African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 18, Num. 4, 2010, pp. 155-164

African Crop Science Journal, Vol. 18, No. 3, 2010, pp. 155 - 164

Tolerance to aluminium toxicity IN Tanzanian sorghum genotypes

J.H. Ringo, E.E Mneney, A.O. Onkware¹, B.A. Were¹, E.J. Too¹, J.O. Owuoche² and S.O. Gudu¹

Mikocheni Agricultural Research Institute, P. O. Box 6226, Dar Es Salaam, Tanzania
¹ Department of Biological Sciences, Moi University, P. O. Box 1125-30100, Eldoret, Kenya
²Department of Biotechnology, Moi University, P. O. Box 1125-30100, Eldoret, Kenya

Corresponding author: ringojustinh@yahoo.com

Code Number: cs10019

ABSTRACT

Aluminium (Al) toxicity is a major abiotic constraint on grain sorghum (Sorghum bicolor L. Moench) production on acid soils in East Africa. Aluminium in acidic soil inhibits water and mineral uptake from and consequently, reduces plant vigour and yield. A study was done to determine genetic diversity of Tanzania's sorghum for response to Al toxicity. Five day old seedlings of 98 sorghum genotypes were subjected to 0, 148 or 222.25 moles of Al³⁺ supplied as Al2 (SO4)3.16H2O in Hoagland's nutrient solution. Seedlings were raised in a growth chamber for five days, after which root lengths were recorded. Net root growth was used to discriminate the germplasm into phenotypic groups. The genotype MCSR T33 exhibited highest net root length and was classified as tolerant. Wahi, MCSR T69 and MCSR T11 were moderately tolerant, while the rest were susceptible.

Key Words: Genetic diversity, root length, Sorghum bicolor

RÉSUMÉ

La toxicité aluminique est une contrainte majeur à la production du sorhgo (Sorghum bicolor L. Moench) sur les sols acides en Afrique de l'est. L'aluminium (Al) des sols acides inhibe l'assimilation d'eau et de minéraux du sol, et réduit par conséquent la vigueur des plantes et le rendement.Une étude était faite pour déterminer la diversité génétique du sorgho de la Tanzanie en réponse à la toxicité aluminique. Les plants agés de 5 jours issus de 98 génotypes de sorgho étaient soumis à 0, 148 ou 222.25 moles de Al³⁺ fournis sous forme de Al2 (SO4)3.16H2O dans une solution de nutriment de Hoagland. Les plantules étaient plantées dans la chambre de croissance pendant 5 jours après lesquels la longueur des racines était mesurée. La croissance nette des racines était utilisée pour séparer les racines en groupes phénotypiques. Le génotype MCSR T33 avait exhibé une longueur nette plus élevée des racines et était classifié comme tolérant. Wahi, MCSR T69 et MCSR T11 étaient modérément tolérant, alors que les restes étaient susceptibles.

Mots Cles: Diversité génétique, longueur des raciness, Sorghum bicolor

INTRODUCTION

Sorghum (Sorghum bicolor L. Moench) is one of the important staple cereals in the semi-arid regions of the world (Rohrbach et al., 2002). It is an important food and feed crop, and is becoming an industrial crop used in biofuels and brewing. It is an appropriate crop for cultivation in semi-arid lands of Eastern Africa because of its relative tolerance to drought. Moreover, it performs better under low soil fertility than the other locally grown cereals. It has been identified as a crop that can improve livelihoods of the vulnerable communities (World Bank, 2005) living in the arid environments. However, lack of superior cultivars arising from limited research on sorghum improvement in the region, coupled with drought and acidity often result in low yields (800 kg^-1 compared to 2-3.0 t ha^-1 world average) (INTSORMIL/ USAID, 2006).

Aluminium toxicity often occurs in acidic soils and is one of major abiotic stresses that limit sorghum productivity worldwide (Magalhaes et al., 2004). Moreover, over 40% of the arable lands are acidic (Von Uexkull and Mutert, 1995). Most sorghum production in East Africa occurs on soils with pH<5.5. In fact, in some parts, Al saturation is high (4-55%) and dramatically affects the availability of phosphorus (Kanyanjua et al., 2002). Acid soils cover more than 15% of the agricultural land in Tanzania (MARI, 2006), and over 7.5% in Kenya of arable land (Kanyanjua et al., 2002).

