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CROP BREEDING, GENETICS & CYTOLOGY-NOTE

A Greenhouse Method to Screen Brachiariagrass Genotypes for Aluminum Resistance and Root Vigor

^a Diversity Arrays Technology (DArT) P/L, GPO Box 3200 Canberra, ACT 2601, Australia
^b Dep. of Renewable Resources, Univ. of Alberta, Edmonton, AB T6G 2E3, Canada
^c Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713 Cali, Colombia

^* Corresponding author (peter{at}DiversityArrays.com)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Brachiaria species are widely sown on the infertile and Al-toxic soils of neotropical savannas. Breeding programs seek to combine edaphic adaptation with other traits in interspecific hybrids. Edaphic adaptation is difficult to assess because it is only manifest in pasture persistence across several growing seasons. We developed and validated a solution-culture technique that uses rooted vegetative propagules from mature plants to assess two key components of edaphic adaptation: root vigor and Al resistance. Root vigor was assessed by measuring growth of adventitious root systems in 200 µM CaCl₂ (pH 4.2). Aluminum resistance was assessed by comparing root growth in this solution vs. root growth in an identical solution that also contained 200 µM AlCl₃. The well-adapted parent (Brachiaria decumbens Stapf cv. Basilisk) was superior to the less-adapted parent (B. ruziziensis Germain & Evrad clone 44-02), and both traits segregated as expected in a set of 44-02 x Basilisk hybrids. A simplified version of this technique, which exclusively relies on visual inspection, has been implemented in our breeding program to facilitate progress toward edaphic adaptation.

Abbreviations: RL, total root length

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
BRACHIARIA SPECIES are the most widely sown tropical forage grasses, occupying up to 70 million ha of South American savannas (Fisher and Kerridge, 1996). The Centro Internacional de Agricultura Tropical (CIAT) and the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) are seeking to develop apomictically reproducing interspecific hybrids by combining traits of three parental species: acid-soil adaptation and spittlebug resistance of B. decumbens cv. Basilisk and B. brizantha (Hochst ex A. Rich) Stapf cv. Marandú, respectively (both tetraploid apomicts), and sexual reproduction of a tetraploidized, sexual biotype of B. ruziziensis clone 44-2 (Swenne et al., 1981), which lacks both agronomic traits (Miles and do Valle, 1996; Miles et al., 2004). Efficient screening methodologies are required to recover the desired traits through stepwise accumulation of favorable alleles in subsequent cycles of recombination and selection. There is a need to develop a greenhouse-based method to assess edaphic adaptation of large segregating populations.

Some Brachiaria spp. such as B. decumbens are well-adapted to the soils of neotropical savannas (Paulino et al., 1987; Rao et al., 1996, 1998). Brachiaria ruziziensis pastures, by contrast, tend to degrade within a few years after establishment (Rao et al., 1996; Miles et al., 2004). The acid and infertile savanna soils are characterized by a combination of nutrient deficiencies (most significantly P, but also Ca, Mg, Mo, and sometimes N and K) and mineral toxicities (Al, occasionally Mn) (Rao et al., 1993; Sánchez, 1997). Edaphic adaptation presumably is an aggregate trait expressed in mature plants which comprises physiological components conferring adaptation to these and possibly other stress components.

The objective of this study was to establish and validate a procedure to evaluate the edaphic adaptation of breeding materials using vegetative propagules (stem cuttings) grown in solution culture. The procedure was designed to quantify two key component traits: root vigor and Al resistance. Vigorous growth of roots, particularly fine roots, increases a plant's nutrient foraging capacity and improves its ability to extract nutrients from infertile soils (Marschner, 1995; Rao et al., 1999). Aluminum toxicity was incorporated as a selection target because previous experiments had confirmed that brachiariagrasses differ for this trait (Wenzl et al., 2001).

