ABSTRACT
A potato breeding population was evaluated for the presence of tolerance and resistance to Verticillium dahliae. Clones were ranked as susceptible or moderately and highly resistant based on symptom expression, and a species-specific polymerase chain reaction (PCR)-based assay was used to determine the Verticillium biomass in the bases of stems. A clone was designated as tolerant if the amount of fungus present in the host tissue was equal to or greater than the collective average amount for all clones in the symptom category above. Tolerant and resistant clones were identified in all trials, but the expression of tolerance was usually unstable across runs. The fungus was detected in some symptomless plants; on the other hand, some symptomatic plants were found to be pathogen-free. These observations emphasize the necessity of accurate quantification of the pathogen biomass in potato during Verticillium resistance breeding programs.
ABSTRACT
The potential of tuber disc culture for chromosome doubling was investigated in somaclonal populations of four dihaploid genotypes and one tetraploid cultivar of potato (Solanum tuberosum). Laser scanning confocal microscopy (LSCM) was used for rapid determination of the ploidy level based on the number of chloroplasts in stomatal guard cells of leaves. Factorial analysis of chloroplast number in 58 clones and two leaf types showed that somaclones were clearly divided in two groups. Clones with 5-7 chloroplasts per cell as observed in tuber derived diploid controls were classified as 2X (not doubled), while those with 9-14 chloroplasts resembled the tuber derived tetraploid controls and were considered 4X (doubled). A high frequency of spontaneous chromosome doubling, 42% - 50%, was detected in 3 dihaploid genotypes, whereas no doubling was observed in one of the dihaploids as well as the tetraploid cultivar Yukon Gold. Effects of leaf type on chloroplast number was also significant. The middle leaf showed significantly higher chloroplast number than the younger leaves.
