Abstract
Seedlessness in fruit is a trait that is much sought after by juice making industries. Close to the city of Sao Sebastiao do Cai, in the state of Rio Grande do Sul (RS), Brazil, a new mutant orange originating from natural mutation was identified and selected as a seedless material. To determine the mechanisms involved in the absence of seeds, the reproductive structures of this new mutant by comparison with a Valencia sweet orange as control, a cultivar with a profusion of seeds, was analyzed in terms of meiotic behavior, meiotic index, pollen viability, in vitro germination, and ovule features to determine the grounds for seed absence. Other morphological analyzes allowed for visualizing the structures of normal appearance and size in both cultivars. Meiotic analysis identified chromosome normal pairing with a predominance of bivalents at diakinesis and metaphase 1. URS Campestre flowers at different developmental stages had anthers and ovaries whose dimensions are typical while pollen grain analysis pointed to a standard developmental pattern, normal meiosis, high viability (84 %) and elevated in vitro pollen tube germination rates (63 %). The cv. Valencia and URS Campestre ovules had a similar shape and morphology, sharing an anatropous orientation, and two integuments. In the internal ovule analyses of Valencia sweet oranges, normal embryo sac cells were identified: presence of one egg cell and two synergids, three antipodes and a bigger and central cell containing two polar nuclei. However, the analysis of ovules from URS Campestre reveals an apparent senescence or non-formation of an embryo sac, where only a few highly stained and collapsed cells could be identified. These results led to the conclusion that female sterility in URS Campestre, with a total absence of a female gametophyte, is the limiting factor for fertilization and seed production.
Highlights
A plant is seedless when it is not able to produce fruit with seeds, or produce the remains of aborted seeds or even fruit with very few seeds
To determine the mechanisms involved in the absence of seeds, the reproductive structures of this new mutant by comparison with a Valencia sweet orange as control, a cultivar with a profusion of seeds, was analyzed in terms of meiotic behavior, meiotic index, pollen viability, in vitro germination, and ovule features to determine the grounds for seed absence
Meiosis of microsporocyte The analyses of meiotic behavior of both cultivars indicate a high percentage of normal cells in bivalent associations at diakinesis and metaphase I, besides typical disjunction at anaphase and telophase I and II (Figure 2AC)
Summary
A plant is seedless when it is not able to produce fruit with seeds, or produce the remains of aborted seeds or even fruit with very few seeds. Absence of seeds in citrus fruit is a pleasing trait to fresh fruit consumers and a needed attribute for the juice industry (Hu et al, 2007; Ye at al., 2009). Ex Tan.), Washington Navels (Citrus sinensis Osb.) and Tahiti lime (Citrus latifolia Tanaka) are leading cultivars in this segment (Breto et al, 2001; Deng et al, 1996; Zhang et al, 2012). Most of these cultivars were obtained by way of selection of mutating buds or plantlets. Despite the availability of seedless cultivars, one of the key targets of citrus breeding programs consists of hitting upon new cultivars with distinguishing attributes in addition to seedlessness to supply growing consumer demands (Wu and Zhao, 2004; Zhang et al, 2012)
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