No description available.

Being the third most cultivated crop in South Africa, potatoes are of great economic importance. As potatoes originated from cooler areas in the world, they do not easily adapt to South African conditions. The main objective of potato breeding is, therefore, to extend the crop's limited genetic base. Progress in crop improvement is slow due to dominance, segregation and other factors caused by the tetraploid character of cultivated potatoes. A new breeding program for rapid progress has been initiated at the Vegetable and Ornamental Plant Institute, Roodeplaat, South Africa, which comprises the combination of conventional and unconventional breeding techniques. The program is based on the reduction of the ploidy level from the tetraploid to the dihaploid level to facilitate crossings with diploid wild species. Anther culture is the preferred technique for the rapid reduction of the ploidy level and has been successfully applied on different members of the Solanaceae. Cultivated potato, Solanum tuberosum is, however, an important exception. In this study various potato genotypes (tetraploid cultivars, dihaploid breeding lines and a diploid wild species) were used in experiments concerning microtechniques, alternative culture methods and medium manipulation. The main objectives were to evaluate and compare the androgenetic ability of the various genotypes used and to try and identify the factors limiting their in vitro response. Regarding microtechnique, the study focussed on the investigation of the frequency of androgenesis - as a function of plant age - and the determination of defined flower bud lenqths representative of the correct microspore developmental stage for optimal androgenetic response. Combined with an extensive histological study on the microspore development within anthers, from the time of flower selection, after a cold-pretreatment and at various time-intervals during the culture period of 42 days, the following conclusions were reached: In vitro androgenetic response proved optimal when flowers of responsive genotypes were selected during the first seven to 21 days of the flowering period. Both microspore derived embryoid- and callus development were visible within responsive anthers after a culture period of only seven days. The flower bud length required for anthers to be in the optimal stage of microspore development, e.g. the uninucleate stage, varied between the different genotypes but could readily be determined with the DAPI (4,6-diamidino-2- phenylindole) technique. It was also concluded that anthers of the tetraploid cultivar Atzimba should be selected later, between the late-uninucleate and the early-binucleate developmental stages. This suggested a limited selection period for Atzimba anthers, as starch depositioning - which prevent embryogenesis - occurs within anthers during the binucleate stage. Histologically, Atzimba showed limited embryoid development with no embryoid release, while the diploid wild species, S. canasense, proved androgenetically unresponsive. Alternative culture methods were applied to study the effect of different culture phases (liquid, double layered and agar solidified) and anther orientations (lateral, dorsal and ventral) on the androgenetic response of the potato genotypes used. Liquid cultures, based on the so-called shed-pollen technique, enhanced the androgenetic response of the tetraploid cultivar Atzimba. Optimal embryogenesis was obtained for responsive breeding line 87.2002/3 with the utilization of agar solidified media, with maximal response when anthers were cultured in the lateral orientation. No response was observed from S. canasense. The effect of medium manipulation on the androgenetic response of the three genotypes was investigated. The utilization of various combinations of different concentrations of indole-3-acetic acid (1M) and benzyladenine (BA), the alteration of the initial time of incubation of anthers on the initiation media and the use of media without growth regulators compared to that containing gibberellic acid (GA[3]), were investigated. BA had to be present in the initiation media and had a major, though not exclusive, effect on embryogenesis compared to 1M. The optimal BA concentration varied between the two trials. IAA also had an increasing effect on anther response, both in the absence of BA and, especially, in addition with relatively high BA concentrations. In this experiment, only breeding line 87.2002/3 responded. The initial culture of anthers, during the first seven to 21 days of the culture period, on media containing growth regulators proved essential for microspore derived embryoid production in the tetraploid cultivar Atzimba. As these growth regulators are metabolized in the culture media, the regular transfer at shorter, two-weekly intervals to media containing metabolically active substances, proved important. GA[3] had no enhancing-effect on embryogenesis in any of the three tetraploid cultivars. The results obtained in this study suggest that the first 21 days is the critical stage in the anther culture period in terms of the optimal time for flower selection, embryoid induction and the increase in embryogenetic response due to growth regulator influence. It is important to pre-determine the developmental stage when most microspores were in the uninucleate stage of development and to correlate this stage with a specific flower bud length. This would assure maximum response of those genotypes amenable to anther culture. It also implies a more practical and economical starting pOint to anther culture experiments. Following the determination of microspore developmental stage and pollen fertility, flowers should be selected from the donor plants only during the first three weeks of the flowering period. The composition of the nutrient media used for potato anther cultures were sufficient with respect to growth regulators. The growth regulators SA, IAA and the amines glutamine and asparagine had to be present in the initiation media, especially during the first three weeks of the culture period. As microspore development within anyone anther was found to be asynchronous, the regular transfer of anthers to fresh media is recommended to assure proper development of all microspores. The use of a slightly higher IAA concentration could be considered, but care should be taken as too-high concentrations would induce callus production. Microspore derived embryoid production is preferred, as the ploidy level of callus derived plantlets normally varies and somaclonal variation can occur. Liquid media should be considered for anther culture of tetraploid genotypes, while embryoid production can be increased by culturing the anthers of responsive genotypes on agar solidified media on the lateral orientation. Finally, the diploid wild species S. canasense seemed androgenetically unresponsive, or the media and culture conditions used did not satisfy the specific requirements of this genotype. Androgenetic amenability should first be transferred by means of interspecific crossings with a responsive dihaploid genotype, such as the breeding line 87.2002/3. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1995.

