Browsing by Author "Lewis, O A M"

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    An 15N study of the effects of nitrate, ammonium, and nitrate + ammonium nutrition on nitrogen assimilation in Zea Mays L.
    (1984) Murphy, Andrew Telfer; Lewis, O A M
    A brief review of the literature on the effects of nitrate and ammonium nitrogen sources on plant growth, and the assimilation of those nitrogen sources, has been presented. A water culture technique for the growth of maize was developed. The use of a gravel rooting medium with nutrients recycling through the rooting medium, and an iron supplement in the form of ferric citrate, resulted in superior growth of nitrate-fed plants in comparison to that observed with a hydroponic growth technique and a standard Long Ashton nutrient solution. The effects of nitrogen source on plant growth, organic nitrogen and inorganic nitrogen contents, and the rates of incorporation into nitrogenous compounds were studied. The observed differences were explained with reference to the effects of the various nitrogen sources on the physiology of the plants. The experimental techniques included assays of the enzymes nitrate reductase and glutamine synthetase, whole plant growth studies, and the analysis of nitrogenous compounds of xylem exudate and those extracted from the leaf blade, leaf base, and root regions of maize plants after feeding with a nutrient solution containing nitrogen as 15N.
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    Amelioration of salinity toxicity in South African crop plants with special reference to maize
    (1992) Tshivhandekano, Thivhulawi Robert; Lewis, O A M
    The effects of ammonium and nitrate nutrition on potassium uptake, photosynthetic gas exchange and growth responses to salinity stress (80 mM) together with the ameliorative roles of three ranges of calcium concentrations (2.5 to 12 mM, 1 to 8 mM and 0.5 to 5 mM) and one range of potassium concentrations (0.2 to 5 mM) were investigated in Zea mays L. var PNR 394. The ameliorative roles of two ranges of calcium concentrations (1 to 8 mM and 0.5 to 5 mM) were also investigated in salt-stressed maize grown at high (35 °C) and low (25 °C) temperature conditions in order to establish if the role of calcium in enhancing salt-tolerance in maize could be temperature-dependent. The criteria chosen to monitor salinity stress were (i) dry plant mass, (ii) plant moisture content, (iii) photosynthetic performance and (iv) nitrogen uptake.
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    An investigation of aspects of the nitrogen physiology of Helianthus annuus L
    (1988) Kaiser, James J; Lewis, O A M
    Helianthus annuus L. plants were grown hydroponically in Long Ashton nutrient solutions containing either 2 mM ammonium, 2 mM nitrate or 1 mM ammonium+ 1 mM nitrate as the nitrogen supply to determine the effect of these nutrients upon physiological processes within the plant. Nitrate fed plants had a larger root mass than ammonium fed plants whereas ammonium fed plants had a larger shoot mass than those fed with nitrate. Ammonium+ nitrate fed plants produced both large shoots and roots. The fruit mass of plants fed nitrate was greater than plants fed ammonium. Ammonium+ nitrate fed plants produced a greater fruit mass than either of the other single nitrogen sources. Nutrient supplied at late stages of fruit filling was shown to be beneficial to the fruit. Sunflower plants supplied with ammonium+ nitrate made more efficient use of water than ammonium fed plants which produced 2.87g dry mass 1 - 1 water consumed while the mixed feed plants produced 4.15g 1- 1 H2 0 lost. Plants supplied with ammonium nutrient showed a lower net leaf photosynthetic rate than nitrate or ammonium+ nitrate fed plants. Root respiration expressed as CO2 exchange per gram fresh mass was significantly higher in both nitrate and ammonium+ nitrate fed plants than those fed with ammonium only. This indicated a higher requirement for ATP for the transport of nitrate into the root as well as the assimilation of part of that nitrate in the root in comparison to those fed ammonium only which would only require ATP for ammonium assimilation via glutamine synthetase. Analysis of the 15N free amino (+ amido) content of the xylem sap showed that the products of 15 NH 4 + contributed 80% to the xylem sap of plants fed ammonium+ nitrate while the products of 15N0 3 - constituted 20%. Of the total nitrogen content of the xylem sap, nitrate constituted 80% in the nitrate-only, 50% in the ammonium+ nitrate and 0% in the ammonium-only fed plants, whereas ammonium was no greater than 2% for any of the three nutrient sources. Glutamine was the principal free amino compound in the xylem sap for all three treatments. It would appear from these findings that sunflower plants fed ammonium+ nitrate, assimilate ammonium primarily in the roots while nitrate is assimilated in the leaves. Ammonium uptake was shown to be more rapid than that of nitrate, and it appeared to suppress nitrate uptake when the two nitrogen sources were combined. Nitrate appeared to aid in the translocation of the products of ammonium which was primarily assimilated in the root. The assimilation of organic compounds by mixed feed plants was greater than that for plants fed either nitrogen source alone. 15N0 3 - vacuum infiltration of mature leaves on 10 week old plants showed that the products of nitrate assimilation were transported mainly to the capitulum. Even at this late stage of fruit filling, the root was shown still to be processing nitrogen as 26% of the bound 15N in the plant was found in the root. Leaf nitrate reductase activity was 8 times that of the root while glutamine synthetase activity was 7 times, which indicated that the leaf was the major site of nitrate reduction. It was demonstrated that the presence of both casein and polyvinylpyrrolidone indicated that both proteases and phenolic compounds inhibit in vitro NRA and GSA in sunflower plants. Ammonium+ nitrate nutrient was shown to be more beneficial to the growth and fruit production of Helianthus annuus L. plants than either nitrogen source singly.
