Nutrient Solution Experiments Research Articles

Antioxidant defense system in leaves of Indian mustard (Brassica juncea) and rape (Brassica napus) under cadmium stress

Plant species capable of hyper-accumulating heavy metals are of considerable interest for phytoremediation, and differ in their ability to accumulate metals from environment. Using two brassica species (Brassica juncea and Brassica napus), nutrient solution experiments were conducted to study variation in tolerance to cadmium (Cd) toxicity based on (1) lipid peroxidation and (2) changes in antioxidative defense system in leaves of both plants (i.e., superoxide dismutase (SOD EC 1.15.1.1), catalase (CAT EC 1.11.1.6), ascorbate peroxidase (APX EC 1.11.1.11), guaiacol peroxidase (GPX EC 1.11.1.7), glutathione reductase (GR EC 1.6.4.2), levels of phytochelatins (PCs), non-protein thiols (NP-SH), and glutathione. Plants were grown in nutrient solution under controlled environmental conditions, and subjected to increasing concentrations of Cd (0, 10, 25 and 50 μM) for 15 days. Results showed marked differences between both species. Brassica napus under Cd stress exhibited increased level of lipid peroxidation, as was evidenced by the increased malondialdehyde (MDA) content in leaves. However, in Brassicajuncea treated plants, MDA content remained unchanged. In Brassica napus, with the exception of GPX, activity levels of some antioxidant enzymes involved in detoxification of reactive oxygen species (ROS), including SOD, CAT, GR, and APX, decreased drastically at high Cd concentrations. By contrast, in leaves of Brassica juncea treated plants, there was either only slight or no change in the activities of the antioxidative enzymes. Analysis of the profile of anionic isoenzymes of GPX revealed qualitative changes occurring during Cd exposure for both species. Moreover, levels of NP-SH and PCs, monitored as metal detoxifying responses, were much increased in leaves of Brassica juncea by increasing Cd supply, but did not change in Brassica napus. These results indicate that Brassica juncea plants possess the greater potential for Cd accumulation and tolerance than Brassica napus.

Leaf senescence and N uptake parameters as selection traits for nitrogen efficiency of oilseed rape cultivars

The cultivation of N‐efficient oilseed rape cultivars could contribute to a reduction of the large N balance surpluses of this crop. To facilitate the breeding process of N‐efficient cultivars, the identification of secondary plant traits correlating with N efficiency is necessary. The objectives of this study were to investigate leaf senescence and N uptake parameters of oilseed rape cultivars and doubled haploid (DH) lines with contrasting N efficiency in a short‐term nutrient solution experiment and to relate these results to their performance in field experiments. In the nutrient solution experiment, genotypes differed in leaf senescence of fully expanded leaves and maximum N uptake rate per unit root length under low N supply. A high maximum N uptake rate seemed to have contributed to delayed leaf senescence by enhancing N accumulation in leaves. Also in the field experiments, genotypes differed in leaf senescence after flowering at limiting N supply. Additionally, the most N‐efficient DH line was able to adapt leaf photosynthetic capacity to the low‐light conditions in the canopy during flowering. N efficiency (grain yield at limiting N supply) was positively correlated with delayed leaf senescence both in nutrient solution and field experiments. It is concluded that important leaf and root traits of N‐efficient cultivars are expressed in short‐term nutrient solution experiments, which may facilitate the selection of N‐efficient cultivars.

Responses to Mg/Ca balance in an Iranian serpentine endemic plant, Cleome heratensis (Capparaceae) and a related non-serpentine species, C. foliolosa

