Somatic cell selection with thaxtomin A as a positive selection agent was used to isolate variants of potato cv. Russet Burbank with strong to extreme resistance to common scab. Glasshouse and field trials identified 51 variants with significantly reduced disease incidence (frequency of infected tubers) and severity (tuber lesion coverage) compared with the parent cultivar. The most promising variants exhibited extreme disease resistance, rarely showing lesions, which were invariably superficial and shallower than those on the parent. Resistance traits were consistently expressed both in 10 glasshouse and two field trials at different locations, with varied inoculum and disease pressure. Disease-resistant variants differed in their response to thaxtomin A in tuber slice bioassays. Of 23 variants tested, 10 showed reduced thaxtomin A susceptibility, with the remaining 13 responding similar to that of the parent. Thus, toxin tolerance was not the only factor responsible for observed disease resistance; however, four of the five most disease-resistant variants had enhanced thaxtomin A tolerance, suggesting that this factor is important in the expression of strong disease resistance. Pathogenicity and toxin tolerance remained stable over a 6-year period, demonstrating that selected phenotypes were robust and genetic changes stable. The majority of disease-resistant variants had tuber yields equivalent to the parent cultivar in glasshouse trials. This suggests that selection for disease resistance was not associated with negative tuber attributes and that certain variants may have commercial merit, worthy of further agronomic testing.

A somatic cell selection approach, developed using thaxtomin A as a positive selection agent, recovered 113 potato cell colonies of cv. Iwa from which 39 regenerated plants within a single subculture period. In glasshouse pathogenicity trials, 13 of these regenerated variants exhibited significantly greater resistance to common scab disease than the unselected parent cultivar, both in mean tuber surface coverage with lesions and the proportion of disease free tubers. The best variant had on average an 85-86% lower disease score (or 91-92% less estimated tuber surface cover) than the unselected parent cultivar. The disease-resistant variants varied in their response to thaxtomin A in detached leaflet and tuber slice bioassays. This suggested that the cell selection procedure and thaxtomin A screening had limited success in selecting for toxin tolerance, and that thaxtomin A tolerance was not necessarily the driving factor for induction of disease resistance observed. Variation in relative necrosis scores between leaflet and tuber assays for individual variants possibly suggests differential expression of thaxtomin A tolerance in the different tissues. Yield estimates from the glasshouse grown potatoes showed the majority of disease-resistant variants had tuber yields equivalent to the unselected parent cultivar. This suggests that selection for disease resistance was not associated with negative tuber yield attributes and that these variants may have commercial merit and are worthy of further agronomic testing.

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Successful flower bud initiation and development was achieved on Boronia megastigma in vitro. The effect of cytokinin on flowering was investigated in environmental conditions that promote flowering as well as under conditions that stimulate vegetative growth (nonfloral promotory conditions). Flower initiation and differentiation was enhanced by cytokinin; however, many flower buds reverted when the media contained high levels of cytokinin. Anthesis occurred only on media that had no cytokinin added and under floral promotory conditions. Boronia megastigma Nees. is a woody understorey shrub 1-2 m in height, bearing a profusion of strongly scented brown, purple, and yellow flowers in early spring. They are initiated in autumn and continue to differentiate during the winter. Floral and vegetative buds are normally initiated from uncommitted primordia located in the axils of nonexpanding leaves, on current season's node-bearing structures, under a range of environmental conditions. More flowers initiate (up to three per leaf axil) under low night temperatures (5-7~ with short days (10 h) and 50-100% full sunlight (floral promotory conditions) (Roberts and Menary 1989a,b).Differentiation and development of preexisting flower buds in vivo is enhanced by exogenous cytokinin under floral promoting conditions. However, when applied under long days (16 h) and moderate night temperatures (> 15~ many flower buds revert to vegetative differentiation rather than abort (Roberts 1989). Successful reversion to vegetative growth depends partly on the stage of flower bud differentiation when the cytokinin is applied. A flowering gradient exists along most laterals where the more mature flowers are usually located in the axils of the fourth leaves below the apex; flower maturity decreases both acropetally and basipetally from this point (Roberts and Menary 1990). The presence of the flowering gradient makes it difficult to carry out investigations on buds that initiate at the same time and have the same degree of sink strength; however, this can be achieved in vitro. The aim of this experiment was to investigate flower bud initiation and development in vitro using different levels of cytokinin and two environments. Materials and Methods Preliminary ExperimentsGenerally for woody species, embryos or young inflorescences must be used to regenerate shoot types or organs in vitro; hence, the explant must be in to a juvenile state (Tran Thanh van 1981).For example, in Citrus Altmen and Goren (1974), Garcia-Luis et al. (1989), andTisserat et al. (1990) used explants that consisted of the whole bud, portions of bark and wood from the branchlet and the stump end of the petiole; Tsujikawa et al. (1990) used axillary leaf buds or growing shoot apices for successful flowering of Japanese pear. Trial experiments were carried out with Boronia using explants derived from a variety of sources, including: shoot proliferations, in vitro grown shoots, and current seasons' laterals. These preliminary experiments showed that flower buds could be initi...