Previous studies indicate that Al tolerance in plants is largely influenced by a putatively orthologous series of at least two major loci that are inherited as major Al tolerance genes in sorghum and wheat (Magalhaes et al., 2007). Tolerance to Al in sorghum is controlled by a major gene Alt_SB, located on chromosome 3 (Magalhães et al., 2004). The quantitative trait locus (QTL) located on chromosome 1 of rice is orthologous to the Alt_SB sorghum gene, while the QTL found on chromosome 3 of rice is orthologous to the Alt_BH wheat genes (chromosome 4DL) and to barley Alp on chromosome 4H (Magalhães et al., 2004). The ALMT1 gene, which encodes a malate transporter activated by Al, was cloned by Sasaki et al. (2004); and was found to be related to Al tolerance in wheat. In rice (Oryza sativa), Al tolerance is a quantitative trait and QTL studies identified Al tolerance loci in all the 12 rice chromosomes (Nguyen et al., 2003).

Several techniques have been developed for more rapid evaluation of tolerance to soil acidity. Among those is bioassay that includes nutrient solutions (Duncan et al., 1983; Magnavaca et al., 1987). Screening sorghum genotypes for tolerance to Al toxicity has been done through Al-induced root growth inhibition (Magalhaes et al., 2004), callose production and Al-content in root tips in nutrient solution (Baligar et al., 1989). Solution culture is cheap, fast and the most commonly used method in Al toxicity screening experiments. It provides easy access to root systems, tight control over nutrient availability and pH, and non-destructive measurement of tolerance (Carver and Ownby, 1995). It has been applied for Al tolerance analysis in alfalfa (Baligar et al., 1989), cowpea (Paliwal et al., 1994); barley (Ma et al., 1997); maize (Conaado et al., 1999); tomato and rape (Luo et al., 1999); Soybean (Villarcia et al., 2001) and in sorghum (Magalhaes et al., 2004). The inhibition of seminal root growth by Al in the nutrient solution is used to quantify Al tolerance in crops. Magnavaca et al. (1987) developed an extensively applied protocol that uses basal nutrient solution for screening for Al tolerance. Root length measurement is the most suitable criterion for Al stress is studies in maize and sorghum. It is also suitable for identifying genotypes with superior alleles for Al tolerance (Hede et al., 2002).

This study was done to identify new sources of Al tolerance in sorghum and determine the level of variation for tolerance to Al toxicity in the Tanzanian sorghum germplasm.

MATERIALS AND METHODS

Sorghum accessions used in the study. Ninety eight sorghum accessions were collected from sorghum growing areas in Tanzania (Table 1a, b, c, d). Five commercial released varieties (Hakika, Macia, Pato, Tegemeo and Wahi) were obtained from Ilonga Research Centre in Morogoro (9^o 4' 0" S and 36^o 51' 0" E) in Tanzania. Sorghum standards for Al tolerance were obtained from International Crops Research Institute for the Semi-Arid (ICRISAT).

The study materials were screened for Al tolerance using nutrient solution as the growth media according to procedure described by Magnavaca et al. (1987). Seedlings were subjected to Al treatments of 0 (control), 148, 222 mM supplied as AlK (SO₄)₂.16H₂O. Sorghum seeds were surface sterilised in 1% sodium hypochlorite (NaOCl) for 8 minutes and then rinsed through 8 times using sterile distilled water. Seeds were then germinated between moistened sterilized 20 cm x 20 cm Velvex ^® paper towels in an incubator at 25 ^oC in the dark for 3 days. Initial root length (irl) was measured before the seedling were put in growth plastic cups (2.5 cm x 3.5 cm). Loaded cups were placed on 32.5 cm x 32.5 cm plastic rafts and transferred to trays containing 8-litre nutrient solutions. The seedlings were raised in a growth chamber with continuous aeration of the nutrient solution aeration pump (FIMA^® air compressor) for 5 days at a pH of 4.2.

Temperature and light were maintained at 26 ^oC and 550 µmol photons per square metre per second, respectively. Final root length (frl) was measured from the root tip to the base on the 5^th day after transfer to nutrient solution. The net root length (nrl) was used to group sorghum into tolerant and sensitive phenotypic classes.

