This thesis describes experiments designed to improve microtuberization efficiency, and to evaluate a range of dormancy-breaking agents for microtubers and minitubers. Provision of continuous darkness, agitation to cultures, mechanical resistance to stolons, or lower levels of medium nitrogen did not improve microtuberization. The 16/8 h d/n cycle at step 1 of microtuberization was significantly better than 12/12 or 8/16 h d/n cycles based on microtuber yield. Cultures exposed to prolonged step 2 or 2 successive harvests, rather than a single harvest at 30--35 d, had significantly improved microtuber yield. In a series of chemical and mechanical treatments applied to microtubers and minitubers, with or without variable periods of cold storage, 500 mg l -1 GA3 was the most efficient in breaking-dormancy and inducing precocious sprouting. GA3 was the only agent that was able to break dormancy of minitubers that had not been cold stored. After 2 weeks of cold storage, minitubers treated with GA3 also broke dormancy, while Signal was less effective in promoting sprouting. (Abstract shortened by UMI.)

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In vitro culture techniques are widely used in potato for rapid production of specific pathogen tested plantlets and microtubers, which are essential for seed certification programs. The limited calcium (Ca) uptake in vitro promotes physiological disorders in many micropropagated species including potato. Factors that may affect Ca uptake by potato plantlets, including cations, osmotic potential, and boron (B) were examined. The possibility of improving microtuber yield by low pH shock or successive harvests, which stimulated greenhouse potato production, were also investigated. Ca uptake by cvs. Norland and Bintje potato plantlets grown on modified Murashige and Skoog (MS, 1962) media was enhanced by diluting the total macroelements in the nutrient medium, except Ca. Increasing Ca:specific cation ratios, by lowering either NH 4, Mg or K, had little effect on Ca uptake by Norland plantlets. At equivalent Ca:total cation, Ca uptake was promoted further when the total macroelement concentration was reduced by half than when the Ca level was doubled (from 3 to 6 mM). This suggested that the increase in the osmotic potential of the medium had a great impact on Ca uptake, probably by affecting the root pressure. B contamination from impurities in the nutrient chemical components always occurred. The addition of the recommended H3BO 3 level of 0.1 mM (1.08 ppm) or more (0.3 mM) to the medium decreased Ca content of leaves and shoots in Norland but not Bintje, while addition of only 25% of this level was sufficient to provide plantlets with adequate B tissue concentration without compromising normal growth. Macroelement dilution along with Ca addition, and lower B levels than commonly used in MS (0.1 mM) are recommended for optimal Ca uptake in potato and possibly other plant species. Microtuber yield was not improved by a low pH shock. Adjusting microtuberization medium pH to less than 5.7 (3.5, 4.3 or 5.2), prior to autoclaving, reduced microtuber number and/or weight

A protocol is presented for the rapid (28 days) induction of microtubers on micropropagated layered potato plantlets of 'Kennebec', 'Russet Burbank' and 'Superior' in medium devoid of growth regulators. With this method the addition of coumarin, 6-(2-chloroethyl)-trimethylammonium chloride and 6-benzylamino-purine to the microtuberization medium either had no effect or significantly reduced microtuber weight per plantlet. Increasing the incubation period from 28 to 56 days significantly increased the weight of microtubers per plantlet and the proportion of microtubers heavier than 1 gram. Increasing the volume of microtuberization medium from 50 to 100 ml significantly increased the number of microtubers per plantlet. Microtuber dormancy periods were cultivar-specific and microtubers $ le$250 mg had longer dormancy periods as compared to microtubers $>$250 mg. A positive correlation was established between endogenous abscisic acid levels and microtuber dormancy periods. Microtubers $ le$250 mg had lower specific gravity, fewer eyes and produce fewer sprouts than microtubers $>$250 mg. Microtuber-derived plants were generally single-stemmed. Severe physiological ageing treatment ($>$2500 degree-days) had no effect on microtuber sprout development, stem number, tuber number and only minimally influenced tuber weight of microtuber-derived plants. Decreasing field in-row planting density from 30 to 10 cm reduced tuber weights and numbers per plant but increased them on a per hectare basis. Economic analysis indicated that optimum planting density varied depending on plantlet cost. The optimum planting density was 10 cm if the cost of the plantlet was $0.10 or less, 20 if plantlet cost were from $0.10 and $0.20 and 30 cm for plantlet cost greater than $0.20. A potato seed tuber certification program adapted to the needs and constraints of Egypt is presented.

