Seedlessness can be obtained through parthenocarpy (i.e., fruit formation without fertilization or embryo abortion). In practice, the actual reduction in seed number in parthenocarpic plants is often exaggerated by coupling parthenocarpy with self-incompatibility or male sterility. Traits related to seedlessness, such as parthenocarpy, can be introduced into genetic accessions through conventional cross-breeding. However, conventional breeding in Citrus L. species faces several limitations. First, these species have long juvenile (nonflowering) periods during which a relatively thick canopy develops, which limits the size of seedling populations that can be maintained for further evaluation. Second, they have a narrow genetic base, which limits the availability of alternative alleles that could be introgressed into other lines for the formation of a particular phenotype such as parthenocarpy. Third, breeding efforts are limited by the lack of knowledge of the mode of inheritance of specific characteristics. Fourth, and last, breeding efforts are limited by the polygenic nature of many important traits. Despite these limitations, conventional breeding in fruit trees has yielded improved cultivars and will most likely continue to be a very important strategy. However, emerging biotechnological approaches should be continuously evaluated for their potential for expediting such breeding efforts. The objective of this review is to present, evaluate, and discuss conventional and emerging biotechnological approaches for the induction and maintenance of seedlessness in a variety of crops. Particular attention will be paid to citrus crops, including the presentation and discussion of some preliminary data on the genetic inheritance of parthenocarpy.

Nucellar callus lines were established from two orange cultivars ('Nucellar Shamouti', 'Shamouti Landau'), three mandarin cultivars ('Murcott', 'Dancy', 'Ponkan') one grapefruit cultivar ('Duncan') and sour orange (Citrus aurantium). These callus lines were initiated from in vitro cultured ovules of young fruits and maintained an embryogenic capacity. The plating efficiencies of protoplasts derived from these calli, as well as those of protoplasts from lemon (cv. 'Villafranca') nucellar callus were differentially affected by the maceration enzymes and by the sugars used as osmotic stabilizers. Plants with normal morphological features were regenerated from cultured protoplasts derived from each of the nucellar callus lines. The establishment of eight new protoplast systems in Citrus paves the way for cell genetics studies and for novel breeding approaches in these economically important orchard trees.

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A physical plastome map was constructed for Citrus aurantium, and the plastomes of species and cultivars of Citrus and of two Citrus relatives were analysed by Southern blot-hybridisation of labelled total tobacco cpDNA to digests of total Citrus DNA. A resemblance was found between the plastomes of cultivars of C. limon (lemon), C. sinensis (orange), C. aurantium (sour orange), C. paradisii (grapefruit) and C. grandis (pomello). The plastomes of other Citrus types such as mandarin (C. reticulata) and citron (C. medico) differed from each other as well as from the plastomes of the aforementioned group. The plastomes of Poncirus trifoliata and Microcitrus sp. are distinct from each other as well as from the Citrus types.

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