Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 193

Most papayas are grown from self-pollinated or hybrid seeds, although micropropagation and other clonal propagation methods are also used (Litz and Conover, 1978a; Reuveni et al., 1990; Drew, 1992; Drew and Vogler, 1993; Yu et al., 2000; Hidaka et al., 2008; Calleja et al., 2017; Estrella-Maldonado et al., 2017; Mastache, 2017). Micropropagation involved decontamination of selected papayas from the field using dilute commercial bleach and culturing on various media that would support shoot growth. The earliest experiments were conducted on field-grown shoots, but sometimes with seedlings. Murashige and Skoog (1962) medium (MS) was most often used, either full-strength or half-strength with low concentrations of 6-benzylaminopurine (BA) and naphthaleneacetic acid (NAA), e.g. 1 μM, (Table 6.1.3) to stimulate multiple shoot growth. Shoots were rooted on medium with 2.5–10 μM indole-3-butyric acid (IBA). Aeration using 0.02 μm filter disks in culture vessels enhances shoot proliferation (Lai et al., 1998) and aeration with vermiculite medium resulted in efficient root formation (Yu et al., 2000). Malaysian ‘Eksotika’ papaya formed thick, callus-like roots using published methods (Table 6.1.3) but IBA (12.3 mM) dipping followed by culture in water agar resulted in normal root morphology (Teo and Chan, 1994; Chan and Teo, 2002).

Table 6.1.3. Summary of papaya micropropagation media and results.

Micropropagated hermaphrodite ‘Rainbow’ were compared with thinned seedlings, rooted cuttings and single hermaphrodite seedlings (identified by PCR) for growth and yield (Fitch et al., 2005a,b). Cloned propagules produced ripe fruit 1–3 months earlier than thinned seedlings and had significantly higher early and cumulative yields. In Taiwan, Hainan and other tropical areas, micropropagation of papayas is efficient and inexpensive (S.D. Yeh, Taiwan, 2014, personal communication; J. Zhu, Hawaii, 2017, personal communication). However, in Hawaii, the protocol is still not efficient; cost per plant is c. US$ 10.

In subtropical regions, e.g. Florida, Queensland, Australia or Kagoshima, Japan, dioecious females are grown because they flower more consistently during the warmer and cooler months and can provide a consistent crop year-round (Storey, 1941, 1953; Arkle and Nakasone, 1984; Nakasone and Paull, 1998; Fukamachi and Kato, 2006; Hidaka, 2006). In the Ryukyu Islands, southern Kyushu to Okinawa, Japan, a low-bearing, heat-tolerant female papaya clone, ‘Ishigaki Sango’ was selected from seedlings of ‘Wonder Bright’ papaya (Fukamachi and Kato 2006; T. Hidaka, Japan, 2018, personal communication). A low-bearing habit was selected because papayas are grown in greenhouses to protect the plants from typhoons. ‘Ishigaki Sango’ (sango means coral) is female, and micropropagules are sold for c. US$ 12 (The Japan Agricultural News, 2015; Futaba, 2018; T. Hidaka, Japan, 2018, personal communication).

Depending on whether they are dioecious or hermaphrodites, papaya seeds segregate as females and hermaphrodites or as females and males, and seedlings are thinned in the field to the preferred sex type. Hybrids (F1) involving hermaphrodites show hybrid vigour (Chan, 2004) and segregate one female:one hermaphrodite when females are pollinated by hermaphrodites. Dioecious papayas segregate one male to one female. Therefore, the normal practice is to plant at least three to five seedlings and roguing at the time of flowering for the desired sex type. Inexpensive protocols to determine sex type by a DNA test using primers specific for hermaphrodites (Deputy et al., 2002) are being developed to assist planting by sex type.

Clonally propagated hermaphrodite ‘Rainbow’ plants produce ripe fruit earlier, lower on the stem and in the first year, and had significantly higher yields (P < 0.05) for two, three, and eight months in three different locations (Fitch et al., 2005a,b). Micropropagated plants had significantly earlier production compared to thinned seedlings and singly planted seedlings that had been identified by PCR.