A soil Ca/Mg quotient greater than unity is generally considered necessary for normal plant growth but some serpentine plants are adapted to much lower Ca/Mg quotients, resulting from a major cation imbalance in their substrata. In order to investigate the growth and tolerance responses of serpentine and non-serpentine species to varied Ca/Mg quotients, controlled nutrient solution experiments were performed using an a newly reported Iranian endemic serpentine plant, Cleome heratensis Bunge et Bien. Ex Boiss. and a related non-serpentine species Cleome foliolosa DC. and a Eurasian Ni-hyperaccumulating species Alyssum murale Waldst. and Kit. Seedlings were grown in modified Hoagland’s solutions with varying Ca and Mg concentrations (0.2–2.5 and 0.5–10 mM, respectively) in a fully factorial randomised block design. The yields of the two serpentine plants increased significantly as Mg concentrations in the nutrient solution were increased from 0.5 to 4 mM but decreased in the 10 mM Mg treatment. For C. foliolosa yields decreased significantly from 0.5 to 10 mM Mg, indicating the sensitivity of this non-serpentine plant, and the relative tolerance of the serpentine plants to extremely high levels of Mg. Shoot and root Mg and Ca concentrations in C. heratensis and A. murale were higher than those in C. foliolosa in the low and moderate Mg treatments, supporting the view that many serpentine plants have a relatively high requirement for Mg. Maximum Mg concentrations were found in the roots of C. heratensis. Yields of C. heratensis and A. murale did not change significantly as Ca levels in nutrient solution increased from 0.2 to 2.5 mM Ca, However the yield of C. foliolosa increased significantly from 0.2 to 1.5 mM Ca, indicating sensitivity in this non-serpentine plant and tolerance of the two serpentine plants to low levels of Ca correlated with tissue Ca concentrations, probably because of a greater ability for Ca uptake at low-Ca availability. Calcium deficiency in the low-Ca treatments could be a reason for reduced yield in the non-serpentine plants.

Effect of Cadmium and Iron on Rice (Oryza Sativa L.) Plant in Chelator-Buffered Nutrient Solution

ABSTRACT To better understand the mechanisms responsible for differences in uptake and distribution of cadmium (Cd), nutrient-solution experiments were conducted with different varieties of rice (Oryza sativa), ‘Khitish’ and ‘CNRH3’. The plants were grown in a complete nutrient solution with different levels of pCd (-log free Cd+2 activity) and pFe [-log free iron (Fe+2) activity]. The required concentrations of chelating agent and metals were determined using a computerized chemical equilibrium model such as Geochem-PC. Experimental treatments included a combination of four pCd activity levels (0, 7.9, 8.2, and 8.5) applied as Cd (NO3)2 4H2O, and two pFe activity levels (17.0 and 17.8) applied as FeCl3. The application of both Cd and Fe in solution culture significantly affected plant growth, yield, and Cd accumulation in plant tissue. In general, yield of rice was decreased by an increase in amount of solution Cd; however, yield response varied among the cultivars. At the 7.9 pCd level, yields of rice cultivars ‘Khitish’ and ‘CNRH3’ were reduced to 69% and 65%, respectively, compared with control plants. Root Cd concentrations ranged from 2.6 mg kg−1 (control plants) to 505.7 mg kg−1 and were directly related to solution Cd concentrations. In rice plants, Cd toxicity symptoms resembled Fe chlorosis. Differential tolerance of varieties to phytotoxicity was not readily visible, but a significant interaction of substrate Cd and variety was obtained from dry-matter yields. Significant interactions indicated that response of tissue Cd concentration, plant Cd uptake, and translocation of Cd to the aerial parts were dependent on variety as well as substrate Cd. Uptake of Cd by roots was significantly higher than by shoots. Higher Cd uptake by rice plants decreased the uptake of other beneficial metals. The effect of Cd and Fe on the rate of phytometallophore release was also studied in the nutrient solution. Among the rice genotypes, ‘Khitish’ was the most sensitive to Cd toxicity. In both genotypes, with the onset of visual Cd-toxicity symptoms, the release of phytometallophore (PM) was enhanced. Among the rice varieties, ‘Khitish’ had the highest rate of PM release. Treatments with the metal ions studied produced a decrease in chlorophyll and enzyme activity. A decrease in concentrations of chlorophyll pigments in the third leaf was observed due to the highest activity level of Cd (pCd 7.9). Activities of enzymes such as peroxidase (POD) and superoxide dismutase (SOD) are altered by toxic amounts of Cd. Changes in enzyme activities occurred at the lowest activity of Cd (pCd 8.5) in solution. Peroxidase activity increased in the third leaf. Results showed that in contrast with growth parameters, the measurements of enzyme activities may be included as early biomarkers in a plant bioassay to assess the phytotoxicity of Cd-contaminated solution on rice plants. Evidence that Cd uptake and translocation are genetically controlled warrants the selection of varieties that assimilate the least Cd and that translocate the least metal to the plant part to be used for human and animal consumption.

