Effects of plant growth regulators on post-cryopreservation recovery and respiratory rate of Green Kangaroo Paw shoot tips

Plant growth regulators can have a significant impact on regeneration and regrowth of shoot tips after cryopreservation. This study investigated four cytokinins, two auxins and gibberellic acid at three different concentrations to assess their effect on the quality of regenerating shoot tips of the cryotolerant species Green Kangaroo Paw (Anigozanthos viridis) over a 400 h (approximately 2 weeks) recovery period. Shoot tip weight, length and respiratory rate were used as the key metrics for shoot tip performance throughout the recovery period. The control treatment with no plant growth regulator (PGRs) exhibited 58.1% regeneration, with a maximum regeneration of 79% observed with the 1 µM gibberellic acid treatment and minimum regeneration of 0% when 25 µM kinetin was applied. Higher concentrations of the PGRs used generally resulted in decreased regeneration and poor regrowth of the shoot tips. The use of zeatin and gibberellic acid at 1 µM in the recovery medium did not improve regeneration but did cause the shoots to grow significantly larger and longer, with a greater final respiration rate. The respiratory rate of regenerating and non-viable shoot tips remained very low until 120–168 h (5–7 d) post-cryopreservation, after which the regenerating shoot tips started to show significant increases in respiratory rate compared to the non-viable shoot tips. These findings suggest that the role of PGRs on increasing oxygen consumption is mostly due to their effects on promoting cell proliferation and tissue growth. Use of plant growth regulators during the early recovery phase can bolster metabolic activity, resulting in increased cryo-survival and regrowth outcomes.