Potato leafroll virus (PLRV) is one of the most destructive potato viruses in South Africa. In order to establish resistance against PLRV in commercial potato cultivars, the coat protein (CP) gene of the virus was previously isolated, cloned and subcloned into the plant expression vector pBI121 in both the sense and antisense orientations (BURGER, unpublished results). The pBI121 constructs containing the PLRV-CP gene were subsequently transferred to Agrobacterium tumefaciens LBA 4404 in a triparental mating process with the helper plasmid pRK2013. Two A. tumefaciens- mediated transformation methods for potatoes were investigated, viz. vacuum infiltration and leaf disk transformation. In addition, optimal transformation and regeneration conditions were identified for potato cultivars Late Harvest and BP[1] In total, 27 transgenic potato lines containing the PLRV-CP, β-glucoronidase (GUS) and nptII (neomycin phosphotransferase II) trans genes were generated under kanamycin selection. Transgenic plants grown in the glasshouse appeared to be phenotypically normal, and no differences in ploidy level in comparison to non-transformed plants could be established. Stable transgene insertion into the genome of the transgenic plants was verified using PCR and Southern blot analysis. Expression of the GUS transgene was investigated using a fluorometric assay (JEFFERSON et al. 1987), and it was found that orientation of the inserted PLRV-CP gene upstream from the GUS gene had a direct influence on the levels of GUS expression. The expression of the PLRV-CP gene was analysed using DAS-ELISA and immunoblot detection. Coat protein could not be detected in either assay. RNA dot blots were used successfully to show PLRV-CP expression in transgenic potato plants at the mRNA level. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1995.

This research was carried out to identify salt tolerant potato genotypes in vitro among 131 tetraploid potato cultivars (Solanum tuberosum), 9 diploid simple hybrid clones (4 clones of S. chacoense $ times$ S. tuberosum, 4 clones of S. phureja/S. stenotomum $ times$ S. tuberosum, and 1 clone of S. tuberosum $ times$ S. tuberosum), 1 primitive cultivated diploid S. phureja/S. stenotomum accession, 12 tetraploid complex hybrids, and 13 diploid S. chacoense accessions. Four levels of NaCl (0, 40, 80, and 120 mM) were used. The cultivars, and the simple and complex hybrids were tested for salt tolerance at the vegetative stage in the nodal cutting bioassay. The thirteen S. chacoense accessions were tested for salt tolerance at the germination and early seedling growth stage, in a seedling bioassay. Eleven of these S. chacoense accessions were further tested at the vegetative stage, in the nodal cutting bioassay. There was a progressive decline in the morphological parameters measured, with increased salt levels, in the nodal cutting bioassay. The parameters were used collectively in ranking the different genotypes, averaged over three NaCl levels (40, 80, and 120 mM). Twenty potato cultivars, two clones of the simple hybrid S. chacoense $ times$ S. tuberosum, and one complex hybrid were all considered salt tolerant at the vegetative stage. Ranking of seven S. chacoense accessions was similar between early seedling growth and later vegetative stage. Two of these accessions were promising as sources of salt tolerance.