Antioxidant defense system and cadmium uptake in barley genotypes differing in cadmium tolerance

Using two barley ( Hordeum vulgare) cultivars (cvs. Tokak and Hamidiye) nutrient solution experiments were conducted in order to study the genotypic variation in tolerance to Cd toxicity based on (i) development of leaf symptoms, (ii) decreases in dry matter production, (iii) Cd concentration and (iv) changes in antioxidative defense system in leaves (i.e., superoxide dismutase, ascorbate peroxidase, glutathione reductase, catalase, ascorbic acid and non-protein SH-groups). Plants were grown in nutrient solution under controlled environmental conditions, and subjected to increasing concentrations of Cd (0, 15, 30, 60 and 120 μmol/L Cd) for different time periods. Of the barley cultivars Hamidiye was particularly sensitive to Cd as judged by the severity and earlier development of Cd toxicity symptoms on leaves. Within 48 h of Cd application Hamidiye rapidly developed severe leaf Cd toxicity symptoms whereas in Tokak the leaf symptoms appeared only slightly. Hamidiye also tended to show more decrease in growth caused by Cd supply when compared to Tokak. The differences in sensitivity to Cd between Tokak and Hamidiye were not related to Cd concentrations in roots and shoots or Cd accumulation per plant. With the exception of catalase, activities of the enzymes involved in detoxification of reactive oxygen species (ROS) were markedly enhanced in Hamidiye by increasing Cd supply. By contrast, in Tokak there was either only a slight increase or no change in the activities of the antioxidative enzymes. Similarly, levels of ascorbic acid and especially non-protein SH-groups were increased in Hamidiye by Cd supply, but not affected in Tokak. The results indicate the existence of a large genotypic variation between barley cultivars for Cd tolerance. The differential Cd tolerance found in the barley cultivars was not related to uptake or accumulation of Cd in plants, indicating importance of internal mechanisms in expression of differential Cd tolerance in barley. As a response to increasing Cd supply particular increases in antioxidative mechanisms in the Cd-sensitive barley cultivar Hamidiye suggest that the high Cd sensitivity of Hamidiye is related to enhanced production and oxidative damage of ROS.

Dynamics of zinc uptake and accumulation in the hyperaccumulating and non-hyperaccumulating ecotypes of Sedum alfredii Hance

Sedum alfredii Hance has been identified as a Zn-hyperaccumulating plant species native to China. The characteristics of Zn uptake and accumulation in the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii were investigated under nutrient solution and soil culture conditions. The growth of HE was normal up to 1000 μM Zn in nutrient solution, and 1600 mg Zn kg−1 soil in a Zn-amended soil. Growth of the NHE was inhibited at Zn levels ≥250 μM in nutrient solution. Zinc concentrations in the leaves and stems increased with increasing Zn supply levels, peaking at 500 and 250 μM Zn in nutrient solution for the HE and the NHE, respectively, and then gradually decreased or leveled off with further increase in solution Zn. Minimal increases in root Zn were noted at Zn levels up to 50 μM; root Zn sharply increased at higher Zn supply. The maximum Zn concentration in the shoots of the HE reached 20,000 and 29,000 mg kg−1 in the nutrient solution and soil experiments, respectively, approximately 20 times greater than those of the NHE. Root Zn concentrations were higher in the NHE than in the HE when plants were grown at Zn levels ≥50 μM. The time-course of Zn uptake and accumulation exhibited a hyperbolic saturation curve: a rapid linear increase during the first 6 days in the long-term and 60 min in the short-term studies; followed by a slower increase or leveling off with time. More than 80% of Zn accumulated in the shoots of the HE at half time (day 16) of the long-term uptake in 500 μM Zn, and also at half time (120 min) of the short-term uptake in 10 μM 65Zn2+. These results indicate that Zn uptake and accumulation in the shoots of S. alfredii exhibited a down-regulation by internal Zn accumulated in roots or leaves under both nutrient solution and soil conditions. An altered Zn transport system and increased metal sequestration capacity in the shoot tissues, especially in the stems, may be the factors that allow increased Zn accumulation in the hyperaccumulating ecotype of S. alfredii.

Phosphorus and aluminum interactions in soybean in relation to aluminum tolerance. Exudation of specific organic acids from different regions of the intact root system.

