Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The export of Japanese plums from South Africa is challenging. Exporting late season plums require fruit to last as long as 8 weeks in cold-storage. Prolonged storage periods can cause some cultivars to develop a shrivelled appearance due to moisture loss. Moisture loss from perishable commodities manifests mainly as shrivelling due to a loss in the turgidity of the surface cells of the fruit, or weight loss. ‘African DelightTM’ (highly susceptible to shrivel), ‘Laetitia’ (shrivel susceptible), ‘Sapphire’ (shrivel susceptible) and ‘Songold’ (not shrivel susceptible) plums were investigated by means of fluorescent microscopy for cracks and openings in the fruit peel. Only ‘African DelightTM’ had open hairline cracks in its peel, and fruit with wider cracks were associated with higher water vapour permeabilities. Open lenticels were found in the peels of ‘African DelightTM’, ‘Laetitia’ and ‘Sapphire’ plums. For ‘Songold’ no peel cracking or open lenticels were observed. The fact that the cuticle of this cultivar is mostly intact may be the reason why it is not susceptible to postharvest shrivel manifestation. The water vapour permeance of the fruit peel determines how easily fruit lose moisture. In this study it was determined to what extent fruit, trees, orchards, harvest date and cultivar contribute to the total variation in plum peel water vapour permeability. The permeabilities of ‘African DelightTM’, ‘Laetitia’, and ‘Songold’ were determined weekly from 4 weeks before harvest until post optimum maturity. Fruit to fruit variation made the largest contribution towards the total variation (> 45%), followed by harvest date (> 20%) and orchard (> 15%) effects. The permeability across all cultivars increased two-fold as fruit became over mature. The contribution of cultivar differences to fruit peel permeability varied greatly between seasons (42% in 2013/2014 and 5% in 2014/2015). Differences between cultivars may include cuticle thickness and composition, micro cracks in the peel and/or open lenticels. Current handling protocols suggest that fruit should be cooled as soon as possible after harvest, but this is not always possible. ‘African DelightTM’ plums were exposed to various handling scenarios in order to determine the handling protocol with the least risk of moisture loss. The control consisted of packaging and cooling the fruit within 6 h of harvest. Fruit quality was comparable or even better than the control when the fruit were pre-cooled to 0 °C and 15 °C for up to 72 h. High vapour pressure deficits caused fruit to lose more moisture, especially when fruit were exposed to ambient temperatures for 48 h and 72 h. It is recommended that handling protocols for plums should be followed stringently in order to reduce mass loss and shrivel manifestation. Since other studies found that silicate (Si) has positive effects on fruit quality, we applied potassium silicate preharvest to ‘African DelightTM’ trees. However, we did not find significant differences between treatments regarding crack width or crack incidence in the fruit peel, shrivel, decay, internal browning, gel breakdown or aerated tissue levels. Currently preharvest potassium silicate applications are not recommended to improve plum quality. / AFRIKAANSE OPSOMMING: Die uitvoer van die Japanese pruime uit Suid-Afrika is 'n uitdaging, omrede daar verwag word dat laatseisoen kultivars tot 8 weke in koelopberging moet bly. Lang opbergingsperiodes veroorsaak dat sommige kultivars 'n verrimpelde voorkoms ontwikkel a.g.v. vogverlies. Vogverlies uit vars produkte manifesteer hoofsaaklik as verrimpeling a.g.v. 'n verlies in die turgiditeit van die selle in en onder die vrugskil, en as massaverlies. ‘African DelightTM’ (hoogs vatbaar vir verrimpeling), ‘Laetitia’ (vatbaar vir verrimpeling), ‘Sapphire’ (vatbaar vir verrimpeling) en ‘Songold’ (nie vatbaar vir verrimpeling) pruime is ondersoek deur middel van fluoressensie mikroskopie vir krake en openinge in die vrugskil. Slegs ‘African DelightTM’ het oop haarlyn krake in sy skil gehad en vrugte met wyer krake het ʼn hoër waterdamp deurlaatbaarheid gehad. Oop lentiselle is gevind in die skille van ‘African DelightTM’, ‘Laetitia’ en ‘Sapphire’ pruime. ‘Songold’ het geen krake of oop lentiselle getoon nie. Die feit dat ‘Songold’ se kutikula meestal ongeskonde was, mag die rede wees waarom hierdie kultivar nie vatbaar vir verrimpeling is nie. Die waterdamp deurlaatbaarheid van 'n vrugskil bepaal hoe maklik vrugte vog verloor. In hierdie studie is bepaal tot watter mate vrugte, bome, boorde, oesdatum en kultivar bydra tot die totale variasie in die pruimskil se waterdamp deurlaatbaarheid. Die deurlaatbaarheid van ‘African DelightTM’, ‘Laetitia’, en ‘Songold’ is weekliks bepaal vanaf 4 weke voor die verwagte oesdatum tot die vrugte oorryp was. Vrug tot vrug variasie het die grootste bydrae tot die totale variasie gemaak (> 45%), gevolg deur oesdatum (> 20%) en boord (> 15%). Die skildeurlaatbaarheid van al die kultivars het verdubbel in die tyd van net voor oes tot die vrugte oorryp was. Die kultivar se bydrae tot die deurlaatbaarheid van die vrugskil het baie gewissel tussen seisoene (42% in 2013/2014 en 5% in 2014/2015). Verskille in skil-deurlaatbaarheid tussen kultivars kan kutikula-dikte en -samestelling, mikro-krake in die skil en/of oop lentiselle insluit. Huidige hanteringsprotokolle stel voor dat vrugte so spoedig moontlik afgekoel word na oes, maar dit is nie altyd moontlik nie. In hierdie studie is 'African DelightTM' pruime is blootgestel aan verskeie hantering scenario's om die hanteringsprotokol met die laagste risiko vir vogverlies te bepaal. Die kontrole vrugte is gepak en onder geforseerde verkoeling geplaas binne 6 ure na oes. Vrugkwaliteit was vergelykbaar of selfs beter in vergelyking met die kontrole wanneer die vrugte voorverkoel is tot 0 °C en 15 °C vir tot 72 uur. Hoë dampdrukverskille het veroorsaak dat vrugte meer vog verloor, veral wanneer vrugte aan kamertemperatuur blootgestel was vir 48 h en 72 h na oes. Dit word aanbeveel dat hanteringsprotokolle vir pruime streng gevolg moet word om massaverlies en verrimpeling te beperk. Aangesien ander studies gevind het dat silikaat (Si) ‘n positiewe uitwerking op vrugkwaliteit het, het ons kaliumsilikaat vooroes aan ‘African DelightTM’ bome toegedien. Daar was egter geen beduidende verskille tussen behandelings met betrekking tot kraakbreedte of kraakvoorkoms in die vrugskil of t.o.v. gehalte eienskappe soos die voorkoms van verrimpeling, bederf, interne verbruining, gelverval of deurlugte weefsel nie. Tans word voor-oes kaliumsilikaat spuite nie aanbeveel om pruimkwaliteit te verbeter nie.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In order to export South African plums to overseas markets strict quality standards must be maintained. Among these quality classifications are specifications about the presence of cavities and pieces of broken stone/pit within the flesh of the fruit. If more than 10% of the fruit in a carton are affected by severely broken stones or large flesh cavities, the fruit have to be marketed as Class 2. A substantial amount of plums destined for export from South Africa is affected by broken stones and thus have to be marketed as Class 2. Lower prices are attained for Class 2 fruit, therefore, the presence of broken stones has a detrimental effect on the income generated from these fruit. The main aim of this study was to gain a better understanding of broken stone development and compare the growth characteristics of Japanese plum cultivars differing in their susceptibility to broken stones. Stone breakage in ‘Laetitia’ was observed as soon as stone hardening was initiated. At the start of stone hardening the parts of the stone that are still ‘soft’ are not strong enough to resist the pulling forces of the growing mesocarp and the stone is subsequently pulled apart. Regression analysis indicated that lengthwise growth of the fruit, fresh weight of the endo- and mesocarp, minimum orchard temperature and orchard night temperature, and relative humidity (RH) early in the growing season could possibly be used to predict the incidence of broken stones at harvest. Differences in the incidence of broken stones were observed between ‘Laetitia’, ‘Sapphire’ and ‘Songold’ plums and between seasons. Furthermore, significant differences were observed in the density of the endocarp in different parts of the stone. For ‘Laetitia’ and ‘Songold’, stone breakage was observed when rapid increases in stone density coincided with rapid increases in fruit growth. The stones broke in positions where an interface