V. M. C. Alves

Genetic diversity for aluminum tolerance in sorghum.

Genetic variation for aluminum (Al) tolerance in plants has allowed the development of cultivars that are high yielding on acidic, Al toxic soils. However, knowledge of intraspecific variation for Al tolerance control is needed in order to assess the potential for further Al tolerance improvement. Here we focused on the major sorghum Al tolerance gene, AltSB, from the highly Al tolerant standard SC283 to investigate the range of genetic diversity for Al tolerance control in sorghum accessions from diverse origins. Two tightly linked STS markers flanking AltSB were used to study the role of this locus in the segregation for Al tolerance in mapping populations derived from different sources of Al tolerance crossed with a common Al sensitive tester, BR012, as well as to isolate the allelic effects of AltSB in near-isogenic lines. The results indicated the existence not only of multiple alleles at the AltSB locus, which conditioned a wide range of tolerance levels, but also of novel sorghum Al tolerance genes. Transgressive segregation was observed in a highly Al tolerant breeding line, indicating that potential exists to exploit the additive or codominant effects of distinct Al tolerance loci. A global, SSR-based, genetic diversity analysis using a broader sorghum set revealed the presence of both multiple AltSB alleles and different Al tolerance genes within highly related accessions. This suggests that efforts toward broadening the genetic basis for Al tolerance in sorghum may benefit from a detailed analysis of Al tolerance gene diversity within subgroups across a target population.

Aluminum Effects on Nitrogen Uptake and Nitrogen Assimilating Enzymes in Maize Genotypes with Contrasting Tolerance to Aluminum Toxicity

Abstract The objective of this study was to investigate the effect of aluminum (Al) on NO3 − and NH4 + uptake and on activity/expression of the N‐assimilating enzymes glutamine synthetase (GS1, GS2, and GSr isoforms, EC 6.3.1.2), glutamate synthase (NADH‐GOGAT, EC 1.4.7.14; ferredoxin‐GOGAT, EC 1.4.7.1), and glutamate dehydrogenase (GDH, EC 1.4.1.2) of maize (Zea mays L.) genotypes contrasting in tolerance to Al toxicity. Seeds were germinated on paper towels, screened for uniformity and allowed to grow for 5–6 d in a modified Steinberg nutrient solution, pH 5.5, in a glasshouse, under natural light. Immediately after transference to a growth chamber (12 h photoperiod, 26°C ± 0.5/18°C ± 0.9 day/night temperature, 72% RH and 540 µ Em−2 s−1 quantum flux density) Al stress was imposed by lowering pH to 4.5 and addition of 166 µmoles L−1 AlCl3. Control plants were grown in the same nutrient solution without Al, pH 4.5. Aluminum inhibited NO3 − and NH4 + uptake and decreased shoot dry matter yield in all genotypes used in this investigation. Irrespective of their tolerance to Al toxicity, Al‐treated plants had less nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), and sulfur (S) in their leaves. In roots, Al treatment did not affect dry matter weight but decreased accumulation of N, Ca, and Mg. Phosphorus and potassium (K) tended to accumulate in roots of Al‐treated plants, suggesting that Al inhibited P translocation to the leaves. In roots, Al induced anaplerotic GDH and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activities, but inhibited GS activity. Moreover, in the apex (0–5 mm) and in the 6–15 mm segment of the elongation zone of most genotypes, Al tended to inhibit expression of GSr and favor expression of GS1. This increase in GS1 expression concomitantly with decreased N uptake and elevated deaminating GDH activity are indicative of metabolic changes usually associated with plant senescence. In leaves, Al did not influence GOGAT and GS activities and expression of GS isoforms suggesting it does not disturb assimilation of nitrogenous compounds translocated from roots to leaves. Nonetheless, lower leaf PEPC activity suggests that Al intoxicated plants had lower CO2 − fixation rates. The inverse effect of Al treatment on the activities of GDH and PEPC in roots and leaves, indicates that whereas increased activities in roots were induced by stress related stimuli, decreased activities in leaves were caused by a reduced nitrogen supply to the leaves. Because the earliest effects observed in this investigation appeared only after 24 h of exposure to Al and were similar in both tolerant and sensitive genotypes, it is concluded that the changes observed in N uptake and assimilation into plant amino acids were downstream of the initial, primary Al toxicity event.

