Aluminum is usually regarded as the determining factor for plant growth in acid soils and nutrient deficiencies are often additional growth‐limiting factors in tropical acid soils. Taking into account the potential interactions between Al toxicity and nutrient deficiencies, the present study investigated sorghum (Sorghum bicolor Moench [L.]) and maize (Zea mays L.) cultivar differences for: (1) Al tolerance (relative growth in a one‐fifth strength nutrient solution [low‐nutrient medium, ionic strength: 4.5 mmol L−1] with Al and without Al), (2) low‐nutrient tolerance (relative growth in a low‐nutrient medium compared with growth in a full‐strength nutrient solution), (3) combined tolerance (relative growth in a low‐nutrient medium containing Al compared with a full‐strength medium lacking Al). The goal of the present 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 15 cultivars of sorghum and 10 cultivars of maize in short‐term assays (2.5 or 20 µmol L−1 AlCl3 in 0.2 mmol L−1 CaCl2 at pH 5.0 or 4.9, respectively, for 24 h) was positively correlated with Al tolerance in long‐term cultures (11.1 or 42.6 µmol L−1 soluble Al in the low‐nutrient medium at pH 4.5 or 4.3, respectively, for 29 days). However, the level of Al tolerance in the short‐term assays was not correlated with the combined tolerance, suggesting that a short‐term screening technique may not be practically useful for estimating cultivar adaptation to a combination of stress factors in tropical acid soils. In sorghum, a less Al‐tolerant plant species, higher Al tolerance was associated with less Al absorption by the roots and greater K translocation into the shoots. In maize, a more Al‐tolerant plant species, there was no correlation between the accumulation or transport of elements and Al tolerance. Standardized partial regression coefficients suggested that low‐nutrient tolerance contributed more to combined tolerance than Al tolerance under most conditions (except for Al‐sensitive sorghum at 42.6 µmol L−1 AlCl3). A greater combined tolerance was associated with a higher K shoot concentration in sorghum and a higher Ca shoot level in maize. 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.
We investigated variations in aluminum (Al) tolerance among rice plants, using ancestor cultivars from the family line of the Al-tolerant and widely cultivated Japonica cultivar, Sasanishiki. The cultivar Rikuu-20 was Al sensitive, whereas a closely related cultivar that is a descendant of Rikuu-20, Rikuu-132, was Al tolerant. These two cultivars were compared to determine mechanisms underlying variations in Al tolerance. The sensitive cultivar Rikuu-20 showed increased permeability of the plasma membrane (PM) and greater Al uptake within 1 h of Al treatment. This could not be explained by organic acid release. Lipid composition of the PM differed between these cultivars, and may account for the difference in Al tolerance. The tolerant cultivar Rikuu-132 had a lower ratio of phospholipids to Delta(5)-sterols than the sensitive cultivar Rikuu-20, suggesting that the PM of Rikuu-132 is less negatively charged and less permeabilized than that of Rikuu-20. We used inhibitors of Delta(5)-sterol synthesis to alter the ratio of phospholipids to Delta(5)-sterols in both cultivars. These inhibitors reduced Al tolerance in Rikuu-132 and its Al-tolerant ancestor cultivars Kamenoo and Kyoku. In addition, Rikuu-132 showed a similar level of Al sensitivity when the ratio of phospholipids to Delta(5)-sterols was increased to match that of Rikuu-20 after treatment with uniconazole-P, an inhibitor of obtusifoliol-14alpha-demethylase. These results indicate that PM lipid composition is a factor underlying variations in Al tolerance among rice cultivars.
We previously developed a new simple technique of methylene blue (MB) staining for the discrimination of aluminum (Al)‐tolerant protoplasts from 4 plant species (rice, oats, maize and pea). The objectives of the present study were to confirm the applicability of this technique to a wider range of plant species, cultivars and lines, and to identify a common strategy for the early stage of Al tolerance. A total of 10 plant species, two Brachiaria spp., two Oryza spp., buckwheat, maize, pea, triticale, wheat and barley, corresponding to 18 different plant samples (species, cultivars, and lines), were used. Al tolerance (relative net root elongation of the longest root), which was screened at 20 μM AICI3 in 0.2 mM CaCl2 (pH 4.9) for 24 h, ranged widely from 10 to 88. Among cultivars and lines within the same species, Al accumulation in the root tip portion was higher in Al‐sensitive plants, corresponding to more severe permeabilization of the plasma membranes (PM). Protoplasts isolated mainly from the epidermis, and outer and central parts of the cortex were stained to different degrees by MB, and the blue color was observed both on the surface and inside the protoplasts. Color pictures obtained after staining for 3 min with 0.1% MB were analyzed by Image Hyper II. The ratio of the heavily stained area at threshold 95 to the entire area stained with MB at threshold 125 was defined as MB stainability. MB stainability was negatively correlated with Al tolerance (y= 48.6e−0.02x, R2= 0.676**) suggesting the common importance of permeation characteristics of PM, in addition to PM negativity for Al tolerance in a wide range of plant species, cultivars and lines. Analysis of the PM lipid composition was proposed as an important topic for future studies on the negativity and permeation of PM.
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