The clone is one of the basic categories of cultivar (4) with extremely important applications in horticulture. “Cloning” can be defined as the vegetative regeneration of a single genotype as represented by a single plant, single growing point, single meristem, or single explant. More recently this term has been expanded to cover regeneration of a fragment of a chromosome or gene. Cloning is a powerful procedure both as a plant selection tool for breeding and as a plant propagation tool for reproduction. This paper focuses attention on the opportunities and problems in propagation as it relates to horticulture.
Effect of genotype on variation in chilling requirement among almond seed populations was studied. Pollen of different varieties used on the same seed tree changed embryo genotype and, subsequently, the chilling requirements of the populations of seeds produced. There was a direct correlation between time of bloom of seed and pollen parents and the length of chilling required by their offspring seeds. Chilling requirements were not significantly different in seed populations resulting from reciprocal crosses involving varieties of long and short chilling. Such seed populations had comparable genotypes but were exposed to different maternal effects. Embryo genotype was a controlling factor in determining seed chilling requirement in almond. A distinction between systems affecting inheritance of time of bloom in almond was shown in that ‘Tardy Nonpareil’, a late-blooming bud mutation of ‘Nonpareil’, transmits later bloom to its seedling offspring but did not transmit longer chilling to the seeds.
Source of buds within individual trees was studied in relation to development of noninfectious bud-failure (BF) in almond. Buds were collected from upper and lower portions of phenotypically normal seedling trees of populations where BF potentiality existed. Comparison of propagated trees over a period of 7 years showed that differences existed in BF potential between upper and lower parts of the tree but not among buds from the same budstick. The differences between location suggested random changes in BF potential occurred within the bud source trees.
The term “clone” is a key biological term that designates a number of horticultural situations. In breeding, many cultivars are designated as clones, originating from consecutive vegetative propagation from individuals within a seedling population, from individual plants of a clone exhibiting “bud mutations,” and, more recently, from genetic engineering and biotechnology. Extensive vegetative propagation of a limited numbers of clones in modern horticultural systems has been accompanied by systemic incorporation by serious pathogens (viruses, viroids, phytoplasmas, etc.), and in some cases by horticultural deterioration (e.g., noninfectious bud-failure in almonds). Control of these problems in clonal propagation is achieved by 1) propagation source selection 2) maintenance of the source in a registered foundation block under protected conditions and 3)multipli-cation in controlled “mother blocks” or “increase blocks” from which commercial material is distributed after a minimum of consecutive generations of vegetative propagation. This system is the basis for Registration and Certification programs and “clean stock” in general. In many crops the selected propagation source is a single plant, its progeny constitutes a “clone,” and the new entity is given a unique name or number. To distinguish this “new” clone from the “original” clone, the designation of FOUNDATION CLONE is suggested. Biological and horticultural significance is illustrated in almond (Prunus dulcis).
A method for growing almond (Prunus amygdalus Batsch) and almond-peach hybrid shoots in vitro is described using dormant shoot buds collected from December to February and stored at 3°C. Shoot tips can be in cultured in 0.7 to 0.8% agar, modified Knop’s macroelement mineral solution, 2% sucrose, FeEDTA, microelements and organic supplements of Murashige and Skoog medium, 6-benzyladenine (BA), and light. At 0.1 mg/liter, BA produced shoot elongation; at 1 mg/liter, lateral shoot proliferation. Limited rooting has been obtained.
In the article “‘Hansen 2168’ and ‘Hansen536’: Two New Prunus Rootstock Clones” by Dale E. Kester and R.N. Asay (HortScience 21:331–332), Fig. 2 was printed upside-down. The correct orientation is shown below.
Two new, patented, vegetatively propagated Prunus rootstocks are being released for commercial use. These rootstocks are hybrids of an almond selection, Almond B, crossed with Peach Selection 1-8-2 by the late Carl J. Hansen. These rootstocks are being released primarily for use with almond but also can be used as rootstocks for peach, plum, and prune.
Almond (Prunus dulcis MIll) and peach (Prunus persica L.) are closely related species with many genetic traits in common. Variation in growth habit shows a consistent pattern among populations of peach, almond and their hybrid offspring. From this material a system of growth habit traits has been identified based upon genetically controlled processes of vegetative shoot elongation and flower bud initiation. All flowers are produced from lateral buds. The classification proposed for their characterization includes:
Class I. Growth from terminal buds on one year old shoots (six morphological groups),
Class II. Growth produced from lateral buds on 1-year old shoots (three morphological groups),
Class III. Combinations of Class I and II These classes cover the entire range of peach and almond phenotypes and probably all Prunus. Class I is precocious and produces flowers by the second year from growth initiation. Class II plants do not produce flowers until the third year. Expression is enhanced by increase in vigor.
Approximately twenty native almond species have been described. Representative germplasm from seven of these are present in UC collections and have been used in crossing. Three specific breeding lines utilizing these species are described. One (1980 series) involved increasing yield potential through selection of high blossom density following gene introgression from Prunus fenzliana. A second involved incorporation of self-fertility, late bloom, smaller tree size, early maturity, high blossom density, and desirable nut characters from Prunus webbii into commercial breeding lines. A self-fertile selection resembling `Nonpareil' has been obtained from this material. The third line involves transmission of a unique thin, netted-surfaced, hard-shell phenotype from Prunus argentea.
Breeding lines have been developed incorporating introgressed genes from three native almond species Prunus fenzliana, Prunus webbii and Prunus argentea. Selected traits include self-fertility and autogamy, late bloom, smaller tree size, early nut maturity, improved cropping potential, and a well-sealed shell (endocarp) with high kernel/shell crack-out percentages. Fertility barriers, while present were easily overcome though linkage to introgressed genes with undesirable phenotypes remains an important obstacle to commercial use. Current breeding results, however, support a general conclusion that the wide diversity present within the range of species related to the cultivated almond (Prunus dulcis) provides an valuable gene pool for variety improvement.