ANNABÉ LOUW-GAUME1, ALAIN GAUME1, IDUPULAPATI RAO2, EMMANUEL FROSSARD1
1Swiss Federal Institute of Technology (ETH), Swiss Centre for International Agriculture (ZIL); Institute of Plant Sciences, Switzerland
2CIAT Cali, Colombia
Brachiaria grasses are the most widely planted tropical forage grass. However, mechanisms of Brachiaria that contribute to successful adaptation to low-fertility acid soils are poorly understood. Since phosphorus availability in soil is the major limitation in many tropical ecosystems, the main focus is on understanding P acquisition and use in these grasses. Comparative studies between the two species, B. decumbens (with good acid soil adaptation) and B. ruziziensis (poorer adaptation), are conducted by following growth in soils as well as under hydroponic conditions. Adaptations for increased phosphate availability and Pi uptake efficiency include mechanisms to expand plant root surface area, enhanced expression of Pi transporters, increased organic acid synthesis and exudation, phosphatase secretion and mycorrhizal symbiosis. Interest in root growth in Brachiaria grasses stems from the observation that root traits appear to be important for genotypic differences in plant adaptation to low-P soils. Studies to compare shoot and root growth responses of both Brachiaria species in acidic Colombian soil treatments, showed that B. decumbens grows better than B. ruziziensis in low-P soil. This was mainly due to the ability of B. decumbens to develop a highly branched actively growing root system. It is possible that this grass could access less available P forms from low-P soil. 31P-NMR studies are undertaken to elucidate the organic-P component in the soils. Results from pot experiments to determine the specific contribution of P fertilisation (from mineral and organic P sources) as a factor for the successful establishment of B. decumbens on the low-P soil will be presented. Low-P adaptation of Brachiaria species has been attributed in part to mycorrhizal associations, therefore, such a contribution cannot be excluded since mycorrhizal infection was particularly evident in B. decumbens in the low-P soil. Plant responses towards conservation of P use include decreased growth rate, increased growth per unit of P uptake and remobilisation of internal Pi. P use efficiency in the Brachiaria species will be addressed in future studies, and, in particular, the distribution of plant P into metabolic pools and physical compartments to understand maintenance of Pi homeostasis.
Keywords: Brachiaria, forage grass, genotypes, phosphorus efficiency, Pi homeostasis, root traits, tropical soils