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    Effects of defoliation on regrowth and carbon budgets of three semi-arid Karoo shrubs
    (1992) Van der Heyden, Francois; Stock, WD; Lewis, O A M
    Plant regrowth, nonstructural carbohydrate utilization patterns, photosynthesis and the partitioning of photosynthetic products in response to foliage removal were studied for the following karoo shrubs: Osteospermum sinuatum, a dwarf deciduous shrub with fleshy leaves, Pteronia pallens, a dwarf evergreen shrub with sclerophyllous leaves and Ruschia spinosa, a dwarf evergreen shrub with succulent leaves. Defoliation adversely affected both vegetative growth and reproductive output for periods up to 26 weeks following foliage removal. A gradient of increasing regrowth capacity with decreasing defoliation intensity and frequency was observed in all species. In terms of biomass production, defoliation was the least detrimental to the deciduous shrub, O. sinuatum, and the evergreen shrub, P. pallens, and the most injurious to the succulent shrub, R. spinosa. All species regrew better during spring and autumn, and no regrowth was recorded in the moderate (40%) or intensely (80%) defoliated plants during summer and winter over the 6-week monitoring periods. Spatial patterns of carbohydrate accumulation were the same for all species, with most of the total nonstructural carbohydrates (TNC) being stored in the twigs and stems. Karoo shrubs can be divided into two distinct groups based on the primary nonstructural polysaccharides accumulated in their plant parts. The Asteraceous plants, O. sinuatum and P. pallens, accumulate predominantly fructans. In contrast, the succulent species, R. spinosa, accumulate starch and fructans in equal proportions. Differences among species in terms of seasonal changes in TNC levels of undefoliated plants reflect the extent to which different species are dependent on stored carbohydrates or photosynthesis for normal vegetative growth processes. Repeated defoliations at a moderate frequency (26-week interval) resulted in the elevation of TNC concentrations of O. sinuatum and P. pallens. In contrast, defoliations at heavy or at leniant frequencies caused decreases in TNC concentrations in all plant parts of Ruschia spinosa. Restoration of plant storage TNC levels in excess of undefoliated plant TNC levels occurred prior to complete vegetative regrowth in the two Asteraceous shrubs which suggests that some factor(s) other than the carbon resource was limiting vegetative regrowth in karoo shrubs. Analyses of short-term changes (2-weekly) in TNC levels in response to defoliation demonstrated the elevation in TNC concentrations of Pteronia pallens plant parts only during the periods when no regrowth was recorded. This phenomenon illustrates that on a short-terms basis, regrowth and over-replenishment of reserves represent two alternate responses to defoliation. However, during periods when regrowth was recorded for P. pallens (autumn and spring), and during all seasons of the year for O. sinuatum and R. spinosa, depressions in TNC concentrations were observed in most plant parts up to six weeks following defoliation. This illustrates the large dependence these shrubs have on stored carbohydrates following defoliation. Defoliation had no effect on the photosynthetic rates of karoo shrubs for at least 11 days following defoliation. Foliage removal resulted in the redistribution of photoassimilates in all plant species. These changes in the allocation of newly produced photosynthates appear to be associated largely with the replenishment of carbohydrate reserves following the initial TNC utilization caused by foliage removal. Comparison of TNC utilization patterns, following defoliation of O. sinuatum in the dark (no photosynthesis) and in the light resolved the question of the relative importance of reserve carbohydrates and photosynthates following defoliation. Reserve carbohydrates were used only for the first 2 weeks following defoliation for respiratory functions while photoassimilates were used for the production of new foliage. The magnitude of nonstructural carbohydrate utilization in the absence of photosynthesis emphasized the importance of continuing photosynthesis to the survival of defoliated karoo shrubs. The differences among species in terms of the timing and the extent of changes (elevations or decreases) in TNC levels in response to defoliation are interpreted as being the result of alterations in plant chemistry which in turn are governed by species specific physiological adaptations to environmental constraints. Rangeland management guidelines are recommended within the framework of the observed short-term and long-term defoliation effects on karoo shrub plant production.
