Thesis (DTech. degree in Agriculture.)--Tshwane University of Technology, 2010. / Low soil N fertility accounts for poor crop yields in Africa. Additionally, chemical fertilizers use is inadequate due to high cost and unavalability. Because crops grown on such nutrient-poor soils are micronutrient deficient, incorporating N2-fixing legumes into cropping systems can sustainably improve crop yields and micronutrient capture. Nodulated cowpea easily forms symbioses with native rhizobial species, and can potentially fix atmospheric N2 into ecosystems. These bio-fertilization/-fortification traits can increase crop yields and nutrient accumulation in edible plant organs. Field experiments conducted in Ghana and South Africa assessed N2 fixation in cowpea in 2005 and 2006 using the 15N natural abundance technique. The genotypes Vuli-1 and TVu11424 contributed 178 and 198 kg N ha-1 in 2005 and 2006, respectively at Wa. At Manga, IT84S-2246 and TVu11424 fixed 157 and 171 kg N.ha- 1 in 2005 and 2006, while Fahari and IT84S-2246 contributed 182 and 131 kg N ha-1 in 2005 and 2006 respectively, at Taung. The effect of genotype x environment interaction on plant growth and N2 fixation were evaluated at Wa and Manga in Ghana, and Taung in South Africa. Whole-plant dry matter was highest at Manga relatve to Taung. The G x E interaction was significant for biomass yield, symbiotic parameters and grain yield. Fahari produced 3.2 and 2.2- fold more biomass at Manga than at Wa and Taung respectively. All the 25 genotypes derived significantly higher levels of N nutrition from symbiotic fixation at Taung compared to Manga and Wa. vi The relationship between symbiotic N nutrition and photosynthetic carbon assimilation in young fully expanded leaves of cowpea was assessed in Ghana, in 2005 and 2006. Benpila, Glenda, IT86D-1951 in 2005, and Sanzie in 2006, showed high photosynthesis, greater stomatal conductance, high transpiration, and increased water-use efficiency. Symbiotic N contribution was greater in Mchanganyiko, Ngonji and IT86D-2075 in 2005. These data show that where symbiotic legumes derive a large proportion of their N from N2 fixation, photosynthetic C yield is increased as a result of elevated Rubisco Analysis of edible leaves and grain of cowpea genotypes revealed significant differences in mineral concentration. Cowpea genotype IT82D-889 exhibited high concentrations of P, K, Ca, Mg and S. Iron concentration in cowpea grain was 137 μg.g-1 in Soronko, while Zn was 65 μg.g-1 in Vuli-1. Leaf concentrations of Cu, Mn, B and Fe were consistently higher in the genotypes Apagbaala, IT84S-2246, Fahari and IT97K-499-39. Compared with spinach, cowpea leaves had 3-fold more Fe which can offer biological solution to micronutrient deficiency in Africa.

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The effect of the time of application of Gibberellic Acid (GA3) and Lipo-Chitin Oligosaccharides (LCOs) on annual production of globe artichoke (Cynara scolymus L.) cultivars 'Imperial Star' and 'Green Globe' was studied in 2000 and 2001. In order for flowering to occur plants must undergo a period of vernalization. In 2000, plants received 54 chilling (vernalizing) hours, an amount sufficient to induce flowering in 100% of the Imperial star and 25% of the Green Globe control plants. Applications of 25ppm GA3 at the 6 and 10-leaf stage reduced the time to flowering by 11 days, increased uniformity of flower bud production, and also increased yield of Imperial Star. GA3 had no significant effect on Green Globe plants. LCO fraction C (an identified and collected nod factor peak in retention time of 32.56 minutes) had minimal effect on growth and development for both cultivars in 2000. In 2001 plants received no vernalizing temperatures and in addition were exposed to 89 hours of temperatures greater than 32°C which could devernalize the plants. (Abstract shortened by UMI.)

Two independent experiments were conducted during 1992 and 1993 using a Randomized Complete Block Design with four replications. The first experiment was designed to study the effect of mulch spectral properties on weed growth and development. The second was conducted to study the effect of polyethylene mulch on aphid population and on growth and yield of bell pepper. Initially, rye-grass germination under polyethylene mulches was significantly higher than that on bare soil, whereas mustard seed germination under mulches did not differ significantly from that on bare soil. However, at the end of 30 days, none of these weeds were observed in the mulched plots. Transmittance, reflectance, and absorbance were recorded in the Near Ultraviolet (390-399 nanometre), Photosynthetically Active Radiation (400-700 nanometre), and Near Infrared (701-1100 nanometre) bands for black, black microperforated, wavelength selective and silver mulches. Aging had little effect on the optical properties of the mulches. The wavelength selective (Infra red transmitting = IRT-76) green mulch had the warmest mean soil temperatures, followed by silver, black, microperforated black mulch, and bare soil. The soil moisture content was higher under plastic mulches than in bare soil. The use of mulches significantly increased both the early and total marketable yields of pepper. Plants grown with polyethylene mulch had significantly lower number of aphids on them than did their bare soil counterparts.

