Harald Kosegarten

The central role of microbial activity for iron acquisition in maize and sunflower

Abstract Maize (Zea mays L.) and sunflower (Helianthus annuus L.) grown on a calcareous soil showed poor growth and/or were chlorotic in spite of abundant Fe in the roots. It has been hypothesized that microbial siderophores chelate Fe (III) in the soil, and that in this form Fe is transported towards the root apoplast. On the calcareous soil, total and apoplastic root Fe concentrations were high, probably because of a high apoplastic pH depressing Fe (III)-reductase activity and thus the Fe2+ supply to the cytoplasm. On the acidic soil, total and apoplastic root Fe concentrations were low, probably because of a low apoplastic pH favouring Fe (III) reduction, hence plants showed no Fe-deficiency symptoms. The main objective of the present work was to investigate the role of microbial soil activity in plant Fe acquisition. For this purpose, plants were grown under sterile and non-sterile conditions on a loess loam soil. Plants cultivated under non-sterile conditions grew well, showed no Fe-deficiency symptoms and had fairly high Fe concentrations in the roots in contrast to plants grown in the sterile medium. Low root and leaf Fe concentrations in the axenic treatments indicated that the production of microbial siderophores was totally suppressed. Accordingly, sunflowers were severely chlorotic and this was associated with very poor growth, whereas in maize only growth was drastically reduced. In maize under sterile conditions, root apoplastic and total Fe concentrations were not as low as in sunflowers, which may have indicated that phytosiderophores produced in maize partly sustained Fe acquisition, but due to poor growth were not as efficient in supplying Fe as microbial activity under natural conditions. It may be therefore assumed that in natural habitats soil microbial activity is of pivotal importance for plant Fe acquisition.

Depressed growth of non-chlorotic vine grown in calcareous soil is an iron deficiency symptom prior to leaf chlorosis

