Cryopreservation of Papaya Germplasm
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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 0oC. 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.