Beneficial effects of silicon on hydroponically grown corn salad (Valerianella locusta (L.) Laterr) plants.

Soil-less cultivation of horticultural crops represents a fairly recent innovation to traditional agriculture which has several advantages including higher water-use efficiency. When plants are grown with this system, their roots come in contact with nutrients solely via the hydroponic solution. Although its beneficial effects have been widely demonstrated, silicon (Si) is mostly omitted from the composition of nutrient solutions. Therefore, the objective of this study was to assess the beneficial effect of Si addition to hydroponic solution on quali-quantitative aspects of edible production of two cultivars of corn salad (Valerianella locusta (L.) Laterr.) grown in soil-less floating system. Impacts on shelf life of this food were also studied. Results show that the supply of Si increased the edible yield and the quality level reducing the nitrate concentration in edible tissues. This result might be attributed to changes either in the metabolism (such as the nitrate assimilation process) or to the functionality of root mechanisms involved in the nutrient acquisition from the outer medium. In fact, our results show for the first time the ability of Si to modulate the root activity of nitrate and Fe uptake through, at least in part, a regulation of gene expression levels of the proteins involved in this phenomenon. In addition, the presence of Si decreased the levels of polyphenoloxidase gene expression at harvest and, in post-harvest, slowed down the chlorophyll degradation delaying leaf senescence and thus prolonging the shelf life of these edible tissues. In conclusion, data showed that the addition of Si to the nutrient solution can be a useful tool for improving quali-quantitatively the yield of baby leaf vegetable corn salad as well as its shelf life. Since the amelioration due to the Si has been achieved only with one cultivar, the recommendation of its inclusion in the nutrient solution does not exclude the identification of cultivars suitable for this cultivation system and the comprehension of agronomical and environmental factors which could limit the Si benefits.

Silicon ameliorates manganese toxicity in cucumber by decreasing hydroxyl radical accumulation in the leaf apoplast.

This work was focused on the role of silicon (Si) in amelioration of manganese (Mn) toxicity caused by elevated production of hydroxyl radicals (·OH) in the leaf apoplast of cucumber (Cucumis sativus L.). The plants were grown in nutrient solutions with adequate (0.5 µM) or excessive (100 µM) Mn concentrations with or without Si being supplied. The symptoms of Mn toxicity were absent in the leaves of Si-treated plants subjected to excess Mn, although the leaf Mn concentration remained extremely high. The apoplastic concentration of free Mn(2+) and H(2)O(2) of high Mn-treated plants was significantly decreased by Si treatment. Si supply suppressed the Mn-induced increased abundance of peroxidase (POD) isoforms in the leaf apoplastic fluid, and led to a rapid suppression of guaiacol-POD activity under excess Mn. The spin-trapping reagent 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide was used to detect ·OH by electron paramagnetic resonance spectroscopy. Although supplying Si markedly decreased the accumulation of ·OH in the leaf apoplast with excess Mn, adding monosilicic acid to the Mn(2+)/H(2)O(2) reaction mixture did not directly affect the Fenton reaction in vitro. The results indicate that Si contributes indirectly to a decrease in ·OH in the leaf apoplast by decreasing the free apoplastic Mn(2+), thus regulating the Fenton reaction. A direct inhibitory effect of Si on guaiacol-POD activity (demonstrated in vitro) may also contribute to decreasing the POD-mediated generation of ·OH.

Is iron phloem mobile during senescence in trees? A reinvestigation of Rissmüller's finding of 1874.

