AN IMPROVED EMBRYO-RESCUE PROTOCOL FOR A CARICA INTERSPECIFIC HYBRID

An improved embryo-rescue protocol was developed for embryos (90 days old) of Carica papaya L. (Clone 2001), and subsequently was utilised f or efficient production of interspecific hybrids of C. papaya x C. cau liflora Jacq. from 90- to 120-day-old embryos. Pre-incubation of C. pa paya embryos for 7 days on a germination medium containing half-streng th De Fossard nutrients supplemented with gibberellic acid (10 mu M), 6-benzylamino-purine (0.25 mu M), alpha-naphthalene-acetic acid (0.25 mu M). 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, allo wed good growth but induced shoot etiolation and callus production. Re ducing the pre-incubation of C. papaya embryos on this medium to 5 day s before transfer to the medium free of plant growth regulator produce d similarly high germination (96%), but allowed for the production of good quality seedlings that were unetiolated and free of unwanted call us. For interspecific hybrids, a 5-day pre-incubation of the embryos o n a liquid formulation was better than the solid formulation as it pro moted better growth and vigour of the normally abortive interspecific hybrid embryos. Using the improved protocol, 1981 of 2100 (94%) inters pecific hybrid embryos consisting of single and multiple forms were ge rminated. In all cases, the germinating multiple embryos underwent fur ther embryogenesis that allowed for the production of 485 (25%) morpho logically normal hybrid plants grown in soil in the glasshouse.