The recognition and understanding of the nature of nutrient interactions is critical to: (a) the diagnosis and correction of nutritional disorders; (b) the attainment of maximum yields; and (c) the development of meaningful models for plant-soil systems. The most interesting interactions reported in the current literature are among Mn, Fe, and plant genotypes. The antagonistic interaction between Fe and Mn in their absorption by plants may result from differences in their reactions within soils, in their absorption and utilization by plants, and the influences of environmental factors and other ions on these reactions.

　Plant absorption and utilization of Fe and Mn differs widely among species. The prediction and correction of Fe deficiency of crops growing in the world's extensive areas of calcareous soils is of continuing concern to plant nutritionists.In addition, Mn toxicity is one of the most important growth-limiting factors in acid soils.

　There is a great need for additional information on the following aspects of the Fe-Mn relationships of plants: (a) mechanisms of the induction of visual symptoms of Fe deficiency and Mn toxicity in barley plants; (b) mechanisms for the acquisition and translocation of metal micronutrients in the xylem sap of Fe-deficient barley plants fed with different metal chelators such as PS with or without excess Mn; and (c) relation of Fe to K in alleviating Mn toxicity and Mn-induced Fe deficiency symptoms in plants. Based on these conceptions, experiments were conducted and following results were obtained:

　1. Iron deficiency, either induced by high levels of Mn or lack of Fe, reduced dry matter yield to a similar extent. The dark brown spots were observed on older leaves and stems of barley plants grown in either Mn-toxic or Fe-deficient treatments. Interveinal chlorosis was more expressive in plants grown under Fe-deficient conditions than under Mn toxicity. The PS content in, and release from, roots of plants grown under Fe-deficient conditions were higher than those of plants grown under Mn toxicity. The Mn-toxic treatments caused a significant decrease in the translocation of Fe.

　2. Barley plants grown under Fe-deficient conditions had brown spots on older leaves and stems, which were related to high concentrations of Mn in the tissues. The decrease in Mn concentration in the nutrient solution repressed or prevented the appearance of these spots. The concentrations of Cu and Zn were higher in Fe-deficient than Fe-sufficient plants, and were affected by Mn concentrations in the nutrient solution. Based on PS release from roots of Fe-deficient plants, the Mn treatments were arranged 0.025 > 0.25 &"62; 0.0025 μM.

　3. The concentration and translocation of PS, Cu, Mn and Zn increased, while those of Fe decreased in xylem sap of the Fe-deficient barley plants compared to the Fe-sufficient plants. Application of excess PS to the nutrient solution increased the concentrations of PS, Cu, Fe, Mn, and Zn in the xylem sap of the Fe-deficient plants. By contrast, Fe-deficient plants treated with excess EDTA showed a decrease in the concentrations of PS and the above nutrients in the xylem sap.

　4. The concentrations of PS, Cu, Fe, and Zn in the xylem sap of the Fe-deficient barley plants were decreased by the addition of excess Mn to the feeding medium. Addition of excess Mn and equimolar amount of PS to the feeding medium increased the concentrations of PS, Cu, Fe, and Zn in the xylem sap, while EDTA decreased the concentrations of PS and the above nutrients. Excess Mn in the feeding medium increased Mn concentration in the xylem sap and this increase was more pronounced with the addition of PS to the feeding medium, while EDTA had a depressing effect. These finding suggest that the roots of Fe-deficient plants can enhance the absorption and translocation of both Mn²⁺and a Mn-PS complex. In the xylem sap of the Fe-deficient plants, the concentrations of metal micronutrients were positively correlated with the concentrations of PS.

　5. Under Fe deficiency, the concentration of Fe in the xylem sap decreased compared with that of Fe-sufficient barley plants, while the concentrations of K, Ca and Mg were not different, and those of Mn, Zn and Cu increased by Fe deficiency. The concentration of PS in the xylem sap of Fe-deficient plants was about 24-fold higher than that of Fe-sufficient plants. Iron deficiency increased the concentrations of citrate, malate, and succinate in xylem sap 7.3-fold, 2.0-fold, and 1.8-fold, respectively. The xylem concentration of total amino acids increased 1.4-fold by Fe deficiency.

　6. Additional Fe (100 μM) in the nutrient solution counteracted Mn-induced Fe deficiency symptoms of Mn-toxic barley plants, recovered total dry matter yield partially, and changed the plant's concentration and total amount of essential mineral nutrients. The Mn-toxic plants grown with additional Fe released PS similar to the control plants. Additional Fe reduced Mn concentrations in shoots and roots of Mn-toxic plants, which were still above critical toxicity levels, indicating that elevation of Fe ameliorated Mn toxicity to some extent.