We initially tested the procedure by monitoring, during up to 3 wk, the growth of the adventitious root system of stem cuttings from the three parental genotypes. Having established its effectiveness, we evaluated a refined version of the procedure with B. ruziziensis, B. decumbens, and a group of 38 B. ruziziensis x B. decumbens hybrids, which were expected to segregate for edaphic adaptation because of the typically high heterozygosity level of apomicts such as B. decumbens (Asker and Jerling, 1992).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Plant Material
The three main parents of the Brachiaria breeding program (B. decumbens cv. Basilisk, B. ruziziensis cv. Marandú, and B. ruziziensis clone 44-02) and 38 different B. ruziziensis x B. decumbens hybrids were used to validate the screening procedure. These 41 genotypes had been propagated vegetatively on soil from an experimental station in Santander de Quilichao (altitude = 990 m; Oxisol–Plinthic Kandiudox; Cauca Department, Colombia) for several years. Soil from the same site was used to grow enough tillers to produce sufficient numbers of stem cuttings for this study. Chemical characteristics of this soil were: pH 4.6 at a soil-to-water ratio of 1:1, 14.7 mg kg^–1 Bray-II extracted P, 1.8 cmol_c kg^–1 KCl-extracted Al, 0.15 cmol_c kg^–1 Bray-II extracted K, 2.3 cmol_c kg^–1 KCl-extracted Ca, and 1.3 cmol_c kg^–1 KCl-extracted Mg.

Before planting, the soil was mixed with sand at a 3:1 ratio, and the mixture was fertilized with (milligram of element per kilogram of soil–sand mixture): 20.6 N (urea), 25.8 P (triple superphosphate), 51.6 K (KCl), 34.0 Ca (dolomitic lime), 18.0 Ca (triple superphosphate), 14.6 Mg (dolomitic lime), 10.3 S (elemental sulfur), 1.0 Zn (ZnCl₂), 1.0 Cu (CuCl₂), 0.05 B (H₃BO₃), and 0.05 Mo (Na₂MoO₄ · 2 H₂O) (Rao et al., 1992). The fertilized soil–sand mixture was distributed into 41 containers, each holding 40 kg. Approximately 10 tillers from each of the 41 genotypes were potted into the containers (one genotype per container).

Forty-five days after potting, vegetative propagules were produced from tillers with three to five leaves and not more than three nodes. The tillers were detached below the lowest node above soil level, and all but the youngest three leaves were removed. The remaining leaves were pruned to approximately 2 cm to reduce transpiration. The resulting stem cuttings were used for solution-culture experiments in the greenhouse. The tillers remaining in the pots were pruned and the soil–sand mixture was fertilized with an identical amount of urea and triple superphosphate as had been used for potting. After 23 to 25 d, a second set of stem cuttings was generated, and the soil–sand mixture was fertilized again with urea and triple superphosphate. After producing a third set of stem cuttings under identical conditions, the remaining tillers were pruned and repotted into a fresh soil–sand mixture that had received a full fertilization. This repotting cycle was repeated four times in the course of this study.

Growth Conditions
All growth experiments with nutrient solutions were performed in the greenhouse at CIAT headquarters (3°30' N, 76°21' W; altitude = 965 m). Typical conditions in the greenhouse were 19 to 36°C, 48 to 96% relative humidity, and 1100 µmol m^–2 s^–1 maximum photon-flux density during the day.

Rooting of Cuttings
The bases of the stem cuttings produced from tillers of potted plants were inserted into 1.5-cm-thick polyurethane foam discs (diam. = 4 cm) and transplanted to racks floating on a large volume of aerated, low-ionic-strength nutrient solution (Fig. 1 ). This solution, known to support close-to-maximum growth of Brachiaria seedlings (Wenzl et al., 2003), contained (in µM): 500 NO₃^–, 50 NH₄⁺, 300 K⁺, 300 Ca²⁺, 150 Mg²⁺, 160 Na⁺, 5 H₂PO4^–, 286 SO₄^2–, 5 Fe³⁺, 1 Mn²⁺, 1 Zn²⁺, 0.2 Cu²⁺, 6 H₃BO₃, 5 SiO₃^2–, 0.001 MoO₄^2–, 5 H₂–EDTA^2–, 332.4 Cl^– (excluding HCl) and 67.8 HCl to adjust the pH to 4.20.

View larger version (81K): [in this window] [in a new window]   Fig. 1. Procedure to identify acid soil-adapted Brachiaria genotypes. Plants were propagated in a mixture of soil and sand (3:1). Vegetative propagules (stem cuttings), excised from these plants, were floated at the surface of a low-ionic-strength nutrient solution to produce adventitious roots. After 9 d, pairs of rooted stem cuttings were selected for homogeneity. One propagule of each pair was transferred to Solution 1 (200 µM CaCl2, pH 4.20), the other to Solution 2 (200 µM CaCl2, 200 µM AlCl3, pH 4.20). Twenty-one days after transfer, roots were separated from stems, stained, and scanned on a flatbed scanner to determine total root length (RL) and average root diameter. Genotypes with vigorous root growth were identified based on RL in Solution 1. Aluminum-resistant genotypes were identified based on RL in Solution 2 after removing the variance component that was due to differences in root vigor measured in Solution 1.

