ROOT INITIATION AND ACCLIMATIZATION OF PAPAYA PLANTS

Papaya (Carica papaya L.) is a major tropical fruit crop worldwide with an annual production of 6 million tonnes. Although it is a model species in terms of application of biotechnology to tropical fruit species, in vitro culture is problematic. Reliable and repeatable protocols for initiation and growth of good quality root systems in vitro and acclimatization of large numbers of papaya plantlets after culture can be difficult to achieve. We micropropagate papaya by growth of axillary shoots from single node sections which have been dissected from apically dominant plants. Axillary shoots, 5 to 10 mm in length are subsequently cultured on medium containing 10 占IBA for 3 days (for root induction) before transfer to growth regulator free medium. Exposure to auxin can be controlled by addition of riboflavin to the medium as it photo-oxidises auxin when exposed to light. Riboflavin can either be added to the medium or overlayed onto the medium. Although 100% rooting can be achieved with these methods, roots produced on solidified medium are thickened and have few lateral branches and no root hairs, making them difficult to acclimatize. Use of vermiculite moistened with mineral solutions, in place of solidified medium produced roots with improved quality and structure. Plantlets produced after cryopreservation of shoot tips are much more difficult to root and acclimatize as they have a tendency to produce more callus on the cut surface compared to non-cryopreserved controls. The use of vermiculite in place of solidified medium as a last in vitro stage has proved very beneficial for acclimatization of cryopreserved shoots.