The effects of excess manganese on photosynthetic rate and concentration of chlorophyll in Triticum aestivum grown in solution culture

Manganese toxicity, which involves a broad array of physiological responses, has been identified as an important factor limiting plant growth on acid soils. In the experiments reported here, we examined the toxic effects of Mn on chlorophyll content, photosynthesis and respiration in two cultivars (Norquay and Columbus) of Triticum aestivum (wheat) which differ in tolerance of Mn. When grown over a range of concentrations of Mn (0–1 000 μ M ), the Mn-tolerant cultivar maintained higher rates of photosynthesis and respiration, and higher concentrations of chlorophyll a and chlorophyll b , than did the Mn-sensitive cultivar, despite greater accumulations of Mn in leaf tissues. After 5 days growth with 1 000 μ M Mn in solution, the photosynthetic rate fell to 25% of control in the sensitive cultivar and to only 75% of control in the tolerant cultivar. The concentration of chlorophyll a fell to 50% of control in the sensitive cultivar, but did not differ from control in the tolerant cultivar. Greater effects were seen on concentrations of chlorophyll b . which fell to 35% and 55% of control in the sensitive and tolerant cultivars, respectively. Rates of photosynthesis decreased in both cultivars as concentrations of chlorophyll decreased; however, the photosynthetic rate per unit chlorophyll remained constant or increased in the tolerant cultivar and decreased in the sensitive cultivar as concentrations of Mn in solution increased. Thus, in the sensitive cv. Columbus, Mn seemed to have a toxic effect on both chlorophyll content and photosynthesis per unit chlorophyll. In the tolerant cv. Norquay, the only clear effect of Mn was a reduction in chlorophyll content, although direct inhibition of photosynthesis could not be discounted.     

Modelling yield losses of aluminium-resistant and aluminium-sensitive wheat due to subsurface soil acidity: effects of rainfall, liming and nitrogen application

Subsurface soil acidity reduces the growth of roots, which can potentially decrease crop yields. However, the magnitude of these yield reductions is dependent on interactions between factors such as the depth and severity of subsurface soil acidity, plant resistance to acidity, and water and nutrient availability. The Agricultural Production Systems Simulator (APSIM) was used to examine effects of these factors and their interactions on wheat yields in the Mediterranean climatic regions of Western Australia. The model was linked to historical meteorological data of the region (up to 90 different seasons), and was run for three locations representing low, medium and high rainfall zones and three constant but contrasting soil acidity profiles in a deep sandy soil with two wheat cultivars differing in aluminium (Al) resistance. The simulated results showed inherently high variability between seasons in grain yield, rooting depth and nitrogen leaching. Subsurface soil acidity could decrease average grain yields by up to 60%, particularly in soil profiles with acidity in deep layers. The adverse effects of acidity on wheat yields were greater in the high than the low rainfall zone. Amelioration of acidity by 75% in the entire profile or in the top 20-cm layer improved the yield of the Al-sensitive wheat cultivar. Growing Al-resistant wheat partially eliminated the negative effects of acidity on yields in soils with severe subsurface acidity and almost fully eliminated these negative effects in soils with moderate subsurface acidity. The yield benefits arising from growing Al-resistant wheat were greater than those from ameliorating acidity in the 0–20 cm layer by liming. Increasing nitrogen input increased yields of both Al-sensitive and Al-resistant wheat grown in acid soils in all the rainfall zones, but the yield increments were much greater in the high than the low rainfall zones. Applications of nitrogen fertilisers mitigate the effect of acidity on yields of Al-sensitive wheat in soils with shallow (10–40 cm) subsurface acidity. Furthermore, the improved yield by growing Al-resistant wheat and amelioration of acidity was correlated with increased rooting depth and was associated with decreased nitrogen leaching. Possible agronomic management options to combat the subsurface acidity problem are discussed.     