Data were subjected to analysis of variance and means separated by Least Significant Difference at 5% probability level using SAS Version 8 (SAS, 2002).

RESULTS

Overall, final root length and net root length differed significantly (P<0.05) with Al concentration (Table 2). The highest root reduction was observed at 222 mM Al treatment and plants grown in this (highest) Al concentration had stunted roots with blackish tips, typical symptoms of Al on the meristematic region. This treatment was too severe even for the cultivars that appeared to tolerate the stress imposed by 148 mM Al.

Genotypic differences in Al tolerance among the screened sorghum germplasm was very clear from the fact that the root growth of the genotypes screened in the solution culture varied. Based on net root growth (nrl), MCSR T33 had nrl of 1.94 and was above the Al tolerant standard check. The standard check, ISCR 110 had nrl of 1.70 cm. Three sorghum genotypes, MCSR T69, T53 and MCSR T 11 were closer to the standard check. On the basis of the same parameter (nrl), sorghum genotypes were grouped into three different classes that is tolerant, medium tolerant and sensitive (Table 3).

DISCUSSION

Although sorghum root growth was impaired by the presence of Al in the nutrient solution, there was differential response of genotypes to Al stress (Table 2). Normally, the root is the plant organ most affected by Al toxicity, and more specifically the root tip is considered to be the main site for Al toxicity (Archambault et al., 1997). As a result, root elongation is considered to be the most sensitive parameter under short-term exposure to Al and, therefore, may represent the whole-plant reaction to Al. The inhibition of root elongation seems to explain the retardation in plant growth through reduced nutrient and water uptake, consequently resulting in poor yield. The variability in Al tolerance has previously been noted in sorghum (Magalhaes et al., 2006), barley (Tamas et al., 2006) and maize (Ligeyo, 2007). This experiment based on net root length to discriminate the genotypes into respective tolerance groups.

The distinct difference in root growths at different levels of aluminium concentration in the nutrient solution indicates that after exposing sorghum roots to aluminum treatments for 5 days, the nutrients uptake by the seedlings was limited due to effect of aluminum on the tips. Root tips are directly involved in nutrients and water absorption by plants. The tolerant genotypes showed little effect of aluminium across the treatments and had better growth.

It was also found that 148 mM Al concentration was sufficient to discriminate tolerant Tanzanian sorghum genotypes from sensitive ones. However, Al concentration at 222 mM was too high and this classified tolerant genotypes into sensitive. Majority of the Tanzanian genotypes screened in this study were sensitive to Al stress. Majority of sorghum growing areas are reported to have soils with pH ranging from 4.5 to 5.5 (MARI, 2006). Therefore, cultivation of the broad germplasm largely aluminium sensitive could be one of the contributing factors to low sorghum production. This justifies the need to breed and select for sorghum cultivar(s) tolerant to Al stress.

Several experiments for selection of genotypes tolerant to Al in the nutrient solution have been successfully conducted in sorghum (Furlani and Clark, 1981; Giaveno et al., 2001). Galvez and Clark (1991) demonstrated that two sorghum genotypes maintained their relative differences to Al toxicity tolerance independently whether they were grown separately or in the same nutrient solution. According to Magalhaes et al. (2006), genetic variation for Al tolerance in plants has allowed the development of cultivars that are high yielding on acidic, Al toxic soils.

Only one accession MCSR T33 of the sorghum genotypes screened for Al tolerance was classified as tolerant. This genotype had relatively higher net root growth in aluminium treatment as compared to the standard check (Table 2). MCSR T33 was collected from the southern Tanzania. The most sensitive genotype, MCSR T60 was collected from Musoma rural in the Mara region of Tanzania. Three genotypes were in medium tolerant class, while the remaining genotypes were sensitive to Al toxicity. The medium tolerant genotype T53 (Wahi) is at the same time a Striga tolerant variety (Mbwaga, 2006) which make it a suitable candidate to be included in breeding programmes for developing a multiple stress varieties of sorghum.

ACKNOWLEDGEMENT

The authors thank Swedish Government (SIDA) through BIO-EARN Project for funding this work. Dr. Mary Mgonja of ICRISAT provided sorghum standard materials.

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