Cryopreservation of papaya germplasm

Shoot tips and seeds of papaya (Carica papaya L.) were successfully cryopreserved. Shoot tips were incubated for 1 to 6 days before vitrification and optimum treatment time was 1-4 days. Duration of exposure to vitrification solution was varied and 70% recovery was obtained from the shoot tips which had been exposed to 100% PVS2 for 20 minutes at 0 degrees C. Treatments for less than 20 minutes or more than 40 minutes resulted in no regeneration after liquid nitrogen (LN) treatment. Shoot tips closer to the apex had almost a two-fold rate of recovery compared with tips excised from the basal part of the shoot. Seeds of a southeast Queensland genotype were desiccated to moisture contents of 40%, 20%, 15%, 10%, and 5% (wet weight basis), followed by freezing in liquid nitrogen (LN). Germination rate of seeds that had been desiccated but not cryopreserved were consistently high (between 70% and 90%) across the range of moisture contents down to 5%. For seeds frozen in LN, the germination rate of seeds at 100%, 40%, 20% and 15% moisture content was 0%, 8%, 6%, and 8% respectively. A substantial increase in germination rate to 48% was achieved at 10% moisture content, declining to 20% at 5% moisture content for cryopreserved seeds. Within 10 weeks of transplanting to the field, growth rates of plants re-grown from cryopreserved seeds were significantly reduced (25%) when compared with the control plants. Substantially retarded growth was observed in one plant re-grown from a cryopreserved shoot tip, after 2.5 years in the field.