As long as 130 years ago Rissmüller reported substantial retranslocation of iron (Fe) from beech leaves (Fagus sylvatica L.) shortly before leaf fall. This rather limited report on Fe retranslocation via the phloem in plants was the reason for this research to study changes in Fe content in individual beech leaves in more detail during the vegetative period. Besides Fe, other micronutrients and particularly Ca and K, well known to differ substantially in phloem mobility, were analysed as mineral nutrient markers. In addition to beech, other deciduous and evergreen species of Angiosperms and Gymnosperms were also studied. As expected, there was no evidence of Ca retranslocation from senescent leaves, while K as a phloem mobile mineral nutrient was retranslocated in fall in deciduous but not in evergreen trees. There was no indication to support Rissmüller's finding of Fe retranslocation in any of the different species studied. From these results, we conclude that natural leaf senescence of trees during late season does not induce retranslocation of Fe and other micronutrients. Possible reasons for the absence of a distinct retranslocation of Fe in the species studied during late season senescence are the lack of a sink activity, as for example the development of seeds in annual plant species (e.g., cereals), or the presence of a root system still active enough to provide Fe and other mineral nutrients for plant demand, and both factors have to be considered in further studies. Reviewing the data in the literature on Fe and Zn retranslocation during senescence, we conclude that in principle both micronutrients are potentially phloem mobile. However, various prerequisites are needed for the occurrence of phloem mobility which were absent in the plant species studied. Regardless of this conclusion, we recommend that in general early published research data need a critical re-evaluation.

Integrated soil-crop system management for food security.

China and other rapidly developing economies face the dual challenge of substantially increasing yields of cereal grains while at the same time reducing the very substantial environmental impacts of intensive agriculture. We used a model-driven integrated soil-crop system management approach to develop a maize production system that achieved mean maize yields of 13.0 t ha(-1) on 66 on-farm experimental plots--nearly twice the yield of current farmers' practices--with no increase in N fertilizer use. Such integrated soil-crop system management systems represent a priority for agricultural research and implementation, especially in rapidly growing economies.

Effect of nitrogen on root release of phytosiderophores and root uptake of Fe(III)-phytosiderophore in Fe-deficient wheat plants.

Root release of phytosiderophores (PSs) is an important step in iron (Fe) acquisition of grasses, and this adaptive reaction of plants is affected by various plant and environmental factors. The objectives of this study were to study the effects of varied nitrogen (N) supply on (1) root and leaf concentrations of methionine, a precursor in the PS biosynthesis, (2) PS release from roots, (3) mobilization and uptake of Fe from (59) Fe-labeled Fe(III)-hydroxide [(59) Fe(OH)(3) ] and (4) root uptake of (59) Fe-labeled Fe(III)-deoxymugineic acid (DMA) by durum wheat (Triticum durum, cv. Balcali2000) plants grown in a nutrient solution. Enhanced N supply from 0.5 to 6 mM in a nutrient solution significantly increased the root release of PS under Fe deficiency. High N supply was also highly effective in increasing mobilization and root uptake of Fe from (59) Fe-hydroxide under low Fe supply. With adequate Fe, N nutrition did not affect mobilization and uptake of Fe from (59) Fe(OH)(3) . Root uptake and shoot translocation of Fe supplied as (59) Fe(III)-DMA were also stimulated by increasing N supply. Leaf concentration of methionine was reduced by low Fe supply, and this decline was pronounced in high N plants. The results show that the root release of PS, mobilization of Fe from (59) Fe(OH)(3) and root uptake and shoot translocation of Fe(III)-PS by durum wheat are markedly affected by N nutritional status of plants. These positive N effects may have important implications for Fe nutrition of human populations and should be considered in biofortification of food crops with Fe.

Processes and factors controlling N2O production in an intensively managed low carbon calcareous soil under sub-humid monsoon conditions.

An automated system for continuous measurement of N2O fluxes on an hourly basis was employed to study N2O emissions in an intensively managed low carbon calcareous soil under sub-humid temperate monsoon conditions. N2O emissions occurred mainly within two weeks of application of NH4(+) based fertilizer and total N2O emissions in wheat (average 0.35 or 0.21 kg N ha⁻¹ season⁻¹) and maize (average 1.47 or 0.49 kg N ha⁻¹ season⁻¹) under conventional and optimum N fertilization (300 and 50-122 kg N ha⁻¹, respectively) were lower than previously reported from low frequency measurements. Results from closed static chamber showed that N2O was produced mainly from nitrification of NH4(+)-based fertilizer, with little denitrification occurring due to limited readily oxidizable carbon and low soil moisture despite consistently high soil nitrate-N concentrations. Significant reductions in N2O emissions can be achieved by optimizing fertilizer N rates, using nitrification inhibitors, or changing from NH4(+)- to NO3(-)-based fertilizers.

