Browsing by Author "Marais, Helen"

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    Aspects of mineral nutrition affecting fruit quality of ‘Nadorcott’ mandarin
    (Stellenbosch : Stellenbosch University, 2017-12) Marais, Helen; Cronje, Paul Jacobus Robbertse; Hoffman, L. (Lynn) (Horticulturalist); Zacarias, Lorenzo; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH SUMMARY: Mineral nutrition of Citrus is one of the key controllable factors influencing fruit quality. The mineral nutrients known to have the greatest impact on Citrus fruit quality, in order of magnitude of effect, are nitrogen (N), potassium (K) and phosphorous (P). The timing of application together with the amount of fertiliser applied is of critical importance in determining the impact it will have on Citrus fruit quality, including any possible long-term effects on tree growth, development and phenology. Three experiments were conducted to study fruit quality of ‘Nadorcott’ mandarin, taking both these above-mentioned considerations into account. The first experiment explored the known influence of P on decreasing citric acid content of Citrus fruit. In production areas with cold winters, harvest dates can be delayed by high fruit acid content, to such an extent that flowering is detrimentally impacted and hence also the fruit yield of the following season. Results obtained indicated that the citric acid content of fruit was significantly decreased by mono-ammonium phosphate (MAP) sprays, applied at seven and/or eight weeks after full bloom (WAFB). The second and third experiments concentrated on the influence of late N application on ‘Nadorcott’ fruit quality when studied in combination with different irrigation regimes and under different climatic conditions, respectively. The second experiment showed that, when the influence of N is considered in combination with irrigation, it has a more pronounced effect on Citrus fruit quality than exerted by N alone. Over-irrigation can have the most serious negative effect on fruit quality when all the factors studied were considered, therefore it is proposed that irrigation is likely to be the main determinant for success when managing N fertilisation. In the third experiment where the effect of late N in different forms on fruit quality was studied, the influence thereof was found to be insignificant. In addition, none of the N applications negatively affected rind quality, including that of colour development or the incidence of disorders. Beneficial effects on flowering for the following season were, however, also not recorded. Currently it can be concluded that since the application of late N had negligible effects on fruit quality in this study it may be used in an orchard with a known N deficiency, later in the season, subject to the prerequisite that the orchard is not over-irrigated. This study confirmed the complexity of mineral nutrition in citriculture, based on the large number of factors that influence fruit quality. Future research should attempt to establish the ideal time of MAP application, also taking different concentrations and production areas into account. Late N application trials should be expanded to an in-depth study of the effect of the N status of the tree on flower initiation, whilst establishing the production conditions under which this practice should be recommended.