Testing the Refined Screening Procedure
Brachiaria ruziziensis, B. decumbens, and the 38 B. ruziziensis x B. decumbens hybrids were included in 10 successive, partly overlapping experiments. For each experiment, one to three pairs of stem cuttings of each genotype were prepared. The cuttings were rooted and transplanted to one of the two solutions, as described in the previous sections. At harvest, after 21 d of growth, roots were separated from aerial parts. The roots were stained for 24 h in an aqueous solution containing 0.1% (w/v) methylene blue and 0.1% (w/v) neutral red, washed, submersed in a thin layer of water and scanned on a flatbed scanner at 300 dpi because pilot experiments had confirmed that this resolution was sufficient to capture even the finest roots (Fig. 1). The images were analyzed with WinRHIZO software (Régent Instruments Inc., Québec, Canada) to measure total root length (RL) for each individual root system. The aerial parts were dried at 60°C for 48 h and their dry weights recorded.

Staining Root Apices of Selected Genotypes with Hematoxylin
During the last experiment, a small number of root apices were excised from adventitious roots of stem cuttings from B. ruziziensis, B. decumbens, and two hybrids with contrasting levels of Al resistance (all grown in Solution 2 for 12 d). The apices were stained with hematoxylin {7,11b-dihydrobenz[b]indeno[1,2-d]pyran-3,4,6a,9,10(6H)-pentol} according to Ownby (1993). Zones of Al-induced damage were visualized by fixing apices in a 1:1 mixture of phormol [37% (v/v) formaldehyde, 1% (v/v) methanol, ph 7.4] and glutaraldehyde (1,3-diformylpropane), and cutting 70-µm-thick longitudinal sections.

Data Analysis
The pooled RL data from the 10 experiments designed to test the refined screening procedure were log transformed (Causton and Venus, 1981) and adjusted for harvest mean and the dry weight of stem cuttings (Beck et al., 2003; Zeegers et al., 2004). This procedure removed 40.5% of the RL variance in Solution 1 and 32.2% in Solution 2. The removed variance components presumably were due to differences among replicate experiments in growth conditions as well as differences in the amount of carbohydrates and nutrients supplied by stem cuttings to roots.

Aluminum resistance was quantified after regressing the adjusted logarithms of the RL values from the Al treatment (Solution 2) on those from the basal treatment (Solution 1) to remove the variance component reflecting the inherent differences in root vigor among the hybrids. The residual values after regression were expected to be a more informative measure of true Al resistance than the original values from the Al treatment if root vigor and Al resistance were not correlated (Zeegers et al., 2004). Lack of correlation between the two traits was independently confirmed by comparing the genotype means for the adjusted logarithm of RL in Solution 1 (root vigor) against the genotype means for an alternative Al-resistance index (the log-transformed ratio of RL in Solution 2 to RL in Solution 1); the two sets of hybrid means were indeed uncorrelated (r = 0.02).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Initial Evaluation of the Main Parents of the Breeding Program
The commercial brachiariagrass cultivars are widely propagated by stem cuttings, a feature that enables breeders to generate genetically identical clonal propagules for evaluation of phenotypic characters (Cardona et al., 1999). When the basal node of stem cuttings was incubated in a low-ionic-strength nutrient solution for 9 d, all tested Brachiaria genotypes typically produced two to four adventitious roots (Fig. 1). We initially used the three parental species (B. ruziziensis, B. decumbens, B. brizantha) to establish the effectiveness of the hydroponic solutions designed to simulate stress factors of the acid-soil syndrome. Solution 1 contained a low concentration of Ca²⁺ to protect root plasma membranes, but lacked other nutrients (200 µM CaCl₂, pH 4.20). Solution 2 was identical to Solution 1, but also contained 200 µM AlCl₃. Root growth in Solution 1 should reflect the plants' ability to produce an extensive root system that explores a large volume of soil for nutrient uptake. A comparison of root growth between the two solutions should provide a measure of Al resistance.