The root-hairless barley mutant brb used as model for assessment of role of root hairs in iron accumulation.

Main components of Strategy II mechanism for Fe uptake are secretion of chelating compounds, phytosiderophores, and specific uptake of Fe(III)-phytosiderophores complex. Since the amount of phytosiderophores secreted correlates positively with plant ability to cope with Fe shortage, a role of root hairs in enhancing root capability to store phytosiderophores under Fe stress might be envisaged. In this study the root-hairless mutant of barley (Hordeum vulgare L.) brb (bald root barley) and the wild-type genotype (cv. Pallas) were compared with respect to their capacity to respond to Fe shortage in nutrient solution. Plants were grown with Fe(III)-EDTA at 0, 0.02 and 0.08 mM, in order to reproduce severe or moderate Fe deficiency, and adequate Fe nutritional status, respectively. Analysis was performed after 11 and 14 days considering leaf Fe content, phytosiderophores release and accumulation in root tips, and 59Fe uptake. Biomass accumulation and chlorophyll content were not reduced in mutant plants as compared to wild-type ones; leaf Fe content was similar in both genotypes after 14 days of growth. Accumulation and release of phytosiderophores showed a similar trend in both genotypes when subjected to Fe limitation. Furthermore, no significant difference between the two genotypes was observed when 59Fe uptake was measured. Results seem to support the idea that the presence of root hairs and their increased production in response to low-Fe availability, while causing major modifications of root geometry, did not necessarily lead neither to an effect on growth nor on Fe uptake and accumulation in barley plants.

The role of root hairs in cadmium acquisition by barley.

The role of root hairs in Cd acquisition from soil was investigated in three pot experiments using a root hairless mutant (bald root barley, brb) and its wild-type (WT) cultivar of barley (Hordeum vulgare). brb had significantly lower concentrations and lower total amounts of Cd in shoots than WT. The Cd uptake efficiency based on total root length was 8-45% lower in brb than in WT. The difference between brb and WT increased with increasing extractable Cd in soil under the experimental conditions used. Additions of phosphate to soil decreased Cd extractability. Both soil and foliar additions of phosphate decreased root length, and root hair formation in WT. These effects resulted in decreased Cd uptake with increasing P supply. Cd uptake in WT correlated significantly with root length, root hair length and density, and soil extractable Cd. Root hairs contribute significantly to Cd uptake by barley.

Extraradical development and contribution to plant performance of an arbuscular mycorrhizal symbiosis exposed to complete or partial rootzone drying.

Sweet potato plants were grown with or without Glomus intraradices in split-root pots with adjacent root compartments containing a soil with a low availability of phosphate. One fungal tube, from which root growth was excluded, was inserted into each root compartment. During 4 weeks before harvest, the soil moisture level in either both or only one of the two root-compartments of each pot was decreased. Controls remained well watered. Low soil moisture generally had a negative effect on the amount of extraradical mycelium of G. intraradices extracted from the fungal tubes. Sporulation in the fungal tubes was much higher compared with the soil in the root compartment, but remained unaffected by the soil moisture regime. Concentrations of P in extraradical mycelium were much lower than usually found in plants and fungi, while P concentrations in associated mycorrhizal host plant tissues were in an optimum range. This suggests efficient transfer of P from the extraradical mycelium to the host plant. Despite the negative effect of a low soil moisture regime on extraradical G. intraradices development, the symbiosis indeed contributed significantly to P uptake of plants exposed to partial rootzone drying. The possibility that extraradical arbuscular mycorrhizal fungal development was limited by P availability under dry soil conditions is discussed.

Glyphosate inhibition of ferric reductase activity in iron deficient sunflower roots.