Influence of aluminum in nutrient solutions on chemical composition in two rice cultivars at different growth stages

Summary A solution culture experiment was conducted using 2 rice cultivars (EEA 304, and CICA 4) to determine the effects of Al concentrations on chemical composition. The treatments consisted of five Al concentrations: 0, 10, 30, 40 and 60 ppm.Aluminum content in plant tissues way increased with increasing levels of Al in two cultivars. Increased Al concentrations in the nutrient solution exerted an inhibiting effect on the uptake of N, P, K, Ca, Mg, S, Fe, B, Cu, Zn, and Mn. Rice cultivars responded differently to Al treatments with respect to nutrients uptake. Tolerant cultivar, EEA 304, absorbed more phosphorus compared to susceptible cultivar CICA 4. Macro and micronutrients inhibiting effect was much lower in this Al tolerant cultivar. These results suggested that one of the Al tolerance mechanism in rice cultivars is associated with more efficient nutrients uptake.     

Aluminium and manganese and their relation to calcium in soil solution and needles in three Norway spruce (Picea abies,L. Karst.) stands of Upper Austria

Soil solution and needles of three mature spruce stands in Upper Austria were analysed in order to investigate the uptake and possible toxic effects of Mn and Al, as these two elements become highly mobilised in the soil due to increasing acidity. The Ca/Al molar ratio in the soil solution was below 0.2 in the most damaged stand during almost the whole vegetation period. Despite different dynamics, Al reaches almost identical values in all stands at the end of the vegetation period in both 1-year (current) and 2-year-old needles, respectively. Therefore, needle analysis is not a useful tool for estimation of free Al in the soil. Needle contents of other elements could provide a better information for understanding the forest decline. Mn in the needles correlates significantly with Mn concentrations in the soil solution. As soil Mn will be mobilised by acidic input, Mn needle content can increase to very high levels. Manganese distribution, its interaction with calcium, and possible toxic effects are discussed.     

Liming and deep ripping responses for a range of field crops

Grain yields were measured over 2 seasons from a range of field crops following liming and deep ripping an acid and compacted soil in north-eastern Victoria. Lime (2.5 t ha^–1) substantially reduced the level of exchangeable Al and exchangeable Mn whilst raising soil pH by about 1.0 unit. The crops grown were 7 cultivars of wheat and one cultivar each of triticale, oats, barley, rapeseed, safflower, field pea, chick pea and lupins. With the exception of lupin, liming the soil increased (p=0.05) the grain yield of all crops and cultivars. With the wheat cultivars there were 2 distinct groups with different tolerance to soil acidity. Wheat, oats, triticale and lupins had higher absolute yields than the other crops. Safflower and chick pea had very low yields without soil amendment. The magnitude of the lime response did not differ between the wheat cultivars (17%) or between any of the crop species (range 9–29%). Deep ripping the soil to break a hard compacted layer resulted in more yield for all the cereals and safflower. The results demonstrate the importance of using crops with tolerance to acid soil conditions as well as gains that can be obtained with ameliorating identifiable soil problems.     

Al-Fe interactions and growth enhancement in Melastoma malabathricum and Miscanthus sinensis dominating acid sulphate soils

Plants growing in acid sulphate soils are subject to high levels of Al availability, which may have effects on the growth and distribution of these species. Although Fe availability is also high in acid sulphate soils, little is known about the effect of Fe on the growth of native plants in these soils. Two species dominating this soil type in Asia, viz. Melastoma malabathricum and Miscanthus sinensis were grown hydroponically in a nutrient solution with different concentrations of Al and Fe. Melastoma malabathricum is found to be sensitive to Fe (40 and 100 microm). Application of 500 microm Al, however, completely ameliorates Fe toxicity and is associated with a decrease of Fe concentration in shoots and roots. The primary reason for the Al-induced growth enhancement of M. malabathricum is considered to be the Al-induced reduction of toxic Fe accumulation in roots and shoots. Therefore, Al is nearly essential for M. malabathricum when growing in acid sulphate soils. In contrast, application of both Fe and Al does not reduce the growth of M. sinensis, and Al application does not result in lower shoot concentrations of Fe, suggesting that this grass species has developed different mechanisms for adaptation to acid sulphate soils.     