exists between high and low density parts in the stone and when rapid radial growth takes place in the direction where the stone is least dense. In contrast, in ‘Sapphire’, stone breakage was observed before the stones had started to lignify, indicating that the endocarp was pulled apart by the expanding flesh because it was too soft to withstand the strong pulling forces created by the flesh. The incidence of broken stones was influenced by environmental factors, as higher temperatures during the stone development and hardening period could lead to more complete endocarp formation (more stone cells are formed under such conditions). Such fruit would thus have higher endocarp density, which, if coupled with rapid radial growth, could lead to a higher incidence of broken stones. Foliar and/or root applications of calcium nitrate and potassium silicate were applied to ‘Laetitia’ plums to determine whether the incidence of broken stones could be reduced by increasing the strength of the endocarp cell walls. However, no such effect was observed. Hence, neither calcium nor silicate treatments can be recommended for reducing broken stones in plums. / AFRIKAANSE OPSOMMING: Om Suid-Afrikaanse pruime na oorsese markte uit te voer moet vrugte aan streng kwaliteitstandaarde voldoen. Daar is ondermeer spesifikasies in verband met die teenwoordigheid van holtes en stukkies gebreekte pit binne die vrug. Indien meer as 10% van die vrugte in ʼn karton deur ernstige gebreekte pit of groot vrugholtes geaffekteer word, moet die vrugte as Klas 2 bemark word. Aangesien ʼn groot hoeveelheid van die pruime wat vir uitvoer bestem is, geaffekteer word deur gebreekte pit en gevolglik as Klas 2 bemark moet word, word aansienlike finansiële verliese gelei. Laer pryse word behaal vir Klas 2 vrugte, en dus het die teenwoordigheid van gebreekte pitte ʼn negatiewe effek op die wins wat deur die uitvoer van hierdie vrugte gegenereer kan word. Die hoofdoel van hierdie studie was om die ontwikkeling van gebreekte pit in Japannese pruime te ondersoek en om die groei-patrone van kultivars wat geneig is tot gebreekte pit te vergelyk met ʼn nie-sensitiewe kultivar. Gebreekte pit in ‘Laetitia’ is opgemerk sodra die pit begin verhard het. Met die aanvangs van pit- verharding is die gedeeltes van die pit wat nog nie volkome verhard het nie, nie sterk genoeg om die sterk trekkragte van die groeiende mesokarp te weerstaan nie en die pit word gevolglik uitmekaar- getrek. Regressie-analise het gewys dat lengtegroei van die vrugte, vars massa van die endo- en mesokarp, minimum boordtemperatuur en boord-nagtemperatuur, asook relatiewe humiditeit gedurende die vruggroei-periode moontlik gebruik kan word om die voorkoms van gebreekte pit by oes te voorspel. Verskille in die voorkoms van gebreekte pit is opgemerk tussen ‘Laetitia’, ‘Sapphire’ en ‘Songold’ pruime, en ook tussen seisoene. Verder is beduidende verskille opgemerk in die digtheid van die endokarp in verskillende dele van die pit. By beide ‘Laetitia’ en ‘Songold’ is gebreekte pit opgemerk wanneer vinnige toename in pit-digtheid saamgeval het met ʼn vinnige toename in vruggroei. Die pitte breek veral in die oorgang tussen dele van die pit met hoë en lae digtheid en as dit gekombineer is met vinnige radiale vruggroei in die rigting waar die pit die minste dig is. In teenstelling hiermee is gebreekte pit in ‘Sapphire’ opgemerk selfs voordat die pitte begin verhard het. Dit dui daarop dat die endokarp uitmekaargetrek is deur die vinnig groeiende mesokarp, omdat dit te sag was om die trekkragte van die groeiende vrugvlees te weerstaan. Die voorkoms van gebreekte pit word ook deur weerstoestande beïnvloed, want hoër temperature gedurende die pit-ontwikkeling en verhardingsperiode, kan lei tot die ontwikkeling van endokarpe met meer steenselle. Hierdie vrugte sal dus ʼn hoër digtheid hê, en as dit saamval met vinnige radiale groei, kan dit lei tot ʼn groter voorkoms van gebreekte pit. Blaar- en/of worteltoedienings van kalsiumnitraat en kaliumsilikaat is gemaak om te bepaal of die voorkoms van gebreekte pit in ‘Laetitia’ verminder kon word deur die versterking van die endokarp-selwande. Geen van hierdie behandelings het tot ʼn vermindering in gebreekte pit gelei nie en nie kalsiumnitraat of kaliumsilikaat kan dus aanbeveel word om gebreekte pit in pruime te verminder nie.