Iron (Fe) deficiency is increasingly being observed in cropping systems with frequent glyphosate applications. A likely reason for this is that glyphosate interferes with root uptake of Fe by inhibiting ferric reductase in roots required for Fe acquisition by dicot and nongrass species. This study investigated the role of drift rates of glyphosate (0.32, 0.95 or 1.89 mm glyphosate corresponding to 1, 3 and 6% of the recommended herbicidal dose, respectively) on ferric reductase activity of sunflower (Helianthus annuus) roots grown under Fe deficiency conditions. Application of 1.89 mm glyphosate resulted in almost 50% inhibition of ferric reductase within 6 h and complete inhibition 24 h after the treatment. Even at lower rates of glyphosate (e.g. 0.32 mm and 0.95 mm), ferric reductase was inhibited. Soluble sugar concentration and the NAD(P)H oxidizing capacity of apical roots were not decreased by the glyphosate applications. To our knowledge, this is the first study reporting the effects of glyphosate on ferric reductase activity. The nature of the inhibitory effect of glyphosate on ferric reductase could not be identified. Impaired ferric reductase could be a major reason for the increasingly observed Fe deficiency in cropping systems associated with widespread glyphosate usage.

Boron uptake by the root cortex symplast of tomato and pea plants: evidence for low-boron-induced active transport.

The objective of this research was to test the hypothesis of the existence of an active boron (B) uptake into the cortical cells induced by low B supply. The uptake of B was characterised in two tomato (Lycopersicon esculentum Mill.) genotypes: B-efficient FER and B-inefficient mutant T3238. In addition, pea (Pisum sativum L.) was used as an anatomically appropriate model for obtaining intact root cortex. Time course uptake studies in tomato indicate that the B-inefficient mutant was defective by the absence of an active low-B-induced uptake system in the cortex. Pea roots showed up to 10-fold higher accumulation of B into the cortex symplast at low (0.5 µm) external B supply in comparison to adequate B (10 µm) supply. Also, low-B-induced uptake of B was strongly inhibited by 2,4-dinitrophenol, indicating a metabolic energy-derived active component of B uptake at low external supply. Uptake of B by the cortical cells of tomato and pea plants appears to be a combination of both passive and active components, with a passive component prevailing at higher external B. An active component of B uptake suppressed by either adequate or high B supply might indicate a downregulation of plasma membrane-associated B transporter(s) in root cortical cells.

Germanium-68 as an adequate tracer for silicon transport in plants. Characterization of silicon uptake in different crop species.

A basic problem in silicon (Si) uptake studies in biology is the lack of an appropriate radioactive isotope. Radioactive germanium-68 ((68)Ge) has been used previously as a Si tracer in biological materials, but its suitability for the study of Si transport in higher plants is still untested. In this study, we investigated (68)Ge-traced Si uptake by four crop species differing widely in uptake capacity for Si, including rice (Oryza sativa), barley (Hordeum vulgare), cucumber (Cucumis sativus), and tomato (Lycopersicon esculentum). Maintenance of a (68)Ge:Si molar ratio that was similar in the plant tissues of all four plant species to that supplied in the nutrient solution over a wide range of Si concentrations demonstrated the absence of discrimination between (68)Ge and Si. Further, using the (68)Ge tracer, a typical Michaelis-Menten uptake kinetics for Si was found in rice, barley, and cucumber. Compared to rice, the relative proportion of root-to-shoot translocated Si was lower in barley and cucumber and especially in tomato (only 30%). Uptake and translocation of Si in rice, barley, and cucumber (Si accumulators) were strongly inhibited by 2,4-dinitrophenol and HgCl(2), but in tomato, as a Si-excluding species, both inhibitors produced the opposite effect. In conclusion, our results suggest the use of the (68)Ge tracer method as an appropriate choice for future studies of Si transport in plants. Our findings also indicate that the restriction of Si from symplast to apoplast in the cortex of Si excluders is a metabolically active process.

Short-term interactions between nitrate and iron nutrition in cucumber.