Relação entre o zinco "disponível", por diferentes extratores, e as frações de zinco em amostras de solos

Many extracting procedures for the plant-available zinc have been developed. Total soil Zn fractionation has been used as an attempt to understand the reactions of this nutrient in soils and the behaviour of the extracting procedures. This work evaluated the influence of soil characteristics on the concentration of Zn extracted by different procedures and on soil Zn fractions. Samples were collected from the 0-20 cm surface layer of twelve soils, representing the great soil groups of Dark-Red Latosol (LE), Red-Yellow Latosol (LV), Yellow Latosol (LA), Red-Yellow Podzolic (PV), and Quartzi Psaments (AQ). The soil samples received 0 and 20 mg dm-3 of Zn and were incubated for 30 days. Soil Zn concentration was determined after extraction with DTPA-TEA-CaCL2, 0.1 mol L-1 HCl, Mehlich-1 (M-1) and Mehlich-3 (M-3). Zinc fractionation was also carried out to determine exchangeable Zn (ExZn), Zn tied up to organic matter (OmZn), to manganese oxide (MnZn), to amorphous iron oxide (AFeZn), and to crystalline iron oxide (CFeZn). DTPA and M-3 were the extractants with larger sensitivity to the soil characteristics related to the capacity factor (buffering power). The M-1 and HCl extractants presented lower sensitivity and lower correlation with these characteristics due to their larger extraction power. The ratio Zn recovered by the extractant/Zn applied to the soil showed the best correlation with soil characteristics related to the Zn capacity factor. Exchangeable Zn fraction was the largest source of this nutrient to the tested extractants. The fractions of ExZn, OmZn, MnZn AFeZn and CFeZn did not account for the zinc recovered by the extractants, in all cases.

Nitrate uptake by corn under increasing periods of phosphorus starvation

Abstract Single cross seedlings of the male progenitor of the double cross hybrid, BR 201, were grown for seven days in a complete nutrient solution prior to evaluating the kinetics of nitrate uptake under varying periods of phosphorus deficiency. Nitrate uptake decreased 60% after a short period of phosphorus starvation (2 days) and ceased when phosphorus was withheld longer than six days. Nitrate uptake resumed after phosphorus (P) was re‐supplied, but the time required and extent of recovery depended on the length of phosphorus starvation.

Acúmulo de nitrogênio e de fósforo em plantas de milho afetadas pelo suprimento parcial de fósforo às raízes

Two split root trials were carried out in nutrient solution to evaluate nitrogen accumulation as a function of phosphorus supply applied in partial contact to maize roots. After a seven day growth period in complete nutrient solution, maize seedlings were transplanted to joint pots containing 1.6 L of nutrient solution. Roots were equally shared between the pots. In the first trial, two phosphorus levels (0.02 mmol L-1 and 0.1 mmol L-1) and phosphorus and nitrogen placement were evaluated. In the second trial, two nitrogen sources (nitrate and ammonium) and the same nitrogen and phosphorus placement design adopted in the first trial were assessed. There was a greater phosphorus accumulation in the shoots as a consequence of phosphorus supplied to the whole root system as compared to just half of it. Phosphorus supplied to part of the root system did not properly provide this nutrient to the other part, a fact which raises the possibility of inner phosphorus cycling problems in maize plants. These results were verified for nitrogen as well. When jointly supplied to the whole root system, nitrogen and phosphorus caused greater accumulation of nitrogen in the shoots.

Mechanisms of phosphorus efficiency in maize

The National Maize and Sorghum Research Center of the Brazilian Agricultural Research Corporation, has been working for the past two decades to increase plant adaptation to low P levels in acid soils. We have studied several mechanisms in order to clarify the differences between genotypes in relation to phosphorus efficiency using maize standards for phosphorus efficiency developed at Embrapa Maize and Sorghum.

Change in root apical protein and peroxidase activity in response to aluminum in tolerant and sensitive maize inbred lines

The effects of a short-term (80 min) exposure to 222 µM aluminum (Al) on the protein content and expression and on peroxidase activity and isoenzymes in the primary root of maize were evaluated. Two inbred lines differing in their level of tolerance to Al were used: Cateto 237 (tolerant) and L36 (sensitive). The apical 20 mm of the primary root was divided into 2-mm-long segments that were analyzed for total protein content and peroxidase activity. These results demonstrate that the total protein content along the root apex was not affected by Al in the tolerant inbred line, but decreased in the sensitive line. In the apical 2 mm of the root of the sensitive line, the expression of low molecular weight proteins (43 kDa or smaller) was decreased. Expression of low molecular proteins increased in the tolerant inbred line, even though total protein content did not increase. This suggests that some of these proteins could play a role in metal tolerance, perhaps as binding peptides. While the peroxidase activity of the tolerant inbred line did not change with exposure to Al, peroxidase activity in the apical 6 mm of the root of the sensitive line decreased. The tolerant inbred line constitutively expressed more anionic peroxidase isoforms. These results demonstrate that maintenance of protein expression may be an important component of the plants resistance to Al stress, and that resistance to Al stress is associated with the higher expression of anionic peroxidase isoforms.