Aluminum (Al) toxicity and phosphorus (P) deficiency often coexist in acid soils that severely limit crop growth and production, including soybean (Glycine max). Understanding the physiological mechanisms relating to plant Al and P interactions should help facilitate the development of more Al-tolerant and/or P-efficient crops. In this study, both homogeneous and heterogeneous nutrient solution experiments were conducted to study the effects of Al and P interactions on soybean root growth and root organic acid exudation. In the homogenous solution experiments with a uniform Al and P distribution in the bulk solution, P addition significantly increased Al tolerance in four soybean genotypes differing in P efficiency. The two P-efficient genotypes appeared to be more Al tolerant than the two P-inefficient genotypes under these high-P conditions. Analysis of root exudates indicated Al toxicity induced citrate exudation, P deficiency triggered oxalate exudation, and malate release was induced by both treatments. To more closely mimic low-P acid soils where P deficiency and Al toxicity are often much greater in the lower soil horizons, a divided root chamber/nutrient solution approach was employed to impose elevated P conditions in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. Under these conditions, we found that the two P-efficient genotypes were more Al tolerant during the early stages of the experiment than the P-inefficient lines. Although the same three organic acids were exuded by roots in the divided chamber experiments, their exudation patterns were different from those in the homogeneous solution system. The two P-efficient genotypes secreted more malate from the taproot tip, suggesting that improved P nutrition may enhance exudation of organic acids in the root regions dealing with the greatest Al toxicity, thus enhancing Al tolerance. These findings demonstrate that P efficiency may play a role in Al tolerance in soybean. Phosphorus-efficient genotypes may be able to enhance Al tolerance not only through direct Al-P interactions but also through indirect interactions associated with stimulated exudation of different Al-chelating organic acids in specific roots and root regions.

Toxic Effect of Cadmium on Rice as Affected by Nitrogen Fertilizer Form

A nutrient solution experiment was conducted to determine the influence of N forms on growth, oxidative stress, and Cd and N uptake in rice plants. The treatments were consisted of two Cd levels (0 and 1 μmol) and three N forms (NH4)2SO4, NH4NO3 and Ca(NO3)2. The results indicated that without Cd addition in the culture solution, the N forms had no significant effect on all measured parameters, including plant growth, photosynthetic traits, malondialdehyde (MDA) concentration, superoxide dismutase (SOD) activity, and Cd and N concentration, while Cd addition in the medium resulted in significant differences in measured parameters among the three forms of N fertilizers. The least inhibition of growth was noted in (NH4)2SO4-fed plants, and the largest in Ca(NO3)2-fed plants, when plants were exposed to Cd stress. The highest photosynthetic rate and chlorophyll content was also recorded in (NH4)2SO4-fed plants. Addition of Cd caused a remarkable increase in SOD activity and MDA content in plants, and the extent of increase varied with N form, with (NH4)2SO4-fed plants being smallest. In comparison with the control plants, the N concentration in roots and shoots was not significantly affected in (NH4)2SO4-fed plants, but significant decrease in root N concentration was found for the NH4NO3 and Ca(NO3)2-fed plants under Cd stress. Moreover, the significant differences were also noted among the three N forms in both root and shoot Cd concentrations, with (NH4)2SO4-fed plants being the lowest. The results indicated that the toxic effect of Cd on rice varied with the form of N fertilizer.

The influence of EDDS on the uptake of heavy metals in hydroponically grown sunflowers

Phytoextraction is an environmentally friendly in situ technique for cleaning up metal contaminated land. Unfortunately, efficient metal uptake by remediation plants is often limited by low phytoavailability of the targeted metals. Chelant assisted phytoextraction has been proposed to improve the efficiency of phytoextraction. Phytoremediation involves several subsequent steps: transfer of metals from the bulk soil to the root surfaces, uptake into the roots and translocation to the shoots. Nutrient solution experiments address the latter two steps. In this context we investigated the influence of the biodegradable chelating agent SS-EDDS on uptake of essential (Cu and Zn) and non-essential (Pb) metals by sunflowers from nutrient solution. EDDS was detected in shoots and xylem sap for the first time, proving that it is taken up into the above ground biomass of plants. The essential metals Cu and Zn were decreased in shoots in the presence of EDDS whereas uptake of the non-essential Pb was enhanced. We suggest that in the presence of EDDS all three metals were taken up by the non-selective apoplastic pathway as the EDDS complexes, whereas in the absence of EDDS essential metal uptake was primarily selective along the symplastic pathway. This shows that synthetic chelating agents do not necessarily increase uptake of heavy metals, when soluble concentrations are equal in the presence and absence of chelates.

