The steps involved in plant reductive assimilation of sulphate to sulphide for incorporation into cysteine are not clear. The aim of the project described in this thesis was the isolation of genes encoding sulphate assimilation enzymes which will provide molecular tools for unravelling this key metabolic pathway. Functional complementation of the Escherichia coli cysteine auxotrophic strain JM15 (serine acetyltransferase deficient) using an Arabidopsis thaliana cDNA library in the expression vector YES resulted in the isolation of at least three members of an A. thaliana multigene family encoding serine acetyltransferase. Characterisation of one clone, Sat-1, showed that it conferred serine acetyltransferase activity (with apparent KM for substrates acetyl CoA and L-serine of 0.043 and 3.47 mM, respectively) on strain JM15. The 1515 bp full-length cDNA encodes a deduced protein of 391 amino acids, SAT-1, that has significant identity with bacterial and plant serine acetyltransferases, and that contains a putative N-terminal organellar targeting peptide. Southern hybridisation indicated that Sat-1 is present as a single copy in A. thaliana, while northern analysis revealed a single message of 1.5 kb. Using the A. thaliana cDNA library in the expression vector YES, cDNAs encoding a novel putative "APS reductase" were obtained by functional complementation of E. coli cysteine auxotrophic strains JM81A (adenosine 5'-phosphosulphate [APS] kinase deficient) and JM96 (3'-phosphoadenosine-5'-phosphosulphate [PAPS] reductase deficient). Retransformation of three clones (Papsr-19, Papsr-26 and Papsr-43) encoding different putative APS kinase isoforms into strain JM96 conferred low PAPS reductase activity on the mutant, although this activity was thioredoxin-independent unlike wild-type bacterial activity. The putative APS reductase has a PAPS reductase-like C-terminal domain, but further analysis demonstrated that the enzyme accepts APS in preference to PAPS as substrate and has a thioredoxin-like C-terminal domain. Isolation and characterisation of these genes invites a new hypothesis for plant reductive sulphate assimilation and provides direction for future research.

Although sulphur is one of the more abundant elements present in plants, its importance as a plant nutrient has been underestimated until comparatively recently. Scientific literature over the past few years, however, shows that interest in the determination of sulphur in natural waters, soil and plant materials has been renewed. Perhaps the main reason for the non-recognition of the importance of sulphur as a plant nutrient is the fact that sulphur-deficiency seldom occurs since sufficient sulphur is usually added to the soil in rainfall (particularly near industrial towns) to supply all the sulphur requirements of plants. In addition, sulphur is frequently present in fertilizers added to soils to counteract deficiency in other elements, especially phosphorus. Superphosphates, for example contain up to 60%, gypsum. Sulphur is also added to the soil in some instances in order to reduce the soil pH, the elemental sulphur being fairly readily oxidised in the soil to sulphuric acid. The organic fraction in the soil also contains sulphur.

The effects of temperature and sulphur nutrition on the growth, yield and mineral composition (N, NO₃-N, S and SO₄-S) of Hordeum vulgare L. cv Olli, Pisum sativum L. cv Dark Skin Perfection and Brassica campestris L. cv Arlo, were investigated in controlled environments. The net CO₂ exchange rates and compensation points were also determined at two S levels (0 and 64 ppm) under various temperature regimes. When barley and rape plants were grown at 0 ppm S, deficiency symptoms developed in about two weeks, whereas pea plants at the same level developed deficiency symptoms in about three weeks. Plants at the lowest S level and the highest temperature took the shortest time to develop S deficiency symptoms. Fresh and dry weights, shoot length, number of nodes and number of fertile fruit increased with increasing S levels. Shoot growth in all three species was more depressed by S deficiency than root growth. Optimum growing temperature regimes for barley and peas were found to be 24/16 at the vegetative stage and 18/10°C at the mature stage as evident from increased weights, maximum fruit set and mineral uptake. Optimum temperature for rape plants was 29/21°C at both stages of growth. Detrimental effects of cotyledon or endosperm removal tended to mask the effects of temperature and S levels. This method was thus found to be unsatisfactory for the study of S nutrition in plants. Higher mineral concentration was observed at the vegetative stage than at the mature stage in peas and rape plants, while in barley the mineral concentration remained constant at both stages of growth. With increase in S supply there was an increase in uptake of both total S and SO₄-S. Uptake also increased with increasing temperatures. This increase was largely due to "concentration effects". Hence the use of SO₄-S level as a criterion for diagnosis of S deficiency may be unsatisfactory, unless plants are grown at optimum temperatures. S deficient plants had increased total N and NO₃-N concentrations in all three species. NO₃-N concentration also increased with an increase in temperature. The total N concentration did not increase appreciably with temperature. Consequently, at low S level (0 and 8 ppm) total N:total S ratios (N:S) tended to increase or decrease depending on low or high growing temperatures respectively. These changes in ratios were independent of actual size of the plants. Furthermore the ratios for all S levels at the vegetative stage were lower than those at the mature stage. Therefore both temperature and stage of growth are important factors to be considered in interpreting S deficiency from N:S ratios in plants. The net C0₂ exchange rates were generally higher at 20 days than at 30 days. At 0 ppm S level and at high temperature, the decline in net C0₂ exchange rate with age was greater. Maximum CO₂ exchange rates were observed at the optimum growing temperatures for both S levels. Increasing the measuring temperature above the growing temperature caused no further stimulation in CO₂ uptake, and at high temperatures there was a decrease in uptake. When CO₂ exchange rates were measured at two 5.5°C intervals above and below the growing temperatures the maximum rates were recorded at or below growing temperatures in all the species at both S levels. The CO₂ compensation values were higher with lower S level in the leaf tissue than at higher S levels. Increase in growing temperatures also caused larger CO₂ compensation values than at lower temperatures. Negative correlations between CO₂ compensation point and leaf tissue S level and positive correlations between CO₂ compensation point and temperature were observed in barley and peas. / Land and Food Systems, Faculty of / Graduate

