[Truncated] Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status; these specialised root structures, together with their functioning, are considered a desirable trait as we face the era when rock P reserves for fertilisers are being depleted. Much research has been conducted on the mechanism of cluster-root functioning in both Lupinus, as crop plants, and some Australian/Chilean Proteaceae species. However, the cause of variation in cluster-root formation among cluster-root forming Lupinus species is unknown. This PhD research project investigated whether the variation in biomass allocation to cluster roots is an environmentally or genetically regulated process among Lupinus species. I aimed to study the relationship between cluster-root formation, plant P status, medium P concentration and plant relative growth rate. I determined if cluster-root-forming Lupinus species with a similar shoot P status and relative growth rate had similar cluster-root investment (Chapter 2). Furthermore, I investigated if cluster-root-bearing Lupinus species grown with a similar P availability but with a higher relative growth rate, as dependent on light availability, showed a greater investment in cluster roots (Chapter 3). Finally, whether cluster-root formation and carboxylate release from cluster roots is increased at higher internal sugar and/or auxin concentration, irrespective of P supply, was investigated in L. albus; the alternative that carboxylate exudation depends on a local low-P status of the roots was also explored (Chapter 4).
Phosphorus treatments caused major effects on cluster-root allocation (Chapter 2), with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg-1 sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20% of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant. Multiple regression analysis showed that cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2014|