Response of soyabeans (Glycine max.L.) to high levels of alluminium in the soil
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Soyabean, Glycine max.L, is the most important oil crop in the Temperate and Sub-tropical regions. The grain consists of twenty percent oil, which makes it the most important crop for producing edible oil. Soyabean is sensitive to high levels of aluminium in the soil, a situation which limits its production. High rainfall areas of Zambia are characterized by acid soils. This study evaluated twenty varieties of soyabean from different seed companies, both in the laboratory and field. The laboratory experiment was done at the University of Zambia, School of Agricultural Sciences. The field experiment was done in the 2012/13 season at Seedco Lusaka West Farm and Liempe Farm. The Lusaka West Farm was treated as an optimum environment with pH 6.5 with aluminium content of 0.11 ppm, and Liempe Farm had 0.44 ppm levels of aluminium with pH 4.3. The laboratory experiment was in four phases: Determination of discriminatory level of aluminum, screening of genotypes, Hematoxylin test and Plant tissue analysis of aluminium content. The determination of discriminatory level of aluminium was carried out in order to determine the level of aluminum which was most injurious to soyabean so that the injurious level can be used to screen the rest of the genotypes. Five genotypes were used and the levels of aluminium used were 0,4,8,12,16 and 20mg/l in hydroponics. The results showed that as the aluminium levels increased, the parameters taproot length, shoot length, shoot biomass and root biomass decreased. The highest reduction was at 16mg/L and this was the discriminatory level that was identified. The level that was used in this experiment was 20mg/L for effective discrimination because it was noticed that even at that level, there was a decrease in all the parameters. The screening of all the genotypes was based on 0mg/L, as the control, and 20mg/L. The results showed highly significant differences, at probability ≤ 0.01, among genotypes. Semeki and Samba were chosen be the most tolerant varieties to Aluminium because their shoot biomass percentage decreases from level 0 to 20 mg/L were 5.77 and 6.74 respectively. While Spike, Hernon 147B and S810/6/10 were identified to be susceptible with shoot biomass percentage decrease of 74.42, 75.51 and 66.77 respectively. The Hematoxylin test was carried out to also select varieties tolerant to aluminium toxicity. The results from Hematoxylin helped group the genotypes into three: tolerant which included Samba, Semeki, Scribe, Squire, Sirocco, Score, Kaleya and Saga, moderate tolerant which included Sovereign, Satelite, Safari, Sequel, Soprano, Lukanga, Dina and Magoye, and susceptible varieties which included S810/6/10, Hernon 147B and Spike. Hematoxylin results confirmed the results found in the screening of genotypes which showed that Semeki and Samba were tolerant while Spike, Hernon 147B and S810/6/10 were susceptible. The plant tissue analysis to determine the amount of aluminium in the plant tissue was carried out to identify the mechanism of tolerance. It was noticed that the tolerant varieties, Samba and Semeki had 97.33 and 108.61mg of Al/kg root sample respectively which was lower than in the susceptible varieties, Spike, S810/6/10 and Hernon 147B with 543.38, 508.21 and 549.22mg of Al/kg of root sample. The mechanism of tolerance was determined to be exclusion type. Single site analyses for both Liempe farm and Lusaka West farm data showed highly significant differences, at p≤0.01, among genotypes. Generally, genotypes performed better at Lusaka West Farm than at Liempe farm with the highest yield at Lusaka West Farm being 4.08t/ha from Squire and 3.17t/ha from Safari at Liempe farm. The genotypes selected to be tolerant in the laboratory, showed some stability in yield at both sites with Samba having a yield reduction percentage between Lusaka West and Liempe of 18.18 and Semeki 17.65 respectively. While the susceptible varieties Spike, Hernon 147B and S810/6/10 had their yield percentage decreases of 79.86, 82.61 and 76.95 respectively. In order to establish the most important parameter affecting yield at Liempe Farm, we determined the cause and effect relationship between yield and other parameters measured at that site. A stepwise multiple regression analysis was done where yield, as a response variable, was regressed on number of pods per plant, number of pink nodules per plant, root biomass, weight of 100 grains, plant height, days to flower, mature and pod shuttering. The results indicated the most important parameter that affected yield was number of pink nodules per plant because it contributed 51.3 %. A comparison of lboratory results to field results was done using and Orthogonal Contrast of the performance between the tolerant and susceptible varieties based on yield and number of pink root nodules per plant at Liempe farm. We discovered that highly significant differences appeared in the comparison between tolerant and susceptible genotypes at p≤0.01 for both yield and number of pink root nodules indicating that the Laboratory results were repeatable in the field. Hence laboratory screening can be used to select genotypes for aluminium tolerance.
- Agricultural Sciences