Cucumber (Cucumis sativus L.) plants were precultured for 7 days in either optimal (10 µm) or low (0.5 µm) Fe conditions and then grown for further 5 days in a N-free nutrient solution with (+Fe) or without (-Fe) 10 µm Fe. Thereafter NO3- (4 mm) was added to the nutrient solution for 24 h and, concomitantly, half of the -Fe plants were treated with 1 µm Fe complexed to water extractable humic substances (WEHS). Supply of NO3- to +Fe-N-deprived plants caused a large induction in the capacity to take up the anion by roots, which was accompanied by a rise in root-shoot NO3- concentration. The -Fe plants showed a lower level of induction of NO3- uptake and hence a lower accumulation of the anion in the tissues, these effects being reversed by supply of Fe-WEHS. Supply of either NO3-- or NH4+-N (+/- Fe-WEHS) to -Fe plants promoted the development of the root FeIII-chelate reductase activity, but the capacity of roots to take up the Fe2+ remained unaffected. Results show that an inadequate Fe supply can limit the acquisition of NO3-, whereas NO3- supply can affect Fe uptake by influencing the development and maintenance of a high FeIII-chelate reducing capacity.

Foliar-applied glyphosate substantially reduced uptake and transport of iron and manganese in sunflower (Helianthus annuus L.) plants.

Evidence clearly shows that cationic micronutrients in spray solutions reduce the herbicidal effectiveness of glyphosate for weed control due to the formation of metal-glyphosate complexes. The formation of these glyphosate-metal complexes in plant tissue may also impair micronutrient nutrition of nontarget plants when exposed to glyphosate drift or glyphosate residues in soil. In the present study, the effects of simulated glyphosate drift on plant growth and uptake, translocation, and accumulation (tissue concentration) of iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) were investigated in sunflower (Helianthus annuus L.) plants grown in nutrient solution under controlled environmental conditions. Glyphosate was sprayed on plant shoots at different rates between 1.25 and 6.0% of the recommended dosage (i.e., 0.39 and 1.89 mM glyphosate isopropylamine salt). Glyphosate applications significantly decreased root and shoot dry matter production and chlorophyll concentrations of young leaves and shoot tips. The basal parts of the youngest leaves and shoot tips were severely chlorotic. These effects became apparent within 48 h after the glyphosate spray. Glyphosate also caused substantial decreases in leaf concentration of Fe and Mn while the concentration of Zn and Cu was less affected. In short-term uptake experiments with radiolabeled Fe (59Fe), Mn (54Mn), and Zn (65Zn), root uptake of 59Fe and 54Mn was significantly reduced in 12 and 24 h after application of 6% of the recommended dosage of glyphosate, respectively. Glyphosate resulted in almost complete inhibition of root-to-shoot translocation of 59Fe within 12 h and 54Mn within 24 h after application. These results suggest that glyphosate residues or drift may result in severe impairments in Fe and Mn nutrition of nontarget plants, possibly due to the formation of poorly soluble glyphosate-metal complexes in plant tissues and/or rhizosphere interactions.

Importance of plant species and external silicon concentration to active silicon uptake and transport.

Here, we characterized silicon (Si) uptake and xylem loading in Oryza sativa, Zea mays, Helianthus annuus and Benincase hispida in a series of hydroponic experiments. Both active and passive Si-uptake components co-exist in all the plants tested. The active component is the major mechanism responsible for Si uptake in O. sativa and Z. mays. By contrast, passive uptake prevails in H. annuus and B. hispida at a higher external Si concentration (0.85 mM), while the active component constantly exists and contributes to the total Si uptake, especially at a lower external Si concentration (0.085 mM). Short experiments showed that Si uptake was significantly suppressed in O. sativa and Z. mays by metabolic inhibitors or low temperature, regardless of external Si concentrations. By contrast, Si uptake in H. annuus and B. hispida was inhibited more significantly by metabolic inhibitors or low temperature at lower (for example, 0.085 mM) than at higher (for example, 1.70 mM) external Si concentrations. It can be concluded that both active and passive Si-uptake components co-exist in O. sativa, Z. mays, H. annuus and B. hispida, with their relative contribution being dependent much upon both plant species and external Si concentrations.

Involvement of auxin and CKs in boron deficiency induced changes in apical dominance of pea plants (Pisum sativum L.).

