True potato (Solanum tuberosum) seed (TPS) is used for preservation of variable genetic lines of wild and cultivated potatoes (Hawkes et al., 2000) and for propagation of food crops in some developing countries. TPS has advantages over seed potato tubers in storage and transportation and favors lower virus infection levels in fields. However, TPS has thermodormancy and will not readily germinate at 25°C and above (D'Antonio and McHale, 1988; Pallais, 1995a, b; Alvarado et al., 2000). TPS can be extremely unreliable when planted directly in fields due to poor emergence related to diseases and soil crusting. Germination tests were conducted with two lots of TPS derived from cvs. EB-8109 and All Blue, respectively, to study dormancy mechanisms. Seeds were germinated under four temperature regimes (10°C, 15°C, 20°C and 25°C). The two lots showed distinctly different germination characteristics. EB-8109 seeds showed only thermodormancy whereas All Blue seeds showed very deep dormancy. A carotenoid synthesis inhibitor, fluridone, which blocks abscisic acid (ABA) synthesis, effectively broke thermodormancy in EB-8109 TPS but did not break primary dormancy in All Blue seeds. Additional treatments, including pre-chilling and hormonal regimes, also failed to break All Blue deep dormancy. When the micropylar region of the endosperm (endosperm cap) was removed from seeds of both seed lots, radicle elongation was observed, suggesting that mechanical resistance from the endosperm cap restrains radicle protrusion, and that weakening of the endosperm cap is requisite for TPS germination. Endo-β-mannanase expression was measured to help characterize mechanism underlying the weakening of endosperm cap tissues. This enzyme is thought to permit radicle protrusion by degrading cell walls thereby weakening the tissues of the endosperm cap (Groot et al., 1988). The coding region of germination-specific mannanase was isolated from the potato genome by use of polymerase chain reaction (PCR) with primers specifically designed for the tomato germination-specific mannanase gene (LeMAN2, Nonogaki et al., 2000). The cDNA of the TPS mannanase was identical to that of LeMAN2. The expression of mannanase mRNA was detected in the endosperm cap of germinating TPS after 72 h of imbibition at 15°C, while no expression was detected at 25°C (thermodormant condition). Fluridone induced mannanase expression in the micropylar region of the endospem at 25°C. Thus, there was a correlation between induction of mannanase and dormancy breakage. A major increase in TPS post-germinative endo-β-mannanase activity was detected by use of gel diffusion assay. Two isoforms of mannanases were detected in the protein extracts of germinated TPS by activity staining of native polyacrylamide gel electrophoresis. The post-germinative mannanase was detected in the whole endosperm of germinated TPS by using tissue printing with the LeMAN1 (Bewley et al., 1997) RNA probe. These results suggest that, as with tomato, TPS also expresses post-germinative mannanase activity. The promoter region of a new tomato mannanase was isolated during this research. This promoter was shown to be involved in anther-specific expression of mannanase. / Graduation date: 2003

Thesis (MScAgric)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The potato can be considered as one of the most important food crops in many African countries. The potential of this crop is reflected in the large increase in area of production where Africa showed the highest rate of growth within the developing world over the past twenty years. The multiplication rate of potatoes is very low compared to other crops. Therefore it is essential to investigate methods of increasing the number of minitubers produced from disease free in-vitro plantlets. There is a number of potato propagation procedures that are currently being used worldwide to multiply seed potatoes. As aeroponics is still a relatively new technique that has not been researched extensively for the production of minitubers, a study relating to the production practises including the nutritional requirements of potato minitubers produced in a aeroponic system were undertaken. Potato plantlets, cv BP1, were grown aeroponically at two different densities (20 and 30 plants/m2) and four harvest intervals (7, 10, 14 and 18 days). The interaction between harvesting intervals and plant densities did not influence plant growth, minituber quality or yield. Best results were realised when harvesting every 7 days with a higher total tuber number over the growing period. Harvest interval also influenced the phosphorus and copper concentration in minitubers. To study the effect of Calcium (Ca) application rate, potato plantlets of cultivars Up-to-date, Mnandi, Buffelspoort and BP1 were grown at four different Ca levels (8.40, 6.75, 5.10 and 3.45 meq/L). The interaction between Ca application levels and cultivars significantly influenced the percentage stolon branching. BP1 had more stolons at the lowest Ca application level and Buffelspoort had more stolons at the full Ca application levels. However, low Ca treatments produced the