The development of iron deficiency symptoms (growth depression and yellowing of the youngest leaves) and the distribution of iron between roots and leaves were investigated in different vine cultivars (Silvaner, Riparia 1G and SO4) grown in calcareous soils. As a control treatment all cultivars were also grown in an acidic soil. Only the cultivars Silvaner and Riparia 1G showed yellowing of the youngest leaves under calcareous soil conditions at the end of the cultivation period. All cultivars including SO4 showed severe shoot growth depression, by 50 % and higher, before yellowing started or without leaf yellowing in the cultivar SO4. Depression of shoot growth occurred independently from that of root growth. In a further treatment the effect of Fe-EDDHA spraying onto the shoot growth of the cultivar Silvaner after cultivation in calcareous soil was investigated. Prior to Fe application plants were non-chlorotic, but showed pronounced shoot growth depression. Spraying led to a significant increase in shoot length, though leaf growth was not increased. Accordingly, depression of shoot growth of non-chlorotic plants under calcareous soil conditions and with ample supply of nutrients and water has been evidenced to be at least partly an iron deficiency symptom. We suggest that plant growth only partially recovered because of dramatic apoplastic leaf Fe inactivation and/ or a high apoplastic pH which may directly impair growth. Since growth was impaired before the youngest leaves showed chlorosis we assume that meristematic growth is more sensitively affected by Fe deficiency than is chlorophyll synthesis and chloroplast development. In spite of high Fe concentrations in roots and leaves of the vines grown in calcareous soils plants suffered from Fe deficiency. The finding of high Fe concentrations also in young, but growth retarded green leaves is a further indication that iron deficiency chlorosis in calcareous soils is caused by primary leaf Fe inactivation. However, in future, only a rigorous study of the dynamic changes of iron and chlorophyll concentration, leaf growth and apoplastic pH at the cellular level during leaf development and yellowing will provide causal insights between leaf iron inactivation, growth depression, and leaf chlorosis. Gehemmtes Wachstum von Reben auf Karbonatboden ist ein empfindlicheres Eisenmangelsymptom als die Chlorose junger Blatter Es wurde die Entwicklung von Eisenmangelsymptomen (Wachstumshemmung und Chlorose bei jungen Blattern) unter dem Aspekt der Eisenverteilung zwischen Wurzel und Blattern bei verschiedenen Rebsorten (Silvaner, Riparaia 1G, SO4) auf Karbonatboden untersucht. Die Kontrollpflanzen wurden auf einem Boden mit niedrigem pH angezogen. Nur die jungsten Blatter der Sorten Silvaner und Riparia 1G wurden am Ende der Vegetationsperiode nach Anzucht auf den Karbonatboden chlorotisch. Alle Sorten, einschlieslich SO4, zeigten auf den Karbonatboden eine erhebliche Hemmung des Sprosswachstums, um 50 % und hoher, bevor sich Chlorosesymptome zeigten (Silvaner, Riparia 1G) bzw. ohne dass sich eine Chlorose ausbildete (SO4). Die Wachstumshemmung des Sprosses erfolgte unabhangig von der Hemmung des Wurzelwachstums. In einem weiteren Versuch wurde die Applikation von Fe-EDDHA auf das Sprosswachstum bei der Sorte Silvaner untersucht. Die Gabe von Fe-EDDHA fuhrte zu einer signifikanten Verbesserung des Langenwachstums von Pflanzen, die nicht-chlorotisch, aber im Wachstum gehemmt waren; das Blattwachstum wurde nicht signifikant gefordert. Damit wurde gezeigt, dass das gehemmte Wachstum von nicht-chlorotischen Pflanzen auf Karbonatboden bei ausreichender Nahrstoff- und Wasserzufuhr, zumindestens teilweise auf einen Eisenmangel zuruckzufuhren ist. Das gehemmte Wachstum beruht sehr wahrscheinlich auf einer massiven Eiseninaktivierung im Blattapoplasten und/oder auf einem direkten pH-Effekt und wurde insofern durch Eisenspritzung nur teilweise wieder hergestellt. Da die Wachstumshemmung vor der Chlorose auftrat, nehmen wir an, dass das meristematische Wachstum empfindlicher durch Eisenmangel gestort wird als die Chlorophyllsynthese und die Chloroplastendifferenzierung. Trotz hoher Eisenkonzentrationen in Blatt und Wurzel bei allen Weinsorten nach Anzucht auf den Karbonatboden zeigten die Pflanzen Eisenmangel. Hohe Eisenkonzentrationen wurden nicht nur in chlorotischen Blattern, sondern auch in jungen grunen, im Wachstum gehemmten Blattern nachgewiesen. Dieser Befund spricht fur eine primare Eiseninaktivierung in Blattern als direkte Ursache des Eisenmangels. Allerdings wird nur eine konsequente Untersuchung der dynamischen Veranderungen der Eisen- und Chlorophyllkonzentration, des Blattwachstums und des apoplastischen pH auf zellularer Ebene wahrend der Blattentwicklung und der Ausbildung von Chlorose die kausalen Beziehungen zwischen der Eiseninaktivierung im Blattapoplasten, der Hemmung des Blattwachstums und der Chlorose klaren.

Effect of different water supply on plant growth and fruit quality of Lycopersicon esculentum