Stem cuttings of all three parental genotypes continued to produce leaves during the duration of the experiment. Leaves of B. ruziziensis, but not the other two parents, tended to become slightly chlorotic toward the end of the experiment. Roots of B. decumbens and B. brizantha continued to elongate in Solution 1 for the entire period of evaluation (3 wk). Those of B. ruziziensis, by contrast, ceased to elongate after approximately 1 wk and were considerably shorter (Fig. 2 , left panel). Presence of Al in Solution 2 strongly inhibited root elongation of B. brizantha, but had only little effect on roots of B. decumbens. Root growth of B. ruziziensis in this solution was negligible (Fig. 2, right panel).

View larger version (32K): [in this window] [in a new window]   Fig. 2. Initial test of the two treatments (without and with Al) using the three parental genotypes of the Brachiaria breeding program. The length of the longest root was recorded for up to 21 d of growth in the basal treatment (left panel) and the Al treatment (right panel).

Evaluation of a Set of Siblings
We next evaluated, in a series of replicate experiments, the 21-d growth performance of a broader range of genotypes. We selected 38 B. ruziziensis x B. decumbens F1 hybrids (full siblings), which were expected to segregate for edaphic adaptation because of the heterozygosity of apomictically reproducing B. decumbens. In this set of experiments we characterized root growth by measuring total root length (RL).

Root Vigor
Total root length in Solution 1 ranged from 0.7 to 6 m. The latter value is quite remarkable considering the biomass of stem cuttings (mean dry weight: 0.42 g) and the fact that the growth medium contained only 200 µM CaCl₂. Not surprisingly, roots of brachiariagrasses in neotropical savannas represent a carbon sink of worldwide significance (Fisher et al., 1994).

The RL values showed a considerable degree of positive transgressive segregation, perhaps as a result of hybrid vigor (Fig. 3 , left panel). The striking difference in RL between B. ruziziensis (and one of the hybrids) and the other genotypes probably was an artifact of the particular subset of hybrids used for this experiment, because a recent screening of a larger group of B. ruziziensis x B. decumbens hybrids has identified hybrids that are intermediate between those two groups of genotypes (data not presented). The performance of individual genotypes was consistent across harvests, as can be deduced from the mean standard error (0.034 log units) compared with the range of RL values measured for the group of 38 hybrids (0.933 log units) (Fig. 3, left panel). This result confirmed the suitability of Solution 1 for quantifying differences in root vigor among a set of full sibs. Root vigor as measured by this method probably comprises different physiological components expressed in plants at a physiologically mature, vegetative stage such as the frequency of initiation of adventitious roots, the tendency of nutrient-deprived plants to allocate carbon to roots rather than shoots, the efficiency with which nutrient reserves in stem cuttings are remobilized to sustain root growth, the branching of adventitious roots, and the growth of fine roots. Any of these components is likely to have an impact on a plant's adaptation to infertile soils.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Segregation of root vigor (left panel) and Al resistance (right panel) in a group of 38 B. ruziziensis x B. decumbens F1 hybrids. Root vigor is the total root length (RL) in Solution 1 (200 µM CaCl2, pH 4.20), adjusted for the effects of stem-cutting biomass and harvest. Aluminum resistance is the stem-cutting-biomass and harvest-adjusted RL in Solution 2 (200 µM CaCl2, 200 µM AlCl3 pH 4.20), after removing the variance component caused by differences in root vigor. The two parents are highlighted by black symbols. The two hybrids with contrasting levels of Al resistance that were used for the hematoxylin-staining test (Fig. 4) are designated as "R" for Al resistant and "S" for Al sensitive.

View larger version (111K): [in this window] [in a new window]   Fig. 4. Hematoxylin-staining patterns for root apices of genotypes identified by the screen as Al-resistant (left side) and Al-sensitive (right side). The two B. ruziziensis x B. decumbens hybrids used for this test are also highlighted in Fig. 3.