No description available.

Thesis (MSc. (Horticulture)) -- University of Limpopo, 2019 / After harvesting litchi fruit, the red pericarp colour is rapidly lost resulting in discolouration and browning during storage and marketing. To mitigate this challenge, the South African litchi industry uses sulfur dioxide fumigation to retain litchi fruit red pericarp colour during extended storage and shelf-life. However, there are health concerns regarding the commercially used (SO2) fumigation for litchi pericarp colour retention due to high levels of SO2 residues in fruit aril. Therefore, this study aimed to explore the possibility of Uvasys slow release SO2 sheets to retain ‘Mauritius’ litchi fruit red pericarp colour when packaged in plastic-punnets and bags. Treatment factors were two packaging materials (plastic-punnets and bags), six SO2 treatments (control; SO2 fumigation and four SO2 sheets viz. Uva-Uno-29% Na2S2O5; Dual-Release-Blue35.85% Na2S2O5; Slow-Release-36.5% Na2S2O5 and Dual-Release-Green-37.55% Na2S2O5) and four shelf-life periods (day 0, 1, 3 and 5). ‘Mauritius’ fruit were assessed for pericarp Browning Index (BI), Hue angle (ho), Chroma (C*) and Lightness (L*). In this study, an interactive significant effect (P < 0.05) between packaging type and SO2 treatments was observed on ‘Mauritius’ fruit pericarp L*, C* and ho during shelf-life. Fruit stored in plastic-bags and treated with SO2 fumigation showed higher pericarp C* and L*, while SO2 fumigated fruit in plastic-punnets had higher pericarp ho. Lower pericarp BI was observed in SO2 fumigated fruit stored in plastic-bags, which showed less pericarp browning than fruit in other treatments. In general, commercial SO2 fumigation resulted in lower pericarp BI, and higher pericarp L*, C* and ho throughout the storage and shelf-life. Our correlation analyses results further showed that litchi fruit red pericarp colour was better preserved as SO2 treatment levels increased, especially in plastic-bags. In retaining ‘Mauritius’ litchi fruit red pericarp colour, Uvasys SO2 sheets were not effective when compared with commercial SO2 fumigation. However, commercially SO2 fumigated fruit were bleached throughout the storage and shelf-life. Furthermore, fruit from all treatments were spoiled due to decay and mould growth after day 5 of shelf-life. Inclusion of pathogen protectants is important in future research to demonstrate whether Uvasys SO2 sheet-packaging technology can retain ‘Mauritius’ litchi fruit pericarp colour. / Agricultural Research Council and National Research Foundation (NRF)

Sulfur (S) deficiencies have become an increasing problem in the United States. A field experiment, in a latin square design, was conducted to investigate effects of S fertilization as 0 and 67 kg/ha as a single or split application on corn (Zea mays L.) forage yield and chemical composition. Sulfur fertilization by either method increased yield of whole plant and grain 7% and increased number of plants with two ears. Total S and sulfate-S concentration in whole corn plants, leaf, stem, and grain were increased with S fertilization. Corn forages were ensiled at harddent stage (35% dry matter). Sulfur fertilized corn silages (N/ S=42 and 43) and non-S fertilized silage (N/ S=62) supplemented at two rates with sodium sulfate (N/ S=l2 and 45) were fed to sheep in metabolism and palatability trials. Both experiments were conducted as a randomized block design with six replications per treatment. All silages were supplemented with urea (6.7 g/ d). Digestibility of dry matter and cell wall components and apparent absorption of Sand N were increased with S fertilization and S supplementation. Nitrogen retention was increased 14% by S supplementation (N/S=l2) and 31% by S fertilization. Sheep fed N/ S=12 silage had lower blood hematocrit and hemoglobin levels then those fed S fertilized or N/ S=45 silages. Blood urea-N levels were higher in sheep fed S fertilized silages. Increasing dietary S by fertilization or supplementation had no measurable effect on dry matter intake. / M.S.