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    An investigation of nitrogen cycling processes in a coastal fynbos ecosystem in the South Western Cape Province, South Africa
    (1985) Stock, William David; Lewis, O A M
    The Cape Floral Kingdom, known locally as "fynbos", is of great scientific and aesthetic interest as well as being economically important as a water source zone, as a source of flowers for the cut flower industry and as a recreation area. Sound ecological knowledge is required in order to manage and conserve fynbos because the extent of this unique, species-rich, endemic flora has been drastically reduced. Nutrients, in particular nitrogen and phosphorus, have been identified as being of crucial importance in the structure and functioning of fynbos ecosystems because of the nutrient poor substrates upon which this vegetation type exists. This study has concentrated on the following aspects of the nitrogen cycle which are considered to be important in describing the functioning of a coastal fynbos ecosystem: a) The nitrogen status of soils supporting coastal fynbos. b) The forms of nitrogen and the seasonal changes in nitrogen concentrations in these soils. c) The influence of fire disturbance on nitrogen form and concentration in these soils. d) An investigation of the nitrogen mineralization process in these soils in relation to control by physical factors, successional age of the stand and the impact of fire on this process. e) The uptake and utilization of different forms of nitrogen by characteristic species of fynbos vegetation. f) The importance of internal recycling of nitrogen as an adaptation to the low nutrient soils of the area. g) The role of atmospheric inputs of nitrogen to the ecosystem, in particular the importance of this source in replacing nitrogen lost during recurrent fires.
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    An investigation of photosynthetic C-fixation in fynbos growth forms and its variation with season and environmental conditions
    (1988) Van der Heyden, Francois; Lewis, O A M
    The seasonal and diurnal patterns of photosynthetic gas exchange and the water relations of seven species of the mediterranean-climate region of South Africa (fynbos) were investigated. The following species, representing the major fynbos elements, were chosen for intensive investigation: Erica plukenetii and Erica hispidula (ericoid element), Thamnochortus lucens and Askidiosperma paniculatum (restioid element), Protea laurifolia and Leucadendron salignum (proteoid element). Metrosideros angustifolia, a shrub of riparian habitats, was also studied.
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    A ¹⁵N study of the effects of nitrate, ammonium and nitrate plus ammonium nutrition on nitrogen assimilation in Hordeum vulgare
    (1985) Chadwick, Susan Glynnis; Lewis, O A M
    A review of the recent literature concerning the assimilation and utilisation of nitrate and ammonium nitrogen within plants has been presented. Barley plants (Hordeum vulgare L.cv. Clipper) were grown hydroponically under controlled environmental conditions. The aerated nutrient solutions contained 2mM inorganic ¹⁴N supplied as either nitrate alone, ammonium alone, a 1:1 nitrate plus ammonium mixture or a 3:1 nitrate plus ammonium mixture. After 20 days of growth ¹⁵N nutrient solutions were substituted. The plant material was harvested four and eight hours after the commencement of the ¹⁵N feeding experiment and prepared for analysis. Xylem sap was also collected for a period of one hour beginning half an hour before each harvest and continuing for half an hour after harvesting. Separate batches of plants were used for harvesting and sap collection. In nitrate-fed plants the shoot was shown to be the main organ of nitrate assimilation. Xylem sap analysis indicated that 66% of the ¹⁵N supply to the shoot was in the form of nitrate and the majority of the absorbed and assimilated ¹⁵N was located in this region. In ammonium-fed plants, however, ¹⁵N-ammonium accumulated in the root with only a very small amount detectable in the xylem sap. Some 93% of the ¹⁵N exported from root to shoot in the xylem stream was in the form of organic nitrogen (mainly glutamine). This indicated that the root was the major organ of ammonium assimilation and that the shoot was the main destination of root assimilated nitrogen.
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    The physiology and biochemistry of the Laminaria Pallida/Carpoblepharis Minima and Ecklonia Maxima/Suhria Vittata associations from South-Western Cape waters, South Africa
    (1984) Stacey, Vivienne Jean; Mitchell, D T; Lewis, O A M
    The two laminarian brown algae Laminaria pallida Grev. ex. J. Ag. and Eoklonia maxima (Osb.) Papenf. are important both economically and as major components of the South-western Cape waters, South Africa. Growing attached to these brown algae are several different species of red algae two of which, Carpoblepharis minima Bart. and Suhria vittata (L.) J. Ag., were chosen and. the L. pallida/C. minima and. E. maxima/S. vittata associations were studied using physiological and biochemical methods. Carpoblepharis minima has only been observed on L. pallida, whereas S. vittata has been found attached to various substrates as well as to E. maxima.