Early Developmental Responses of White Clover Roothair Lengths to Calcium, Protons, and Aluminum in Solution and Soil Cultures

Acidic soils tend to limit the nodulation of forage legumes and roothairs are important for Rhizobia binding and the initiation of nodule formation. This study examined the effects of Ca, pH, and Al on the length of roothairs in nutrient solutions and soils with white clover (Trifolium repens L., cultivar Huia) seedlings. The lengths of roothairs of 1‐ to 2‐d‐old seedlings was not affected by increases in solution Al up to 24 μM in nutrient solution experiments, but showed a pronounced optimum at a predicted root surface pH of 3.8. Roothair lengths of white clover seedlings 2 to 10 d after planting were also assessed in a Gilpin series silt loam collected from New, WV, and amended with various levels of lime. Soils ranged in pH from 4.6 to 5.3 and soil base saturation of Al from 46 to 8%. No significant differences in roothair lengths as a function of soil pH and/or soil available Al were observed. Previous results conducted under similar experimental conditions demonstrated that soil pH–soil‐available Al adversely affected on root elongation and nodulation. When results from this and the previous reports are considered together, the combined results suggest that root elongation and nodulation were more sensitive than roothair length to acidic soil conditions.

Early vigorous growth is a major factor influencing nitrogen uptake in wheat.

A field trial, a lysimeter system study and a nutrient solution experiment were conducted to determine the genotypic differences in nitrogen (N) uptake among wheat (Triticum aestivum L.) genotypes differing in vigour of early growth. Plant growth and N uptake of Vigour 18, a breeding line with early vigour, and the commercial cultivars Westonia, Tincurrin, Camm and Janz were compared. Shoot biomass of Vigour 18 was higher than that of the other genotypes, except for Westonia at booting when 50 kg N ha-1 was applied 3 d after wheat emergence. Vigour 18 had significantly higher efficiency of fertiliser-N uptake than the other four cultivars at tillering when 50kg N ha-1 was applied. Fertiliser-N uptake efficiency at booting was similar in Vigour 18 and Westonia, but significantly higher than in three other commercial cultivars. Vigour 18 had higher root dry matter, root-length density and root surface area than Janz when examined in columns of soil. The greater root growth of Vigour 18 occurred across all soil layers to a depth of 0.6 m. Differences in total N uptake between Vigour 18 and Janz were apparent from tillering (Z14,22) to booting (Z19,24,49). Vigour 18 also had significantly higher shoot biomass and N uptake than Janz when grown in nutrient culture. Nitrate reductase activity (NRA) expressed on a whole-plant basis was higher for Vigour 18 than for Janz, and was related to total N uptake. However, NRA expressed on a per-unit-fresh-weight basis was not significantly different across the cultivars tested. It is concluded that vigorous early root and shoot growth in Vigour 18 was the main driving force for higher N uptake.

Does high bicarbonate supply to roots change availability of iron in the leaf apoplast

The role of the leaf apoplast in iron (Fe) uptake into the leaf symplast is insufficiently understood, particularly in relation to the supposed inactivation of Fe in leaves caused by elevated bicarbonate in calcareous soils. It has been supposed that high bicarbonate supply to roots increases the pH of the leaf apoplast which decreases the physiological availability of Fe in leaf tissues. The study reported here has been carried out with sunflower plants grown in nutrient solution and with grapevine plants grown on calcareous soil under field conditions. The data obtained clearly show that the pH of the leaf apoplastic fluid was not affected by high bicarbonate supply in the root medium (nutrient solution and field experiments). The concentrations of total, symplastic and apoplastic Fe were decreased in chlorotic leaves of both sunflower (nutrient solution experiment) and grapevine plants in which leaf expansion was slightly inhibited (field experiment). However, in grapevine showing severe inhibition of leaf growth, total Fe concentration in chlorotic leaves was the same or even higher than in green ones, indicative to the so-called `chlorosis paradox'. The findings do not support the hypothesis of Fe inactivation in the leaf apoplast as the cause of Fe deficiency chlorosis since no increase was found in the relative amount of apoplastic Fe (% of total leaf Fe) either in the leaves of sunflower or grapevine plants. It is concluded that high bicarbonate concentration in the soil solution does not decrease Fe availability in the leaf apoplast.

QTL × N-level interaction for plant height in rice (Oriza Sativa L.)

