Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Potatoes is the fourth most important food source in the world. To provide in the ever increasing demand by the growing world population for this staple food, alternative ways will have to be found to produce a cheaper and a healthier end product. The tubers mainly consist of carbohydrates and starch, although 9% of the dry weight of the tubers consist of proteins. In its natural state the potato tuber is low in sodium and high in potassium and vitamin C. These proteins can provide in all the amino-acid requirements of humans and make it an outstanding balanced diet. The most important factor by which the production and quality of potatoes can be increased, is to provide better propagation material to producers. Minituber production is a relatively new method of producing better quality propagation material for producers. It is, however, very expensive because of the high capital investment needed. By producing more tubers per plant, or more tubers per square meter, the unit price per tuber may be lowered. The system commonly used for minituber production is where in vitro plants are planted in a greenhouse, usually in a vermiculite/peat mixture as medium. As in a hydroponic system, nutrients are added to the irrigation water. As possible strategy to increase the production of mini-tubers, the effect of nutrition was investigated. No significant differences were found in tuber production with small deviations in phosphorous and calcium nutrition. Researchers who reported significant differences in tuber production with nutrition, either corrected extreme shortages or induced it. By subjecting the root and tuber medium of potato plants to a pH-shock (nutrient solution with a pH of 3.5 for 10 hours), large amounts of tubers were initiated. A new production technique was developed where plants were grown in a hydroponic way without any root medium. In this way the right sized tubers could be removed without damaging the plant. Potato plants do not initiate tubers easily where stolons are not subjected to mechanical resistance. This problem was solved with the help of a pHshock, where significantly more tubers were formed per plant. The action of the pH-shock could be explained by a temporary inaccessibility of nutrients to the plant. Both calcium and magnesium absorpsion are hampered at a low pH, but calcium is also poorly translocated in the plant. Considering that a nutrient solution without calcium produced the same results as the pH-shock treatment in this study, one may make the assumption that an induced calcium shortage may be responsible for the low pH effect. The same effect may even be induced by giving the plants clean water for some time. Such a treatment was, however, not tested but should receive attention in further studies. The correct timing for a low pH-shock may need more research. A precise number of days after transplanting can not be prescribed since many factors such as light intensity, daylength and cultivar difference affect plant growth and tuber initiation. Environmental conditions, the physiological age and the biomass of the plant may affect the timing of the shock. A low pH-shock could not at any stage induce tubers in potato plants where unfavorable environmental conditions occurred. It seems as if the plants need to be ready for tuber initiation before the pH-shock can be successfully applied. By removing/harvesting the tubers at the required weight, the burden on the plant may be relieved enabling more tubers to form. It was experienced that the reproductive system of the potato plant is complex with a number of factors affecting it. The new production system has potential, but more developmental work is still needed. / AFRIKAANSE OPSOMMING: Aartappels is die vierde belangrikste voedelsoort in die wereld, Om te kan voorsien aan die