Aluminum resistance among the hybrids seemed to vary quantitatively. In agreement with the results from a seedling-based root elongation assay, Al resistance of B. decumbens was significantly superior to Al resistance of B. ruziziensis (Wenzl et al., 2001). In contrast to root vigor, the two parents were close to the two extremes of the Al-resistance distribution (Fig. 3, right panel). Though based on a limited number of segregants, this segregation pattern would be consistent with multiple genes contributing to Al resistance of adventitious root systems (a mixture of several root types), perhaps a similar situation as in species such as rice and probably maize (Nguyen et al., 2002; Kochian et al., 2004). The absence of positive transgressive segregation suggests that, in this collection of hybrids, B. decumbens provided most of the alleles that contributed importantly to Al resistance. A recent evaluation of 200 additional siblings has indeed confirmed these tentative conclusions (unpublished data).

Aluminum toxicity not only inhibits root elongation, but also induces lateral swelling of roots (Taylor, 1989). Aluminum-sensitive genotypes, therefore, should not only be characterized by a decrease in RL, but also an increase in root diameter. We found that the RL-based Al-resistance index was indeed negatively correlated with a similar index based on root diameter, which was also quantified by the image analysis software (r = –0.75; data not presented). We also validated our method of quantifying Al resistance against the well-established hematoxylin-staining method, using the two parents and two hybrids that were close to the extremes of the range of Al-resistance levels (see arrows in Fig. 3). In agreement with our classification, root apices of Al-sensitive genotypes stained strongly; while those from Al-resistant genotypes remained clear (Fig. 4 ).

Implementation in a Breeding Program
From the combined results of this study we conclude that our procedure screens brachiariagrass genotypes for traits that contribute to adaptation to infertile and acid soils (root vigor, Al resistance) at a physiologically mature, vegetative stage. This conclusion was further verified by testing Brachiaria hybrid ‘Mulato’, a recently released cultivar that had been selected for edaphic adaptation in field trials. The glasshouse screen revealed good root growth and intermediate Al resistance, consistent with its good vigor and responsiveness to applied nutrients on acid soils (data not presented).

We characterized root growth and root-system morphology in detail to test the effectiveness of the two experimental treatments (Solutions 1 and 2) in revealing genetic differences in edaphic adaptation. Preliminary data from a larger hybrid population suggest that this approach is also useful for identifying QTLs contributing to acid-soil adaptation (unpublished data). To maximize the number of segregants that can be screened, the procedure may be simplified. Plants could be cultivated in Solution 1 (basal treatment), but transferred to Solution 2 (Al treatment) a day or two before harvest, followed by hematoxylin-staining of root apices. This would enable breeders to assess separately root vigor (size of root system) and Al resistance (absence of staining), exclusively by visual inspection. Alternatively, plants could be cultivated in Solution 2 only, thus simultaneously selecting for both component traits (Fig. 2, right panel).

The latter approach may exclude potentially useful segregants for component traits. Yet this approach has significantly increased the efficiency of the Brachiaria breeding program at CIAT by enabling breeders to discard quickly a large number of nonadapted genotypes. In a more recent breeding cycle, 745 sexual segregants were screened for both edaphic adaptation and spittlebug resistance in the course of 6 mo, and the best 5% that combine both traits were used for further genetic recombination and improvement. Several hybrid-derived sexual genotypes that are markedly superior to the original sexual tetraploid B. ruziziensis have been identified using this procedure (data not presented).

Comparison with Other Screening Methods
Root vigor of mature plants does not appear to be a widely-used selection criterion in breeding programs targeting edaphic adaptation (see Annicchiarico and Piano, 2004, for an example in the context of drought tolerance). Root vigor of seedlings has received some breeding attention (Price et al., 1997), yet is unlikely to bear much relevance for edaphic adaptation of Brachiaria, which is manifest in the persistence of pastures across several growing seasons. Stem cuttings from mature plants are probably a more suitable material to assess long-term edaphic adaptation because the dramatic differences in root vigor between B. ruziziensis and B. decumbens were not expressed at the seedling stage (Wenzl et al., 2001).

Aluminum resistance is usually assessed in seedling-based assays, either by quantifying root elongation or apical callose concentrations, or by staining root apices with hematoxylin (Kerridge and Kronstad, 1968; Polle et al., 1978; Ruiz-Torres et al., 1992; Llugany et al., 1994; Cançado et al., 1999). Although seedling-based assays have been successfully applied to brachiariagrasses (Wenzl et al., 2001), poor germination of Brachiaria seeds at the surface of nutrient solutions and the poor viability of hydroponically-grown seedlings on transplantation to soil limit their applicability in a breeding program. The Al-resistance screen based on stem cuttings circumvents the transplantation problem and enables the concurrent assessment of root vigor of mature plants as a second component trait contributing to edaphic adaptation. Vegetative propagation also permits simultaneous assessment of a single genotype (clone) for other traits such as insect or disease resistance, nutritional quality, and seed production.