Nine-week-old yellow-poplar and green and white ash were exposed to various concentrations of O₃ (0.00 to 0.15 ppm) and/or SO₂ (0.08 ppm), 4 hr/d, 5d/wk in combination with simulated rain (pHs 5.6, 4.3, 3.0), 1 hr/d, 2 d/wk, for 5 or 6 wk under controlled laboratory conditions. Pollutant exposures resulted in alterations in seedling biomass accumulation, growth rates, changes in carbon allocation among plant parts and modification in physiological processes associated with gas exchange. Ozone (0.010 ppm) and SO₂ together caused a significant decrease in height growth and biomass and an increase in leaf area ratio (LAR) in yellow-poplar. Ozone and SO₂ exposures resulted in linear decreases and increases, respectively, in root dry weight, leaf area increase, relative growth rates of all yellow-poplar plant parts and unit leaf rate with decreasing rain pH. Chlorophyll content increased in both O₃ and SO₂ treatments with increasing rain acidity. In green and white ash experiments height growth was inhibited by O₃, SO₂ and O₃ + SO₂ for green ash, whereas only leaf dry weight was decreased by O₃ exposure in white ash. Decreasing rain pH resulted in linear decreases in root/shoot ratio (RSR) and LAR, for white ash. In green ash, a quadratic response to rain pH occurred with these growth variables. Ozone and O₃ + SO₂-treated green ash exhibited a significant quadratic response in leaf weight ratio with increasing rain acidity. Leaf area ratio and RSR exhibited linear increases and decreases, respectively, for O₃ and rain acidity. In SO₂-treated white ash with increasing white ash and yellow-poplar seedlings exposed to various O₃ concentrations and simulated rain for 5 and 6 weeks, respectively, increasing O₃ concentrations caused linear decreases in height and biomass of white ash. Linear decreases in root growth rate and biomass and RSR occurred with decreasing rain pH, across O₃ treatments. Ozone (0.05 or 0.10 ppm) caused linear decreases in these variables in combination with increasing rain acidity. For yellow-poplar, increasing O₃ concentrations caused linear increases in RSR and specific leaf area. At 0.05 and 0.10 ppm O₃, stem and leaf biomass, their relative growth rates and leaf area all decreased with decreasing rain pH. Ozone (0.10 ppm) exposure caused a decrease in stomatal conductance, and decreasing variable. rain pH resulted in a linear decrease in this A linear decrease in net photosynthesis also occurred with increasing rain acidity in O₃-treated (0.10 ppm) plants. These results demonstrate that gaseous pollutants in combination with simulated acid rain can have detrimental effects on growth of three forest tree species, under controlled laboratory conditions. / Ph. D.

Legume establishment is difficult on many moderately acid, infertile soils in the humid northeastern United States. Legume seedling vigor as influence by P, S, and Mo fertilization was studied in order to determine fertilizer needs for improved establishment. A Gilpin silt loam was fertilized with 0, 22, 67, and 201 mg P kg⁻¹ in combination with 0, 22, 67, and 201 μg Mo kg⁻¹ in a greenhouse experiment in 1983. Legumes studied in this experiment were red clover (<i>Trifolium pratense</i>), white clover (<i>T. repens</i>), and birdsfoot trefoil (<i>Lotus corniculatus</i>). Field experiments with red clover, birdsfoot trefoil, and flatpea (<i>Lathyrus sylvestris</i>) were initiated in the spring of 1983 and 1984 using 0, 50, 150, and 450 kg P ha⁻¹, 0 and 60 kg S ha⁻¹, and 0 and 874 g Mo ha⁻¹. Seedling vigor as measured by plant height, trifoliate leaf count, dry weight, and trifoliate leaf area increased the greatest in the greenhouse using 22 mg P kg⁻¹ relative to the higher rates of P fertilization. Seedling vigor without applied P was poor, which clearly indicated the essential need for P in the early stages of legume growth. Molybdenum applied at 201 μg kg⁻¹ increased the growth of the greenhouse grown legumes the greatest above the 0 μg Mo kg⁻¹ rate within each added P treatment. Seedling vigor however, was not enhanced with Mo fertilization until the P deficiency was corrected. Field established legumes increased in height and dry matter yield the most using 50 kg P ha⁻¹ with respect to the additional increments of applied P, but the increase was not as great as that obtained in the greenhouse using 22 mg P kg⁻¹, This was attributed to a higher extractable P level before fertilization within the field experiments. Sulfur fertilization generally did not enhance seedling vigor in the field studies; although, red clover yield was increased using 60 kg S ha⁻¹ in the 1984 field experiment, Molybdenum applied at 874 g ha⁻¹ increased seedling vigor as measured by plant height and yield in the field experiments with or without P or S fertilization. Phosphorus uptake and Mo concentrations were increased in the plant tissues with either P or Mo fertilization. / Master of Science / incomplete_metadata