Konrad Mengel

Iron availability in plant tissues-iron chlorosis on calcareous soils

The article describes factors and processes which lead to Fe chlorosis (lime chlorosis) in plants grown on calcareous soils. Such soils may contain high HCO3- concentrations in their soil solution, they are characterized by a high pH, and they rather tend to accumulate nitrate than ammonium because due to the high pH level ammonium nitrogen is rapidly nitrified and/or even may escape in form of volatile NH3. Hence in these soils plant roots may be exposed to high nitrate and high bicarbonate concentrations. Both anion species are involved in the induction of Fe chlorosis.Physiological processes involved in Fe chlorosis occur in the roots and in the leaves. Even on calcareous soils and even in plants with chlorosis the Fe concentration in the roots is several times higher than the Fe concentration in the leaves. This shows that the Fe availability in the soil is not the critical process leading to chlorosis but rather the Fe uptake from the root apoplast into the cytosol of root cells. This situation applies to dicots as well as to monocots. Iron transport across the plasmamembrane is initiated by FeIII reduction brought about by a plasmalemma located FeIII reductase. Its activity is pH dependent and at alkaline pH supposed to be much depressed. Bicarbonate present in the root apoplast will neutralize the protons pumped out of the cytosol and together with nitrate which is taken up by a H+/nitrate cotransport high pH levels are provided which hamper or even block the FeIII reduction.Frequently chlorotic leaves have higher Fe concentrations than green ones which phenomenon shows that chlorosis on calcareous soils is not only related to Fe uptake by roots and Fe translocation from the roots to the upper plant parts but also dependent on the efficiency of Fe in the leaves. It is hypothesized that also in the leaves FeIII reduction and Fe uptake from the apoplast into the cytosol is affected by nitrate and bicarbonate in an analogous way as this is the case in the roots. This assumption was confirmed by the highly significant negative correlation between the leaf apoplast pH and the degree of iron chlorosis measured as leaf chlorophyll concentration. Depressing leaf apoplast pH by simply spraying chlorotic leaves with an acid led to a regreening of the leaves.

Soil pH increase due to biological decarboxylation of organic anions

Abstract Organic matter incorporated into soils may influence soil pH. Until now the detailed processes responsible for a change in soil pH after the incorporation of organic matter were not completely understood. We studied the effects of organic anions, glycine as a representative of amino acids, and glucose as a representative of carbohydrates on soil pH in aerobic incubation experiments. Addition of malate and citrate resulted in an immediate soil pH increase associated with additional CO 2 release. Addition of acetate had no major effect on soil pH but if applied at higher rates decreased soil CO 2 release. Malate added together with ammonium resulted in an immediate soil pH increase, which was followed by a pH decrease associated by an increase in nitrate after 10 days. Also glycine gave an immediate pH increase which was associated with an equivalent decrease of soil organic N extractable with CaCl 2 solution. Addition of glucose gave a slight pH decrease in the first days of incubation followed by an increase to the initial pH value. The results are interpreted in terms of decarboxylation of organic anions, a process which requires one proton per carboxylic group decarboxylated. Under aerobic soil conditions decarboxylation is a major process in organic matter decomposition. The decomposition of carbohydrates in the glycolytic pathway produces carboxylic groups which, after dissociation, may decrease soil pH. As soon as these groups are decarboxylated in the citrate cycle an equivalent amount of protons is required inducing a rise in soil pH. For this reason addition of glucose brought about only a transient soil pH decrease.

Turnover of organic nitrogen in soils and its availability to crops

Major known fractions of soil nitrogen are amino nitrogen (proteins, peptides), polymers of amino sugars, and NH4 + fixed in interlayers of 2:1 minerals. Only a small percentage of the total soil organic N is easily mineralizable and contributes to the pool of mineral soil N. Predominant sources of mineralization are amino-N and polymers of amino sugars present in the soil microbial biomass. Influx into this pool occurs with the application of organic matter (green manure, straw), organic carbon released by plant roots, N2 assimilation by leguminous species and inorganic nitrogen. Microbial metabolization of green manure proteins results in a partial mineralization of the applied organic N, microbial metabolization of straw in the assimilation (immobilization) of inorganic nitrogen.

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.

Bicarbonate, the most important factor inducing iron chlorosis in vine grapes on calcareous soil

SummaryIn pot experiments grape vine was grown on a calcareous and on a non calcareous soil with a low and with a high water saturation. During the growing period soil solution samples were collected and analyzed for their pH and for HCO3−, phosphate, Fe, and Ca. High water saturation resulted in a pH increase and in an increase of the HCO3− concentration in both soils. The level in pH and HCO3−, however, was much higher in the calcareous soil than in the non calcareous soil. The Fe concentration varied much throughout the experimental period, but there was no major differences between soils and water saturation treatments. The Ca concentration of the soil solution increased with time in the calcareous soil; for the non calcareous soil rather the reverse was true. The phosphate level in the soil solution of the non calcareous soil was about 10 times higher than in the calcareous soil.After 3 weeks growth all plants of the calcareous soil with the high water saturation showed first symptoms of Fe deficiency. These became more intense from day to day. Plants of the other treatments did not show any chlorotic symptoms. In the treatment with the chlorotic plants the HCO3− concentration of the soil solution was the highest, the phosphate concentration the lowest from all treatments. It is therefore concluded that HCO3− and not phosphate is the primary cause for lime induced Fe chlorosis. Despite the low phosphate concentration in the soil solution, the P concentration in the chlorotic leaves was more than twice as high as the P concentration in green leaves grown on the same soil. It is thus assumed that the high P content frequently found in chlorotic leaves is the result and not the cause for Fe chlorosis.

