Browsing by Author "Harris, Sue-Re"

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    Phenotypic plasticity in Mercurialis annua : the role of signal detection and response in sex determination
    (2006) Harris, Sue-Re; Pannell, John; Harris, Mark; Midgley, Jeremy J
    Mercurialis annua has been found to show phenotypic plasticity in terms of their sexual strategies. In androdioecious populations (males coexisting with cosexes), genetically labile males function as cosexes at low densities. At high densities, the frequency of males increases, while cosexes respond by increasing biomass allocation to the female function. Phenotypic plasticity in this species is likely a result of environmental detection. This study investigated possible mechanisms for density detection. Greenhouse experiments showed that Mannua responded to an above ground signal by increasing male frequency and male frequency was related to the proportion female allocation in cosexes. Individuals that were isolated from each other below ground and treated with leachate (collected from high density populations) responded by increasing allocation to female function while male frequency decreased. There is most likely an interaction between below and above ground sensing and response to the environment in M annua.
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    Phosphorus sensitivity in species of Proteaceae (Protea obtusifolia, Leucadendron coniferum and Leucadendron salignum) from different soil habitats : possible candidates for growth on former agricultural soils high in P
    (2006) Harris, Sue-Re; Cramer, Michael D; Hawkins, Heidi-J
    The phosphorus (P) uptake capacities and tolerance to high P of three Proteaceae species from acid (Leucadenron coniferum, Leucadendron salignum) and alkaline (Protea obtusifolia, Leucadedndron meridianum) soils in the Cape Floristic Region (CFR) were compared. These species were also investigated as potential candidates for growth on nutrient enriched, post-agricultural soils. In parallel, two crop plant species (Lupinus albus, with cluster roots; and L. angustifolius, without cluster roots) were also compared. The cluster-root forming species (L. albus) was more sensitive to higher P levels and showed toxicity in terms of photosynthesis, efficiency of PSII, transpiration and stomatal conductance. Growth, gaseous exchange and efficiency of photosystem II (as a measure of stress) were determined at 1, 10 and 100 μg P g⁻¹ sand for both lupins and Proteaceae. Kinetics of P uptake (in hydroponics) was determined at a range ( 4 to 2000 μM P) of P concentrations. Phosphorus sensitivity was relatively high in Ld. coniferum as evident from reduced growth, net photosynthetic rate, stomata} conductance and transpiration rates. Protea obtusifolia showed the least sensitivity to increased phosphorus concentrations under acidic and native alkaline conditions. The inability to regulate P uptake rates at elevated phosphorus concentrations and excessive P accumulation were major determinants of P sensitivity. The relatively higher P sensitivity of Lupinus albus compared to L. angustifolius is likely related to the efficient P uptake by cluster roots in the former.