Breeding Alfalfa for Aluminum Tolerance in Southern Brazil
Miguel Dall'Agnol; João H.S.Caetano and Daniel P. Montardo
UFRGS, Faculdade de Agronomia, Dep. Plantas Forrageiras e Agrometeorologia,
91540-000 Porto Alegre, RS, Brazil
Alfalfa is a very sensitive species to acid soils containing aluminum (Al), requiring high amounts of limestone to produce under these conditions. A breeding program was started 4 years ago in order to solve or at least decrease this problem.
The base germplasm for this program was a widely-used landrace, brought by the European immigrants many years ago, called Œ‚Crioula". There were tree breeding strategies, described previously (Caetano, 1998): (i) screening under soil conditions, using acid soil partially limed; (ii) in solution culture containing only calcium (Ca) and Al and (iii) a combination of both methods. There were two selection cycles for each breeding strategy. For (i), the soil was limed in order to obtain an Al saturation of 2.5%. The selection was made using cups with 700g of soil containing 10 plants each, inoculated with an Al-tolerant strain (Hartel & Bouton, 1989). The selection was realized after six weeks of growth and it was based on root and shoot growth. The solution culture (ii) had only calcium (0.62 mmol dm-3) and Al (3.7
: mol dm-3) and a pH of 4.5. Three-day old seedlings were transferred to the solution containing Ca and Al. The selection was done after 12 days of growth and it was based on visual estimates of growth. For (iii), the first selection cycle was realized in soil and the second one in solution culture.After the screening procedure, the progress made was assessed by testing the selected materials under greenhouse conditions in soil with a pH of 6.6 and in solution culture as previously described. The variables evaluated were root weight and length and shoot weight and height.
The results from the solution culture evaluation showed that all the selected populations had a greater initial and final root length than the base population (p<0.05), ranging from 18 to 51%.
In relation to the soil testing at a pH of 6.6, the results showed that all the selected populations had a better top growth than the base population (p<0.05), either expressed as shoot height or weight. The root weight and length presented a similar trend with the populations showing a greater weight and at least the same length than the base population. These results indicated that the selection for Al tolerance did not decrease the yield potential in the absence of the stress but actually increased.
Therefore all the methods were efficient in improving the aluminum tolerance without decreasing the yield potential and that there was still enough genetic variation in the base population for selection for aluminum tolerance.
The seeds of the best populations are being increased in order to allow for testing under field conditions.
References
Caetano, J.H.S. 1998. Alfalfa selection for acid soils. Porto Alegre: UFRGS, 129p. Master Dissertation (in Portuguese).
Hartel, P.G. and Bouton, J.H. 1989. Rhizobium meliloti inoculation of alfalfa selected for tolerance to acid, aluminum-rich soils. Plant and Soil 116: 283-285.