It is well known from earlier work that water stress and salinity results in depressed plant growth and high fruit quality of tomato (e.g. increased sugar and acid levels), but generally is associated with a low marketable fruit yield. In the present work we investigated whether even a small reduction in water supply (without visible symptoms of water stress) also results in a high fruit quality together with high marketable fruit proportions. To characterize fruit quality sugars (glucose and fructose), titratable acids, odour-active aroma volatiles and vitamin C were investigated. Tomato plants (Lycopersicon esculentum Mill. cv Vanessa) were grown in soil and with the onset of fruit development water supply was varied (70% and 50% water capacity). In the treatment with lower water supply plant growth, and in particular the number of fruit settings were depressed and the sugar and vitamin C concentrations in the fruits were significantly increased, especially during fruit ripening. Furthermore, with lower water supply the concentrations of titratable acids and of C6 aldehydes (hexanal, (Z)-3-hexenal and (E)-2-hexenal) were significantly increased in the red fruits. Fruit growth was identical in both treatments. The higher levels of sugars, titratable acids, aroma volatiles and vitamin C are responsible for the higher fruit quality under conditions of lower water supply. Since not all fruits of the well watered plants became mature, the marketable yield in both treatments was rather similar and hence, together with a higher fruit quality in the treatment with lower water supply, high proportions of marketable fruits can be harvested. Einflus einer unterschiedlichen Wasserversorgung auf das Pflanzenwachstum und die Fruchtqualitat von Lycopersicon esculentum Es ist durch andere Arbeiten bekannt, dass Wasserstress und Salinitat zu einem gehemmten Pflanzenwachstum und hoher Fruchtqualitat bei Tomaten (z. B. erhohten Zucker- und Saurengehalten) fuhren; allerdings ist der marktfahige Fruchtertrag im allgemeinen reduziert. In der vorliegenden Arbeit wurde untersucht, ob bereits eine geringfugige Einschrankung der Wasserzufuhr (ohne sichtbare Symptome von Wasserstress) ebenfalls zu einer hohen Fruchtqualitat bei gleichzeitig hohem marktfahigen Fruchtertrag fuhrt. Zur Charakterisierung der Fruchtqualitat wurden Zucker (Glukose und Fruktose), titrierbare Sauren, fluchtige Aromastoffe und Vitamin C untersucht. Tomatenpflanzen (Lycopersicon esculentum Mill. cv Vanessa) wurden im Boden mit Beginn der Fruchtentwicklung bei optimaler und eingeschrankter Wasserversorgung (70% und 50% Wasserkapazitat) angezogen. Bei geringerer Wasserversorgung war das Pflanzenwachstum und insbesondere die Zahl der Fruchtstande erniedrigt und die Konzentrationen an Zuckern wie auch an Vitamin C in den Fruchten waren signifikant erhoht, und zwar insbesondere zur Reife. Daruberhinaus war bei reduzierter Wasserzufuhr in den reifen Fruchten die Konzentration der titrierbaren Sauren und der C6-Aldehyde (Hexanal, (Z)-3-Hexenal und (E)-2-Hexenal) signifikant erhoht. Das Fruchtwachstum war durch die unterschiedliche Wasserzufuhr nicht beeinflusst. Die hoheren Konzentrationen an Zuckern, titrierbaren Sauren, an fluchtigen Aromastoffen und an Vitamin C sind fur die hohere Fruchtqualitat bei geringerer Wasserversorgung verantwortlich. Da nicht alle Fruchte in der gut gewasserten Variante zur Ausreife gelangten, war der marktfahige Ertrag bei beiden Wasserstufen praktisch identisch, so dass also in der weniger gewasserten Variante qualitativ hoherwertige Fruchte ohne Ertragsdepression geerntet werden konnten.

Differential Ammonia-Elicited Changes of Cytosolic pH in Root Hair Cells of Rice and Maize as Monitored by 2[prime],7[prime]-bis-(2-Carboxyethyl)-5 (and -6)-Carboxyfluorescein-Fluorescence Ratio

Intact hair cells of young rice (Oryza sativa L.) and maize roots (Zea mays L.), grown without external nitrogen, were specifically loaded with 2[prime],7[prime]-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein acetoxymethyl ester to monitor fluorescence ratio cytosolic pH changes in response to external ammonia (NH4+/NH3) application. In neutral media, cytosolic pH of root hairs was 7.15 [plus or minus] 0.13 (O. sativa) and 7.08 [plus or minus] 0.11 (Z. mays). Application of 2 mM ammonia at external pH 7.0 caused a transient cytosolic alkalization (7.5 [plus or minus] 0.15 in rice; 7.23 [plus or minus] 0.13 in maize). Alkalization increased with an increase of external pH; no pH changes occurred at external pH 5.0. The influx of 13N-labeled ammonia in both plant species did not differ between external pH 5.0 and 7.0 but increased significantly with higher pH. Pretreatment with 1 mM 1-methionine sulfoximine significantly reduced the ammonia-elicited pH increase in rice but not in maize. Application of 2 mM methylammonia only caused a cytosolic pH increase at high external pH; the increase in both species compared with the ammonia-elicited alkalization in 1-methionine sulfoximine-treated roots. The differential effects indicate that cytosolic alkalization derived from (a) NH3 protonation after passive permeation of the plasma membrane and, particularly in rice, (b) additional proton consumption via the glutamine synthetase/glutamate synthase cycle.