Soil chemistry and plant distributions in rock habitats of southern Sweden

The vascular plant flora of open land on superficial bedrock in southem Sweden (northwards to 59°N) is described and related to soil chemical properties. including soil acidity (pH), exchangeable Al, Ca, Fe, Mn, Mg, and phosphate, as well as to soil solution pH and concentrations of Al, Ca and Mg, and to the contents of soil organic matter, soil depth and bedrock types (sandstone, gneiss, granites, various dark igneous rocks and limestones). About 120 localities with totally 652 sites (4 m²) have been examined. Experimental evidence for the toxicity of acid soils and mineral nutrient deficiency of neutral and alkaline soils is related to field data. Hydrogen and Al ion toxicity in acid soils and low phosphate solubility in neutral - alkaline soils are identified as major factors limiting the field distributions of rock habitat plants. Some species (e.g., Rumex acetosella and Sedum telephium ) were limited by phosphate also in acid soils. The relative importance of H and Al ion concentrations to plant performance under variously acid soil conditions is discussed, and strong evidence is given for a decisive influence of Al ion toxicity on species diversity at pH-KCI > 4.5. The importance of grazing and former land use is considered briefly and the floristic differences between the western and the eastern half of the study area are discussed originating from differences in general distribution patterns of species and soil chemical properties.     

Temporal changes of selected chemical properties in three manure - amended soils of Hawaii

Soil amendment with organic materials (crop residues animal manure, and green manure) reportedly has positive effects on soil properties, from acidity to plant-nutrient availability. To examine that hypothesis, an incubation study was conducted to assess the changes in some chemical properties of three different tropical soils (Andisol, Ultisol, and Oxisol) amended with chicken manure and green manure (Leucaena leucocephala) at the rate of 10tha(-1). The results showed that organic amendments raised soil pH and EC, regardless of the type of manure used. Manuring lowered the concentrations of Mehlich-3 extractable Ca, P, Mn and Si in all soils and decreased the concentration of Mg in the Ultisol and Oxisol. However, manure amendment led to increases in the concentrations of Mg and K in the Andisol. Organic amendments caused a decrease in KCl extractable Al. Initial soluble C levels were highest in the Oxisol (60mumolg(-1)) and lowest in the Andisol (20mumolg(-1)). The concentration of soluble C decreased exponentially with duration of incubation. Three low molecular weight organic molecules (acetic acid, catechol and oxalic acid) out of the eight tested were found in all manure-amended soils. This study quantified the release of some Al chelating organic acids, the reduction of exchangeable Al, and the changes in major plant-nutrients when organic materials were added to nutrient poor, tropical acid soils.     

Aluminium effects on growth, nutrient net uptake and transport in 3 rice (Oryza sativa) cultivars with different sensitivity to aluminium

Varietal differences in net nutrient uptake rate and transport efficiency in the presence of aluminium have seldom been investigated in rice. Therefore, effects of Al on growth, uptake and transport of macronutrients (K, P, Ca, and Mg) and micronutrients (Fe, Zn, Cu, and Mn) were evaluated in 3 rice cultivars (BG35, DA14 and IR45) with different Al sensitivity. The plants were grown in nutrient solution at pH 4.1. An initial growth was completed in the time interval 1 to 5 days immediately before the addition of Al. The final growth period with Al (0, 140, 280 or 560 μ M ) was completed on day 26. With Al, a comparatively high P accumulation occurred in shoots and roots of the Al tolerant cultivar BG35. In contrast, the Al sensitive cultivar IR45 maintained a relatively high Ca accumulation during the Al treatment. A reduced total net uptake rate of P and Ca by IR45 in the time period 5 to 26 days was due to both a reduced root fresh weight and a reduced net uptake rate per g fresh weight of root. Moreover, net Ca transport to the shoots higher than net uptake rate in DA14 and IR45 at > 140 μ M Al during the test period suggests restricted Ca uptake by the roots in combination with a continuous net loss of Ca from the roots to the shoots as time proceeds. In the case of Mg and Mn, there was a general reduction of net uptake rates, irrespective of Al sensitivity of cultivars. With Al treatment, comparatively high accumulation of Fe, Zn and Cu occurred in the roots of IR45, concomitant with a high net Zn and Cu uptake rate. It is concluded that differences in Al sensitivity among rice cultivars BG35, DA14 and IR45 are not primarily linked to the depressed internal Mg or Mn status of the plants but rather to changes in the uptake and distribution of Ca and P.     

