Master of Science (Life Sciences)

The pseudobulb of epiphytic orchids has been shown to be an integral organ in the survival and growth of orchids. These 'false bulbs' are known to function as storage organs for water, carbohydrates and minerals accumulation and photosynthesis of green pseudobulbs (GPSB) can contribute positively to carbon balance. In our study, epiphytic C3 Oncidium Golden Wish was subjected to mild and severe DS (MDS and SDS) and rewatering (RW) by shading the GPSB to investigate the roles of photosynthetic GPSB in drought response and recovery to alter the source/sink ratio in relation to the main photosynthetic sources of green leaves (GL).

There were significant decreases in water content of GPSB as well as GL after SDS unlike after MDS. After 3 weeks of RW, the water content and RWC both fully recovered. The FW and length of new shoot was significantly lower in MDS and the lowest on SDS plant treatments. Interestingly, results were lower for FS-GPSB than FI-GPSB for both MDS and SDS groups. Data for Chl fluorescence parameters such as Fv/Fm ratio, ΔF/Fm’, qP and qN used to describe the photosynthetic state of the plant showed that SDS had a profound effect on the plant as compared to MDS. When exposed to MDS, midday Fv/Fm ratio of GL was not affected but in SDS plants, this was apparent and despite 3 weeks of RW, did not fully recover. The ETR results coincides well with data of Fv/Fm ratio ratio, qP and qN of GL and GPSB after MDS and SDS whereby values of SDS+FS-GPSB plants were the lowest after SDS and following 3 weeks of RW. SDS resulted in significant decreased in total Chl content of GL with greater decrease in FS-GPSB plants that was not observed in GL of MDS plants. Both MDS and SDS resulted in decreases of total Chl content of GPSB with greater decreases in FS-GPSB plants. No significant differences in total carotenoids content in both GL and GPSB among the different treatments after MDS and RW were observed. Less GL SS and IS was observed in FS-GPSB treatments as the photosynthetic role of GPSB had been removed, making less carbohydrate readily available as compared to FI-GPSB treatments during DS. The impact was more discernible after SDS. Our findings support that GPSB is more susceptible to DS when shaded. It takes a longer period of RW for GPSB to fully regain photosynthetic capacity, highlighting the crucial role of GPSB photosynthesis in drought tolerance for Oncidium Golden Wish plants.

Water and plant tissue mineral nutrient of the macrophytes H verticillata and N. indica in Lower Seletar Reservoir (LSR), Singapore were studied. Concentrations of four macronutrients (P, K, Ca and Mg) as well as six micronutrients (Fe, Cu, Zn, Mn, Mo and B) were monitored at three sites within the reservoir. NO3^- assimilation product, Total Reduced Nitrogen (TRN) concentration was also determined. In comparison with standard Netherland solution used in soilless culture, NO3^-, P, K, Ca, Mg, Fe, Mn, Mo, and B concentrations in the water of the reservoir were much lower than the required concentrations for plant growth. P concentrates seemed to be the lowest among the macronutrients although macrophyte growth in LSR did not appear to be nutrient limited. Water concentration of Cu was slightly higher whilst Zn was five folds higher than the required concentrations for plant growth.

Plant tissue analyses showed that both macrophytes were able to accumulate very high concentrations of eleven nutrients measured (NO3^-, TRN, P, K, Ca, Mg, Fe, Cu, Zn, Mn, Mo, and B) without any symptoms of mineral toxicity. The only exception being Mg concentration in H verticillata at 0.16% DW which is below the adequate tissue levels of 2%. Low concentrations of minerals in the water were compensated by its large volume, resulting in a high total available supply of minerals.

Najas indica dominated Sites A and B while Hydrilla. verticillata dominated Site C, and concentrations of water mineral content were similar between Sites A and B but differ from Site C. A greenhouse study was conducted to investigate the effects of water from three sampling sites and vegetation density on mineral nutrient uptake of H verticillata. Phosphorus, TRN and Fe concentrations in water were too low to be reported. Significant uptake of all macronutrients except Ca and Mg from waters of all three sites were observed. High density plant treatments resulted in greater reductions in water mineral concentrations than low density treatments.

No significant changes in micronutrient concentrations of Zn, Mn, Mo and B in OT of H verticillata and waters were observed across different treatments. Iron concentration in OT and YT across all treatments decreased significantly due to very low concentrations of Fe in the water. Manganese concentrations of water in all treatments increased significantly rather than decrease as expected. Higher concentrations of micronutrients in OT than YT is linked to low mobility of the elements.

The macrophytes in LSR play a beneficial role in maintaining water clarity by reducing sediment resuspension, keeping phytoplankton growth low via active accumulation and removal of minerals and enabling proper nutrient cycling between the soil and water medium. Partial removal of macrophytes at regular intervals is encouraged to lower the net amount of metals such as Cu and Zn circulating in the reservoir. The uptake and significance of high levels of micronutrients and the mechanism(s) associated with hyperaccumulation of minerals in the macrophytes, as well as the use of benthic mats to prevent their establishment is recommended for further study.