The paramount influence of nitrate in increasing apoplastic pH of young sunflower leaves to induce Fe deficiency chlorosis, and the re‐greening effect brought about by acidic foliar sprays

The main objective of the present work was to clarify the causal relationship between leaf apoplastic pH increase and Fe chlorosis under alkaline growth conditions. It has been shown that nitrate supply in contrast to ammonium supply induced a pH increase in the apoplast of young green leaves of Helianthus annuus which was followed within 12 hours by leaf yellowing. Hence nitrate nutrition is the primary cause of a high leaf apoplastic pH which induces Fe deficiency chlorosis and not the impaired provision of ATP for plasmalemma H+ pumps in yellow leaves. Supply of bicarbonate in physiological concentrations had virtually no influence on leaf apoplastic pH. Spraying leaves with diluted acids (citric acid, sulphuric acid) resulted in a decrease of apoplastic pH followed by leaf re-greening. Interestingly, the Fe concentrations remained the same in the yellow control leaves and in the sprayed green leaves. From this it follows that Fe efficiency in leaves is mainly related to the Fe distribution between apoplast and symplast. It was demonstrated that Fe chlorosis induced by nitrate nutrition begins from the base of the youngest leaves, presumably from growing interveinal microsites showing high nitrate uptake rates. Leaf yellowing spread gradually from the leaf base to the tip and after seven days of nitrate supply the leaf was almost completely yellow (98%). Leaf yellowing was measured by means of a video imaging technique. Leaf apoplastic pH recordings were conducted after loading the fluorescent dye FITC-Dextran (4000 D) into the leaf apoplast of intact plants thus simulating in vivo conditions. It was also shown using the new loading technique that the fluorescent dye did not penetrate the leaf symplast. Die herausragende Wirkung von Nitrat auf die Erhohung des Apoplasten-pH in jungen Blattern von Sonnenblumen und damit auf das Auslosen von Eisenmangelchlorose sowie die Wiederergrunung nach Bespruhung mit Sauren Das wesentliche Ziel der vorliegenden Untersuchung war, die kausalen Zusammenhange der pH-Erhohung im Blattapoplasten und das Auftreten von Fe-Chlorose unter alkalischen Bedingungen zu klaren. Es wurde gezeigt, dass Nitraternahrung, im Unterschied zur Ernahrung mit Ammonium, eine pH-Erhohung im Blattapoplasten gruner Blatter von Helianthus annus auslost, gefolgt von einer Gelbfarbung des Blattes innerhalb weniger Stunden (12 h). Deshalb ist Nitraternahrung die primare Ursache der pH-Erhohung im Blattapoplasten, die dann Eisenmangelchlorose auslost; die pH-Erhohung im Apoplasten ist damit nicht die Folge einer unzureichenden Versorgung der H+-Pumpen mit ATP durch das bereits chlorotische Blatt. Die Zugabe von HCO3— in physiologisch vertretbaren Konzentrationen hatte keinen Einfluss auf den pH-Wert im Blattapoplasten. Blattbespruhungen mit verdunnten Sauren (Citronensaure, Schwefelsaure) bewirkten einen pH-Abfall im Apoplasten, gefolgt von einer Wiederergrunung der Blatter. Interessanterweise waren die Fe-Konzentrationen in den wiederergrunten und in den chlorotischen Kontrollblattern identisch. Hieraus wird gefolgert, dass die Fe-Effizienz in den Blattern im wesentlichen von der Fe-Verteilung zwischen Apoplast und Symplast abhangt. Es wurde gezeigt, dass die durch Nitraternahrung ausgeloste Fe-Chlorose an der Basis des jungsten Blattes beginnt, ausgehend vermutlich von „microsites“, die hohe Nitrat-aufnahmereraten zeigen und zum meristematischen Gewebe des wachsenden Blattes gehoren. Die Gelbfarbung breitet sich allmahlich von der Blattbasis zur Blattspitze hin aus, und nach sieben Tagen Nitraternahrung war das Blatt fast vollstandig gelb (98%). Die Gelbfarbung der Blatter wurde mittels Video-Imaging Technik gemessen. Zur pH-Messung im Blattapoplasten unter in vivo Bedingungen wurde der Fluoreszenzfarbstoff FITC-Dextran (4000 D) in den Blattapoplasten intakter Pflanzen geladen. Es konnte gezeigt werden, dass auch bei dieser neuen Beladungstechnik der Fluoreszenzfarbstoff nicht in den Symplasten eindringt.

