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Thomas M. Gradziel and Mary Ann Thorpe

Somatic mutations in shoot growing points, while considered relatively common in many horticultural clones, cannot be detected unless the mutation results in a distinguishable change and comes to occupy sufficient shoot area to be observable. Noninfectious bud-failure (BF) in almond, a genetic mutation which results in failure during early development of vegetative but not flower buds, behaves as a chimera in its incidence within an affected tree and in vegetative progeny from bud-failure prone clonal sources (i.e., vegetative lineage). Early stages of BF development are thought to occur as very limited and so undetected sectorial or mericlinal chimeras. Detection of BF during these early stages would be very valuable for the selection of low-BF source clones for nursery increase. Flower symmetry, as measured by differences in the size of each of the five petals of an almond flower, was evaluated as an indicator of the relative fitness of the individual cell lineages from which different petals were derived. Several hundred flowers from individual clonal sources of the almond variety `Nonpareil', known to produce either very low, medium, or very high levels of BF in their vegetative progeny, were tested over 3 separate years. Significant reductions in flower symmetry were consistently observed for medium BF potential clonal sources relative to either low- or high-BF sources despite the lack of any observable BF symptoms in the medium-BF trees tested. Associated with asymmetric-flower-prone sources was a greater number of an easily distinguishable distorted petal morphology. Medium BF-potential sources consistently produced 2- to 3-fold greater numbers of this petal morphology relative to low BF-sources, although the occurrence of distorted petals in both low and medium BF sources limits its use as an efficient field selection tool. Research findings, however, are allowing a more detailed understanding of the developmental ontogeny of “bud-sport” mutations and may have application in the analysis of otherwise hidden chimeras resulting from either somatic mutations or genetic transformation/regeneration schemes.

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Thomas M. Gradziel, Mary Ann Thorpe and Diane M. Barrett

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Pedro Martinez-Gomez, Mary Ann Thorpe and Thomas M Gradziel

Micrografts have proven a very useful technique when the early propagation of plant material is desired either to invigorate weak material, multiply selected genotypes, or obtained virus-free shoots. This is the situation with the recovery of haploid almond embryos, which occur at low frequency with sexual embryos in twin seeds (i.e., multiple embryos within the same seedcoat). Often these haploid plants show weak growth due to their haploid condition and their poorly developed state within twin seeds. Very little information is presently available, however, concerning the effectiveness of different micrografting techniques for almond. In this work, we examine the success of in vivo micrografting of `Nonpareil' almond seedlings under different conditions. Variables included type of micro-scion, the rootstock genotype, and the growth stage of the rootstock. Microscions tested included small (3 mm) micro-wedges from either unsprouted or recently sprouted buds. Rootstocks evaluated included the `Hanson' (peach × almond) hybrid, and Nemared and Nemaguard peach rootstocks. Rootstocks were grafted after either ≈3 weeks of growth, when the tissue was still herbaceous, or after ≈3 months of growth, when the tissue had become woody. Results show significant differences between the treatments. Findings will be discussed both in terms of effectiveness of different approaches and the advantages and disadvantages of their use in breeding programs.