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Gloxinia (Sinningia speciosa Lodd.) seedlings were grown in 2 media to determine the maximum percentage N that can be supplied as NH₄ and the minimum concentration of a soluble complete fertilizer that can be applied without reducing plant quality under different light and temperature conditions. In one group of experiments, plants were supplied modified Hoagland's solutions in which the N source and level were varied. Shoot dry weight of gloxinias grown in an organic medium in the winter was lower with 100% NH₄ than with 100% NO₃ but was unaffected by N level, while in the fall shoot dry weight was lower with 100% NH₄ than with 0 or 50% NH₄, and was reduced at 16 rather than 8 meq N liter⁻¹. In the summer, shoot dry weight did not vary with N source or concentration when treatment solutions were collected and reused for 1 week. Flower bud count at harvest did not vary with N source in the winter but was reduced at 100% NH₄ in the fall and summer. Time to flower bud set was unaffected by N source, but was increased by applying 16 rather than 8 meq N liter⁻¹. In the winter, leaves of plants receiving 75-100% NH₄ were dark green early in the studies, but became chlorotic, twisted and quilted before plants matured; in the summer and fall these symptoms of NH₄ toxicity were absent. Leaf chlorophyll content was greater for plants grown in quartz sand when given 100% NH₄ rather than 100% NO₃. In related studies, gloxinias were treated at every watering with a soluble 20N-8.7P-16.7K fertilizer. Shoot dry weight was greater and time until flower bud set shorter at 200 than at 0, 400 or 600 ppm N in the summer; shoot dry weight and flower bud count were greater at 100 than at 200-400 ppm N in the winter. Days until flowering in the summer and days until bud set in the winter were unaffected by fertilizer levels. Tissue composition varied with N source and level and Ca(OH)₂ treatment although lime application did not affect any external plant characteristics. / Ph. D.

Flooding cycle in wetland rice (Oryza sativa L.) production systems is often subject to seasonal and cultural variations which may affect the availability and uptake of nitrogen in different ways. These factors may more or less influence the physiological and growth responses of the plant. In an effort to improve productivity in rice cropping systems, two controlled environment studies and a field trial were conducted to evaluate the growth and yield responses of rice to different flooding regimes and nitrogen fertilizer management strategies. In the first glasshouse trial, an upland cultivar (GM-1) was used to study the effects of four flooding regimes and three nitrogen application rates on the tillering, yield components and grain yield of rice. The field study determined the applicability of the results of the glasshouse trial to out-door environmental conditions, with the aim of gaining further insight into the impact of nitrogen application strategy on tiller and grain qualities. Two wetland cultivars (FKR-19 and N-19) and GM-1 were used to evaluate the effects of two flooding regimes and two nitrogen topdressing patterns. The second glasshouse trial determined the effect of hydro-priming on the establishment of direct seeded rice, and the effect of flooding on aerenchyma formation in rice roots. Results of the studies showed that flooding with standing water of 5 cm above the soil surface, irrespective of when it occurred, and nitrogen application increased the number of tillers and panicles, above ground dry matter, nitrogen uptake and grain yield of rice. However, late flooding and high nitrogen application rate of 220 kg ha-1 were found to encourage the production of late tillers, thereby reducing the efficiency of nitrogen use for grain production. Nitrogen application in three split doses tended to increase plant nitrogen content at heading; it slightly increased the protein content of the grains at maturity, but reduced the amylose content of the starch granules. Nitrogen application in two split doses led to increased grain yield in non-flooded plants, while the three-split treatment increased nitrogen uptake and grain yield in the flooded plants. Flooding significantly increased aerenchyma formation in the cortical tissues of rice roots, particularly at 50 mm behind the root tips. Hydro-priming seeds for 48 h improved plant establishment by shortening the germination and emergence times, and increasing the height and dry matter accumulation of seedlings, thereby ameliorating the susceptibility of rice to flooding stress. Overall, the thesis affirmed that controlled flooding is beneficial to rice production since it enhanced the growth and yield of the plant. It further revealed that early flooding and appropriate timing of moderate nitrogen application can ensure the conservation of water and nitrogen resources, including the quality of the environment, with no significant consequence for yield and productivity of the crop. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.