Nitrogen compounds extracted by electroultrafiltration (EUF) or CaCl2 solution and their relationships to nitrogen mineralization in soils

The objective of the investigation was to identify the most important organic N-containing fractions extracted from soils by electroultrafiltration (EUF) or a CaCl2 solution, respectively, and their importance for nitrogen mineralization. The investigation comprised 19 agricultural and one forest top soil. Net N mineralization was tested in Mitscherlich pot experiments with three treatments: (1) fallow soil without N fertilizer, (2) soil cultivated with rye grass without N fertilizer, (3) soil cultivated with rye grass with N fertilizer. The highest proportion of N in the extracts was the amino N fraction (amino acids + peptides) amounting to approximately 60% of the total N extracted by CaCl2 and to about 40% of the total N extracted by EUF. The proportion of amino sugars from total N extracted was in average 10% for the CaCl2 and 5.2% for the EUF extracts. The proportion of heterocyclic N bases derived from nucleic acids amounted in average to 4.8% and 3.6% for the CaCl2 and EUF extract, respectively. Amino N (amino acids + peptides) were correlated best with net N mineralization (EUF, r = 0.81***, CaCl2, r = 0.86***). The correlation between amino sugars and net N mineralization was r = 0.55* for the EUF extract and r = 0.49* for the CaCl2 extract. The heterocyclic N bases did not correlate with net N mineralization. Correlations between Norg extracted by CaCl2versus net N mineralization were higher than those obtained by the EUF extract. Net N mineralization was about four times higher in the fallow soils than in the treatment with grass and no N fertilizer. In the treatment with grass + N fertilizer on average no net N mineralization occurred, moreover there was a tendency of N immobilization. It is assumend that in the treatments with grass cultivation, organic C released by roots stimulated the assimilation of mineral N and amino acids by soil microorganisms resulting in a low net N mineralization. Net N mineralization led to a highly significant depletion in the Norg pools and particularly in the amino N and amino sugar pools in the treatment with grass and without N fertilizer. This depletion was particularly evident in the CaCl2 extracts. The results justify the conclusion that the Norg obtained with both extraction methods originates from a dynamic N pool into which N flows in and out. The amino N extractable with EUF or CaCl2 is a reliable indicator for the net N mineralization potential of soils. Stickstoffverbindungen, mittels Elektroultrafiltration (EUF) oder mittels CaCl2-Losung extrahiert, und ihre Beziehungen zur Stickstoffmineralisation in Boden Ziel der Untersuchung war es, die wichtigsten N-haltigen Stoffgruppen, die mittels Elektroultrafiltration (EUF) oder CaCl2-Losung extrahierbar sind, zu identifizieren und deren Bedeutung fur die Netto-N-Mineralisation in Boden zu ermitteln. Die Untersuchungen umfasten 19 landwirtschaftlich genutzte Boden und einen Waldboden. Ihre Netto-N-Mineralisation wurde in einem Mitscherlich-Gefasversuch mit nachstehenden drei Varianten untersucht: (1) unbewachsener Boden ohne N-Dungung, (2) Boden mit Weidelgras ohne N-Dungung, (3) Boden mit Weidelgras mit N-Dungung. Die Stoffgruppe mit hochstem Anteil am N der Extrakte war der Amino-N (Aminosauren + Peptide). Ihr Anteil betrug im Durchschnitt 60% des gesamten mit der CaCl2-Losung und 40% des gesamten mit EUF extrahierten N. Der Anteil an Aminozuckern betrug durchschnittlich 10% im CaCl2-Extrakt und 5,2% im EUF-Extrakt vom gesamten extrahierten N. Die Anteile an heterocyclischen