manzanillo', and `Picual'. The design was a randomised block design with two or three treatments (control with no Fe fertiliser and vivianite at one or two rates). A vivianite suspension (0.05 kg dm<sup>–3</sup> water) was injected into the soil at 10–20 points around the tree at the depth of maximum root density (25–35 cm). The rates (and times of application) were 0.5 and 1 kg tree<sup>–1</sup> for `Hojiblanco' (March 1997), 1 kg tree^–1 for `Manzanillo' (March 1998), and 2 kg tree<sup>–1</sup> for `Picual' (March 1998). The leaf chlorophyll content index (CCI) was estimated on the youngest expanded leaves by means of a Minolta apparatus (SPAD units). The colour index of the olives was estimated by visual comparison with a scale ranging from 1 (pale yellow) to 8 (normal green). For the period studied (July 1997–November 1999), the CCI of fertilised trees was, in general, significantly higher than that of control trees, and so was the case with the olive colour index. Olive yield, measured in the experimental fields with `Hojiblanco' (in 1999) and `Manzanillo' (in 1998 and 1999), was higher for the fertilised than for the control trees but differences were only significant in 1999. These results suggest that vivianite is effective to reduce Fe chlorosis for more than two seasons. Such effectiveness is probably due to the poorly crystalline Fe(III) oxides (which are good sources of Fe to plants) that result from the slow oxidation and incongruent dissolution of vivianite.     

Dinitrogen fixation in the world's oceans

The surface water of themarine environment has traditionally beenviewed as a nitrogen (N) limited habitat, andthis has guided the development of conceptualbiogeochemical models focusing largely on thereservoir of nitrate as the critical source ofN to sustain primary productivity. However,selected groups of Bacteria, includingcyanobacteria, and Archaea canutilize dinitrogen (N₂) as an alternativeN source. In the marine environment, thesemicroorganisms can have profound effects on netcommunity production processes and can impactthe coupling of C-N-P cycles as well as the netoceanic sequestration of atmospheric carbondioxide. As one component of an integrated Nitrogen Transport and Transformations project, we have begun to re-assess ourunderstanding of (1) the biotic sources andrates of N₂ fixation in the world'soceans, (2) the major controls on rates ofoceanic N₂ fixation, (3) the significanceof this N₂ fixation for the global carboncycle and (4) the role of human activities inthe alteration of oceanic N₂ fixation. Preliminary results indicate that rates ofN₂ fixation, especially in subtropical andtropical open ocean habitats, have a major rolein the global marine N budget. Iron (Fe)bioavailability appears to be an importantcontrol and is, therefore, critical inextrapolation to global rates of N₂fixation. Anthropogenic perturbations mayalter N₂ fixation in coastal environmentsthrough habitat destruction and eutrophication,and open ocean N₂ fixation may be enhancedby warming and increased stratification of theupper water column. Global anthropogenic andclimatic changes may also affect N₂fixation rates, for example by altering dustinputs (i.e. Fe) or by expansion ofsubtropical boundaries. Some recent estimatesof global ocean N₂ fixation are in therange of 100–200 Tg N (1–2 × 10¹⁴ g N)yr^–1, but have large uncertainties. Theseestimates are nearly an order of magnitudegreater than historical, pre-1980 estimates,but approach modern estimates of oceanicdenitrification.