Six of 120 species of Annona have pomological significance. Cherimoya (A. cherimola Mill.), sugar apple (A. squamosa L.), and the hybrid between the two, atemoya, which has been assigned horticultural species status (A. atemoya Hort.), are the most popular ones (Batten, 1990; Peña et al., 2002). The other less important annona species producing edible fruits are bullock's heart (A. reticulata L.), Ilama (A. diversifolia Staff), and sour sop (A. muricata L.).
The common constraint limiting commercial production of annonaceous fruits is low flower to fruit ratio resulting from inadequate pollination (Gazit et al., 1982; Peña et al., 2002). Neither self- nor cross-pollination takes place effectively. Self-pollination is hindered by the presence of marked protogyny (Kahn et al., 1994), whereas cross-pollination by insects is limited because the flowers are not brightly colored and lack fragrance and nectar (Escobar et al., 1986; Peña et al., 2002). Because pollen grains occur in clumps and are sticky, wind pollination is not common (George et al., 1989). Moreover, annona pollen at dehiscence generally contains a good proportion of dyads and tetrads intermingled among individual viable and nonviable pollen grains (Saavedra, 1977).
To circumvent poor fruit set, either hand pollination (Escobar et al., 1986; Melo et al., 2004; Saavedra, 1977) or use of insect pollinators (Gazit et al., 1982; George et al., 1989; Peña et al., 2002) is recommended, but the latter gives erratic results and is not efficient when pollination from a specific pollen source is desired. Hand pollination improves fruit set and substantially boosts fruit yield per tree by increasing the fruit size and number, the major yield components (Pritchard and Edwards, 2006). However, it is labor-intensive and should be adopted in varieties that produce premium quality fruits by choosing an efficient pollen source. The influence of pollen on seed (xenia) and fruit (metaxenia) is well known in many crop species (Denney, 1992), including annona, in which Kahn et al. (1994) observed that fruit set, morphology, and quality are dictated by choice of pollen parent. The rewards will more than compensate if desirable xenic effects manifested at the Fo level, like heterosis manifested at the F1 level, is exploited.
‘Arka Sahan’, the annona variety on which different pollen species were tested, is an outstanding recombinant isolated from the family of ‘Island Gem’ (A. cherimola × A. squamosa) × ‘Mammoth’ (A. squamosa) and released for cultivation (Jalikop and Kumar, 2000). However, like other annonas, it yields fewer fruits with variable shape and size. Our hybridization program showed that most of the edible Annona spp. were crosscompatible (Kumar and Jalikop, 2000), and some interspecific crosses exhibited good set producing big, attractive Fo fruits. This motivated us to test allied annona species as pollenizers. The objective of the present research was to study the effects of Annona spp. pollen on the set, size, shape, and quality of ‘Arka Sahan’ fruits and to find the most suitable pollen source for hand pollinating this variety.
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