	ACKNOWLEDGMENTS

 
We thank Jaumer Ricaurte, Ramiro García, María Eugenia Recio, Jarden Molina, and Martín Otero for their research support, and Myriam Cristina Duque for inspiring discussions about the data analysis.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
This work was performed at CIAT and funded by the Colombian Ministry of Agriculture and Rural Development, the Commission of Developmental Issues of the Austrian Academy of Sciences (Project No. 78), and the German Ministry of Economic Cooperation and Development (BMZ/GTZ 2000.7860.0-001.0)

Received for publication July 17, 2005.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Annicchiarico, P., and E. Piano. 2004. Indirect selection for root development of white clover and implications for drought tolerance. J. Agron. Crop Sci. 190:28–34.[CrossRef]
• Asker, S.E., and L. Jerling. 1992. Apomixis in plants. CRC Press, Boca Raton, FL.
• Beck, St.R., W.M. Brown, A.H. Williams, J. Pierce, St. S. Rich, and C.D. Langefeld. 2003. Age-stratified QTL genome scan analyses for anthropometric measures. BMC Genetics 4:S31.
• Cançado, G.M.A., L.L. Loguercio, P.R. Martins, S.N. Parentoni, E. Paiva, A. Borém, and M.A. Lopes. 1999. Hematoxylin staining as a phenotypic index for aluminum tolerance selection in tropical maize (Zea mays L.). Theor. Appl. Genet. 99:747–754.[CrossRef]
• Cardona, C., J.W. Miles, and G. Sotelo 1999. An improved methodology for massive screening of Brachiaria spp. genotypes for resistance to Aeneolamia varia (Homoptera: Cercopidae). J. Econ. Entomol. 92:490–496.[ISI]
• Causton, D.R., and J.C. Venus. 1981. The biometry of plant growth. Edward Arnold, London.
• Hede, A.R., B. Skovmand, J.-M. Ribaut, D. Gonzáles de León, and O. Stølen. 2002. Evaluation of aluminium tolerance in a spring rye collection by hydroponic screening. Plant Breed. 121:241–248.[CrossRef]
• Fisher, M.J., and P.C. Kerridge. 1996. The agronomy and physiology of Brachiaria species. p. 43–52. In J.W. Miles et al (ed.) Brachiaria: Biology, agronomy, and improvement. CIAT, Cali, Colombia.
• Fisher, M.J., I.M. Rao, M.A. Ayarza, C.E. Lascano, J.I. Sanz, R.J. Thomas, and R.R. Vera. 1994. Carbon storage by introduced deep-rooted grasses in the South American savannas. Nature (London) 371:236–238.[CrossRef]
• Kerridge, P.C., and W.E. Kronstad. 1968. Evidence of genetic resistance to aluminum toxicity in wheat (Triticum aestivum Vill., Host). Agron. J. 60:710–711.[Abstract/Free Full Text]
• Kochian, L.V., O.A. Hoekenga, and M.A. Piñeros. 2004. How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorus efficiency. Annu. Rev. Plant Biol. 55:459–493.[CrossRef][Medline]
• Llugany, M., N. Massot, A.H. Wissemeier, C. Poschenrieder, W.J. Horst, and J. Barcelo. 1994. Aluminum tolerance of maize cultivars as assessed by callose production and root elongation. Z. Pflanzenern. Bodenk. 157:447–451.
• Marschner, H. 1995. Mineral nutrition of higher plants. 2nd ed. Academic Press, London.
• Miles, J.W., and C.B. do Valle. 1996. Manipulation of apomixis in Brachiaria breeding. p. 164–177. In J.W. Miles et al (ed.) Brachiaria: Biology, agronomy, and improvement. CIAT, Cali, Colombia.
• Miles, J.W., C.B. do Valle, I.M. Rao, and V.P.B. Euclides. 2004. Brachiaria grasses. p. 745–783. In L.E. Moser et al (ed.) Warm-season (C₄) grasses. Agron. Monogr. 45. ASA, CSSA, and SSSA, Madison, WI.
