Browsing by Author "Stevens, Gary"
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- ItemDiversity of burial rates in convergent settings decreased as Earth aged(Springer Nature, 2016) Nicoli, Gautier; Moyen, Jean-François; Stevens, GaryThe evolution and the growth of the continental crust is inextricably linked to the evolution of Earth’s geodynamic processes. The detrital zircon record within the continental crust, as well as the isotopic composition of this crust, indicates that the amount of juvenile felsic material decreased with time and that in geologically recent times, the generation of new crust is balanced by recycling of the crust back into the mantle within subduction zones. However it cannot always have been so; yet the nature of the crust and the processes of crustal reworking in the Precambrian Earth are not well constrained. Here we use both detrital zircon ages and metamorphic pressure-temperature-time (P-T-t) information from metasedimentary units deposited in proposed convergent settings from Archaean, Proterozoic and Phanerozoic terrains to characterize the evolution of minimum estimates of burial rate (km.Ma−1) as a function of the age of the rocks. The demonstrated decrease in burial rate correlates positively with a progressive decrease in the production of juvenile felsic crust in the Archaean and Proterozoic. Burial rates are also more diverse in the Archaean than in modern times. We interpret these features to reflect a progressive decrease in the diversity of tectonic processes from Archaean to present, coupled with the emergence of the uniquely Phanerozoic modern-style collision.
- ItemMaking granites : understanding the melting of Earth's crust(Stellenbosch : Stellenbosch University, 2005) Stevens, GaryInaugural address delivered by Gary Stevens during October 2005.
- ItemThe regulation of Phosphoenolpyruvate (PEP) metabolism via Phosphoenolpyruvate Carboxylase (PEPC) in P-deficient roots and nodules of Virgilia divaricata(Stellenbosch : Stellenbosch University, 2015-12) Stevens, Gary; Valentine, Alexander; Steenkamp, Emma; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: Plants exhibit a flexible array of morphological, physiological and biochemical adaptations during phosphorous limitation. Legumes are vulnerable to P deficiency, because it affects their ability to fix atmospheric nitrogen (N2). In particular, legumes from nutrient-poor ecosystems, such as the Fynbos in the Cape Floristic Region (CFR) evolved on P deficient soils and may therefore display unique adaptations to soil P stress. In general, very few studies on legumes have focussed on the belowground structures of nodules as a plant organ. Moreover, even less is known about the P stressed responses in nodules from legumes in nutrient-poor ecosystems. The aim of this research was to investigate the metabolic flexibility of organic acid and amino acid metabolism in the nodulated root system of the Fynbos legume Virgilia divaricata, during low P stress. Virgilia divaricata, which grows in the Cape Floristic Region, was used in this study to enhance our knowledge regarding the vital role that the cytosolic enzyme, phosphoenol pyruvate carboxylase (PEPC) plays in phosphoenol pyruvate (PEP) metabolism, in roots and nodules of this legume during phosphate stress. V. divaricata was grown under glasshouse conditions (20 - 25°C) in sterilized quartz sand for 2-3 months whilst being inoculated with the nitrogen fixation bacteria, Burkholderia phytofirmans, which was isolated from V. divaricata nodules grown in fynbos soil. Two phosphate treatments, 5 μM and 500 μM, were applied simulating low-phosphate and high phosphate conditions respectively using a modified Long Ashton Nutrient Solution to simulate a low nutrient ecosystem such as the Cape Floristic Region. Roots and nodules were then analysed for growth kinetics, nutrient acquisition and distribution, enzyme activity and genetic responses. It was shown that during phosphate deficiency, V. divaricata nodules experienced less Pi stress than roots, due to increased metabolic phosphate conservation reactions during organic acid synthesis via an increased PEPC activity. The increased PEPC activity resulted in an increase in downstream metabolic products such as organic acids, (malic acid and citric acid), and amino acids (glutamate, aspartate and asparagine). Although the biological nitrogen fixation (BNF) declined, the high efficiency of BNF may be underpinned by these altered phosphate conservation pathways and enhanced resource allocation during growth particularly under low phosphate (LP) conditions. Therefore, it can be concluded that the efficiency of the nodules via an increased allocation of resources and P acquiring mechanisms in V. divaricata may be the key to the plant’s ability to adapt to poor P environments and thus sustaining its reliance on BNF. From the data obtained as well as previous findings, it has been established that the phosphate conservation mechanisms in roots and nodules, involve the non-adenylate requiring PEPC-bypass route. 13C Nuclear magnetic resonance (NMR) gave us a better understanding regarding the incorporation rates of the PEPCderived C into malate, α-ketoglutarate and asparagine. It therefore is suggested that V. divaricata nodules may use their large PEPC-derived malate pool to prevent large declines in BNF under low phosphate conditions. The nodules of V. divaricata were able to offset an excessive drop in BNF, despite a decline in inorganic phophosphate (Pi) levels. It therefore appears that nodules have evolved to acquire different mechanisms than roots to adapt to phosphate deficiency in order to maintain their function. This was achieved via increased regulation of nodule PEPC and its downstream products. This implies that compared to roots under low P, nodules alter the metabolism of PEPC derived C, in order to maintain nodule respiration and amino acid synthesis. This trait could be observed in the synthesis of larger 13C malate pools of nodules compared to roots, from PEPC, which was underpinned by their different regulation mechanisms of enzyme activity, of the same protein isoform. Since malate is a potent inhibitor of PEPC activity, roots appear to have invested in more PEPC protein compared to nodules. In contrast, nodules with lower PEPC protein, achieved greater enzyme activity than roots, possibly due to higher phosphorylation in order to reduce the malate effect. The subsequent metabolism of this PEPCderived malate, caused roots and nodules to synthesise asparagine via different pathways. These findings imply that roots and nodules under P stress, synthesise their major export amino acid, asparagine, via different routes. This research has generated new knowledge regarding the physiological impact of the organic and amino acid metabolism, derived from PEPC-C in the roots and nodules of legumes growing in nutrient poor ecosystems. It has demonstrated for the first time that the nodules of legume from a nutrient-poor ecosystem rely on improved resource allocation, Pi distribution, and PEPC-derived organic acids to maintain the efficient functioning of N assimilation under P stress. This may be a consequence of having evolved in a nutrient-poor ecosystem, so that nodule-bacteroid respiration and N metabolism can be maintained in P-poor soils such as the Fynbos.