Potassium in crop production.

Publisher Summary This chapter reviews three main aspects of K+ in crop production—namely, K availability in the soil, the function of K+ in the plant, and potash fertilizer application. The soil is considered as a source of K+ to plant roots. The use of K+ in practical crop production is also emphasized in the discussions on the physiological role of K+ in the plant and in fertilizer application. The potassium status of a soil may be assessed on its content of K+-bearing minerals because the amount of these minerals present in a soil gives some indication of the potential source of K+ to plants. However, in terms of the ability of the soil to supply K+ to plant roots, the quantity of K+-bearing minerals plays only an indirect role. More important in determining the K+ supply to plants are the soil K+ fractions. These fractions, which have been established experimentally using different extraction techniques, are soil solution K+, K+ adsorbed to clay minerals or humus, and K+ present in minerals.

Relationship between leaf apoplast pH and iron chlorosis of sunflower (Helianthus annuus L.)

Abstract A hypothesis has been presented and tested that bicarbonate (HCO3) and nitrate (NO3) are the most important anions inducing iron (Fe) chlorosis because these anions increase the pH of leaf apoplast which in turn depresses ferric‐iron [Fe(III]) reduction, and hence, the uptake of Fe into the symplasm. Experiments with young sunflower (Helianthus annuus) plants showed that nutrition with NO3 as the sole nitrogen (N) source induced chlorosis whereas ammonium nitrate (NH4NO3) did not. Monohydrogen carbonate (bicarbonate) also favoured the development of chlorosis. The degree of chlorosis was not related to the Fe concentration in the leaves. Both anion species, NO3 and HCO3, increased the pH of the leaf apoplast which was measured by means of the fluorescence dye 5‐carboxyfluorescein. A highly significant negative correlation between leaf apoplast pH and chlorophyll concentration in the leaves (r = ‐0.97) was found. Ferric‐Fe reduction in the apoplast—measured by means of ferrocene—provided evidence ...

Effect of low pH of the root medium on proton release, growth, and nutrient uptake of field beans (Vicia faba)

The effect of low root medium pH on growth and proton release of field beans (Vicia faba L. cv. Kristall) was studied in soil and nutrient solution experiments. Decrease of soil pH due to proton release by roots strongly depended on the proton buffer capacity of 8 different soil types tested in a pot experiment. Whereas in soils of high proton buffer capacity no pH decrease during the growth period was detectable, in soils of low buffer capacity pH in the bulk soil dropped from about pH 7.3 to 6.5, 6.3 or 5.8 during growth until maturity. This decrease in pH was closely correlated with an inhibition of plant dry weight production (Y=1.06×+3.33, r=0.94***). Growth reduction was not due to direct inhibition of nitrogen fixation. In short term experiments vegetative growth and proton release were inhibited at pH<6. At pH 5 or lower proton uptake was observed in 1 mM CaSO4. Low pH (4.0 relative to pH 7.0) decreased uptake of all major ions except for Cl the exclusion of which was disturbed. It is concluded that the sensitivity of field beans to low pH is related to a lack of capability to release protons by ATPase activity. This sets limits to nutrient uptake and possibly cytoplasmic pH regulation.

Soil pH changes during legume growth and application of plant material

During cultivation of legumes soil is acidified due to proton release from roots. As a consequence of proton release, plants accumulate organic anions which may, if returned and decomposed in the soil, neutralize the soil acids. Until now the detailed processes responsible for the change in soil pH after incorporation of plant material have not been completely understood. Using a pot experiment we studied the changes in acid and base in soil during growth of field beans (Vicia faba L. cv. Alfred) and after incorporation of the plant material into the soil. Soil pH was significantly decreased by field beans from 6.00 to 5.64 in a cultivation period of 45 days. Proton release amounted to 32.7 mmol H+ pot-1, which was approximately equivalent to the accumulated alkalinity in the plant shoots (34.4 mmol). Return of field bean shoots caused a significant soil pH increase from 5.64 to 6.29. Within 7 days more than 90% of the added alkalinity was released. After 307 days incubation, soil pH decreased to 5.86 due to nitrification. In a second experiment, maize leaves (Zea mays L.), containing various concentrations of nitrogen and at various alkalinities, were incorporated into the soil. Soil pH change was positively correlated to alkalinity and malate concentration and negatively correlated to total nitrogen and water-soluble organic nitrogen of incorporated leaves. It is concluded that the soil acidification caused by legume cultivation can be partly compensated for if crop residues are returned to the soil. Addition of plant material may initially cause an increase in soil pH due to decomposition of organic anions and organic nitrogen. Soil pH may decrease if nitrification is involved. The concentrations of nitrogen and alkalinity of added plant material are decisive factors controlling soil pH change after incorporation of plant material.

Iron chlorosis on calcareous soils. Alkaline nutritional condition as the cause for the chlorosis

Abstract Iron chlorosis of plants grown on calcareous soils is not induced by an absolute Fe deficiency but rather results from a physiological disorder which affects the mobility of Fe in the entire plant. Evidence is provided that this Fe immobility is caused by an alkaline nutrition which means NO‐ 3”; as the major, if not as the sole N source combined with HCO‐ 3. Such alkaline nutritional conditions prevail in calcareous soils. It is supposed that alkaline nutrition results in high pH levels in the leaf apoplast which may bring about a precipitation of Fe. It is also feasible that a high leaf apoplast pH inhibits the plasmalemma located FeIII reductase which is responsible for the Fe transfer across the plasmamembrane. Measurements which decrease the apoplastic pH such as NH4 + nutrition, the application of indole acetic acid or fusicoccin resulted in a regreening of Fe chlorotic leaves.