Responses of maize grain yield to changes in acid soil characteristics after soil amendments

An experiment was conducted from 1997 to 2000 on an acid soil in Cameroon to assess the effectiveness of cultivating acid tolerant maize (Zea mays L.) cultivar and the use of organic and inorganic fertilizers as options for the management of soil acidity. The factors investigated were: phosphorus (0 and 60 kg ha^?1), dolomitic lime (0 and 2 t ha^?1), organic manure (no manure, 4 t ha^?1 poultry manure, and 4 t ha^?1 of leaves of Senna spectabilis), and maize cultivars (ATP-SR-Y – an acid soil-tolerant, and Tuxpeño sequia – an acid susceptible). On acid soil, maize grain yield of ATP-SR-Y was 61% higher than the grain yield of Tuxpeño sequia. Continuous maize cultivation on acid soil further increased soil acidity, which was manifested by a decrease in pH (0.23 unit), exchangeable Ca (31%) and Mg (36%) and by an increase in exchangeable Al (20%). Yearly application of 60 kg ha^?1 of P for 3 years increased soil acidity through increases in exchangeable Al (8%) and H (16%) and a decrease in exchangeable Ca (30%), Mg (11%) and pH (0.07 unit). Lime application increased grain yield of the tolerant (82%) and susceptible (208%) cultivars. The grain yield increases were associated with a mean decrease of 43% in exchangeable Al, and 51% in H, a mean increase of 0.27 unit in pH, 5% in CEC, 154% in exchangeable Ca, and 481% in Mg contents of the soil. Poultry manure was more efficient than leaves of Senna producing 38% higher grain yield. This yield was associated with increases in pH, Ca, Mg and P, and a decrease in Al. The highest mean grain yields were obtained with lime added to poultry manure (4.70 t ha^?1) or leaves of Senna (4.72 t ha^?1). Grain yield increase was more related to the decrease in exchangeable Al (r = ?0.86 to ?0.95, P<0.01) and increase in Ca (r = 0.78–0.94, P<0.01), than to pH (r = ?0.57 (non-significant) to ?0.58 (P<0.05)). Exchangeable Al was the main factor determining pH (r = ?0.88 to ?0.92, P<0.01). The yield advantage of the acid tolerant cultivar was evident even after correcting for soil acidity. Acid soil-tolerant cultivars are capable of bringing unproductive acid soils into cultivation on the short run. The integration of soil amendments together with acid soil-tolerant cultivar offers a sustainable and comprehensive strategy for the management of acid soils in the tropics.     

Honeylocust (Gleditsia triacanthos L.) root response to aluminium and calcium

Honeylocust (Gleditsia triancanthos L.) root growth response to varying levels of Al and Ca in soil solutions was examined in two horizons each of two forest soils. With results from all four horizons combined, multiple regression analysis indicated that both Ca and Al were significant (p<0.01) factors affecting root elongation, branching and biomass production. Over a wide range of Al and Ca concentrations in soil solutions from four different soil horizons, the Ca:Al ratio was a significantly better predictor of honeylocust root response to acid soils than Al or Ca alone.     

