Thesis (MScAgric (Horticulture))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The size of the navel-end opening is an important parameter for external fruit quality in navel oranges [Citrus sinensis (L.) Osbeck]. The application of 2,4-dichlorophenoxyacetic acid (2,4-D) to increase the percentage of closed navel-ends and reduce the size of the navel-end opening was conducted on six different navel orange cultivars. Treatments were applied at full bloom (FB), 100% petal drop (PD), as well as 2 weeks (2 WAPD) and 4 weeks after 100% petal drop (4 WAPD), at 15 mg·L-1 to 45 mg·L-1, to determine the most effective timing and concentration. The application of 2,4-D at FB consistently decreased the average navel-end size (all fruit) and increased the percentage of closed navel-ends in all the cultivars, with later applications at PD, 2 WAPD and 4 WAPD being generally ineffective, regardless of the concentration applied. There were no major negative side effects on internal and external fruit quality, except for the reduction in juice content (%), especially with the later treatments. Therefore, 15 mg·L-1 2,4-D at FB can be applied to increase the percentage of closed navel-ends and possibly increase export packouts. Navel oranges have a small secondary fruit located inside the primary fruit at the stylar-end and an opening at the stylar-end called the navel-end opening or the stylar-end aperture. Fruit growth and development was studied in three navel orange cultivars by measuring the primary fruit diameter, the secondary fruit diameter and the navel-end opening fortnightly, using both destructive and non-destructive sampling methods. The relationships between the primary fruit size, the secondary fruit size and the navel-end opening size were studied using correlation analysis. In addition, the effect of 2,4-D on fruit morphology, when applied as a treatment to reduce the size of the navel-end opening, was also evaluated on the same cultivars. The primary fruit, the secondary fruit and the navel-end opening followed a similar developmental pattern, although the navel-end opening developed later, about six weeks after FB. The primary fruit size was not related to the size of the secondary fruit or the navel-end opening. Similarly, the size of the navel-end opening was not related to the size of the secondary fruit. No negative effects were noted on the primary fruit morphology when 2,4-D was applied. Fruit splitting is a major physiological disorder of ‘Marisol’ Clementine mandarin (Citrus reticulata) fruit. The effect of application of 2,4-D on fruit splitting and fruit quality was evaluated on ‘Marisol’ Clementine mandarin trees grafted on Troyer citrange rootstock. Treatments included an untreated control, 2,4-D applied at 15 mg·L-1 or 25 mg·L-1 at FB and 15 mg·L-1 or 25 mg·L-1 at PD. The application of 2,4-D reduced fruit splitting in ‘Marisol’ Clementine fruit. Internal fruit quality was not affected by the treatments, however, the fruit developed a coarse rind due to enlarged oil glands and the styles stayed attached on the fruit until harvest. Therefore, although 2,4-D reduced fruit splitting, it cannot be recommended at the timings and concentrations evaluated. / AFRIKAANSE OPSOMMING: Die grootte van die nawel-ent opening is ‘n belangrike parameter vir eksterne vrugkwaliteit van nawel lemoene [Citrus sinensis (L.) Osbeck]. Die toediening van 2,4-dichlorofenoksie asynsuur (2,4-D) om die persentasie geslote nawel-ente te vermeerder en die grootte van die nawel-ent opening te verklein is uitgevoer op ses verskillende nawel lemoen kultivars. Behandelings is toegedien by volblom (FB), 100% blomblaarval (PD), asook 2 weke (2WAPD) en 4 weke na 100% blomblaarval (4 WAPD), teen 15 mg·L-1 tot 45 mg·L-1, om die mees effektiewe tyd van toediening en konsentrasie te bepaal. Die toediening van 2,4-D by FB het herhaaldelik die gemiddelde nawel-ent grootte (alle vrugte) verminder en die persentasie geslote nawel-ente vermeerder in al die kultivars, terwyl die later toediening by PD, 2 WAPD en 4 WAPD oor die algemeen nie effektief was nie, ongeag die konsentrasie toegedien. Daar was geen noemenswaardige negatiewe effekte op interne en eksterne vrugkwaliteit nie, behalwe vir ‘n verlaging in die sapinhoud (%) van vrugte, veral by die later behandelings. Dus kan 15 mg·L-1 2,4-D by FB toegedien word om die persentasie geslote nawel-ente te vermeerder en moontlik ook die uitvoerpersentasie te verhoog. Nawel lemoene het ‘n klein sekondêre vrug binne die primêre vrug aan die styl-ent en ‘n opening by die styl-ent wat die nawel-ent opening of die styl-ent opening genoem word. Die vruggroei en ontwikkeling van drie nawel kultivars is bestudeer deur die primêre en sekondêre vrugdeursnit en die nawel-ent opening elke twee weke te meet, deur gebruik te maak van destruktiewe en niedestruktiewe monsterneming. Die effek van 2,4-D op vrugmorfologie, toegedien as ‘n behandeling om die nawel-ent grootte te verklein, is ook ge-evalueer op dieselfde kultivars. Die primêre vrug, die sekondêre vrug en die nawel-ent opening het dieselfde ontwikkelingspatroon gevolg, alhoewel die nawel-ent opening later ontwikkel het. Daar was geen sterk verwantskap tussen die primêre vruggrootte en die sekondêre vruggrootte of die grootte van die nawel-ent opening nie. Daar was ook nie ‘n vewantskap tussen die grootte van die nawel-ent opening en die sekondêre vruggrootte nie. Geen negatiewe effekte op vrugmorfologie as gevolg van die 2,4-D toediening is waargeneem nie. Vrugsplit is ‘n belangrike fisiologiese abnormaliteit van ‘Marisol’ Clementine (Citrus reticulata) vrugte. Die effek van 2,4-D op vrugsplit en vrugkwaliteit is ge-evalueer op ‘Marisol’ Clementine mandaryn bome op Troyer citrange onderstamme. Die behandelings het ‘n onbehandelde kontrole, 2,4-D toegedien teen 15 mg·L-1 of 25 mg·L-1 by FB en 15 mg·L-1 of 25 mg·L-1 by PD ingesluit. Die toediening van 2,4-D het vrugsplit verminder. Interne vrugkwaliteit was nie geaffekteeer deur die behandelings nie, maar die vrugte het ‘n growwe skil ontwikkel as gevolg van vergrote oliekliere en die style het aangeheg gebly aan die vrugte tot oestyd. Dus, alhoewel 2,4-D vrugsplit verminder het, kan dit nie aanbeveel word teen die tyd van toediening en konsentrasie soos ge-evalueer in hierdie studie nie.