Dissertation (PhD)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Ph.D. (Botany) / Please refer to full text to view abstract

D.Sc. (Botany) / Please refer to full text to view abstract

Thesis (PhD(Agric) (Horticulture))--University of Stellenbosch, 2005. / Bitter pit fruit in commercial consignments of apples still poses an economic threat to exporters from South Africa. Bitter pit develops pre-harvest, but gets progressively worse during storage and is only traceable once the lesions appear after storage. Accurate, early indications of bitter pit incidence will allow for remedial pre-harvest measures in the field, e.g. Ca foliar applications, to reduce the potential losses. Similarly, the automatic detection of a bitter pit fruit during packing will reduce financial losses by identifying unacceptable fruit before shipping. Fluorescence imaging is a fast, non-destructive technique, able to evaluate numerous fruits individually. Results of pre-harvest imaging on apples to identify fruit susceptible to bitter pit showed that pitted fruit were correctly classified, but misclassification of non-pitted fruit with fluorescence imaging was still too high. NIR-spectroscopy point meter readings could distinguish visible bitter pit lesions from healthy tissue. Important wavelengths associated with visible bitter pit were identified. This technique could also identify immature apples, more prone to bitter pit development. It could however not distinguish between bitter pit and non-pitted fruit when applied randomly on the calyx end of apples at harvest. Pre-harvest foliar applications to increase fruit Ca content and reduce bitter pit incidence, is a standard practice world wide. External Ca uptake by fruit was monitored to determine the efficacy of applications during different stages of fruit development. Two periods of efficient uptake of external Ca were identified, viz., cell division and the last few weeks before harvest. Foliar Ca applications from 40 days after full bloom were more effective in increasing fruit Ca content and reducing bitter pit incidence than at 80 days after full bloom, which was recommended previously. Mineral analysis of fruit has been used with variable success to predict bitter pit prior to harvest. The possibility of increasing the accuracy of existing predictive models by using analysis of individual fruit rather than pooled samples, was investigated. By improving the normality of different mineral distributions and decreasing the overlap between pitted and non-pitted fruit classes, it was attempted to improve the reliability of predictions based on variable threshold values. The Ca distribution showed a variation between pitted and nonpitted classes, but still a significant overlap between classes reduced the accuracy of the predictive capacity of this distribution. Even though our results produced a correct classification of 85% for non-pitted fruit, which can be useful, this was still below the required tolerance, of less that 2%, expected on the market. The effect of pruning and fruit bearing position on two-year-old wood on dry mass and Ca allocation of fruit was determined. ‘Golden Delicious’ fruit set was the lowest at the basal bearing position compared to the other positions evaluated and was contrary to expectations. Fruit in a terminal bearing position was superior to the basal position regarding total dry weight and fruit size. Distal wood possibly inhibited growth and set on the basal position via auxin distribution. Ca allocation differed between seasons and cultivars and could either be influenced by bearing position or presence or absence of re-growth.

Se estudia la nutrición mineral en manzanas libres y afectadas por bitter pit, de las variedades: Fuji Royal, Granny Smith y Fuji. Los elementos esenciales determinados, en cáscara y pulpa, son: Cu, Zn, Fe, Mn, Ca, Mg, K y P. También se incluye Sr, elemento no esencial, del cual se dispone poca información sobre este tema. Inicialmente los frutos fueron evaluados determinando: peso del fruto, firmeza de la pulpa, concentración de sólidos solubles y acidez titulable. El análisis de los datos se centró especialmente en las diferencias entre frutos libres y afectados por bitter pit. Se detecta una menor concentración de calcio en frutos afectados en comparación con los libres del desorden fisiológico, confirmando lo concluido por otros autores. Sin embargo, respecto al magnesio solo la variedad Fuji presenta una relación con el daño. También se detecta y confirma lo obtenido anteriormente para la variedad Fuji, donde unido a la deficiencia de calcio se presenta una mayor concentración de potasio en frutos afectados por el daño, lo cual no fue observado en las otras variedades. El estroncio, se encuentra en frutos en un rango de concentraciones de 1 a 4 µg/gps. Sin embargo, el contenido de estroncio es significativamente menor en cáscara de frutos afectados que en los libres tanto en variedad Fuji Royal como en Granny Smith. De acuerdo a los datos obtenidos, y en relación a la nutrición mineral equilibrada de las plantas, las relaciones binarias y terciarias resultan ser mejores indicadores para incidencia de bitter pit que la concentración de cada mineral en forma independiente. Estas relaciones son: Mg/Ca, K/Ca y (Mg + K)/ Ca. Finalmente, es necesario destacar que la cáscara es el tejido más adecuado para estudiar la nutrición mineral en manzana respecto al bitter pit.

In a series of related experiment: some aspects of the histology and physiology of the disorder bitter pit in apple (Malus domestica Borkh.) were studied. A two year field study was conducted to induce consistent bitter pit development in type ’Delicious’ (D) and ’Golden Delicious’ (GD) apple fruit. Multiple spray treatments of CaCl₂ and MgCl₂, combined with paper bag fruit covers, were applied and subsequent bitter pit development examined. The main effect of bags in both cultivars was increased pit development, decreased Ca in fruit and increased fruit K. CaCl₂ sprays resulted in less pit development, increased Ca in fruit, and less fruit Mg. MgCl₂ sprays resulted in increased bitter pit, decreased fruit Ca, and increased fruit Mg. Overall, field treatments provided a wide range of bitter pit incidence after storage, from 100% (bag and MgCl₂ spray combined) to 3% (CaCl₂ spray alone) in both D and GD. The cellular morphology of pitted apple fruit from field trees was examined using transmission and scanning electron microscopy. The overall tissue morphology of both cultivars was similar, but in pitted tissues differences were observed in tannin localization, starch hydrolysis, and cell wall morphology. Cation levels in the tissues were examined using X-ray microanalysis. High Mg levels were localized in pit cells, while K levels were similar in both healthy and pitted cells. Ca levels in both tissue types were too low to be detected by this method. Using ’Golden Delicious’ fruit from the field study, the relationship between pyruvate kinase activity, fruit cation concentration and bitter pit was investigated. Pyruvate kinase activity during early fruit growth was higher in fruit which developed 100% bitter pit after storage (MgCl, spray + bag), than in fruit that developed 3% bitter pit (CaCl₂ spray). Fruit with a high bitter pit incidence had a lower Ca: Mg + K ratio than fruit with a low level of the disorder. There was a strong positive correlation between enzyme activity early in the season and bitter pit incidence alter storage. An assay for pyruvate kinase may be valuable for early prediction of postharvest bitter pit development. Finally, the qualitative electrophoretic patterns of soluble fruit proteins from each treatment were examined starting early in the season and continuing until termination of fruit storage. Patterns from all treatments were almost identical throughout the season. / Ph. D.