Nutritional characteristics in leaves of native plants grown in acid sulfate,peat, sandy podzolic,and saline soils distributed in Peninsular Thailand

Acid sulfate soils, peat soils, sandy podzolic, and saline soils are widely distributed in Peninsular Thailand. Native plants adapted to such problem soils have grown well, and showed no symptom of mineral deficiency or toxicity. Dominant plants growing in low pH soils (acid sulfate and peat) were Melastoma marabathricum and Melaleuca cajuputi. Since M. marabathricum accumulated a huge amount of aluminum (Al) in leaves, especially in new growing leaves, it can be designated an Al accumulator plant. While M. cajuputi did not accumulate Al in shoot, it can be designated an Al excluder plant. Both plant species adapted well to low pH soils, though a different strategy was used for Al. On the other hand, in acid sulfate and peat soils, M. cajuputi, Panicum repens, Cyperus haspan, and Ischaemum aristatum accumulated large amounts of Na in the leaves (or shoots), even in soil with low exchangeable Na concentration. Thus, when growing in the presence of high Al and Na concentration in soils, plant species have developed two opposite strategies: (1) Al or Na accumulation in the leaf and (2) Al or Na exclusion from the leaf. Al concentration in leaves had a negative relationship with the other mineral nutrients except for N and Mn, and Na concentration in leaves also had a negative relationship with P, Zn, Mn, Cu, and Al. Consequently, Al and Na accumulator plants are characterized by their exclusion of other minerals from their leaves.     

Deschampsia cespitosa and soil acidification: general and trait‐specific responses to acid and aluminium stress in a solution experiment

Genetically based adaptation and phenotypic plasticity represent important means of coping with natural or human‐induced increases in soil acidity. In the present study, we examined the role of phenotypic plasticity in the grass Deschampsia cespitosa by testing for general and trait‐specific responses to acid and aluminium (Al) stress. We sampled tussocks (genets) from sites in southern Sweden differing in their exposure to acid deposition, and quantified the performance of each genet under low pH and high Al levels in a solution experiment using the length and biomass of both shoots and roots as response variables. In agreement with results from a previous solution experiment, the overall performance (expressed as total biomass) declined under acid and Al stress, and there was no evidence for local genetic adaptation with respect to acidity. Three Öland populations showed signs of being stimulated by high Al levels, despite originating from relatively basic soils. We observed a significant increase in root length under high Al levels and hypothesize that this response may be adaptive in the natural soil environment, allowing growing roots to “escape” patches of soil with toxic concentrations of this element. Our results for D. cespitosa indicate that phenotypic plasticity has the potential to mitigate the negative effects of soil acidity in this species.     

The role of arbuscular mycorrhizas in decreasing aluminium phytotoxicity in acidic soils: a review

Soil acidity is an impediment to agricultural production on a significant portion of arable land worldwide. Low productivity of these soils is mainly due to nutrient limitation and the presence of high levels of aluminium (Al), which causes deleterious effects on plant physiology and growth. In response to acidic soil stress, plants have evolved various mechanisms to tolerate high concentrations of Al in the soil solution. These strategies for Al detoxification include mechanisms that reduce the activity of Al³⁺ and its toxicity, either externally through exudation of Al-chelating compounds such as organic acids into the rhizosphere or internally through the accumulation of Al–organic acid complexes sequestered within plant cells. Additionally, root colonization by symbiotic arbuscular mycorrhizal (AM) fungi increases plant resistance to acidity and phytotoxic levels of Al in the soil environment. In this review, the role of the AM symbiosis in increasing the Al resistance of plants in natural and agricultural ecosystems under phytotoxic conditions of Al is discussed. Mechanisms of Al resistance induced by AM fungi in host plants and variation in resistance among AM fungi that contribute to detoxifying Al in the rhizosphere environment are considered with respect to altering Al bioavailability.     