Water is integral for the survival of life and sustainable development. With the rapid advancement of economic progress, urbanization and an ever-increasing global population, pollution and climate change has undoubtedly placed a strain in the scarcity of freshwater resources. Over the past decades, tremendous water treatment technology efforts such as thermal distillation, advanced oxidation and reverse osmosis have been dedicated to address these global issues. Recently, solar vapor generation has been attracting increasing attention for its minimum environmental footprint by utilizing renewable solar energy for solar-driven steam generation as an effective water purification technology. Thus, this project seeks to address the challenges of plastic pollution by upcycling different common grades of household plastic waste to produce high value carbon nanomaterials through a simple and effective two-step solvothermal synthesis as well as designing a solar still prototype using the synthesized carbon materials as potential solar absorbers in desalination application. Amongst the various materials tested, NTUC plastic bag (PB) demonstrated the steepest mass evaporation loss of 1.50 kg m−2 h-1 with a solar-to-vapor conversion efficiency of 89.8% owing to abundant porosity for fast water transport and excellent solar absorption for photothermal conversion. A thorough study into its synthesis procedure, photothermal properties, characterization techniques, solar vapor generation performance and solar still desalination application will be further explored. To conclude the project, several existing limitations, potential solutions, and future directions will also be provided based on literature knowledge.

The cross coupling of nucleophiles with electrophilic aromatic halides using transition metal salt is a robust synthetic method that is widely employed in the manufacture of medicinal agents, bioactives and polymeric materials. The traditional methods for generating C-N bonds have been accomplished using copper catalysts or the palladium-mediated Buchwald-Hartwig method. In most cases, the reactions must be conducted under stringent inert conditions and anhydrous solvents which limit industrial applications. In some instances, specially designed ligands have to be synthesized which further adds cost to the system. In this context, the catalytic systems should be economical and experimentally simple to be useful for large scale applications from an industrial viewpoint.

In this dissertation, we have shown an economical strategy for N-arylations of methane sulfonamides with a range of differently substituted aromatic bromides using copper catalysis under ligand-free condition. Reports on the use of aryl bromides under ligand free catalysis are very limited owing to lower reactivity compared to the iodo-counterparts. Moreover, the use of aryl bromides offers many advantages including lower toxicities and cost. This method was shown to afford product yields up to 80% under the best reaction conditions and have the following features: (i) highly efficient; (ii) high generality and applicable to a wide variety of substrates; (iii) uses cheap and sustainable copper catalyst at low loading to reduce overall cost and (iv) requires non-inert reaction conditions and simple to carry out.

Food security maintenance, in terms of quantity and quality, has always been a challenge for Singapore where land and water are limited. In land-scarce Singapore, vertical farming is growing to strengthen food supply. However, being one of the water-stressed nations meant that food production can be threatened anytime. Growing halophytes using saline water would be a feasible solution to the vulnerable water supply in Singapore. In this study, two nutritious halophytes, Mesembryanthemum crystallinum and Portulaca oleracea, were grown under increasing saline conditions at a PPFD of 200 μmol m-2 s-1 (12 h). Productivity and leaf growth of both species grown in nutrient solution with 100 mM NaCl were elevated than without NaCl, indicating that both halophytes require some salt to perform optimal growth. However, productivity decreased with increasing NaCl concentration >100 mM in both species. Grown with 500 mM NaCl, M. crystallinum appeared healthy albeit smaller while P. oleracea, was only able to survive at 300 mM NaCl, indicating that P. oleracea is more sensitive to salt stress. Those high NaCl salinities negatively affected root morphology. In both species grown with high salinities, the lower concentrations of nitrate, dietary minerals (K, Ca and Mg) could be attributed to the stunted roots which limited water and mineral uptake. However, the correlation between root morphology and Fe uptake was not clear in P. oleracea. Lower specific leaf area of both species grown with high NaCl concentrations were due to their high dry matter content, lower water content and lower leaf succulence. Grown with 300 and 500 mM NaCl, P. oleracea and M. crystallinum respectively had the highest concentrations of photosynthetic pigments and Chlorophyll a/b ratios than with lower NaCl concentrations. All plants had Fv/Fm ratios ~0.8, implying that salinity did not damage PS II in any plants. However, electron transport rate and photochemical quenching were negatively affected by high NaCl salinity in both species. CAM was induced from C3 and C4 photosynthesis in M. crystallinum and P. oleracea respectively, grown with high NaCl salinity coupled with higher non-photochemical quenching. Reduced Cytb6f concentration was found in P. oleracea grown with 300 mM NaCl. All P. oleracea had comparable PS II concentrations. However, salinity did not affect Cytb6f concentration for M. crystallinum. CO2 assimilation rate (A), stomatal conductance (gs) and transpiration rate (Tr) reduced as salinity in M. crystallinum increased. For both species, although high salinity reduced leaf NO3− concentration, all plants had similar amounts of Rubisco protein as nitrogen deficiency did not occur in any plants. Low absorption of dietary minerals may result from the excessive accumulation of Na in both species. For M. crystallinum, high salinity enhanced proline, total soluble sugar, total ascorbic acid and total phenolic compounds. However, only proline content was increased in P. oleracea grown with high salinity. In conclusion, it is feasible to grow both halophyte species nutrient solution with 100 mM NaCl to obtain greater productivity and better nutritional values. However, accumulation of phytochemicals with increased salinity may depend on not only salinity but also species.