Browsing by Author "Newton, R"

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    Ultraviolet irradiation effects on serotinous shape Leucadendron laureolum seeds: altered seed physiology and ultrastructure, and seedling performance.
    (Springer, 1998) Musil, C; Newton, R; Farrant, J
    Dry seeds of Leucadendron laureolum (Lam.) Fourc. (Proteaceae) were exposed for different intervals (range: 7 to 84 days) to visible, UV-A and UV-B radiation of different biologically effective dose (range: 0 to 11.43 kJ m-2 d-1). Changes in seed germination, physiology and ultrastructure, and residual UV effects on seedling performance, were examined. Germination was depressed in seeds following short (7-day) exposures to UV radiation. This depression was intensified with increased UV exposure dose, and most pronounced at shorter UV-B wavelengths. Also glutathione reductase (GR) activities increased in seeds exposed to shorter UV-B wavelengths, but these were unaffected by irradiation dose level in the UV-B range. Electrolyte leakage rates from UV-irradiated seeds were unaltered, which indicated that germination depression did not result from intrinsic membrane damage. The reversal of germination depression (UV-induced dormancy) in UV-irradiated seeds by red light pointed to the possible involvement of phytochrome in this photo-response. Germination depression disappeared in seeds after 56-days irradiation, possibly due to photoreceptor damage by excess UV light. At this stage, all UV irradiated seeds, irrespective of treatment wavelength or dose level, exhibited increased electrolyte leakage rates, which indicated membrane perturbation. Also, increased GR activities were observed in irradiated seeds, but these were proportionately smaller in seeds exposed to shorter wavelength UV-B radiation (9.1 to 35.8% increase) than longer wavelength UV-A (73.4% increase) and visible (97.7% increase) radiation. This implied a metabolic limitation for scavenging of free radicals and peroxides in aging seeds exposed to UV-B radiation, which pointed to accelerated seed deterioration. It was indirectly supported by ultrastructural evidence of sub-cellular damage (lipid coagulation and plasmalemma withdrawal from cell walls) in embryonic tissues of seeds after 84 days UV-B exposure, and reflected in decreased leaf numbers, photochemical efficiencies, and foliar chlorophyll a and carotenoid levels in seedlings cultured from these seeds.
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    Will water abstraction from the Table Mountain Aquifer threaten endemic species? A case study at Cape Point, Cape Town
    (2004) February, E C; Bond, W J; Taylor, R; Newton, R
    THE PRINCIPAL SOURCE OF CAPE TOWN’S future water needs is the Table Mountain Group aquifer. The floral characteristics of the region overlying this aquifer are unique with five endemic and one near-endemic families. Many of these endemics are restricted to areas such as wetlands and marshes that are supplied by aquifer water, making them vulnerable to groundwater abstraction. Here we report the results of a study undertaken at two sites in the Cape Point Nature Reserve using stable hydrogen isotopes to determine the water source of plants to establish dependence on a permanent water supply. The results from the Suurdam site suggest that Erica labialis has a more diverse rooting strategy than Erica multumbellifera, with two plants sourcing water close to the surface, whereas others extract water much deeper down. The results from the Anvil Hill site, however, suggest that the rare Mimetes hirtus uses water very close to the surface that has undergone extensive isotopic fractionation. These results indicate that this species is shallow-rooted and dependent on surface soil water for survival. As such, it would be vulnerable to an even slight lowering of the water table during the dry summer months, when the surface soil water would be replenished mainly from the permanently saturated zone of the wetland. These results have implications for other endemic plant families with the same water requirements.