ewigdurende verhoogde vraag na hierdie stapelvoedel deur die groeiende wereld-bevolking, sal daar maniere gevind moet word om die eindproduk goedkoper en in 'n gesonder toestand op die mark te plaas. Die knolle bestaan hoofsaaklik uit koolhidrate en stysel, alhoewel 9% van die droe gewig van knolle uit proteiene bestaan. Die aartappelknol is in sy natuurlike toestand laag in natrium en hoog in kalium en vitamien C. Hierdie protei'ene kan in al die aminosuur-behoeftes van mense voorsien wat dit dus 'n uitstekende gebalanseerde dieet maak. Die belangrikste faktor waarmee die produksie en kwaliteit van aartappels verhoog kan word is om beter voortplantingsmateriaal aan produsente te verskaf. Miniknolproduksie is 'n relatief nuwe manier om gesonde en groeikragtige voortplantingsmateriaal vinnig by die produsente te kry. Dit is egter 'n duur metode omdat die kapitaal-investering groot is. Deur meer knoUe per plant, of meer knolle per vierkante meter te produseer kan die eenheidsprys per moer verlaag word. Die sisteem wat algemeen vir miniknolproduksie gebruik word is om in vitro plantjies in 'n kweekhuis, gewoonlik in 'n vermikulietldennebas mengsel te plant. Voedingstowwe word dan soos in 'n hidrokultuurstelsel saam met die besproeiingswater toegedien. As moontlike strategie om die produksie van miniknolle by aartappels te verhoog, is die invloed van voeding daarop ondersoek. Daar is gevind dat daar nie groot verskille in knolproduksie met klein afwykings in fosfaat- en kalsium-voeding veroorsaak is nie. Navorsers wat wel betekenisvolle verskille III knolproduksie met voedingsbehandelings verkry het, het ekstreme tekorte reggestel of gemduseer. Deur die wortel- en knolmedium van aartappelplante aan 'n pH-skok (voedingsoplossing met 'n pH van 3.5 vir 10 ure) bloot te stel, is daar gevind dat 'n groot hoeveelheid knolle geinisieer word. Verder is 'n nuwe produksietegniek ontwikkel waar die plante met 'n suiwer waterkultuur metode geproduseer word, sodat knolle wat oesgereed is verwyder kan word. Aartappelplante inisieer nie maklik knolle onder hierdie omstandighede waar stolons geen meganiese weerstand ervaar nie. Met behulp van die pH-skok het daar betekenisvol meer knolle per plant gevorm. Die werking van die pH-skok kan moontlik aan 'n tydelike ontoeganklikheid van voedingstowwe toegeskryf word. Beide kalsium en magnesium word moeilik by 'n lae pH opgeneem maar kalsium word boonop moeilik deur die plant getranslokeer. Aangesien 'n voedingsoplossing sonder enige kalsium dieselfde resultate as die pHskok gelewer het, lyk dit asof 'n geinduseerde kalsium-gebrek vir die lae pH effek verantwoordelik mag wees. Die skok kan makliker en veiliger toegepas word deur vir 'n tydperk net skoon water aan die plante te gee. So 'n behandeling is egter nie beproef nie maar behoort in opvolgwerk aandag te ontvang. Die optimum tyd waarop so 'n skok toegedien moet word is nog onduidelik. Daar kan nie op 'n eksakte aantal dae na uitplant gewerk word nie, aangesien daar te veel faktore soos ligintensieteit, daglengte en kultivar is wat plante se groei en knolinisiasie kan beinvloed, Omgewingstoestande, die fisiologiese ouderdom van die plant en die hoeveelheid biomassa wat gevorm het mag bepaal wanneer die plante geskok moet word. Waar ongunstige omgewingstoestande geheers het, kon 'n lae-pHskok op geen stadium knolinisiasie induseer nie. Dit wil voorkom asof die plant vir 'n knolinisiasie fase gereed moet wees voordat 'n lae-pH-skok doeltreffend is. Dit verhoog dan bloot die aantal knolle wat per plant ontwikkel. Deur die knolle op die verlangde grootte te pluk kan die las op die plante verlig word sodat meer knolle mag vorm. Daar is ervaar dat die reproduktiewe stelsel van die aartappelplant kompleks is en dat daar baie faktore is wat dit mag beinvloed. Die nuwe produksiestelsel toon potensiaal, alhoewel verdere ontwikkelingswerk nog nodig is.