The effect of salinity on phosphorus (P) absorption kinetics by 'Russet Burbank' cv. potato propagated by tissue culture technique was studied in the laboratory. Nine clones of the cultivar were evaluated for their salt tolerance at the vegetative stage by using in vitro culture. Four concentrations of NaCl (0, 40, 80, and 120 mM) were used in the experiments. The first experiment was carried out to study the effect of salinity on P absorption by using radiophosphorus ($ sp{32}$P) as a marker. P absorption showed a quadratic relationship with time. In general, there was a significant reduction in P absorption as the NaCl concentration increased in the nutrient medium. However, the root tip concentration of $ sp{32}$P increased with increased salt concentration. When P concentration in the nutrient medium was increased, $ sp{32}$P activity was also increased with time at 120 mM salt concentration. The activity of radiophosphorus decreased on root tips when P concentration in the nutrient medium was increased to 2.5 mM P. The second experiment was conducted in tissue culture using nodal cutting bioassay. The plant morphological parameters of shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, and root dry weight were reduced for all nine clones as salinity increased. The clone PEI#1 (tu) exhibited high salt tolerance characteristics.

This study investigates respiration rate of potatoes (Solanum tuberosum L.) under real and experimental storage conditions. Real storage conditions were obtained by building a small-scale storage facility equipped with two independent storage bins including all controls. Each bin was filled with 2.5 metric tons of potatoes and these were stored for five months. Temperature, relative humidity, air flowrate and other operational parameters were collected. Also, mass loss and CO2 analyses were performed. / A heat and moisture balance was applied to quantify heat and moisture rates produced by potatoes and were converted to respiration rates as CO 2 produced (ml.kg-1.h-1). Evaporation rates were also determined (g.kg-1.h -1). The balance was mainly applied on data collected from specific periods in which steady conditions were imposed; heaters and humidifiers were turned off and fans were operated to circulate air at a given flowrate for more than 8 h/day over a two month period. Respiration rates were determined as rates of heat produced (W) with stability and acceptable accuracy for a relatively large facility. Respiration rates were converted to CO2 production (ml.kg-1.h-1), and these agreed fairly with ranges reported in the literature. Simultaneously, respiration rates were measured by in-store CO2 analysis, and later they were measured under laboratory conditions using a closed gas analysis system. Comparing these with rates obtained by the heat and moisture balance, the later were slightly higher. Mass losses were also obtained by both weight losses and moisture balance; results from the two methods were quite similar and comparable with those reported in the literature. / Under experimental conditions, respiration rates were measured for healthy, diseased and sprouted tubers using a gas analysis method. This experiment demonstrated the need for a more extensive investigation of disease development and its relation to respiration rates, and the need for a faster and more convenient gas analysis system. (Abstract shortened by UMI.)

