Aluminum (Al) tolerance is usually regarded as the determining factor for plant growth in tropical acid soils. However, nutrient deficiencies are often additional growth-limiting factors in these soils. Little investigations have ever reported to confirm the primary stress factor on plant growths under high aluminum in low nutrient conditions with considering low-nutrient tolerance. At first, I carried out the following experiments: 1) to know whether single concentration of Al is enough for screening of several crop plant species differing in Al tolerance, and 2) to establish the short-term screening technique to know the tolerances of the cultivars of sorghum, soybean and maize. Next, considering the potential interactions between Al toxicity and nutrient deficiencies, I compared the following points by long-term water culturing using many sorghum and maize cultivars : (a) Al tolerance (relative growth in a one-fifth strength nutrient solution [low-nutrient medium, ionic strength: 4.5 mM] with Al or without Al), (b) low-nutrient tolerance (relative growth in the low-nutrient medium to the growth in a full-strength nutrient solution) and (c) combined tolerance (relative growth in the low-nutrient medium containing Al to the growth in the full-strength medium lacking Al). The main goal of this study was to identify the predominant growth-limiting factor using a solution culture medium that simulates the nutrient status of tropical acid soils.

　Differential Al tolerance among 29 cultivars of sorghum ( Sorghum bicolor Moench [L.]), 22 cultivars of soybean ( Glycine max L.) and 18 cultivars of maize ( Zea mays L.) was conducted in short-term experiments (2.5 or 20 μM AlCl₃ in 0.2 mM CaCl₂ at pH 5.0 or 4.9, respectively, for 24 h). Among several other short-term screening methods ever reported, measurement of root elongation is still the most popular in spite of inherent complexity. Wide range of Al tolerance among cultivars was observed in sorghum, soybean and maize. Al tolerance in low Al conditions was positively correlated with that in high Al conditions for all crop plant species. However, for sensitive crop plant species (sorghum), screening of Al tolerance study was considered to be reasonable in low Al conditions. On the other hand, tolerance study for Al-tolerant crop plant species (maize) should be done in higher Al concentrations. The level of Al tolerance for soybean showed similar tendency under both Al conditions and it ranked as intermediate-Al tolerance. Considering these results, I decided to use sorghum as extreme sensitive and maize as tolerant crop plant species for further study in long-term glass-house experiment after selecting 15 and 10 cultivars, respectively.

　Relationship between long-term Al tolerance and short-term Al tolerance was checked. Short-term screening technique for Al tolerance was suggested to be useful for the estimation of Al tolerance in long-term culturing with nutrients. Even though the concentrations of soluble Al in long-term culturing medium were maintained as 11.1 μM in low Al conditions and 42.6 μM in high Al conditions similarly for both nutrient concentrations, i.e., full nutrients and low-nutrients which equals to one-fifth strength of full nutrients, Al in roots for both plant species was proportionally and positively correlated with activity of whole Al ion species (Al [OH]₃⁰ + AlSO₄⁺+ Al[OH]₂⁺ + AlOH²⁺ + Al³⁺) in medium. Additionally , the slope of the correlation equation for sorghum was more than twice higher than for that for maize. Greater Al tolerance for maize was considered to be ascribed to the less Al absorption by roots. However, the level of Al tolerance in short-term assays was not correlated with the combined tolerance as defined above. Short-term screening technique was finally considered not to be practically useful for estimating cultivar adaptation to the combination of stress factors in tropical acid soils. Al in nutrient solution decreased DW and all nutrients measured (P, K, Ca, Mg, Fe and Mn) for sorghum more considerably than those for maize. Greater inhibitory effect of Al on DW indicates also the greater Al tolerance for maize as compared with sorghum. Among sorghum cultivars, higher Al tolerance was associated with less Al absorption by roots, however among maize cultivars, there was no correlation between them. Positive correlation was only recognized between Al tolerance and K in sorghum shoots, however no correlations were recognized between Al tolerance and all nutrients in shoots for maize. Significance of higher potential of K absorption / translocation for better growth of sorghum in the medium with Al was suggested.

　In all present experimental conditions, combined tolerance was significantly correlated both with Al tolerance and low-nutrient tolerance for both plant species. To evaluate the relative importance of these two factors on combined tolerance, I calculated standardized partial regression equation among combined tolerance, Al tolerance and low-nutrient tolerance. Greater contribution of low-nutrient tolerance was finally suggested to combined tolerance than Al tolerance under most conditions ( comparisons of standardized partial regression coefficients [SPRC] between for low-nutrient tolerance in the left-side term and for Al tolerance in the right-side term were as follows: 0.46 > 0.39 for sorghum in low Al conditions, 0.69 > 0.57 for maize in low Al conditions, and 0.69 > 0.64 for maize in high Al conditions) except for Al-sensitive sorghum at high Al conditions (0.67 [SPRC for Al tolerance] > 0.02 [SPRC for low-nutrient tolerance]). Among all nutrients, combined tolerance was positively correlated with K for sorghum shoots and with Ca for maize shoots. In high Al and low-nutrient conditions, K or Ca was considered as a primary nutrient for higher production of sorghum or maize, respectively. It was proposed that plant nutritional characteristics linked to low-nutrient tolerance should be evaluated as an important strategy for plant production in tropical acid soils, both for Al-tolerant plant species and for Al-sensitive plant species under low-Al conditions.