Basen der Nukleinsauren beliefen sich im Durchschnitt auf 4,8% im CaCl2- und 3,6% im EUF-Extrakt. Von diesen drei N-haltigen Stoffgruppen korrelierte der Amino-N am engsten mit r = 0.81*** (EUF-Extrakt) und mit r = 0.86*** (CaCl2-Extrakt) mit der Netto-N-Mineralisation. Die Korrelationskoeffizienten fur die Beziehung Aminozucker versus Netto-N-Mineralisation betrugen 0.55* fur den EUF-Extrakt und 0.49* fur den CaCl2-Extrakt. Die heterocyclischen N-Basen korrelierten nicht mit der Netto-N-Mineralisation. Norg des CaCl2-Extraktes war enger mit der Netto-N-Mineralisation korreliert als Norg des EUF-Extraktes. Im Durchschnitt war die Netto-N-Mineralisation in der unbewachsenen Variante etwa vierfach hoher als in der Variante mit Gras und ohne N-Dungung. In der Variante mit Gras + N-Dungung lag im Durchschnitt keine Netto-N-Mineralisation, sondern die Tendenz zur N-Immobilisierung vor. Es wird angenommen, das in den Varianten mit Grasbewuchs organischer C, der von den Wurzeln freigesetzt wurde, zur Assimilation von mineralischem N und Aminosauren durch Mikroorganismen beitrug, wodurch die Netto-N-Mineralisation vermindert wurde. In der bewachsenen Variante ohne N-Dungung bewirkte die Netto-N-Mineralisation eine hoch signifikante Abnahme des N in den Norg-Pools und besonders eine Abnahme des N in den Amino-N und Aminozucker-Pools. Diese Abnahme war am starksten im N des CaCl2-Extraktes ausgepragt. Die Ergebnisse berechtigen zu dem Schlus, das der Norg beider Extraktionsmethoden einen dynamischen Pool darstellt, in den N hinein- und herausfliest, und das die mittels EUF oder CaCl2-Losung extrahierbaren Amino-N-Verbindungen einen zuverlassigen Indikator fur das N-Mineralisationspotential von Boden darstellen.

Effects of NH(4)(+), NO(3)(-) and HCO(3)(-) on apoplast pH in the outer cortex of root zones of maize, as measured by the fluorescence ratio of fluorescein boronic acid

Abstract. A fluorimetric ratio technique was elaborated to measure apoplastic pH in the outer root cortex of maize (Zea mays L.) grown hydroponically. A newly synthesized fluorescent probe, fluorescein boronic acid (pKa = 5.48), which covalently binds to the cell wall of the outer cell layers, was used. Under conditions of saturating ion concentrations the apoplastic pH was determined along the root axis ranging from 1 to 30 mm behind the root tip. Apoplastic pH was recorded for root segment areas (1 mm2), and pH values of high statistical significance were obtained. With an external solution of pH 5, the apoplastic pH was about pH 5.1 in the division zone, between pH 4.8 and 4.9 in the elongation region and about pH 4.9 in the root hair zone. At an external pH of 8.6, the difference between the external pH and the apoplastic pH was considerably more, with a pH of 5.2–5.3 in all root zones. Addition of 1 mM NH4+ caused a small apoplastic pH decrease (0.05 of a pH unit) in all root zones. Apoplastic alkalization upon application of 6 mM NO3− was highest (0.3 of a pH unit) in the zone where root hairs emerge; in the division and early elongation zones, apoplastic pH increased only transiently. In the presence of 10 mM HCO3−, NO3− elicited a higher and persistent alkalization (0.06–0.25 of a pH unit) in all root zones. Application of fusicoccin reduced apoplastic pH from 4.85 to 4.75 in the elongation zone, while inhibition of the H+-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences along the root axis upon differential N supply and application of HCO3− provide evidence that this new pH technique is a useful tool with which to measure apoplastic pH, and in future may permit measurements at microsites at the cell level by use of microscope imaging.