highest yield. The minituber number and weight harvested were three times more for Mnandi. An aeroponic study on the irrigation frequency (20, 30, 40 and 50 minutes interval) was conducted on four potato cultivars (Up-to-date, Mnandi, Buffelsoort and BP1). Significant differences were noted in the interaction between irrigation frequencies and cultivars for the percentage tuberised plants and stolon and tuber dry mass. When irrigated every 40 minutes, 48% of the Buffelspoort plants produced tubers. Plant height was also significantly affected by the interaction between irrigation frequencies and potato cultivars, with Mnandi producing taller plants when irrigated every 30 minutes. Total tuber number and tuber fresh and dry weight was higher at the irrigation frequency of 20 minutes. The interaction between irrigation frequencies and cultivars on the response to macro and trace elements was not significant for sodium and iron but was for phosphorus, potassium, calcium, zinc and aluminium. A field study was conducted in a greenhouse where potato seed of BP1 obtained from the first trial were graded into different sizes (Small: >20, medium: 20–40 and large: > 40 mm of diameter) and stored at 3 different temperatures (3, 16 and 25oC) for 2 supplementary months before being planted. Sprouting capacity was mostly influenced by temperature regardless of other factors applied to potato seed minitubers such as harvest intervals and sizes. The higher storage temperature of 25oC resulted in tubers with a higher number of sprouts, longer sprouts and with a higher sprouting capacity. / AFRIKAANSE OPSOMMING: Aartappels is een van die belangrikste voedselgewasse in baie lande in Afrika. Die potensiaal van die gewas word gereflekteer in die groot toename in produksie areas, met Afrika wat die vinnigste van al die ontwikkelende lande gegroei het die laaste 20. In vergelyking met ander gewasse is die tempo van planvermeerdering by aartappels baie stadig. Dit is dus essensieel om metodes te ondersoek wat sal help om die aantal miniknolle wat per in vitro plantjie verkry kan word te verhoog. Daar is heelparty plant vermeerderings prosedures wat tans wêreldwyd gebruik word om saad aartappels te vermeerder. Aangesien aeroponika nog steeds ʼn relatiewe nuwe tegniek is wat nog nie ekstensief ondersoek is vir die verbouing van miniknolle nie, is ‘n studie geloods om te kyk na die produksie praktyke, wat insluit die voedingsbehoeftes van aartappel miniknolle in ʼn aeroponika sisteem. Aartappel plantjies, kultivar, BP1, is aeroponies verbou by twee plant digthede (20 en 30 plante/m2) en vier oesintervalle (7, 10, 14 en 18 dae). Die interaksie tussen oesintervalle en plantdigtheid het geen effek gehad op plant groei, miniknol kwaliteit of opbrengs nie. Die beste resultate is verkry waar die knolle elke 7 dae geoes is met ‘n hoër totale aantal knolle oor die groeiseisoen. Die oesinterval het ook ‘n effek gehad op die fosfaat en koper konsentrasie van die miniknolle. Om die effek van die Kalsium (Ca) toedieningspeil te ondersoek is aartappel plantjies; kultivars Up-to-date, Mnandi, Buffelspoort en BP1 gekweek by vier verskillende Ca peile (8.40, 6.75, 5.10 en 3.45 meq/L). Die interaksie tussen Ca toedienings peile en kultivars het ‘n beduidende effek gehad op die persentasie stolon vertakking. BP1 het meer stolons gehad by die laagste Ca toedieningspeil en Buffelspoort het meer stolons gehad by die volle Ca toedieningspeil. Die hoogste opbrengste is egter waargeneem by die laagste Ca toedieningspeil. Die aantal miniknolle en oes massa was drie keer meer vir Mnandi. ‘n Aeroponiese studie op die besproeiingsfrekwensie (20, 30, 40 en 50 minuut intervalle) is gedoen met vier aartappel kultivars (Up-to-date, Mnandi, Buffelsoort en BP1). Beduidende verskille is opgemerk in die interaksie tussen besproeiings frekwensie en kultivars vir die persentasie plante met knolle en stolon en knol droë massa. Met besproeiings elke 40 minute het 48% van die Buffelspoort plante knolle produseer. Plant hoogte is ook beduidend beïnvloed deur die interaksie tussen besproeiingsfrekwensie en aartappel kultivar met Mnandi plante wat hoër was wanneer dit elke 30 minute besproei is. Die totale aantal knolle en knol vars- en droë massa was hoër wanneer daar elke 20 minute besproei is. Die interaksie tussen besproeiings frekwensie en kultivars op die makro- en mikro element inhoud van die knolle was nie beduidend vir natrium en yster nie, maar wel vir fosfaat, kalium, kalsium, sink en aluminium. ‘n Potproef is gedoen in ‘n kweekhuis waar aartappel saad van BP1 verkry vanaf die eerste proef nadat knolle verdeel is in verskillende grootte klasse (klein: < 20mm, medium: 20-40mm en groot: >40mm) en gestoor is by drie verskillende temperature (3, 16 en 25oC) vir 2 addisionele maande voor plant. Spruit ontwikkelings kapasiteit was meestal beïnvloed deur temperatuur ten spyte van ander behandelings soos oes intervalle en knol grootte. Die hoër bergings temperatuur 25oC het aanleiding gegee tot knolle met ‘n hoër aantal spruite, langer spruite en ‘n hoër spruit ontwikkelings kapasiteit.

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