Matéria seca e acúmulo de nutrientes em genótipos de milho contrastantes quanto a aquisição de fósforo

Com o intuito de avaliar o efeito do estresse de P sobre atributos morfologicos do sistema radicular de genotipos de milho contrastantes quanto a eficiencia na aquisicao de fosforo, conduziu-se um experimento em casa-de-vegetacao da Embrapa Milho e Sorgo, Sete Lagoas, MG, onde foram testados oito genotipos de milho, provenientes do programa de melhoramento da Empresa. O delineamento experimental utilizado foi o inteiramente casualizado, em esquema fatorial 8 x 2, com tres repeticoes, correspondendo a oito genotipos (tres linhagens: L1 = ineficiente, L2 e L3 = eficientes; e cinco hibridos: H1, H2 e H3 = eficientes, H4 e H5 = ineficientes) e dois niveis de P (baixo e alto). As plantas foram crescidas em solucao nutritiva com duas concentracoes de P equivalentes a 2,3 µM e 129 µM. A composicao da solucao nutritiva foi a seguinte, em mg L-1: N-NO3 152; N-NH4 18,2; Ca 141,1; K 90,1; Mg 20,8; S 18,8; Fe 4,3; Mn 0,5; B 0,27; Cu 0,04; Zn 0,15; Mo 0,08; Na 0,04 e HEDTA 20,06. Aos 18 dias do transplantio, verificou-se que, sob condicoes de estresse de P em solucao nutritiva, os hibridos H5, H1 e H2 apresentaram os maiores valores de massa seca total e da parte aerea, em relacao aos demais. De modo geral, tanto os hibridos como as linhagens, quando foram crescidos em solucao com baixo nivel de P, apresentaram maior relacao raiz/parte aerea, do que os materiais que cresceram em solucao com alto nivel do nutriente. A concentracao de N na parte aerea das plantas foi significativamente superior no tratamento com baixo nivel de P na solucao nutritiva. Os hibridos H1 e H2 e a linhagens L3 acumularam as maiores quantidades de P na parte aerea, independentemente do nivel de P na solucao. Esses resultados foram influenciados pelas maiores producoes de producao de materia seca apresentadas pelos referidos materiais. Independente do nivel de P houve variacao no conteudo do nutriente na parte aerea dos hibridos, nao sendo observado o mesmo comportamento para as linhagens.

Genetic variability in sorghum for P efficiency and responsiveness

The complex of acidity in tropical soils includes low pH, toxic levels of Al, low availability associated with high adsorption of phosphorus (P), and a generalized low level of plant nutrients. This soil acidity complex often creates a chemical barrier to root growth and development, decreasing root contact with soil nutrients and water, resulting in even greater water and nutrient stress. The objective of this research was to identify genetic resources of sorghum both efficient and inefficient in P acquisition and responsive and non-responsive to applied P.Thirty six sorghum lines, 12 traditional lines representing both tolerance and susceptibility to Al toxicity and 24 lines derived from crosses between elite B-lines and a source of tolerance to Al toxicity, SC 283 (IS7173C), were evaluated for P acquisition efficiency on a Dark Red Dystrophic Latosolo under Cerrado vegetation at the National Maize and Sorghum Research Center of Embrapa at Sete Lagoas, MG, Brazil. The soil was limed to raise the pH to a range between 5.5 and 6.0. Two levels of soil phosphorus were used, 50% (5ppm) and 100% (10ppm) critical level with no additional P added the second year. The 36 entries were classified into four groups; efficient and responsive to P (ER), non-efficient and responsive to P (NR), efficient and not responsive to P (EN), and non-efficient and not responsive to P (NN), according to their field performance for grain yield. Genetic variability for both P use efficiency and P responsiveness was identified within the set of 36 sorghum line representing both tolerance and susceptibility to Al toxicity.

Fluxo difusivo de zinco em amostras de solo influenciado por textura, íon acompanhante e pH do solo

The diffusive flux (diffusion) is the main form of zinc transport in soil, due to its low concentration in solution. This work aimed to evaluate the diffusion of Zn in samples of three soil types: a loamy texture Dark-Red Latosol (LE), a medium texture Red-Yellow Podzolic (PV), and a sandy texture Red-Yellow Latosol (LV). The diffusion was assessed in samples with the natural pH for the three soil samples and at pH 5.40 for the LE and 6.00 for the PV. Three sources of Zn (ZnCl2, ZnEDTA and ZnSO4) at the rates of 0, 20 and 40 mg dm-3 of Zn were tested. The experimental units were constituted of 400 cm3 of soil placed in PVC rings, 10 cm of diameter and 5 cm high. At the depth of 2.5 cm, a slide of a strong acid exchange cation resin (CR61CZR IONICS, INC.)as a sink for Zn, with the dimensions of 2.0 x 5.0 cm was set. The soil samples were moistened to the field capacity and incubated for 15 days. Then, the resin slides were removed and submitted to the extraction of Zn. Soil pH was an important factor controlling Zn diffusion, which decreased with pH increase. In general, Zn diffusive flux was lower with the increase of the soil clay content. The diffusive flux of Zn was greater in the three soils, when the source was ZnCl2.