Friday, 14 July 2006

Development and Validation of a Hydroponic Screening Method to Identify Acid Soil Adapted Genotypes of the Tropical Forage Grass Brachiaria.

Peter Wenzl, Alba Chaves, Maria Buitrago, Gloria Patino, John Miles, and Idupulapati Rao. CIAT, A. A. 6713, Cali, Colombia

The highly weathered acid soils of the South American savannas are characterized by a combination of nutrient deficiencies (phosphorus, calcium [Ca], magnesium, molybdenum; sometimes nitrogen, potassium) and mineral toxicities (aluminum [Al]; occasionally manganese). Perennial brachiariagrasses (Brachiaria spp.) are the most widely sown forage grasses in these areas, occupying up to 70 million hectares. The Centro Internacional de Agricultura Tropical (CIAT) and the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) are developing apomictically reproducing interspecific hybrids to combine traits of three parental species: acid-soil adaptation of B. decumbens and spittlebug resistance of B. brizantha (both tetraploid apomicts), and sexual reproduction of a tetraploidized, sexual biotype of B. ruziziensis, which lacks both agronomic traits. 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. Edaphic adaptation is difficult to assess because it is only manifest in the persistence of pastures over several growing seasons. Our main objective was to establish and validate a high-throughput hydroponic screening method to evaluate the edaphic adaptation of breeding materials using vegetative propagules (stem cuttings) grown in solution culture. The screening procedure was designed to quantify two key component traits: root vigor and Al resistance. The three main parents of the Brachiaria breeding program (B. decumbens cv. Basilisk, B. brizantha cv. Marandú, tetraploid B. ruziziensis clone 44-02) and a group of 38 B. ruziziensisx B. decumbens hybrids were used to validate the procedure. Root vigor, which determines a plant's nutrient-foraging ability, was assessed by measuring growth of adventitious roots during 21 days in 200 µM CaCl2 (pH 4.2) (solution 1). Aluminum resistance was evaluated by comparing root growth in solution 1 with root growth in an identical solution that also contained 200 µM AlCl3 (solution 2). Scanned root images were analyzed with WinRHIZO software to measure total root length (RL) and the average root diameter (RD) for each individual root system. The pooled RL and RD data were log-transformed and adjusted, by linear regression, for harvest mean and the dry weight of stem cuttings. Aluminum resistance was quantified after regressing the adjusted logarithms of the RL (or RD) values from the Al treatment (solution 2) on those from the basal treatment (solution 1) to remove the variance component reflecting differences in root vigor among the hybrids. Roots of B. decumbens and B. brizantha continued to elongate in solution 1 for the entire period of evaluation (three weeks). Those of B. ruziziensis, by contrast, ceased to elongate after approximately one week and were considerably shorter. 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. For both traits (RL, RD), the acid soil-adapted parent (B. decumbens) was significantly superior to the less-adapted parent (B. ruziziensis). Both traits segregated in the group of 38 B. ruziziensis x B. decumbens hybrids, with root vigor but not Al resistance showing transgressive segregation. Although seedling-based assays have been successfully applied to brachiariagrasses, poor germination of Brachiaria seeds at the surface of nutrient solutions and the poor viability of hydroponically-grown seedlings upon 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. Implementation of a simplified version of this hydroponic screening method, which allows simultaneous assessment of both traits based on visual inspection, has facilitated progress toward edaphic adaptation in the Brachiaria breeding program at CIAT.

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