Quantitative trait loci (QTLs) for plant height in rice were mapped onto the molecular marker linkage map of a double-haploid (DH) population derived from a cross between IR64 and Azucena under low and high N levels, in both nutrient solution and soil culture experiments. Two QTLs, one on chromosome 1 and the other on chromosome 8, were detected at high N levels in soil and/or nutrient solution culture experiments. A total of 8 QTLs were identified at low N level in the soil and/or nutrient solution culture experiments, which located on chromosomes 1, 2, 3, 4, 5 and 6, respectively. The QTL flanked by the molecular markers RZ730 ∼ RZ801 on chromosome 1 was identified in all experimental conditions. This position corresponds to sd-1, a semi-dwarfing gene. The QTLs on chromosomes 2, 3, 4, 5 and 6 were only detected at low N level and the QTL on chromosome 8 was only detected at high N level in the nutrient solution culture experiment. Based on the differing responses to low N stress between two parents and the QTL×N-level interaction observed in this study, it is suggested that the expression of several QTLs associated with plant height could be induced by low N stress.

The root growth response to heterogeneous nitrate supply differs for Lupinus angustifolius and Lupinus pilosus

Little is known about the ability of legume root systems to respond to the heterogeneous supply of nitrate. A split-root nutrient solution experiment was set up to compare the root growth response of 2 lupin species, Lupinus angustifolius L. (dominant tap root and primary lateral system) and L. pilosus Murr. (minor tap root and well-developed lateral root system), to differentially supplied nitrate. These 2 species represent the extremes of the root morphology types present across the lupin germplasm. Nutrient solution containing low (250 M) or high (750 M) nitrate was supplied either uniformly, or split (high and low) between the upper and lower root system. The average growth rate and total root length of L. pilosus was 1.7 times that of L. angustifolius. For both species, the increased proliferation of roots in a high nitrate zone was accompanied by a decrease in root growth in the low nitrate zone, giving approximately the same total growth as the uniform low nitrate treatment. This correlative growth rate response was 15% larger for the first-order branches of L. pilosus than L. angustifolius. While few second-order branches grew for L. angustifolius, the second-order laterals of L. pilosus showed a 2-fold correlative root growth and branching response to the split treatments, with no difference in growth between the uniform high and low nitrate treatments. The second-order laterals thus proliferated in response to the differential supply of nitrate and not the absolute concentration. While the growth rate and branching of the second-order laterals of L. pilosus exhibited a typical correlative response, first-order branching was inhibited in all split treatments, regardless of whether the roots were in the high or low nitrate zone. This response was not seen in L. angustifolius. The difference in the root growth response of the 2 root system types to differentially supplied nitrate suggests a potential in the lupin germplasm for developing a line capable of greater nitrate capture from the soil profile.

Aluminium: The Need for a Re-Evaluation of Its Toxicity and Solubility in Mature Forest Stands

Aluminium (Al) is a key element in critical loadcalculations for forest. Here, we argue for re-evaluating theimportance of Al. Effects of two levels of enhanced Alconcentrations and lowered Ca:Al ratios in the soil solutionin a field manipulation experiment in a mature spruce stand(1996–1999) on tree vitality parameters were tested. Inaddition, Al solubility controls were tested. Various loads ofAl were added to forest plots by means of an irrigationsystem. Potentially toxic Al concentrations and criticalratios of Ca to inorganic Al were established. The ratio of Cato total Al was not a suitable indicator for unfavourableconditions for plant growth. No significant effects on crowncondition, tree growth and fine root production were observedafter three years of treatment. In 1999, foliar Mg content inthe highest Al addition treatment had declined significantly.This agreed with the known response to Al stress of seedlingsin nutrient solution experiments. No support was found forusing the chemical criterion Ca:Al ratio in soil solution,foliar and root tissue as an indicator for forest damage dueto acidification. Al solubility was considerably lower thanimplied by the assumption of equilibrium with gibbsite,particularly in the root zone. The gibbsite equilibrium iscommonly used in critical load models. Substitution of thegibbsite equilibrium with an Al-organic matter complexationmodel to describe Al solubility in soil water may have largeconsequences for calculation of critical loads. The resultsindicate that critical load maps for forests should bereconsidered.