Potato (Solanum tuberosum L.) cultivars Atlantic, BelRus, Kennebec, Superior, and Yukon Gold, grown at six locations in Southwest Virginia, were analyzed for yield, percent dry weight, specific gravity, soluble protein, and ascorbic acid. Thereafter. tubers were cold stored at 3 C for six weeks, reconditioned at 25 C for two weeks, and analyzed for percent dry weight, specific gravity, soluble protein, and ascorbic acid. Potato grown in one location was analyzed for glucose, fructose, sucrose, and total sugars. Another experiment was conducted to evaluate the effect of seed type and in-row spacing on yield and quality of Yukon Gold. Cultivar and location influenced yield and quality of potato at harvest, after cold storage, and following reconditioning. Kennebec, Atlantic, and Yukon Gold produced higher yield of US No. 1 tubers than Superior and BelRus. Atlantic and BelRus had higher percent dry weight and specific gravity than the other cultivars. Specific gravity increased during cold storage and following reconditioning. Percent dry weight increased during reconditioning but did not change during cold storage. / Master of Science

Lack of effective storage facilities to mitigate post harvest losses threatens the profitability of organic farming. In rural KwaZulu-Natal, small scale farmers use traditional storage and sequential harvesting to keep potatoes post maturity while waiting to sell. The effect of these practices on potato quality has not been studied and documented. This study set out to investigate if traditional practices (sequential harvesting and traditional farmer’s store) of the Ezemvelo Farmers Organisation in Umbumbulu affect the quality and marketable quantity of organically grown potatoes. Specifically the study set to investigate the effect of traditional farmer’s store, in situ and controlled storage on the carbohydrate content and sensory quality of potatoes organically grown in Embo by EFO farmers; determine consumer quality expectations of organically and conventionally grown potatoes; investigate the effect of sequential harvesting on the potato quality expectations of consumers and to investigate the produce and income losses experienced by small scale organic farmers at harvest and during storage. Experiments were conducted to compare the effect of sequential harvesting, farmer’s store and controlled cold storage (7oC and 90% relative humidity) on the sensory, appearance and keeping quality of organically grown potatoes. A survey of 100 consumers (40 organic and 60 conventional consumers) was conducted to ascertain consumer appearance and keeping quality expectations of potatoes. In addition, a survey of 101 farmers investigated the storage practices of the EFO farmers who grew potatoes and the type of losses incurred in storage. Preference ranking was used to investigate if time of harvesting post maturity affected potato quality. Produce losses experienced by the farmers were quantified. A co-research group of three seasoned farmers of the EFO participated in the research. They produced potatoes used in the study and provided valuable input to ensure that the study adhered to storage practices of the farmers. The lowest and highest sugar levels were observed in potatoes stored in situ and under controlled conditions, respectively. Potatoes left in situ also recorded higher starch content. Potatoes stored in situ were significantly preferred by sensory panellists (p<0.05) over those stored in both farmer’s store and in controlled storage. Preference rank scores were negatively correlated to total sugar content and positively correlated to starch content. Consumers in the study highlighted five desirable appearance qualities in potatoes: absence of greening, absence of sprouting, smooth skin texture, absence of blemishes and light skin colour. No significant differences in the quality expectations between participating organic and conventional potato consumers were found. The majority of consumers expected potatoes to store for at least three weeks post purchase. Sequentially harvested potatoes met this expectation when potatoes were left in situ for a maximum of six weeks post maturity. Potatoes in situ also maintained good appearance and sensory quality. The highest produce losses were experienced in summer owing to soft rot problems. Production in the drier seasons (autumn and winter) increased the proportions of potatoes too small to be sold as table potatoes. With the exception of completely rotten potatoes, poor quality potatoes were consumed, used as seed potatoes and sold to the local market as seed and for food. Poor potato quality resulted in reduced income for the farmers. This investigation pioneered research into the effect of sequential harvesting on the quality of organically grown potatoes. The findings demonstrate that sequential harvesting provides resource-poor small scale organic farmers with an efficient storage option where other storage methods and technologies may be inappropriate, ineffective or unaffordable. It is recommended that government and other players in the agricultural sector plan initiatives to educate small scale potato farmers on the benefits of sequential harvesting as an effective short term method of potato storage. Research with other potato cultivars in different agro-ecological settings is needed to optimise sequential harvesting. Government policy aimed at training and developing farmer capacity in organic seed potatoes production is essential to ensure that farmers access disease and pest free seed. Farmers also need assistance to access to irrigation resources to improve production. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.