The effects of early season and late season drought stress on various carbohydrates, calcium and/or nitrogen content at apical, central and basal tuber locations were studied for Russet Burbank and A082260-8 potatoes. Drought stress which occurred early in the stage of tuber development appeared to have more detrimental effect than later season stress. Interactions between treatment and sampling date and variety x position x date were significant during early season stress. Generally, percent total solids increased during potato development for both varieties. The central portion of tuber had the lowest total solids. Total reducing sugar content generally decreased during potato development for both varieties with significant (P<.05) differences at early season stress due to the interaction effects of treatment x date and variety x position. No significant difference in reducing sugar at later season stress was found regardless of treatment. Variety difference in reducing sugar content occurred at the apical end. Russet Burbank had more reducing sugars than A082260-8 at this end. The exploration of fructose, glucose and sucrose individually showed the same developing pattern as total reducing sugar. Sucrose made up over 50% of the total sugars with glucose and fructose the next in order of importance. The apical end had more sucrose and glucose than the basal end. The interaction of variety and position for both nitrogen and dietary fiber may be a result of growth pattern differences in the two treatments. Total dietary fiber content was generally the highest at the basal portion for both varieties. Russet Burbank appeared to have higher total dietary fiber than A082260-8 at this end. The level of calcium in Russet Burbank was significantly higher than that in A082260-8. Basal and apical portions had higher calcium content than the central portion. / Graduation date: 1990

Starch residue samples from two Australian sweet potato varieties (Beauregard and Northern Star) and two Indonesian sweet potato varieties (Bis192 and Bis183), and commercial sample of sweet potato starch residue, were studied for their pectins. Pectins were extracted using 0.1M HCl, 0.05M NaOH, 0.1M HCl/0.75%SHMP, and 0.05M NaOH/0.75% SHMP. Hydrolysis of residual starch in the cell wall of sweet potato using heat stable α-amylase and amyloglucosidase was employed prior to pectin extraction to eliminate starch contamination. Pectins were characterised for yield, moisture, ash, galacturonic acid content (GA), degree of esterification (DE), neutral sugars content and molecular weight (MW). Selected pectins were also characterised for their dispersion and gel properties. Pectin with the highest yield was compared to commercial low methoxyl pectin (GRINSTED??YF 450) for its application in milk pudding. The pectin characteristics are dependent on variety and extraction process; however, the extraction methods were dominant. Yields were between 7 and 30 % of the cell wall. Ash varied from 2.9 to 6.8%. NaOH/SHMP extraction gave higher yields and ash contents. Moisture was between 5.5 and 6.7. GA varied from 27 to 80% with the highest found in Bis192 extracted using NaOH/SHMP. DE varied between traceable and 57%. HCl extraction gave higher DE, while NaOH/SHMP caused demethylation. Neutral sugars varied from 10 to 25% where galactose was the predominant sugar, followed by arabinose, rhamnose, glucose and xylose. Pectin MW was very heterogeneous. HCl extraction gave higher MW than the others. Dispersion of sweet potato pectins displayed shear-thinning. Consistency coefficient (K) increased, while flow behaviour index (n) decreased with increasing pectin concentration. HCl-extracted pectins formed stiff sugar-gels at concentrations higher than 2%. The optimum Ca2+ concentration for NaOH/SHMP-extracted pectin gels was 20 mg/g pectin. Pectin with the highest yield and stiffness (Norther Star extracted using NaOH containing SHMP) was used for milk pudding preparation. This pudding had comparable texture to pudding prepared from commercial low methoxyl citrus pectin (GRINSTED??YF 450). Overall, this study demonstrated that pectin from sweet potato starch residue is mainly low methoxyl pectin with possible chemical structure of rhamnogalacturonan I (RGI) that has potential for food applications.

Thesis (Ph. D.)--Illinois State University, 1969. / Title from title page screen, viewed Aug. 24, 2004. Dissertation Committee: Robert M. Chasson, M.J. Nadakavukaren (co-chairs), Arthur D. Bond, John L. Frehn, David F. Weber. Includes bibliographical references (leaves 106-114). Also available in print.

Thesis (M.S.) in Plant, Soil and Environmental Science--University of Maine, 2000. / Includes vita. Advisory Committee: Gregory A. Porter, Assoc. Prof. of Agronomy, Advisor; Timothy Griffin, Extension Sustainable Agriculture Specialist and Assoc. Prof., Sustainable Cropping Systems; C. Wayne Honeycutt, Lead Scientist, USDA-ARS New England Plan, Soil and Water Laboratory. Includes bibliographical references (leaves 88-96).