Lupinus angustifolius has a plastic uptake response to heterogeneously supplied nitrate while Lupinus pilosus does not

Uptake rates calculated from plants uniformly supplied with a nutrient will underestimate uptake under heterogeneous conditions. A split-root nutrient solution experiment was set up to compare the uptake rate of 2 lupin species (Lupinus angustifolius L., L. pilosus Murr.) under conditions of uniform and heterogeneous nitrate supply. Nitrate was supplied uniformly to the root system at 250 M (low), 750 M (high), or 1500 M (high), or in a split low/high or high/low combination between the upper and lower root system. While L. pilosus had a greater total nitrate uptake over the treatment period due to a higher total root length, L. angustifolius had 1.5–2.5 times greater nitrate uptake rate per unit of root length. L. angustifolius also had the capacity to increase the nitrate uptake rate in sections of the root system supplied locally with high nitrate, compared with a root system uniformly supplied with high nitrate. This increased uptake rate under heterogeneous supply enabled the plant to take up 74–94% of the total nitrate taken up when uniformly supplied with high nitrate, while only 58–72% would have been taken up without such a compensation mechanism. L. pilosus did not show this response. The difference between the response of these 2 species suggests that a range of nitrate uptake responses may exist across the lupin germplasm, and that it may be possible to select a lupin species with an enhanced ability to capture nitrate from the profile, thus decreasing nitrate losses from leaching.

Specific Fe2+ uptake system in strategy I plants inducible under Fe deficiency

The aim of this study was to examine more in detail the uptake of Fe2+ in non‐graminaceous (Strategy I plants) and graminaceous species (Strategy II plants). FeSO4 (2 μM) labelled with 59Fe was supplied in short‐term nutrient solution experiments (30 min, pH 5.0) to cucumber (Cucumis sativus L.), tomato (Lycopersicon esculentum Mill.), barley (Hordeum vulgare L.) and maize (Zea mays L.). A mutant of tomato (Tfer, defective in Fe deficiency inducible reductase) was also examined. Plant Fe nutritional status was modified by different Fe preculture (Fe adequate and Fe deficient, respectively). The Fe2+ uptake rates varied within a narrow range (0.25–0.40 μmol 59Fe g−1 root dry wt 30 min−1) in both, Strategy I and Strategy II plants adequately supplied with Fe. Under Fe deficiency, the Fe2+ uptake rates were markedly increased (4–10‐fold) in the Strategy I plants (tomato and cucumber). In contrast, in the graminaceous species (barley and maize) as well as in the Fe inefficient tomato mutant (Tfer), this increase was significantly less pronounced (1.3–1.6‐fold). Our results indicated that a Fe deficiency induced transport system for Fe2+ is operating in Strategy I plants. In the graminaceous species this inducible system is absent or is less expressed. The kinetic studies on Fe2+ uptake in cucumber plants gave indications for two saturable uptake systems that were activated under Fe deficiency, a high‐affinity system with a Km of 4.7±1.6 μM and Vmax of 7.5±1.9 μmol 59Fe g root dry−1 wt 30 min−1, and a low‐affinity system that operated at Fe2+ concentrations higher than 50 μM. Studies with cucumber plants on the specificity of the Fe deficiency activated high‐affinity Fe2+ transport system showed that a 10‐ and 100‐fold excess of Zn resulted in 42–83% inhibition of Fe2+ uptake, suggesting that Zn could also be transported by the Fe2+ transporter, although less efficiently. A lower inhibition of Fe2+ uptake was found in the control cucumber plants as well as in the control and Fe deficient barley plants at both Zn concentrations. Cadmium inhibited by 90% the Fe2+ uptake in Fe deficient cucumber plants that may be partly related to its toxicity. Fe2+ transport was markedly less affected (by 12%) in Fe deficient barley plants even at 100‐fold excess of Cd. In conclusion, it appears that Fe2+ is transported by an uptake system with distinct preference for Fe2+ (Fe2+ transporter) rather than by a general transport system for divalent cations.

The chlorosis paradox: Fe inactivation as a secondary event in chlorotic leaves of grapevine