Further Elements of Importance

In the previous chapters all elements which have been considered, with the possible exception of Na, are essential plant nutrients. Without any one of these essential elements the plant would be unable to complete its life cycle. All these elements too play a part in plant metabolism although their essential roles are in some cases still incompletely understood.

Differential pH restoration after ammonia-elicited vacuolar alkalisation in rice and maize root hairs as measured by fluorescence ratio

Abstract. Changes of vacuolar pH in hair cells of young rice (Oryza sativa L.) and maize (Zea mays L.) roots were measured after ammonia application at various levels of external pH. After loading the pH-sensitive, fluorescent dye Oregon green 488 carboxylic acid 6-isomer into the vacuoles of root hairs, ratiometric pH data of high statistical significance were obtained from root hair populations comprising hundreds of cells. The pH of the vacuole at external pH 5.0 was 5.32 ± 0.08 (±SD, n= 15) and 5.41 ± 0.13 (±SD, n= 15) in rice and maize, respectively. A moderate external ammonia concentration of 2 mM led to vacuolar alkalisation at both, low (pH 5.0) and high (pH 7.0–9.0) external pH, presumably due to NH3 permeation into the vacuole. With increasing external pH, ammonia application did not cumulatively increase vacuolar pH. In rice, the increase in vacuolar pH ranged from 0.1–0.8 pH units; in maize a more constant increase of 0.5 pH units was observed. The vacuolar pH increase was efficiently depressed in rice (especially at high external pH), but not in maize. Inhibition of the tonoplast H+-ATPase by concanamycin A raised vacuolar pH and increased the ammonia-elicited vacuolar alkalisation in both species, proving that vacuolar H+-ATPase activity counters the ammonia-elicited alkalisation effect. However, even under conditions of vacuolar H+-ATPase inhibition, rice was still able to restore an ammonia-elicited pH increase. High vacuolar pH levels as found in maize under conditions of high NH3 influx may derive from inefficient cytosolic ammonia assimilation and tonoplast proton pumping. Thus, in maize, prolonged reduction of the proton gradient between the cytosol and the vacuole may play an important role in NH3 toxicity.

Comparative investigation on the susceptibility of faba bean (Vicia faba L.) and sunflower (Helianthus annuus L.) to iron chlorosis

Abstract Comparative physiological studies on iron (Fe) chlorosis of Vicia faba L. and Helianthus annuus L. were carried out. High internal Fe contents in Vicia cotyledons (16–37 μg) were completely used for plant growth and Fe chlorosis was not inducible by the application of nitrate (with or without bicarbonate). In Helianthus, low quantities of Fe in the seeds (4 μg) were insufficient for normal growth and without Fe in the nutrient solution, Fe chlorosis was obtained in all treatments. This chlorosis was an absolute Fe deficiency. Also, the treatment with 1 μM Fe in the nutrient solution and nitrate (with or without bicarbonate) led to severe chlorotic symptoms associated with low leaf Fe concentrations and high Fe concentrations in the roots. In contrast, Helianthus grown with NH4NO3 and 1 μM Fe had green leaves and high leaf Fe concentrations. However, with NO3 supply (with or without bicarbonate), Fe translocation from the roots to the upper plant parts was restricted and leaves were chlorotic. Chl...