　7. Rice plants grown in excess Mn with additional K (10 mM) had higher dry matter yield and lower tissue Mn than plants grown with adequate K (1.0 mM) and did not show leaf injury symptoms, but still showed root browning; however, the total amount of Mn in shoots and roots was about the same. These findings indicated that additional K increased the tolerance of the plant to excess Mn, which were related to the alleviation of Mn-induced Fe deficiency, leaf rolling, and brown spots.

　8. The symptoms of Mn toxicity and Mn-induced Fe deficiency in barley plants grown at adequate Fe (10 μM) were less pronounced than those of plants grown at suboptimal Fe (1.0 μM). Application of additional K in the presence of adequate Fe counteracted the Mn toxicity symptoms, while the decrease in the severity of brown spots, the prevention of leaf desiccation, and the increase in chlorophyll content were observed in the presence of suboptimal Fe. Both concentration and total amount of Mn in plants were decreased by the application of additional K, indicating that additional K reduced Mn absorption. Our results showed close correlations between chlorophyll content and shoot Fe concentration and PS release from roots.

　9. The total amount and the concentration of ⁵⁴Mn in feeding experiments were higher in Fe-deficient barley plants than in nutrient-sufficient plants. Plants grown either deficient or toxic levels of Mn had higher amount of ⁵⁴Mn than nutrient-sufficient plants; reduction in dry weight did not fully compensate for these differences. Additional PS in the feeding media had no effect on the absorption of ⁵⁴Mn. The distribution of ⁵⁴Mn in shoots of all the investigated plants was <10%. Further investigations need to be conducted with different levels of Mn and PS to evaluate the role of PS in the absorption of Mn in plants. Additional K in the feeding media was effective in the decrease of the total amount and the concentration of ⁵⁴Mn irrespective of the conditions of plant growth. This effect was most pronounced in nutrient-sufficient plants. The amount of ⁵⁴Mn in Mn-toxic plants was higher than that of K-ameliorated Mn-toxic plants. This indicated that the damages suffered at the membrane level were so severe that the membrane could not avoid Mn influx even in the physiological status of Mn toxicity.

　栄養素の相互関係の特性の認識と理解は(a)養分障害の診断と矯正(b)最大収量の達成 (c)植物－土壌系の有意義な模式図の開発にとって重要である。最近の文献における最も興味深い相互関係は Mn、Fe、植物種の関係である。植物の吸収におけるFeとMnの拮抗的相互関係は土壌中でのそれらの反応、 植物による吸収と利用、これら反応における環境要因と他のイオンの影響の違いに起因するだろう。

　植物のFeとMnの吸収と利用は種間で大きく異なる。世界の広大な石灰質土壌地域で育つ 作物の鉄欠乏の予見と矯正は植物栄養学者の継続的関心事である。さらに、Mn過剰は酸性 土壌における重要な生育制限要因の一つである。

　そのため、植物のFe－Mn関係は次のような面におけるさらなる知見が要望されている。 (a)オオムギの鉄欠乏とMn過剰の可視的症状の誘導の機構(b)phytosiderophores (PS)（ムギネ酸類） のような金属微量必須元素のキレーターを過剰Mnの有る無しの条件下で与えたときの鉄欠乏 オオムギの道管液における金属微量必須元素の取り込みと移行の機構　(c)オオムギとイネのMn過剰 とMn過剰誘導性Fe欠乏症の軽減におけるFeとKの関係。上記の関係に基づき実験を行い、以下の 結果を得た。

1．高濃度のMnまたはFe除去により誘導されたFe欠乏は同程度に乾物生産を減少させた。 褐色斑点はMn過剰又はFe欠栽培植物の古葉と茎に見られた。葉脈間クロロシスはMn過剰より Fe欠植物でより明瞭であった。Fe欠栽培したオオムギの根中又は分泌PS濃度はMn過剰植物の それより高かった。Mn過剰処理はFeの移行を有意に減少させた。

2．Fe欠栽培したオオムギは古葉と茎に褐色斑点を生じたが、それは、組織の高いMn濃度 と関連した。培地のMn濃度の減少はこの斑点を減少させた。CuとZnの濃度はFe供給オオムギ よりFe欠オオムギで高く、培地のMn濃度により影響を受けた。Fe欠オオムギ根の分泌PSの量は Mn処理濃度において0.025>0.25>0.0025 μMの順であった。