Foliar and wood chemistry of sugar maple along a gradient of soil acidity and stand health

The decline of sugar maple (Acer saccharum Marsh.) in forest of north-eastern North America is an important environmental issue. In this study, relationships between, soil, wood and foliar chemistry were assessed for 17 stands distributed within a large area of the Quebec sugar maple forest and that were growing on soils with a strong gradient of acidity and base saturation. There were many significant relationships between variables describing the acid-base status of the top-B soil (Ca and Mg concentrations, exchangeable acidity and base saturation) and Ca and Mn concentrations and Ca/Mn and Mg/Mn in tree tissues. Manganese was the element that showed the strongest inverse non-linear relationships with top-B soil base saturation with variance explanation of 71 and 65%, for wood and foliage, respectively. The 17 sites were divided in two groups according to their level of decline. The declining stands had significantly higher wood Mn and Mg concentrations and lower Ca/Mn ratios and significantly higher foliar Mn and lower Ca and Al concentrations. It was impossible to determine if these differences were a cause or a symptom of sugar maple health. However, the increase in Mn concentrations in tree tissues with increasing soil acidity, as well as the higher Mn concentrations in declining as compared to healthy stands suggest that Mn, as well as low Ca availability, could be an important contributing factor in the sugar maple decline.     

成土母质和种植制度对土壤pH和交换性铝的影响

研究了我国南方 3种不同成土母质发育的酸性自然土壤的 p H与交换性铝含量状况和种植不同作物后它们的变化。研究结果表明 ,自然土壤的 p H平均值大小顺序为雷州半岛地区玄武岩发育砖红壤≤粤中花岗岩发育赤红壤 <粤北石灰岩发育红壤 ,土壤交换性铝含量的平均值的大小顺序为粤北石灰岩发育红壤 <雷州半岛地区玄武岩发育砖红壤 <粤中花岗岩发育赤红壤。种植茶树将使这 3种土壤的 p H显著降低 ,交换性铝的含量则将不同程度升高 ,最为明显的是种茶后石灰岩发育的茶园土的 p H在这 3种土壤中变为最低 ,交换性铝数量的增加幅度最大。种植水稻将提高土壤的 p H和交换性铝的含量。轮作花生后土壤的 p H也将上升 ,但受施用石灰的影响土壤交换性铝的含量将降低     

Soil aluminium effects on uptake,influx, and transport of nutrients in sorghum genotypes

Sorghum [Sorghum bicolor (L.) Moench] is the fifth most important cereal crop of the world. In South America, it is grown mainly on acid soils, and its production on these soils is limited by deficient levels of available P, Ca, Mg, and micronutrients, and toxic levels of Al and Mn. A greenhouse experiment was undertaken to evaluate the genotypic differences in sorghum for uptake (U), inhibition (IH), influx (IN) into roots, and transport (TR) to shoot for nutrients at three levels of soil Al saturation (2, 41, 64%). Overall shoot nutrient U, IN, and TR showed a significant inverse correlation with soil Al saturation and shoot Al concentration, and a significant positive correlation with shoot and root dry weight. The nutrient uptake parameters differentiated genotypes into most and least efficient categories at various levels of soil Al saturation. The nutrient uptake parameters showed significant differences with respect to soil Al saturation, genotypes, and their interactions. In the current study, Al tolerant genotypes recorded higher IN and TR for P, K, Ca, Mg, Zn, and Fe than Al-sensitive genotypes. Therefore, these U, IN, and TR traits could be used in selection of sorghum plants adaptable to acid soils. Sorghum genotypes used in this study showed intraspecific genetic diversity in U, IN, and TR for essential nutrients. It was concluded that selection of acid soil tolerant genotypes and further breeding of acid (Al) tolerant sorghum cultivars are feasible.IICA/EMBRAPA/World BankIICA/EMBRAPA/World BankIICA/EMBRAPA/World Bank     

Substrate conditions,foliar nutrients and the distributions of two canopy tree species in a Costa Rican secondary rain forest

