Characterisation of nutrition responses in three genotypes of white clover (Trifolium repens L.) selected for tolerance to low phosphorus : a thesis presentation in partial fulfilment of the requirements for the degree of Master of Science in Plant Molecular Biology at Massey University, Palmerston North, New Zealand

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This study focused on the differences in root system architecture (RSA) of three white clover (Trifolium repens L.) genotypes, designated 43-7, 45-14 and 47-9 that were isolated from breeding lines selected to tolerate low phosphorus (P). In this study, genetically identical white clover stolons were cut from stock plants at the fourth node, and primary roots emerged (post excision) from either of the two primordia at nodes three or four. The main objective was to establish if differences in RSA were observed in response to a limited P-supply (10 μM KH2PO4) compared with a sufficient P-supply (1 mM KH2PO4). Each of the cut stolons had a dominant primary root that was nominated to be used for the analysis of root growth in terms of elongation and lateral root emergence. The analysis was standardised by designating the branching zone as the region of the primary root that contained visible lateral roots. The remaining region was designated the elongation zone, and did not contain visible lateral roots. In P-sufficient media, the branching zone was approximately 50% of the primary root length by 15 days post excision and approximately 80% by 25 days in the three genotypes. After 30 days post excision, the branching zone was maintained between 85% and 90% of the primary root length. The response to low P was measured after the emergence of visible lateral roots in experiment I and prior to the emergence of visible lateral roots in experiment II. A third treatment group with a reduced sulfur supply (in experiment II) tested the specificity of the P-stress response. In summary, the morphological responses to P-stress were characteristic for each genotype; the changes to the primary and/or lateral roots occurred within seven days from the reduction in P-supply; and the timing of the reduction in P-supply influenced the degree of the response that was observed with respect to the branching zone. The responses to P-stress included a stimulation of the primary and lateral roots in 47-9 with a reduced root biomass under S-stress; a decrease in the number of visible lateral roots in 45-14; and the decreased elongation of lateral roots in 43-7, which reduced the initiation of tertiary roots. The branching zone decreased in 45-14 only when the P-supply was changed after lateral root emergence. In contrast, the branching zone increased in 47-9 only when the P-supply was changed before lateral root emergence. The results suggest that the P-stress response in the roots is specific and utilisation of P and S may differ in the three genotypes.
White clover, Trifolium repens, Root system architecture (RSA), Phosphorus, White clover roots, Phosphorus stress response