It has previously been shown that boron (B) deficiency inhibits growth of the plant apex, which consequently results in a relatively weak apical dominance, and a subsequent sprouting of lateral buds. Auxin and cytokinins (CKs) are the two most important phytohormones involved in the regulation of apical dominance. In this study, the possible involvement of these two hormones in B-deficiency-induced changes in apical dominance was investigated by applying B or the synthetic CK CPPU to the shoot apex of pea plants grown in nutrient solution without B supply. Export of IAA out of the shoot apex, as well as the level of IAA, Z/ZR and isopentenyl-adenine/isopentenyl-adenosine (i-Ade/i-Ado) in the shoot apex were assayed. In addition, polar IAA transport capacity was measured in two internodes of different ages using 3H-IAA. In B-deficient plants, both the level of auxin and CKs were reduced, and the export of auxin from the shoot apex was considerably decreased relative to plants well supplied with B. Application of B to the shoot apex restored the endogenous Z/ZR and IAA level to control levels and increased the export of IAA from the shoot apex, as well as the 3H-IAA transport capacity in the newly developed internodes. Further, B application to the shoot apex inhibited lateral bud growth and stimulated lateral root formation, presumably by stimulated polar IAA transport. Applying CPPU to the shoot apex, a treatment that stimulates IAA export under adequate B supply, considerably reduced the endogenous Z/ZR concentration in the shoot apex, but had no stimulatory effect on IAA concentration and transport in B-deficient plants. A similar situation appeared to exist in lateral buds of B-deficient plants as, in contrast to plants well supplied with B, application of CKs to these plants did not stimulate lateral bud growth. In contrast to the changes of Z/ZR levels in the shoot apex, which occurred after application of B or CPPU, the levels of i-Ade/i-Ado stayed more or less constant. These results suggest that there is a complex interaction between B supply and plant hormones, with a B-deficiency-induced inhibition of IAA export from the shoot apex as one of the earliest measurable events.

Root exudation, phosphorus acquisition, and microbial diversity in the rhizosphere of white lupine as affected by phosphorus supply and atmospheric carbon dioxide concentration.

White lupine (Lupinus albus L.) was used as a phosphorus (P)-efficient model plant to study the effects of elevated atmospheric CO(2) concentrations on (i) P acquisition, (ii) the related alterations in root development and rhizosphere chemistry, and (iii) the functional and structural diversity of rhizosphere microbial communities, on a P-deficient calcareous subsoil with and without soluble P fertilization. In both +P (80 mg P kg(-1)) and -P treatments (no added P), elevated CO(2) (800 micromol mol(-1)) increased shoot biomass production by 20 to 35% and accelerated the development of cluster roots, which exhibit important functions in chemical mobilization of sparingly soluble soil P sources. Accordingly, cluster root formation was stimulated in plants without P application by 140 and 60% for ambient and elevated CO(2) treatments, respectively. Intense accumulation of citrate and increased activities of acid and alkaline phosphatases, but also of chitinase, in the rhizosphere were mainly confined to later stages of cluster root development in -P treatments. Regardless of atmospheric CO(2) concentrations, there was no significant effect on accumulation of citrate or on selected enzyme activities of C, N, and P cycles in the rhizosphere of individual root clusters. Discriminant analysis of selected enzyme activities revealed that mainly phosphatase and chitinase contributed to the experimental variance (81.3%) of the data. Phosphatase and chitinase activities in the rhizosphere might be dominated by the secretion from cluster roots rather than by microbial activity. Alterations in rhizosphere bacterial communities analyzed by denaturing gradient gel electrophoresis (DGGE) were related with the intense changes in root secretory activity observed during cluster root development but not with elevated CO(2) concentrations.

Silicon uptake and transport is an active process in Cucumis sativus.