A 28 yr old secondary lowland rain forest in Costa Rica was dominated by two tree species of contrasting ecologies, Vochysia ferruginea - a species typical of well-drained but infertile soils of high Al saturation, and Cordia alliodora, a species requiring soils of at least moderate fertility. The two species exhibited markedly different spatial distributions in the forest and we sought to determine whether or not these different distributions were related to variation in substrate conditions and the nutrient requirements of the two species. Two soil types were present: Ultisols (Typic Haplohumults) and Inceptisols (Typic Dystropepts) and topography was of low hills. The study formed part of an analysis of site quality for timber production in the two species and therefore used standard forestry methods for such analyses. 36 plots of 20 m×20 m were established using predetermined criteria of site uniformity, presence of at least four dominant trees of at least one of the study species, and absence of disturbance. The abundance (number of individuals 10 cm dbh) of each species was determined in each plot. Soil samples were taken in 27 of the 36 plots, 9 plots being considered to replicate conditions already sampled, and 14 soil chemical and physical variables were measured using standard methods. Foliar nutrient analyses were carried out for trees of both species with crowns fully exposed to sunlight, distributed across the gradient of soil conditions. Variations in species abundances were not related to soil type, but plots in which Vochysia ferruginea was more abundant tended to be on steeper slopes with soils of higher exchangeable acidity and lower concentrations of Mn; Cordia alliodora was more abundant on gentler topography where soils had much lower exchangeable acidity but higher Mn. Differences between the two species in foliar nutrient concentrations were marked and supported previous interpretations of their nutrient requirements; foliar nutrients of Vochysia ferruginea were typical of tree species of moist tropical forest on infertile soils, and those of Cordia alliodora typical of a species requiring more fertile soils. Factors such as the distributions of seed trees at site abandonment may affect the spatial distributions of tree species in secondary forests such as that studied. The relationships of the distributions of the two species to substrate variation, however, and their evidently different nutrient requirements, support the hypothesis that variation in the composition and structure of the forest studied is at least partially related to exchangeable acidity and its dominant cation, Al.     

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum

Background and Aims

Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance.

Methods

Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared.

Key Results

Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar ‘Chuan’ was significantly lower than that in the most Al-sensitive cultivar ‘Liuku2’. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in ‘Chuan’ and ‘Liuku2’ in the absence of Al, but Al treatment resulted in increased PME activity in ‘Liuku2’ compared with ‘Chuan’. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in ‘Liuku2’.

Conclusions

Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.     

Monitoring of aluminium-induced inhibition of root elongation in four maize cultivars differing in tolerance to aluminium and proton toxicity

Four maize cultivars, which differ in tolerance to acid soils under field conditions ( Zea mays L., acid soil-tolerant C 525 M, BR 201 F and Adour 250, and acid soil-sensitive HS 7777) were used to study the influence of pH (4.3 and 6.0) and Al (0, 20 and 50 μ M ) on the elongation of seminal roots in nutrient solution. Root elongation was inhibited by high H⁺ concentrations (pH 4.3) in cultivars C 525 M, Adour 250 and HS 7777 but not in BR 201 F. After 20 h exposure to Al, root elongation rates were more inhibited in cultivars BR 201 F and HS 7777 than in C 525 M and Adour 250. The use of a computerized linear displacement transducer system with high resolution (1 μm) allowed the monitoring of short-term responses of root elongation to Al. In the three cultivars affected by H⁺ toxicity, but not in the acid-tolerant BR 201 F, Al supply caused an immediate, but transient increase of relative root elongation rates. This result supports the hypothesis that Al-induced growth stimulation is caused by amelioration of proton toxicity. The time required for 20 μ M Al to induce a 5% decrease of root elongation rates was shorter in the Al-sensitive BR 201 F (33 min) and HS 7777 (86 min) than in the Al-tolerant C 525 M (112 min) and Adour 250 (146 min) cultivars. However, the response-time to Al may be overestimated in the proton-sensitive cultivars, due to the transient stimulation of root elongation rates induced by Al. According to our results, experiments intended to investigate primary mechanisms of Al toxicity should be started after less than 30 min exposure to toxic Al concentrations, using pH conditions which avoid Al-induced growth stimulation due to amelioration of proton toxicity.