3．Fe欠オオムギの道管液のFe濃度と移行はFe供給植物に比べ少ない一方、PS、Cu、Mn、Zn の濃度が増加した。培地に過剰のPSを加えるとFe欠植物の道管液のPS、Cu、Fe、Mn、Zn濃度が 増加した。逆に過剰のEDTAを加えた植物は道管中のPSや上記の元素の濃度が減少した。

4．処理培地に過剰Mnを加えるとFe欠オオムギの道管液のPS、Cu、Fe、Zn濃度が減少した。 過剰のMnと等モルのPSを培地に加えると道管液のPS、Cu、Fe、Zn濃度が増加した一方EDTAは PSと上記元素濃度を減少させた。培地の過剰Mnは道管液のMn濃度を増加させ、この増加は培地への PS添加でより顕著であった一方EDTAは濃度抑制効果があった。この発見はFe欠オオムギが Mn²⁺とMn²⁺-PS複合体の吸収、移行を高めうることを示唆する。Fe欠 オオムギの道管液において金属微量必須元素の濃度はPS濃度と正の相関があった。

5．Fe欠条件下では道管液のFe濃度はFe供給オオムギのそれより低かったが、K、Ca、Mg濃度 には差がなかった。そして、Mn、Zn、CuはFe欠乏により増加した。Fe欠植物の道管液のPS濃度は Fe供給植物のそれの24倍高かった。Fe欠乏は道管液のクエン酸、リンゴ酸、コハク酸濃度を7.3、2.0、 1.8倍に増加させた。道管液の全アミノ酸濃度は鉄欠乏により1.4倍に増加した。

6．培地へのFeの添加（100 μM）はMn過剰誘導Fe欠症状を抑制し、一部乾物重を回復させ、 Mn過剰オオムギの必須元素の濃度と全量を変化させた。Fe濃度を増加させたMn過剰植物のPS分泌は 対照区と同程度であった。Fe濃度を増加させたMn過剰植物の地上部と根のMn濃度は有意に減少したが、 臨界過剰濃度よりなお高かった。これらのことは、Fe濃度の増加がMn過剰を幾分軽減することを示した。

7．高濃度のK（10 mM）を与えたMn過剰イネは、通常濃度のK（1.0 mM）で育てた場合より 高い乾物量と組織の低いMn濃度を示し、葉の障害が表れなかったが、根の褐色化は示した。しかし、 地上部と根のMnの全量はほぼ同じであった。この発見はK濃度の増加が植物のMn耐性を高め、それが Mn誘導性Fe欠症状である葉の巻き上がり、褐色斑点の軽減に関連することを示した。

8．適正量のFe（10 μM）で生育したオオムギのMn過剰又はMn誘導性Fe欠乏の症状は低濃度の Fe（1.0 μM）で生育したもののそれより明瞭ではなかった。適正量のFe条件下でのKK濃度の増加は Mn過剰症を抑制した一方、低濃度のFe条件下では褐色斑点を減少させ、葉の枯死を防ぎ、葉緑素含量を 増加させた。植物のMn濃度と全量はK濃度の増加により減少し、このことはK濃度の増加がMn吸収を 減少させることを示した。我々の結果は葉緑素含量、地上部Fe濃度、根のPS分泌の間には強い相関が あることを示した。

9．投与実験において、正常オオムギよりFe欠オオムギの⁵⁴Mnの全量と濃度が高かった。 Mn欠乏又は過剰で栽培した植物は正常植物より高い⁵⁴Mnを示した。乾物重の減少ではこの 違いを説明できなかった。投与培地へのPS添加は⁵⁴Mnの吸収に影響しなかった。⁵⁴Mn の地上部への分配はすべての植物で10%以下であった。オオムギのMn吸収におけるPSの役割を解明する ために異なるMnとPS濃度条件でさらに検討する必要がある。培地のK濃度の増加は植物の生育条件に 関わらず、⁵⁴Mn全量と濃度の減少に効果的であった。この効果は正常植物で最も大き かった。Mn過剰植物の⁵⁴Mn量はKで障害が軽減されたMn過剰植物より高かった。この ことは、膜レベルで受けた損傷が激しく、膜がMn過剰の生理状態であるにも関わらずMn流入を阻止 できないことを示唆していた。