Cucumis sativus is a species known to accumulate high levels of silicon (Si) in the tops, though the mechanism for its high Si uptake is little understood. In a series of hydroponic experiments, we examined uptake and xylem loading of Si in C. sativus along with Vicia faba at three levels of Si (0.085, 0.17 and 1.70 mm). Measured Si uptake in C. sativus was more than twice as high as calculated from the rate of transpiration assuming no discrimination between silicic acid and water in uptake. Measured Si uptake in V. faba, however, was significantly lower than the calculated uptake. Concentration of Si in xylem exudates was several-fold higher in C. sativus, but was significantly lower in V. faba compared with the Si concentration in external solutions, regardless of Si levels. Silicon uptake was strongly inhibited by low temperature and 2,4-dinitrophenol, a metabolic inhibitor, in C. sativus but not in V. faba. It can be concluded that Si uptake and transport in C. sativus is active and independent of external Si concentrations, in contrast to the process in V. faba.

Root-derived cytokinins as long-distance signals for NO3--induced stimulation of leaf growth.

Leaf growth of many plant species shows rapid changes in response to alterations of the form and the level of N supply. In hydroponically-grown tomato (Lycopersicon esculentum L.), leaf growth was rapidly stimulated by NO(3)(-) application to NH(4)(+) precultured plants, while NH(4)(+) supply or complete N deprivation to NO(3)(-) precultured plants resulted in a rapid inhibition of leaf growth. Just 10 microM NO(3)(-) supply was sufficient to stimulate leaf growth to the same extent as 2 mM. Furthermore, continuous NO(3)(-) supply induced an oscillation of leaf growth rate with a 48 h interval. Since changes in NO(3)(-) levels in the xylem exudate and leaves did not correlate with NO(3)(-)-induced alterations of leaf growth rate, additional signals such as phytohormones may be involved. Levels of a known inhibitor of leaf growth, abscisic acid (ABA), did not consistently correspond to leaf growth rates in wild-type plants. Moreover, leaf growth of the ABA-deficient tomato mutant flacca was inhibited by NH(4)(+) without an increase in ABA concentration and was stimulated by NO(3)(-) despite its excessive ethylene production. These findings suggest that neither ABA nor ethylene are directly involved in the effects of N form on leaf growth. However, under all experimental conditions, stimulation of leaf growth by NO(3)(-) was consistently associated with increased concentration of the physiologically active forms of cytokinins, zeatin and zeatin riboside, in the xylem exudate. This indicates a major role for cytokinins as long-distance signals mediating the shoot response to NO(3)(-) perception in roots.

Nitrate does not result in iron inactivation in the apoplast of sunflower leaves.

It has been hypothesized that nitrate (NO(3)(-)) nutrition might induce iron (Fe) deficiency chlorosis by inactivation of Fe in the leaf apoplast (H.U. Kosegarten, B. Hoffmann, K. Mengel [1999] Plant Physiol 121: 1069-1079). To test this hypothesis, sunflower (Helianthus annuus L. cv Farnkasol) plants were grown in nutrient solutions supplied with various nitrogen (N) forms (NO(3)(-), NH(4)(+) and NH(4)NO(3)), with or without pH control by using pH buffers [2-(N-morpholino)ethanesulfonic acid or 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid]. It was shown that high pH in the nutrient solution restricted uptake and shoot translocation of Fe independently of N form and, therefore, induced Fe deficiency chlorosis at low Fe supply [1 micro M ferric ethylenediaminedi(O-hydroxyphenylacetic acid)]. Root NO(3)(-) supply (up to 40 mM) did not affect the relative distribution of Fe between leaf apoplast and symplast at constant low external pH of the root medium. Although perfusion of high pH-buffered solution (7.0) into the leaf apoplast restricted (59)Fe uptake rate as compared with low apoplastic solution pH (5.0 and 6.0, respectively), loading of NO(3)(-) (6 mM) showed no effect on (59)Fe uptake by the symplast of leaf cells. However, high light intensity strongly increased (59)Fe uptake, independently of apoplastic pH or of the presence of NO(3)(-) in the apoplastic solution. Finally, there are no indications in the present study that NO(3)(-) supply to roots results in the postulated inactivation of Fe in the leaf apoplast. It is concluded that NO(3)(-) nutrition results in Fe deficiency chlorosis exclusively by inhibited Fe acquisition by roots due to high pH at the root surface.