　Conclusively, I demonstrated the relative greater contribution of low-nutrient tolerance as a primary strategy for better crop production in tropical acid soils based on the comprehensive experimental design for long-term water culturing contrary to the common agreement on the greater contribution of Al tolerance. This research will supply not only the more useful prescription to the tropical agriculture but also the development of the new research aspects on the effective plant nutritional characteristics, i.e., absorption, translocation, and recycling of nutrients.

＜要約＞

アルミニウム（Al ）耐性は熱帯酸性土壌での作物生育の決定要因と通常みなされている。 しかしながら、これらの土壌では養分欠乏も別の生育制限要因となっている。 養分欠乏耐性を考慮したうえでの、高Al・低養分条件下での作物生育支配主要因を特定する報告は、現在ほとんどない。 そこで、以下のように実験を行った。 先ず初めに、Al耐性の異なる幾つかの植物種を用いて、短期間(24時間)でのAl耐性スクリーニング条件を検討するために、 ソルゴー29品種、ダイズ22品種、トウモロコシ18品種を用い、Al耐性を低Al (2.5 μM Al＋0.2 mM Ca) と 高Al (20 μM Al＋0.2 mM Ca )の両条件で調べた。 次に、Al過剰害と養分欠乏の間での相互作用を考慮し、長期間( 29 日) 6処理区を設け、 短期Alスクリーニングの結果に基いて選抜したソルゴー15品種とトウモロコシ10品種を用いて、水耕栽培を行った： ①対照区（完全培養液、pH 5.2 ）、②完全＋低Al区( 11.1μM可溶性Al, pH 4.5 ) 、 ③完全＋高Al区( 42.6 μM 可溶性Al, pH 4.3 ) 、④低養分区（完全区の1/5強度[ 4.5mM ], pH 5.2 ）、 ⑤低養分＋低Al区( pH 4.5 ) 、⑥低養分＋高Al区( pH 4.3 ) 。 これによって、Al耐性、低養分耐性、複合耐性（Al耐性と低養分耐性）のソルゴーやトウモロコシ品種間比較と、 収穫後の植物体分析によるAl・養分吸収の特性解析を行った。

　ソルゴー、ダイズ、トウモロコシ品種間に広範囲の短時間Al耐性差が認められ、 すべての植物種で高Al条件と低Al条件の両条件でのAl耐性に正の相関が認められた。 以上の結果、ソルゴーはAl感受性植物で、低Al条件が好適であり、他方、トウモロコシはAl耐性種で、 高Al条件が好適で、ダイズは両植物種の中間であることを明らかにした。

　長期間Al耐性と短期間Al耐性の両結果の関係を比較した。 その結果、両者に有意な正の関係が認められ、実際に全養分の共存するAl培地での長期間生育条件でも、 短期間の単純組成培地でのAl耐性スクリーニング技法が有用であることを明らかにした。 ソルゴー、トウモロコシのいずれにおいても、培地の全Alイオン活動度と根部Al含有率の間に直線的な有意な正の相関が認められ、 また、ソルゴーでの直線式の勾配がトウモロコシよりも2倍以上大きかった。 それゆえ、トウモロコシのAl耐性が強い原因は、根でのAl吸収性の小ささによるものと考えられた。 一方、短期間Al耐性結果は、長期間複合耐性との関係が認められず、この短期スクリーニング技法は、 実際の熱帯酸性土壌での適用に有用でないことが明らかとなった。

　長期間複合耐性は、いずれの植物種においてもAl耐性と低養分耐性の両方と有意な正の関係があった。 そこで、標準偏回帰係数を計算し、両耐性の複合耐性への貢献度合いの比較を行った。 その結果ほとんどの培地条件で、低養分耐性はAl耐性よりも複合耐性に、より大きく貢献していることが明らかとなった （低Al条件のソルゴー、高低両Al条件のトウモロコシ）。高Al条件のソルゴーでのみ、Al耐性が複合耐性の主要因であった。 またソルゴーでは、複合耐性は地上部K含有率、トウモロコシではCa含有率と正の相関が認められ、 高Al・低養分条件では、ソルゴーはK吸収能の高いこと、トウモロコシではCa吸収能の高いことが重要であると示唆された。

　以上の結果、熱帯酸性土壌での作物生産向上のためには、低養分耐性を、より重視すべきであることを本研究で明らかにした。 また、本研究の成果は、新しいこの観点に基づく今後の研究展開への有用な方向性も提供するものである。