Various aspects of Ca2+ uptake into greenhouse-grown potato plants and micropropagated potato plantlets were examined, including the movement of Ca2+ into potato plants and tubers, identification of Ca-packing genotypes and assessment of the effect of improved medium Ca2+ level on different parameters such as plantlet growth, microtuber induction, yield and tissue Ca concentration. The effect of exogenous vitamin Ds and UV-elicited endogenous vitamin D synthesis on regulation of 45Ca 2+ uptake into plantlet shoots was also examined. Safranin dye was not as suitable as the tracer 45Ca2+ for monitoring translocation of Ca2+ into tubers. Uptake of Ca2+ into tubers occurred indirectly through the main basal roots and stolon roots and directly through tuber periderm. Liquid scintillation counting and flame atomic absorption spectrophotometry were used to screen six potato cultivars and two wild species for ability to take up Ca2+ from treatment solutions containing high or low Ca2+ levels. In vitro potato micropropagation, microtuberization, and tissue calcium concentration were compared for six cultivars when Murashige-Skoog basal medium Ca2+ level was increased from 3, to 5 or 15 mM. All aspects of growth including shoot dry weight, early microtuber induction, microtuber yield and tissue Ca concentration were improved when medium Ca2+ level was 15 mM. Cultivar Bintje was the most efficient genotype at accumulating Ca 2+ from treatment solutions or growth media containing high or low Ca2+ levels. Vitamin Ds improved 45Ca 2+ uptake into shoots of micropropagated potato plantlets and D 3 was more efficient in this regard than D2 or a combination of D3 and D2. Plantlets had increased 45Ca 2+ uptake when exposed to UV irradiation compared with the untreated control plantlets. Plantlets given a 24 hour interval in the dark after UV exposure had even greater 45Ca2+ uptake, suggesting that vitamin D, metabolites, specifically calcitriol (1, 25(OH)2-v

Micropropagated potato (Solanum tuberosum L.) plantlets are routinely used for producing specific pathogen tested minitubers in the Canadian seed tuber certification industry. In vitro hardening methods for micropropagated 'Norland' were investigated, involving full and half strength Murashige and Skoog (1962) based propagation medium supplemented with NaCl, KCl, CaCl2, polyethyleneglycol, or paclobutrazol. Ten different media treatments were compared for their effect on stomatal function and early transplant performance using porometry, microscopy, and direct ex vitro transplanting. PEG, NaCl and 1/2 MS + 3 mM Ca treatments did lead to decreased leaf water losses. However, these treatments did not improve ex vitro transplant performance compared to controls. Minituber production was investigated using ex vitro plantlets in a rockwool-based hydroponic system. Productivity was evaluated for treatments involving photoperiod pretreatment, planting orientation, planting density, and hilling. No difference in total yield was detected when plantlets were exposed to 12 compared with 16 hour photoperiod pretreatments. However, short photoperiod pretreatment increased the number of minitubers in the most desirable size range. Increased planting density reduced yield per plant. However, small increases in yield per m2 occurred with increased planting density. Hilling, pinching, and planting orientation had no effect upon minituber size, number, or overall fresh weight yield. Significant differences in minituber yield occurred in field experiments. Larger minitubers (10--40 g) had larger yields compared with smaller minitubers (1--5 g). These results will contribute to improved minituber production technology for the Canadian certified potato seed tuber industry.

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