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Nitrogen is the most important nutrient for plant growth and development. This is due to the reason that nitrogen is a key constituent of amino acids, nucleic acids, lipids and chlorophyll. Stress caused by nitrogen deficiency has a large impact on plant growth and development. In most plants, older leaves become chlorotic and eventually fall off if nitrogen stress is not relieved. Chlorophyll concentration is important to determine the nitrogen concentration in a plant as nitrogen is a constituent element of chlorophyll and also of various proteins that are a part of the photosynthetic apparatus. Cytokinins are said to mediate nutrient information status in plants, especially that of nitrogen. The purpose of this research was to measure chlorophyll and cytokinin concentrations to compare the response of Plectranthus parviflorus and Plectranthus ambiguus to nitrogen stress as well as to sufficient nitrogen and to determine if cytokinins were the reason behind the difference in their response. It would be expected that P.parviflorus would have higher concentration of cytokinins at 0 N as this plant does not lose its leaves and re-greens upon re-introduction of nitrogen. Both the species were grown in hydroponics for 6 weeks for each experiment. The plants received either 0 mg/L N or 150 mg/L N for week 1 to 3 and then there was a switch of treatments with plants receiving either 0-0 mg/L N, 0-150 mg/L N, 150-150 mg/L N or 150-0 mg/L N. Leaves were traced to determine leaf area and then assayed to determine chlorophyll and cytokinin concentration of the plants. At the end of the experiment plants were harvested, dried in an oven and their dry weights were measured.
The chlorophyll concentration in plants of both species receiving 0 mg/L N decreased while those receiving 150 mg/L N increased. Also when the plants were switched from 0 to 150 mg/L N they showed an increase in chlorophyll concentration in both species while those switched from 150 to 0 mg/L N showed a decline in their chlorophyll concentrations. Thus, in general, for both species chlorophyll concentration increased when sufficient nitrogen was given to plants while, it decreased when plants were grown under nitrogen stress. Plectranthus ambiguus always had a higher chlorophyll concentration than Plectranthus parviflorus throughout the experiment. Concentrations of the biological precursors, physiologically active and storage forms of cytokinin were lower in plants growing under 0 mg/L N while those plants growing under 150 mg/L N had higher concentrations. Thus when P.ambiguus plants receiving 0 mg/L N lost its leaves or when P.parviflorus plants receiving 0 mg/L N stopped its growth, all of these forms of cytokinins were present in low concentration in the plants. It was also observed that when during nitrogen stress P.ambiguus shed its leaves while P. parviflorus did not. Thus, there was no key difference in their respective cytokinin concentrations. It therefore can be concluded that the cytokinin concentration alone is not responsible for the difference in the response of these two species to nitrogen stress.
Advisor: Ellen T. Paparozzi