• Nguyen, V.T., B.D. Nguyen, S. Sarkarung, C. Martinez, A.H. Paterson, and H.T. Nguyen. 2002. Mapping of genes controlling aluminum tolerance in rice: Comparison of different genetic backgrounds. Mol. Genet. Genomics 267:772–780.[CrossRef][ISI][Medline]
• Ownby, J.D. 1993. Mechanisms of reaction of hematoxylin with aluminum-treated wheat roots. Physiol. Plantarum 87:371–380.
• Paulino, V.T., D.P. Anton, and M.T. Colozza. 1987. Problemas nutricionais do género Brachiaria e algumas relações com o comportamento animal. Zootecnia (São Paulo) 25:215–263.
• Polle, E., C.F. Konzak, and J.A. Kittrick. 1978. Visual detection of aluminum tolerance levels in wheat by hematoxylin staining of seedling roots. Crop Sci. 18:823–827.[Abstract/Free Full Text]
• Price, A.H., A.D. Tomos, and D.S. Virk. 1997. Genetic dissection of root growth in rice (Oryza sativa L.). 1. A hydroponic screen. Theoret. Appl. Genet. 95:132–142.[CrossRef]
• Rao, I.M., D.K. Friesen, and M. Osaki. 1999. Plant adaptation to phosphorus-limited tropical soils. p. 61–96. In M. Pessarakli (ed.) Handbook of plant and crop stress. Marcel Dekker, New York.
• Rao, I.M., P.C. Kerridge, and M.C.M. Macedo. 1996. Nutritional requirements of Brachiaria and adaptation to acid soils. p. 53–71. In J.W. Miles et al (ed.) Brachiaria: Biology, agronomy, and improvement. CIAT, Cali, Colombia.
• Rao, I.M., J.W. Miles, and J.C. Granobles. 1998. Differences in tolerance to infertile acid soil stress among germplasm accessions and genetic recombinants of the tropical forage grass genus, Brachiaria. Field Crops Res. 59:43–52.[CrossRef]
• Rao, I.M., W.M. Roca, M.A. Ayarza, E. Tabares, and R. García. 1992. Somaclonal variation in plant adaptation to acid soil in the tropical forage legume Stylosanthes guianensis. Plant Soil 146:21–30.
• Rao, I.M., R.S. Zeigler, R. Vera, and S. Sarkarung. 1993. Selection and breeding for acid-soil tolerance in crops. Upland rice and tropical forages as case studies. Bioscience 43:454–465.[CrossRef][ISI]
• Ruiz-Torres, N.A., B.F. Carver, and R.L. Westerman. 1992. Agronomic performance in acid soils of wheat lines selected for hematoxylin staining patterns. Crop Sci. 32:104–107.[Abstract/Free Full Text]
• Sánchez, P.A. 1997. Changing tropical soil fertility paradigms: From Brazil to Africa and back. p. 19–28. In A.C. Moniz et al (ed.) Plant–soil interactions at low pH. Brazilian Soil Sci. Soc., Campinas, Brazil.
• Swenne, A., B.-P. Louant, and M. Dujardin. 1981. Induction par la colchicine de formes autotétraploïdes chez Brachiaria ruziziensis Germain et Evrard (Graminée). Agron. Trop. 36:134–141.
• Taylor, G.J. 1989. Aluminum toxicity and tolerance in plants. p. 327–361. In D.C. Adriano and A.H. Johnson (ed.) Acidic precipitation. Vol. 2. Biological and ecological effects. Springer, New York.
• Wenzl, P., L.I. Mancilla, J.E. Mayer, R. Albert, and I.M. Rao 2003. Simulating infertile acid soils with nutrient solutions: The effects on Brachiaria species. Soil Sci. Soc. Am. J. 67:1457–1469.[Abstract/Free Full Text]
• Wenzl, P., G.M. Patiño, A.L. Chaves, J.E. Mayer, and I.M. Rao 2001. The high level of aluminum resistance in signalgrass is not associated with known mechanisms of external aluminum detoxification in root apices. Plant Physiol. 125:1473–1484.[Abstract/Free Full Text]
• Zeegers, M., F. Rijsdijk, and P. Sham. 2004. Adjusting for covariates in variance components QTL linkage analysis. Behav. Genet. 34:127–133.[Medline]

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