Annual Report 2006/07 - ETH - North-South Centre North-South ...

ZIL research projectsLivestocksystemsresearchFodder resourcesZIL research project, Phase IVProject leaderEmmanuel FrossardContact personsAnnabé Louw-GaumeAlain GaumeSergio Mejía KerguelénCollaboratorsIdupulapati Rao, Joe Tohme,Carlos Lascano, CIAT, Colombia /Michael Kreuzer, ETH ZurichDurationAugust 2003 – August 2007Adaptation of Brachiaria species to low-P soilsmagnitude of the tissue P concentration required for theinduction of these root-mediated rhizospheric mechanismsfor P acquisition. We found evidence that the manifestationof these biochemical markers preceded the appearance ofP-limited plant growth in B. ruziziensis. In relation to functionality,it is proposed that: (i) oxalic acid exudation mightprovide a dual ecological solution to alleviate the effectsof P deficiency and Al toxicity, two major co-existing soil constraintsfor Brachiaria pasture productivity; (ii) acid phosphatasesinvolved in the hydrolysis of organic P might alsooffer an important way to enhance P availability, particularlyas a large proportion of tropical soil P occurs in organic forms;and (iii) functional synergism between oxalic acid and acidphosphatases in Brachiaria species for P acquisition cannotbe excluded.Functional relationships between plant morphology and root physiologyand the plant P nutritional status have been identified. Changes inplant growth and root biochemical mechanisms for enhanced P acquisitionare activated upon decreasing plant P concentrations.Investigations at the ETH Zurich focused on possible relationshipsbetween the phosphorus (P) nutritional status andphysiological markers for P stress in hydroponically grownB. decumbens and B. ruziziensis. The contribution of root exudates,both organic acids and acid phosphatase enzymes,during the development of plant P deficiency was examined.In addition, changes in biomass and root length productionas well as biomass allocation, acting as morphological markersfor P deficiency, were studied. Results indicate that thetemporal induction of oxalic acid exudation and acid phosphatasesecretion are linked to decreases in internal plantP concentrations. Species differed with regard to theEvaluation of a segregating population of 191 Brachiariahybrids for shoot and root traits (work done at CIAT) permittedthe selection of 18 genotypes (8 each considered to bewell-adapted and poorly-adapted in addition to the twoparental lines, B. decumbens and B. ruziziensis) using P-deficientMatazul soil. Further testing included targeted screeningfor root morphological parameters e.g. root length/diameter and root biomass and the identification of a developmentalstage where genotypic differences in these traitsare maximised. The improved screening criteria are currentlyapplied and plant material that is suitable for gene expressionstudies and, subsequently, for candidate gene identificationcould be collected.Progress made during the past year in the identificationof morphological and physiological markers reiterates theimportance of root morphology and root physiology in theelucidation of the adaptive mechanisms for low-P adaptationin Brachiaria grasses.34