Numerous studies in high-Si accumulators found physiological and biochemical adaptation processes were enhanced by Si application in crops stressed by cold conditions. Meanwhile, there is relatively little information on the impact of Si fertilization on low-Si accumulator crops (dicots) in respect of physiological and biochemical adaptation to stress, including citrus, which is classified as a chilling-sensitive species. The objectives of this study were: 1) To validate the method for Si analysis in citrus plants; 2) To determine factors that influenced Si uptake in citrus and its uptake mechanism; 3) To establish the impact of Si fertilization and cold stress on the photosynthetic efficiency of citrus plants; 4) To determine the impact of Si fertilization and cold stress on fruit quality and yield of citrus plants; 5) To determine the impact of Si fertilization and cold stress on the levels of key sugars, and proline in citrus trees.
The method validation showed that the ICP-OES analytic method was fast and sensitive, with a detection limit five times lower than a colorimetric analysis.
The uptake study showed that Si uptake in citrus increased significantly (P< 0.05) with the duration of application, leaf age and concentration applied, with 1000 mg L-1 being the optimum concentration tested. Si fertilization onto the roots of citrus trees was shown to be the best method of application compared to foliar application. Dry matter production was significantly increased by Si application.
Electron microscopy studies suggested that Si accumulates in the adaxial epidermal cell regions of leaves and constitutes a double Si layer. The levels of Si in citrus leaves increased from young leaves<mature leaves<roots. Winter uptake of Si was close to zero, whereas summer uptake was substantially higher, which suggested that the uptake and transfer of Si into tissues was an active process depending on the prevailing temperature conditions and the physiological activity of the roots.
The investigation of the impact of Si fertilization on the physiological adaptations of citrus to cold stress suggested that an improvement in photochemical efficiency occurred in both citrus cultivars (Delta and Nules).
Si fertilization of citrus trees made them more cold stress tolerant via physiological factors as well as improvements in the osmotic balance of their sugar (sucrose and fructose) and proline content, resulting in improved membrane rigidity during cold stress. Both factors impacted on water use efficiency via osmoregulation processes.
Overall, the current study provided substantial information on the response of citrus to Si fertilization under both normal and lower temperature conditions. The present study also identified crucial parameters in citrus adaptation to cold stress that may be useful in improving the tolerance of citrus crops to abiotic stress and particularly in frost-prone production areas in South Africa, or similar environments.
Thesis (PhD (Horticultural Science))--University of Pretoria, 2020.