The effect of level and timing of nitrogen (N) fertilizer application on grain yield (YLD), grain protein (GPC), and breadmaking ability of four hard red spring wheat cultivars and on soil residual nitrate was investigated. Nitrogen fertility caused an increase in YLD, lodging, several yield components, GPC, and breadmaking quality and caused a reduction in N harvest index and grain ash, and N use efficiency (NUE) resulting in an increase in soil residual NO$ sb3$-N. Split N application reduced lodging, tillers and spikes m$ sp{-2}$ and caused an increase in grain weight, GPC, loaf volume and NUE. Grain yield increases were largely due to increases in the grain spike$ sp{-1}$ and tillers n$ sp{-2}$. Absolute protein content was found to be critical in determining GPC. Cultivars showed plastic responses to N. Despite its high YLD and flour yield, Hege 155-85 may be risky to produce because of its high dependence on N. Mineralization of N occurred during winter. Marked differences existed between the sites.

The yield of short-season hybrids is lower than long-season hybrids, mainly as a result of the low final leaf area developed by the short-season plants. This is due to the smaller plant stature and smaller leaf number and size of short-season adapted hybrids. In addition, in short-season areas the thermal-time available may be insufficient to mature grain of current maize hybrids. Maize hybrids which accumulate leaf area quickly, mature earlier, yield well and tolerate higher population densities better than the currently available hybrids would be more suitable for production in short-season areas. The "Leafy reduced-stature" maize hybrids, which have only recently been developed, have traits which address these criteria. There has been no previous effort to evaluate the effects of more than two plant population densities or the effects of planting patterns on the yield, yield components and vegetative growth of these hybrids. in 1995, field experiments were conducted at two sites near Montreal to compare the response of leafy reduced-stature (LRS), non-leafy normal stature (NLNS), and non-leafy reduced-stature (NLRS) maize hybrids to plant population densities and planting patterns. LRS maize hybrids showed the most rapid growth of the first ear, and the highest yield per single plant and per hectare at high plant population densities in paired rows. LRS hybrids also had longer grain filling periods, lower grain moisture contents and higher harvest indices than conventional (NLNS) hybrids. Rapid growth of the first ear and a higher harvest index are indications that LRS hybrids should be more tolerant of higher population densities than currently available hybrids. Therefore, LRS hybrids show promise for production in short-season areas at high plant population densities where maize cultivation is not currently economical due to shortness of the growing-season.

The study sought to identify the effects of deficit irrigation on the yield and quality of tomatoes. A greenhouse experiment was conducted during the summer of 1999 and repeated in winter 2000 using nine treatments. Two threshold soil moisture deficit levels, 65% and 80% depletion of plant available water, were factorially combined with 5 irrigation timing patterns: (i) no water stress (ii) stress throughout season, (iii) stress during flowering and fruit set, (iv) stress during fruit growth and (v) stress during fruit ripening. The treatments were set up in a randomized complete block design with 4 replicates. Crop yields, maximum and minimum equatorial diameter and fruit heights were measured. The quality parameters included: soluble solids, pH and the color index. Water stress throughout the growing season significantly reduced yield and fruit size but increased the level of soluble solids. No water stress throughout the growing season or stress only during the flowering stage provided highest tomato yield.

South Africa is the largest producer of sugar in Africa and one of the ten largest sugarcane producers in the world. Sugarcane in South Africa is grown under a wide range of agro-climatic conditions. Climate has been identified as the single most important factor influencing sugarcane production in South Africa. Traditionally, sugarcane mill committees have issued forecasts of anticipated production for a region. However, owing to several limitations of such committee forecasts, more advanced technologies have had to be considered. The aim of this study has been to develop, evaluate and implement a pertinent and technologically advanced operational sugarcane yield forecasting system for South Africa. Specific objectives have included literature and technology reviews, surveys of stakeholder requirements, the development and evaluation of a forecasting system and the assessment of information transfer and user adoption. A crop yield model-based system has been developed to simulate representative crops for derived Homogeneous Climate Zones (HCZ). The system has integrated climate data and crop management, soil, irrigation and seasonal rainfall outlook information. Simulations of yields were aggregated from HCZs to mill supply area and industry scales and were compared with actual production. The value of climate information (including climate station networks) and seasonal rainfall outlook information were quantified independently. It was concluded that the system was capable of forecasting yields with acceptable accuracy over a wide range of agro-climatic conditions in South Africa. At an industry scale, the system captured up to 58% of the climatically driven variability in mean annual sugarcane yields. Forecast accuracies differed widely between different mill supply areas, and several factors were identified that may explain some inconsistencies. Seasonal rainfall outlook information generally enhanced forecasts of sugarcane production. Rainfall outlooks issued during the summer months seemed more valuable than those issued in early spring. Operationally, model-based forecasts can be expected to be valuable prior to the commencement of the milling season in April. Current limitations of forecasts include system calibration, the expression of production relative to that of the previous season and the omission of incorporating near real-time production and climate information. Several refinements to the forecast system are proposed and a strong collaborative approach between modellers, climatologists, mill committees and other decision makers is encouraged. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.