No abstract

No abstract

An improved embryo-rescue protocol was developed for embryos (90 days old) of Carica papaya L. (Clone 2001), and subsequently was utilised for efficient production of interspecific hybrids of C. papaya × C. cauliflora Jacq. from 90- to 120-day-old embryos. Pre-incubation of C. papaya embryos for 7 days on a germination medium containing half-strength De Fossard nutrients supplemented with gibberellic acid (10 μM), 6-benzylamino-purine (0.25 μM), alpha-naphthalene-acetic acid (0.25 μM). sucrose (58 mM) and agar (8 g L-1) supported 100% germination. Subsequent transfer of germinated embryos to a nutrient medium that was identical, except that it was free of plant growth regulator, allowed good growth but induced shoot etiolation and callus production. Reducing the pre-incubation of C. papaya embryos on this medium to 5 days before transfer to the medium free of plant growth regulator produced similarly high germination (96%), but allowed for the production of good quality seedlings that were unetiolated and free of unwanted callus. For interspecific hybrids, a 5-day pre-incubation of the embryos on a liquid formulation was better than the solid formulation as it promoted better growth and vigour of the normally abortive interspecific hybrid embryos. Using the improved protocol, 1981 of 2100 (94%) interspecific hybrid embryos consisting of single and multiple forms were germinated. In all cases, the germinating multiple embryos underwent further embryogenesis that allowed for the production of 485 (25%) morphologically normal hybrid plants grown in soil in the glasshouse.

Papaya ringspot virus type P (PRSV-P) is a major threat to the papaya industry worldwide. F1 hybrids have been produced when Carica papaya L. female flowers have been pollinated with pollen of the PRSV-P resistant species Vasconcellea quercifolia. F1 plant production required embryo rescue 90 days post-pollination, and plantlet regeneration in vitro. Three hundred F1 hybrids were grown to maturity in the field and had morphological characteristics that were identical to 1 or both parents, were intermediate between those of the parents, or were greater than either parent. They had a sex ratio of 2 (male) : 49 (hermaphrodite) : 49(female). Eighteen plants (7 male and 11 hermaphrodite) produced some viable pollen. Pollen viability of these plants as measured by germination on agar medium varied between 1.1 and 6.1% (mean of 3.37 ± 0.18) compared with >90% for C. papaya. Cytogenetic studies showed limited homology between C. papaya and V. quercifolia genomes. The intergeneric hybrids varied in their reaction to strains of PRSV-P in Australia and the Philippines. Of the hybrids, 75% were resistant to PRSV-P while 25% produced virus symptoms.

Carica papaya, C. cauliflora and interspecific hybrids of these species were screened for resistance to two Australian isolates (338, 445) of papaya ringspot virus‐type P (PRSV‐P). Plants were manually inoculated with PRSV‐P in the glasshouse and the reaction assessed 30 days later by back‐inoculation to susceptible Cucurbita pepo and by a plate‐trapped antigen‐enzyme linked immunosorbent assay (PTA‐ELISA). Both parents and interspecific hybrids were also planted adjacent to infected C. papaya and 30 days later tested for PRSV‐P by PTA‐ELISA. All interspecific hybrid and C. cauliflora plants manually inoculated in the glasshouse or planted in the field failed to become infected, whereas C. papaya plants, in both situations, were infected by PRSV‐P. In addition, the surviving interspecific hybrid and C. cauliflora plants tested negative, while all C. papaya plants were positive for PRSV‐P in both the back‐inoculation and PTA‐ELISA tests. Thus, the interspecific hybrid and C. cauliflora plants were resistant to the Australian PRSV‐P isolates.