Iron chlorosis is a wide‐spread disorder of plants, in particular of those on calcareous soils. Elevated bicarbonate concentrations in the soil solution are considered as a main cause of this chlorosis. In nutrient solution culture experiments a supply of bicarbonate results in inhibited Fe acquisition and a subsequent decreased concentration of Fe in the leaf dry matter. This is indicated by a close positive relationship between chlorophyll and total Fe concentration in the upper leaves. In contrast to nutrient solution experiments, in some pot experiments particularly with calcareous soil, and in field experiments under certain conditions, no such close correlation can be observed and a higher Fe concentration can even be found in young chlorotic leaves than in green leaves. This phenomenon is called “the chlorosis paradox” and it has thus been concluded that, Fe chlorosis might be caused by an Fe inactivation in the plant, in particular in the leaf apoplast, e.g. by an alkalinization process. Reconsideration of published data on this phenomenon of enhanced Fe concentrations in chlorotic leaves, coupled with investigations of grapevine grown on calcareous soil reveal that “the chlorosis paradox” can only be observed in soil culture where severe shoot growth inhibition is already present at an early stage; presumably as a consequence of changes in phytohormone metabolism in connection with inhibited root growth. Obviously, the higher Fe concentration in chlorotic leaves with inhibited expansion growth is a consequence of the diminished dilution of normal high Fe concentrations in young leaves. This high concentration in chlorotic leaves can be observed in spite of a distinct lower Fe content in the individual leaves. The declined concentration of HCl‐extractable Fe in chlorotic leaves is presumably not the cause, but rather the consequence, of Fe chlorosis.

Alumínio e a absorção de cálcio por mudas de pimenta do reino

Um experimento com solução nutritiva foi conduzido em casa de vegetação com o objetivo de verificar os efeitos da nutrição de cálcio na toxicidade de alumínio em mudas de pimenta do reino. Os tratamentos constituíram-se de três concentrações de cálcio (10; 40 e 160 mg L-1) adicionados como CaCl2 em combinação com três concentrações de alumínio (0; 10 e 20 mg L-1) adicionados como AlCl3.6H2O. A solução nutritiva continha as seguintes concentrações dos íons expressos em mg L-1: N-NO3 (80,5); N-NH4 (10,5); P(1,5); K(39,0); Mg(24,0); S-SO4(32,0); B(0,5); Cu(0,06); Fe(5,0); Mn(1,0); Mo(0,03) e Zn(0,10). Os tratamentos com alumínio retardaram o crescimento das mudas de pimenta do reino, induziram sintomas de toxicidade de alumínio na parte aérea e principalmente aumentaram o diâmetro das raízes. O acúmulo de cálcio aumentou com o incremento da concentração de cálcio na solução na ausência de alumínio, nas raízes o conteúdo de cálcio foi diminuído com o aumento da concentração de alumínio na solução. A absorção de alumínio diminuiu com o aumento da concentração de cálcio na solução.

Influence of varied zinc supply on re-translocation of cadmium (109Cd) and rubidium (86Rb) applied on mature leaf of durum wheat seedlings

Effect of varied zinc (Zn) supply (0, 0.1, 1, 5 μM) on re-translocation of radio-labeled cadmium (109Cd) and rubidium (86Rb) from mature leaf to root and other parts of shoot was studied in 11-day-old durum wheat (Triticum durum cv. C-1252) plants grown in nutrient solution under controlled environmental conditions. Application of 109Cd and 86Rb was carried out by immersing the tips (3 cm) of mature leaf in radio-labeled solutions for 10 s at three different times over a 42 h period. Differences in Zn supply for 11 days did not affect plant growth nor did it cause visual leaf symptoms, such as necrosis and chlorosis, at either the lowest or the highest Zn supply. Only at the nil Zn supply (0 μM), shoot and root dry weights tended to decrease and increase, respectively, causing a lower shoot/root dry weight ratio. Partitioning of more dry matter to roots rather than shoots, a typical phenomena for Zn-deficient plants in nutrient solution experiments, indicated existence of a mild Zn deficiency stress at the nil-Zn treatment. Irrespective of Zn supply, plants could, on average, retranslocate 3.8% and 38% of the total absorbed 109Cd and 86Rb from the treated leaf to roots and other parts of shoots within 42 h, respectively. At nil-Zn treatment, 2.8% of the total absorbed 109Cd was re-translocated from the treated leaf, particularly into roots. The highest re-translocation of 109Cd (6.5%) was found in plants supplied with 0.1 μM Zn. Increases in Zn supply from 0.1 μM reduced 109Cd re-translocation from 6.5% to 4.3% at 1 μM Zn and 1.3% at 5 μM Zn. With the exception of the nil-Zn treatment, the proportion of re-translocated 109Cd was greater in the remainder of the shoot than in the roots. Contrary to the 109Cd results, re-translocation of 86Rb was not (at 0, 0.1 and 1 μM Zn), or only slightly (at 5 μM), affected by changing Zn supply. The results indicate an inhibitory action of increased concentrations of Zn in shoot tissues on phloem-mediated Cd transport. This effect is discussed in relation to competitive inhibition of Cd loading into phloem sap by Zn.